QUANTIFYING SURFICIAL PROCESSES IN MALAKOFF DIGGINS

Transcription

QUANTIFYING SURFICIAL PROCESSES IN MALAKOFF DIGGINS,
A HISTORIC HYDRAULIC MINE
____________
A Thesis
Presented
to the Faculty of
California State University, Chico
____________
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
in
Environmental Science
____________
by
Keith Landrum
Summer 2014
A Thesis
by
Keith Landrum
Summer 2014
APPROVED BY THE DEAN OF GRADUATE STUDIES
AND VICE PROVOST FOR RESEARCH:
_________________________________
Eun K. Park, Ph.D.
APPROVED BY THE GRADUATE ADVISORY COMMITTEE:
______________________________
Sharon A. Barrios, Ph.D.
_________________________________
Carrie Monohan, Ph.D., Chair
Graduate Coordinator
_________________________________
David L. Brown, Ph.D.
ACKNOWLEDGMENTS
This project was successfully completed with the generous help and support
of many people. I would like to thank, the department chair, Dr. David L. Brown for
providing me with the opportunity to work on this project and for providing support
along the way from the beginning both in the field and in the office. Many thanks go to
my thesis committee members Dr. Carrie Monohan and Jenny Curtis for the research
initiative, and their help developing the project. I would like to thank my fellow graduate
students: Harihar Nepal, Susan Miller, David H. Demaree, Kathleen Berry-Garrett, and
Rebecca Bushway who helped me with their support and encouragement throughout.
Most importantly, I would like to thank Ashley Meese, for all her support, love and
patience throughout the journey of this project.
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TABLE OF CONTENTS
PAGE
Acknowledgments ......................................................................................................
iii
List of Tables..............................................................................................................
vi
List of Figures.............................................................................................................
vii
Abstract.......................................................................................................................
viii
CHAPTER
I.
Introduction ..............................................................................................
1
Significance ..................................................................................
4
Literature Review .....................................................................................
6
Geomorphic Context ....................................................................
Lithofacies Classification of Eocene Sediment ............................
Surficial Processes........................................................................
Mass Wasting ...............................................................................
Needle Ice and Dry Ravel.............................................................
Surface Wash Erosion ..................................................................
Overland Flow ..............................................................................
Deposition in the Pit Floor ...........................................................
8
9
10
11
12
13
14
15
III.
Objectives .................................................................................................
17
IV.
Methodology.............................................................................................
18
Erosion..........................................................................................
Deposition.....................................................................................
Sediment Budget (Discharge Calculation) ...................................
Hydrology.....................................................................................
18
26
32
33
II.
iv
CHAPTER
V.
PAGE
Results ......................................................................................................
34
Erosion..........................................................................................
Deposition.....................................................................................
Sediment Budget ..........................................................................
Hydrology.....................................................................................
34
43
46
46
Discussion.................................................................................................
50
Sediment Budget Discussion........................................................
Comparison of Discharge Estimate to Suspended
Sediment Load ......................................................................
Potential Bias and Uncertainty in Erosion and Deposition
Measurements...............................................................................
52
54
VII.
Conclusions ..............................................................................................
57
VIII.
Recommended Future Works ...................................................................
63
References Cited.........................................................................................................
66
VI.
v
53
LIST OF TABLES
TABLE
PAGE
1.
Erosion Bridge Plot Location, Slope and Aspect .....................................
22
2.
Location of Deposition Posts Within Pit Floor ........................................
28
3.
Erosion Bridge Plot Average Elevation Change, Location,
Slope and Aspect...............................................................................
38
Annual Volume Calculations from Erosion Plots and
Area Polygons ...................................................................................
42
Annual Deposition Volume Calculations.................................................
44
4.
5.
vi
LIST OF FIGURES
FIGURE
1.
PAGE
Location of Malakoff Diggins Within California (Left) and
Aerial View of Mining Pit (right) .....................................................
2
2.
February and March Freeze/Thaw Observed Impacts..............................
13
3.
Relative Soil Elevation Was Measured Using Erosion Bridge
Pictured in Situ..................................................................................
20
Location of Erosion Bridge Sampling Locations Within
Malakoff Diggins Mining Pit............................................................
23
Locations of Deposition Trail Markers (Blue and Green)
and Fence Posts (Red).......................................................................
27
Simplified Sediment Budget Calculation Used in This
Study to Quantify Annual Sediment Discharge................................
32
Digitized Outline of Pit Rim in 1952 and 2012 with Areas
Cliff Retreat Due to Landslides and Gully Headcutting
Represented in Yellow......................................................................
35
Mean Elevation Change Measured with Erosion Bridge
at Each Plot Location During 2012/2013 Water Year ......................
37
Erosional Areas Morphologically Similar to Conditions
at Erosion Plot Locations Used for Volumetric Calculations
of Surface Erosion.............................................................................
41
10.
Depositional Areas by Quadrant (North, South, East, West) ...................
44
11.
Precipitation Events by Incremental Magnitude Measured
at Camptonville Ranger Station ........................................................
47
Detail of Spring 2013 Precipitation Intensity and Duration
with Sampling Events at Erosion Plots.............................................
48
4.
5.
6.
7.
8.
9.
12.
vii
ABSTRACT
by
Keith Landrum
Master of Science in Environmental Science
California State University, Chico
Summer 2014
Malakoff Diggins State Park, California contains one of the largest historic
(1849-1910 gold rush) hydraulic gold mines in California. Chronic erosion within the
200’deep, mile long pit resulted in a State water quality 303(d) listing of Humbug Creek
for sediment, mercury, copper and zinc. In this study, quantification of annual erosion
and deposition was performed using a combination of GIS analysis, and direct field
measurements to create an estimate of annual sediment discharge.
Measurements of material lost due to cliff retreat were made over a 60 year
period using ArcGIS 10 by comparing aerial photographs from 1952 and 2012. During
the 60 year time span, 100,600 ± 25 m2 of pit rim surface area was lost to mass wasting
and gully erosion. Estimates of sediment volume associated with changes in pit rim area
was 10,900 ± 100 m3/ yr. Direct field measurements of elevation change from surface
wash were made with seven, 1-meter erosion bridges in representative areas of the pit.
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The average annual erosion rate measured from erosion bridges in 2012/2013 water year
was -15.4 ± 8.41mm/yr. The annual volume contributed by surface wash erosion was
estimated as 9,100 ± 50.5 m3/yr. Field measurements of the annual deposition rate were
made using sequential measurements of trail markers and t-posts placed in 2005 on trails
and in transects on the pit floor. Deposition was measured as 47.5 ± 4.5mm/yr, and the
estimate of annual volume of deposition throughout the pit was 12,700 ± 889 m3/yr.
Annual sediment yield discharged from the mine was estimated using a simplified
sediment budget, where cliff retreat erosion (10,900 ± 100) m3/ yr + surface wash erosion
(9,100 ± 50.5 m3/ yr) – deposition (12,700 ± 889 m3/ yr) = discharge (7,300 ± 1,039)
m3/yr. It is estimated that 7,300 m3/yr of sediment is discharged from the pit as suspended
and bed load.
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CHAPTER I
INTRODUCTION
Sediment and debris related to gold mining have affected the morphology and
water quality of streams and rivers of the Sierra Nevada foothills since the California
Gold Rush began in 1849. With the invention of hydraulic placer mining, the impact
became more widespread and intensive, and sediment contributions to streams increased
dramatically. Mining era sediment contributions to streams continue to be remobilized
from aggraded streambeds (James, 2005). Historic mines that were never remediated,
such as Malakoff Diggins have steep slopes of exposed weathered Eocene alluvium that
continue to erode at an accelerated rate compared to nearby forestlands, continuing the
legacy of gold mining related pollution to rivers from sediment and metals in the Sierra
Nevada foothills.
Malakoff Diggins State Park is located in northern California, north of
Sacramento, 28 miles north of Nevada City California, via Highway 49 and Tyler Foote
Road, or 14 miles by North Bloomfield Road (Figure 1). Adjacent to the historic mining
town of North Bloomfield is the historic placer gold mine pit created with hydraulic jets
of water during the Gold Rush.
At Malakoff Diggins, the North Bloomfield Mining Company exposed
approximately 1,600 acres of hillside using jets of pressurized water during the late
1800’s (Jackson, 1967). Mining of the auriferous Eocene alluvium left a large open pit
1
2
Figure 1. Location of Malakoff Diggins within California (left) and aerial view of
mining pit (right).
roughly 200 ft deep and a mile long in the gravelly clay deposit (Senter, 1987). The
washing away of auriferous gravels affected miles of downstream waterways with
hydraulic mining debris. Hydraulic mining debris inundated the Yuba River farmland and
properties of Marysville, and filled navigable waterways throughout the Bay-Delta
(Adler, 1980; James, 2005). The legal fight over property damage from this pollution led
to the regulation of hydraulic mining in California via the Sawyer decision in 1885
(James, 2005).
After the Sawyer decision, debris dams were created to trap sediment in the
pond in the pit before flowing through Hiller Tunnel and into Diggins Creek (Jackson,
3
1967). Previous studies identified the effectiveness of the sediment trap as being quite
low by 1979. Deposition of sediment in the pit raised the mine floor considerably since
the debris dam was constructed, and the pond was nearly full by 1980 (Peterson, 1980).
The streams flowing from the east side of the mine flow directly to the Hiller Tunnel
inlet, essentially bypassing the settling pond (Senter, 1987). The mining pit at Malakoff
Diggins continues to erode at an accelerated rate, contributing fine sediment to nearby
Humbug Creek and subsequently, the South Yuba River (Senter, 1987). Willows and
other vegetation have grown in the pit floor, trapping cobble, gravel, and sand sized
sediment (Peterson, 1980). The short residence time of waters within the pit floor and
pond combined with long settling times of weathered fines such as kaolinite results in the
discharge of highly turbid water to Humbug Creek via Hiller Tunnel during winter and
spring storms (NCRCD, 1979a; Nepal, 2013).
Auriferous gravels of the pit walls are composed primarily of quartz gravels
and weathered, fine clay as kaolinite, and to a lesser degree, smectite (Yeend, 1974). As
the steep walls of the pit erode, sand and larger sized sediments of the auriferous gravels
settle on the pit floor as alluvial deposits, while finer clays and silts are transported in
suspension to the pond, and through Hiller Tunnel to Diggins Creek, and its confluence
with Humbug Creek roughly 500m to the south. Clay bonds ionically to water and metals
allowing it to remain in suspension for very long periods. The high concentration of fine
particulates (1.5-62 µm) negatively affects aquatic organisms by clogging gills and
blanketing aquatic vegetation and benthic habitat (Krumbein, 1941; NCRCD, 1979a).
Fish health is particularly impaired by suspended solids as their food source and health
are directly impacted by sediment and siltation (Wilber & Clarke, 2001).
4
Fine clay particulates are known to bond with and transport positively charged
metals including mercury (Xu and Allard, 1991). Mercury transport is of special concern
in this location, since it was a hydraulic placer gold mine where mercury was extensively
used to trap gold in sluice boxes (Jackson, 1967). During mining operation, 1,147.5 lbs of
mercury was scattered on the gravel to be mined, and in the sluices before each run in
order to attract and amalgamate fine gold within the riffles of the sluice (Jackson, 1967).
The large quantity of suspended clays contained in the runoff from the mine continues to
threaten habitat, and species downstream, both physically and chemically (NCRCD,
1979a). The chemical weathering of the deposit over time has created mineral deposits of
naturally occurring metals such as iron, zinc, and copper. The metals within the regolith,
when exposed and disturbed by mining and erosion are free to oxidize within the pit and
the waters of Humbug Creek. The California State Water Resource Control Board
(SWRCB) has included Humbug Creek downstream from Malakoff Diggins on the 303d
list as impacted by sediment and siltation, mercury copper and zinc (SWRCB, 2002).
Significance
Quantifying the magnitude of the sediment budget is the first step necessary to
mitigating the sediment pollutant discharge from Malakoff Diggins. Quantification of the
annual sediment load discharged from the Malakoff Diggins Pit will inform the design
and engineering of remediation techniques to abate erosion and sediment transport. The
State Water Quality Control Board considers the discharge of high concentrations and
volumes of sediment to waterways a type of pollution due to its negative impact on
aquatic life. In addition, the property owner, Department of Parks and Recreation (DPR)
5
has interest in reducing the concentration of sediment discharge, yet needs to know where
to focus erosion control efforts in order to reduce the sediment delivered to Humbug
Creek without impacting the cultural and biological resources within the mining pit.
The California Department of Parks and Recreation mission statement is “to
provide for the health, inspiration, and education of the people of California by helping to
preserve the State’s extraordinary biological diversity, protecting its most valued natural
and cultural resources, and creating opportunities for high-quality outdoor recreation”
(California State Parks, 2013). Most traditional mine remediation techniques would not
be conducive with the mission statement of DPR, as it would likely entail large scale regrading of the cliff walls, thus destroying the historic landscape created by mining which
it is tasked with preserving. Therefore, erosion control efforts that focus on retaining
sediment before it reaches Humbug Creek rather than reducing primary erosion are likely
to be a better alternative for mitigating existing discharge.
CHAPTER II
LITERATURE REVIEW
Previous studies at Malakoff Diggins identified a range of erosion rates using
different techniques. Peterson (1980) utilized erosion plots consisting of dowels, and
analysis of aerial photographs to estimate an erosion rate of 16,000-35,000 m3/yr. During
the winter of 1979/1980, Peterson installed six erosion plots consisting of meter by meter
wooden dowel arrays located on eroding slopes throughout the pit which measured an
average elevation change of -2.6 cm/yr. Landslide contributions were estimated with
ground measurements in the winter of 1979/1980 for recent failures. Older landslides
were estimated using historical accounts (Jackson, 1967) as well as aerial photo analysis
between 1966 and 1980.
Yuan (1979) utilized photogrammetry techniques to measure the hillslope
degradation of slopes at 40-170mm/yr or 23,000 m3/yr. Historical terrestrial photos taken
in 1909, and 1953 were compared to photos taken in 1978 for analysis. The comparison
of historical to contemporary photos taken from the same relative locations utilized
permanent markers such as trees or boulders to measure the change in slope elevation,
assuming parallel slope retreat. The study identified 9 erosion rates providing a range,
with the average at 90mm/yr. Yuan found that erosion rates were decreasing over time, as
upper slopes eroded at a faster rate than lower slopes due to slump block failures, as
slopes approached the angle of repose. Yuan identified the cliff scarps at Malakoff
6
7
Diggins as being primarily within the upper unit of auriferous gravels described by
Yeend (1974), and as such contain a higher percentage of sand, silt and clay than the
lower unit which contains a larger percent of gravel and cobble. The fining upward trait
of the gravels combined with higher erosion rates at the upper reach of slopes leads to the
introduction of large amounts of fines to the system.
The Nevada County Resource Conservation District (NCRCD) produced four,
phased reports related to erosion and sedimentation problems at Malakoff Diggins State
Historic Park. The NCRCD used sediment concentrations in streams at Hiller tunnel,
Diggins Creek, and Humbug Creek to calculate suspended load based on flow for
individual storms. They noted that clay deposits in and around the pond may be
periodically re-eroded by intense runoff events.
The Phase I Progress Report reviewed policies relating to the NCRCD study,
natural resources and hydrology information for the study area and field survey
methodology. Phase I recommended that an in depth study of erosion was needed to
identify sources and create a management plan (NCRCD, 1978a)
The Phase II Progress Report discussed important physical features,
monitoring methodology and field data collected during Phase II. The study concluded
that water quality in Humbug Creek is severely degraded by large amounts of suspended
silt from Malakoff Diggins Creek during periods of high flow. As a result of the siltation,
the trout fishery of Humbug Creek is severely impacted by destruction of spawning
grounds, microhabitats, and possibly reduction of the available food supply (NCRCD,
1979a).
8
Phase III Progress Report encompassed three topics. A discussion of data
collected during phase II for water quality parameters, stream flow measurements,
chemical constituents in the water, particle size distribution in runoff and in bedload, and
precipitation for the area. The results identified a number of project alternatives necessary
to control sedimentation. The conclusion of report III states that
Tremendous sediment loads, largely in the form of colloidal material, are
transported from the Diggings area throughout winter. As a result, water quality is
affected downstream in Humbug Creek and the South Yuba River. Sedimentation is
a factor in the decline of fisheries in Humbug Creek. (NCRCD, 1979b, p. 35)
Phase IV involved a recommended best management practice to control
sedimentation from Malakoff Diggings, a cost analysis and an implementation schedule.
(NCRCD, 1980)
In 1987, the California Department of Water Resources (DWR) summarized
the findings of Peterson, Yuan, and the NCRCD and made recommendations to reduce
the sediment discharge to Humbug creek. These suggestions included: 1) de-watering of
failing slopes, and the routing of low turbidity waters entering the pit via an upland ditch
system; 2) increasing sediment trapping within the system with the installation of check
dams containing flocculant gel “logs”; and 3) raising the outlet structure to reduce scour
of deposits and increase residence time for settling. Presently, none of the actions to
reduce sediment have been implemented (NCRCD 1980; Senter, 1987).
Geomorphic Context
Throughout the San Juan Ridge where Malakoff Diggins is located, Eocene
ancestral Yuba river sediment deposits exist as auriferous gravels in a matrix of gravel
and partially developed sandstone and claystone (Whitney, 1880). The auriferous gravels
9
making up the walls of the pit are generally poorly consolidated; containing nearly equal
amounts of sandstone and conglomerate, with clay beds becoming more common upsection (Senter, 1987). These placer deposits of the mine contain primarily white quartz
gravels and conglomerates as well as sandstones silt and clay as kaolin (Yeend, 1974).
Much of the clays are initially transported as clayballs of various sizes which contribute
clay sized particles as they weather within the channels inside the pit (Wood, 1991). As
the cliffs erode, fine clay particulates are mobilized into overland flow as rills converge
into gullies. Coarse materials such as cobble gravel and sand are deposited on the pit
floor, where they are re-worked and transported towards the pond and outlet by surface
runoff. During this action of reworking, surface runoff remobilizes fine sediments,
holding them in suspension until they are deposited in the pond, or further downstream,
depending on particle size/residence time, and water velocity.
Lithofacies Classification of Eocene Sediment
Eocene auriferous sediments at Malakoff Diggins were originally deposited
by the Ancestral Yuba River, a laterally shifting river system in a depositional
environment (Selby, 1993). The aggrading stream increased in thickness as it formed
deltaic floodplains composed of finer grained sediments, leaving coarser materials in
deep channels and sand, silt, and clay in surrounding floodplains and oxbows during
formation (Whitney, 1880). Peterson observed bedding in cut slopes of the pit exposed by
mining activity which indicated that deposition occurred within a west-southwest flowing
river system. The deposits become generally finer up-section as bedrock and cobble give
way to larger amounts of gravel, sand and claybeds become more common. Colluvial
10
quaternary deposits above the auriferous bench gravels are apparent in the east end of the
pit, and are associated with landslides (Peterson, 1980). These colluvial deposits are
separated from the bench gravel stratigraphic section below by 2 clay beds, which act as a
slip layer for chronic landslides. Peterson noted that landslides from the east end of the
pit were an important source of clay and silt sized sediment within the pit as well as the
source of clayballs seen in gullies below (Peterson, 1980).
Identification within the pit walls of location and extent of stratigraphic
horizons (lithofacies) and their properties have been described in the pit, although not
completely or extensively (NCRCD, 1978; NCRCD, 1979a; Peterson 1980). To date, the
primary focus has been on the east slump area of the pit, which contributes high sediment
loads per unit area (Peterson, 1980). Peterson measured cohesion at 5 locations within the
pit with a soil penetrometer. The unconfined strength (cohesion) was found to vary
widely throughout the layers of the pit, from 0.5 to 4.5 kg/cm2 (Peterson, 1980).
Surficial Processes
The oversteepened walls or cliffs of the mine are prone to failure by mass
wasting and gully erosion which contribute colluvium and alluvium, respectively, to the
base of the slope. However, this does not mean that all of the material will reach the pit
floor or the outlet. Sediment deposited at mid and lower slopes is dissected and reworked
by surface flow. Rainsplash mobilizes soil particles downslope, and as slope length
increases individual rainsplash becomes sheet flow. As the flow becomes turbulent, rills
form, concentrating flow and increasing flow depth, and ultimately entrain larger material
as bedload. Some of the mass failures associated with small slumps at gully headscarps,
11
become supersaturated flows which move all of their material down through the gully
system, where subsequent rainfall moves the material to the pit floor.
All of the erosional processes within the pit operate in concert to transport
material from the upper slopes and deposit it on lower slopes. While all this erosion takes
place, water flowing from the slopes carries sand as bedload and silt and clay in
suspension toward the pond and Hiller Tunnel. Ultimately, erosion from overland flow is
dependent on soil cohesion, particle size, and velocity.
There are many erosional processes taking place within the pit, many of which
overlap spatially and temporally. The different processes will be described separately to
clarify how erosion takes place within the pit. Location of processes, extent of conditions
and properties of sediments leading to increased erosion are discussed below.
Mass Wasting
Along the northeast side of the pit, landslides of various sizes and age have
greatly contributed to erosion (Peterson, 1980). Recurring landslides dominate the
morphology in the east end of the pit. Older landslides are evidenced by large downslope
deposits which are now stable, vegetated by various stages of vegetative succession, and
are bisected by deep gullies. The complex landslides appear be related to clay interbeds at
3,250 ft. and associated localized visible seeps (Senter, 1987). The headscarps of the
landslides continue to migrate upslope, with their arcuous concave shape concentrating
flows, and delivering large amounts of sediment into gullies below. Gullies continue to
mobilize and transport material from the clay rich landslide deposits, and are a major
source of fines within the pit (NCRCD, 1979b). Landslide deposits in the east end of the
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pit are dissected by deep gullies. The walls of these gullies have multiple small failures
which fall into the main channel and are washed downstream. A large volume of
disturbed material in an area with high amounts of overland flow creates accelerated rates
of erosion (Selby, 1993). At the mouth of these gullies, on the edge of the pit floor, are
significant deposits of cobble and gravel fining downstream. Numerous clayballs of
cobble size are visible here. Peterson (1980) performed a point count of clasts in this area
and found that “35 to 45 percent of the cobble sized material was composed of clay
transported as clayball fragments” (p. 48).
Needle Ice and Dry Ravel
Needle Ice and subsequent dry ravel have been widely observed during winter
months within the walls of the pit. The growth of frost columns or needle ice lifts the
upper surface material by as much as 3 cm. As the ice melts, the lifted layer appears
“fluffed up” on the surface, allowing gravity to move the loose material downhill as dry
ravel (Peterson, 1980). The loose material fills gullies and deposits scree cones at the
base of hillsides, where it can be easily re-mobilized during rain events (Selby, 1993).
Peterson noted that this erosional process did not contribute substantial volumes of
material to overland flow within the pit (Peterson, 1980). However, observations in
2012/2013 show that material lost due to frost wedging and needle ice was temporarily
stored mid-slope within rills and gullies. This storage in the upper area of accumulation is
prone to remobilization by subsequent runoff events.
13
Figure 2. February and March freeze/thaw observed impacts. Dry ravel associated
with frost at Erosion Plot 4 flows down existing rills depositing in the gully below
(left). Frost wedging and needle ice at Erosion Plot 6 disaggregate gravel soils
leading to dry ravel upon melting (right).
Surface Wash Erosion
At Malakoff Diggins, rainsplash on unvegetated hillslopes mobilizes fines for
transport by overland flow. Peterson noted that the hortonian “belt of no erosion” at the
upper reach of slopes is non-existent within the pit (Horton, 1945). This may be due to
the extreme angle of the cliffs at the pit edge or a process of overland flow entering the
pit from above (Peterson, 1980). “Surface flow and rainsplash are the dominant erosion
processes” at Malakoff Diggins (Senter, 1987). “The coarse sediment of the pit walls is
particularly susceptible to erosion by rainsplash, while the clay interbeds are much more
cohesive” (Peterson, 1980, p. 27) but more susceptible to the production of sheetflow.
14
Rainsplash influences not only the upper reaches of the mine but all areas of the pit which
are not armored by organic matter, vegetation, rock, and cohesive layers.
Surface elevation changes represent erosion caused by processes that include
rainsplash, sheetwash, ice wedging, dry ravel, and rilling (Sirvent et al., 1997). Posts or
rods inserted into the ground have been used successfully at other locations in studies of
erosional and depositional ground surface elevation changes for many years as an
inexpensive method of measurement (Schumm, 1964; Sirvent, 1997). An array of stakes
or posts within an aggrading plain can be used to create a soil profile and measure change
relative to a stationary reference by progressive covering of the stakes and posts
(Schumm, 1964). Measurement of soil elevation change over time periods of a year or
more creates more reliable data as effects of individual erosive episodes are smoothed out
into a cumulative change
Overland Flow
Inter-rill erosion due to sheet flow and rainsplash occurring on coarse grained
sediment is evidenced by pebbles on soil pedestals. As sheet flow becomes deeper and
more turbulent, rills begin to form, further mobilizing sediment as they cut into the
hillside with increased power. Convergence of overland flow leads to deeper, more
erosive flows where the depth of flow within the rill increases the rate of incision.
Overland flow in the form of rills receives sediment mobilized from rain splash. Turbid
runoff from sheet flow and rills on pit walls has been observed directly contributing
sediment to clear streams originating outside of the pit, suggesting that cliff areas
contribute a substantial amount of sediment to overland flow within the pit (NCRCD,
15
1979a). As sheet flow washes material down from the walls, the network of rills and
gullies accepts and transports material from the steeper hillslopes toward the pond and
Hiller Tunnel (Senter, 1987).
As rills expand and converge, gullies form downstream, further incising and
carrying mobilized sediments of larger sizes. Headcutting of gullies due to plunge pool
action and seepage at cut faces deepens gullies as they migrate upslope. The deepening of
gullies further accelerates erosion by over steepening slopes and undercutting banks
leading to mass wasting of gully sides directly into the stream where it is easily
transported by the stream (Selby, 1993). All contributing streams within the pit exit
gullies at the pit floor before reaching the pond and Hiller tunnel.
Deposition in the Pit Floor
Overland flow is the mechanism of transport for large amounts of sediment
from cliff walls to the bottom of the pit, as it moves toward the outlet at Hiller Tunnel. As
the gradient of the streams decreases, so does the velocity of the streams, allowing the
larger coarse material to settle in alluvial fans. Along the eastern edge of the pit floor,
three or more streams converge in a large braided alluvial fan fining westward toward the
outflow. The interface between the pit floor and the steep walls of the pit is characterized
by numerous alluvial fans associated with gullies and large rills as they deposit the coarse
bedload. The low-lying flats with vegetation and ponding are depositional in nature. An
alluvial fan at the confluence of 3 or more channels on the east side of the pit floor has
aggraded considerably since 1946, extending westward into the pond area (Senter, 1987).
The pond is now nearly full of sediment and vegetation, and has aggraded to a level at or
16
above the inlet to Hiller Tunnel, potentially increasing the sediment load and bedload out
of Hiller Tunnel, especially during high flows (Peterson, 1980; Senter, 1987).
CHAPTER III
OBJECTIVES
The objective of this study was to quantify process-based erosion and
deposition at Malakoff Diggins to quantify the total contribution to Humbug Creek. The
measurements of process based erosion and deposition using a combination of GIS
analysis, and direct field measurements within the pit will be used to develop a simplified
sediment budget. In order to create a sediment budget for Malakoff Diggins Pit, the
amount of sediment eroded and deposited within the pit was estimated based on the
analysis of erosional and depositional changes by location, and process type. The net
difference between erosion and deposition estimates represents the volume of material
that is discharged from the pit.
The research questions include:
1.
What are the major sediment sources within the pit. This includes identifying
and characterizing the primary processes and locations of major sediment sources of silt
and clay eroded within the pit.
2.
What is the amount of annual erosion and deposition within the pit erosional
system?
3.
What is the annual sediment discharge from the pit ?
4.
Is this sediment budget a more complete description of surficial processes
than previous estimates made at Malakoff Diggins?
17
CHAPTER IV
METHODOLOGY
Erosion
Cliff Retreat Erosion
The total mine pit size, change in pit area over time and the area of
unvegetated zones within the pit indicative of surface erosion and deposition were
estimated with in a Geographic Information System (GIS) using a time series of aerial
photographs spanning 1952 to 2012. Utilizing measured values of area, width, and length
of pit rim expansion, the quantity of erosion over time was estimated.
Utilizing GIS (ArcGIS version 10.0), measurements of material lost were
made using fixed geologic and anthropologic features as reference points (Kirchner,
2002) Using aerial photographs from 1952 and 2012, the cliff edge was digitized, and the
amount of recession over time was measured to quantify surface area of material eroded
over the 60 year time span. Separate layers containing polygons for 1952 and 2012 were
created by tracing the topographic signature of the interface between the natural forest
and the pit rim. The pit rim is conveniently apparent as denuded areas high in quartz
making up the mine walls are bright white. The surface area of the pit was calculated
using the attribute table. Subtraction of pit surface area in 1952 from that of 2012
revealed the total amount of surface area lost to erosion between photo periods.
18
19
Subtraction of pit area in 1952 from that of 2012 revealed the total amount of
area lost to erosion between photos. The area was divided into 33 polygons to represent
the individual landslide complexes and gullies. Rim retreat was assumed to be a function
of erosion parallel to slope. The crown of most failures around the rim are arcuate,
although recession due to block failure and surface erosion have left irregular recession in
some places. The geometrical dimensions and method for calculating an estimated
volume are taken from Cruden and Varnes (1996) (Wieczorek et al., 2002). The geometry
of each polygons were measured using ArcGIS, allowing for the calculation of volume
using a semi elliptical cone shape for each mass wasting scar. To represent the shape of
observed slide scars that are one quarter of an ellipse in area, or crescent shaped, 1/12
was used as an operator in place of 1/3 for a true cone. The 33 polygons were
individually calculated for their volume, and summed to create a total volume lost from
pit rim erosion over the 60 year span. Dividing the volume lost by 60 years yielded the
average amount of material lost annually. Equation 1 shows the operations, where
L=length of slide from toe to rim, W= width at the rim, and D= estimated depth of the
failure plane.
(1)
The erosional processes of the cliff walls and gullies were measured using
direct field measurements and extrapolated over representative areas using GIS. Direct
field erosion measurements were made using a 1-meter erosion bridge to find relative soil
elevation change at locations throughout the pit (Blaney and Warrington, 1983). An
20
erosion bridge is a 1-meter level spanning two rebar pins above the soil surface with
measurements made at horizontal intervals of 50mm by thin aluminum rods (see Figure
3). The erosion bridge was constructed of a 4 foot carpenters level flush mounted to a
Figure 3. Relative soil elevation was measured using erosion bridge pictured in situ.
Level and rods are removed between measurements and replaced on semipermanent pins at each location.
wooden clamp brace with holes drilled at 50 mm for measurement rods. The clamps have
marks at 1 meter to ensure accurate and consistent mounting to the rebar pins. Rebar pins
were installed level with one another at seven locations on the pit walls where primary
surface erosion occurs (Blaney and Warrington, 1983). The pins remained undisturbed in
the ground at each location for the duration of the study, and were covered with safety
caps which were painted grey to minimize visual impacts. The horizontal bubble of the
level was used to monitor and adjust for any changes in the level of the rebar pins during
21
the study. The change loss/gain of elevation represents the magnitude of erosion or
deposition, and creates a soil contour profile which can be repeated and held against the
previous measurement at that location (Ypsilantis, 2011).
At each location, 20 measurements were made from a level, local datum down
to the soil surface at intervals of 50mm by thin aluminum rods. Each rod was carefully
placed through a hole on the apparatus and allowed to rest gently on the ground surface.
Each rod was carefully marked in place with a pencil, removed, and measured with a
steel measuring tape to find the distance to ground surface from the top of the erosion
bridge, and the pencil mark was removed from the rod after it was recorded. The
accuracy of the measurements made with the rods of the erosion bridge is ± 0.5mm. The
measurements for each plot were recorded in a field notebook along with the date of
measurement, the conditions, and observations at each location. Field measurements of
rods were made in the same order (from left to right, 1-20), and recorded in the same
column at each location on every visit to reduce errors.
The net change in elevation between measurements represents the magnitude of
erosion or deposition, and creates a contour profile. Repeat measurements can be
compared to previous measurement (Ypsilantis, 2011) to develop a time series of
elevation change.
Erosion bridges. In order to represent the different slope and substrate
conditions erosion bridges were installed at seven representative areas of the pit on
actively eroding slopes or cut faces in order to quantify elevation changes for the
different stratigraphic units and slope ranges. Each plot was representative of a dominant
22
erosion process within the zone of depletion, zone of translocation, or zone of
accumulation within the individual slope (Selby, 1993).
Each location was chosen based on its slope, aspect, substrate type, and
dominant morphological attributes, i.e., steep compact clay prone to slumping, or
medium slope coarse sandy gravel dominated by rills. Table 1 shows the location
(lat/long), slope and aspect of each erosion plot. Figure 4 shows the erosion plot locations
TABLE 1. EROSION BRIDGE PLOT LOCATION, SLOPE AND ASPECT
Name
Lat/Long
Slope°
Aspect
Plot 1
39°22’27.59”N 120°54’52.05”W
55°
East
Plot 2
39°22’25.17”N 120°54’19.09”W
53°
West
Plot 3
39°22’23.80”N 120°54’18.89”W
7°
WSW
Plot 4
39°22’12.71”N 120°54’28.85”W
48°
SSE
Plot 5
39°22’13.29”N 120°54’46.98”W
19°
N
Plot 6
39°22’12.14”N 120°54’47.04”W
48°
N
Plot 7
39°22’19.29”N 120°54’24.27”W
44°
SSW
within the pit relative to streams and other natural features. Two erosion plots were
placed on upper slopes (50-70 degrees) three on mid (30-50 degree) and two on lower
slopes (10-30 degrees) (Selby1993). Vertical slopes along the pit rim were not directly
measured for surface wash erosion due to their inaccessibility, but instead were measured
as cliff edge retreat using GIS.
Plot 1 was located on the north side of the pit on slopes to the west, and above
the confluence of two small inflowing streams (Figure 4). The plot was set up on a 55
degree slope of a compacted grey colored clay unit above the “white cliffs”
(characterized by high quartz gravel content and knife ridges) roughly half way up the
cliff. The overlying clay appears to armor layers below, as is evidenced by its near
vertical faces often void of deep gully cuts which exist in layers above and below. A
23
Figure 4. Location of erosion bridge sampling locations within Malakoff Diggins
mining pit.
cement layer composed of iron and quartz exists above the clay, but is inaccessible due to
steep slopes.
Plot 2 was installed on the far east side of the pit, near the base of the most
recent slide within a recurring landslide complex. The 53 degree slope is composed of
sandy clay with some gravel and hematite cobbles. Soil creep and smaller associated
failures such as rotational slumps are evident in this area. Due to landslide processes,
cobble sized clay blocks are transported into the fluvial streams slowly breaking up as
they migrate downstream. This location was chosen to represent deposition and the
24
ongoing reworking of sandy colluvium deposits within the zone of accumulation and
reworking in the east side of the pit. Although mass wasting in this area is the primary
erosional process, it is episodic, and leaves an arcuious shaped wall which concentrates
flow, increasing erosion, and transport of material from landslide deposits toward plot 3.
Plot 3 was installed on the east side of the pit, downslope from plot 2 where
the alluvial fan with a 7 degree slope where episodic landslide deposition and erosion
occur simultaneously in a braided stream as flows converge and diverge. This location
was chosen in order to quantify storage and re-mobilization in the upper reaches of the
pit. The alluvial fan is actively accepting and reworking material from landslides above,
leaving behind sand and clay blocks of 2-8 cm in diameter.
Plot 4 was located on the 48-degree shoulder slope within the translocational
zone of an amphitheater shaped gully in the southeast corner of the pit roughly 30 m
north of the shooting range. The clayey brown gravel with cobbles and sandstone is one
of the more mineralogically complex stratigraphic units within the pit. It also seems to be
eroding more quickly than other units, with more than one active erosional process. This
area was chosen, due to its representation of the translocational zone, and its sediment
unit type, which is a source area for sediment entering a stream on the southeast side of
the pit.
Plot 5 was located on a debris fan below the southern trail overlook and plot 6.
The 18 degree slope is made up of lobate rock debris deposits with down slope fining
bisected by shouldered rills indicating supersaturated flows (Selby 1993). This location
was chosen to represent deposition and erosional reworking of the coarse deposits at the
base of the pit walls below actively eroding hillsides.
25
Plot 6 was installed on the 48-degree slope along the central south rim of the
pit near the lookout point, above and to the right of plot 5. The deposits at this location
are primarily mine waste, possibly tailings and are made up of uniformly sorted gravels,
sand, and clay. The location was chosen in order to represent actively eroding mine
waste/tailings, as well as to represent slopes with a north facing aspect which experience
larger, more frequent frost wedging.
Plot 7 was installed on a 44-degree slope at the upper reach of a dry gully on
the northwest side of the pit. The soil here is composed of red, silty clay with small
volcanic boulders of andesite. The layer has evidence of many small landslides
throughout the north side of the pit, suggesting low cohesion. Soil creep appears to be the
dominant erosion process, slumping as the stream below undercuts the toe of the clay
hillside.
Erosion bridges were installed in October of 2012, and monitored throughout
the water year as erosion took place. Initial measurements were made in October of 2012
at the beginning of the hydrologic year. Once the winter rains began, measurements were
made intermittently following major precipitation events. The final measurements were
made at the end of September 2013. The annual erosion rate for each plot was determined
by comparing the sequential measurements from the datum (Erosion Bridge) to the
ground surface.
Surface wash area measurement and volume estimate. The volume of erosion
associated with surface wash was calculated by delineating areas of similar stratigraphic
units observationally within the pit. The area of each stratigraphic unit was based on
observation in the pit of sediment properties such as color, texture, and cohesion. Areas
26
of morphology similar to that of erosion bridge locations were delineated on the aerial
photos (Figure 9 in Chapter V). The area associated with each erosion bridge were
multiplied by the elevation change measured at each erosion bridge respectively to find
the volume of sediment eroded from each area. The volumes were then aggregated to
create a total annual sediment eroded via surface wash within the pit (Table 4 in Chapter
V).
Deposition
Deposition Posts
Direct field deposition measurements were made using existing trail markers
and 6-foot t-posts installed by State Parks. Trail markers and t-posts installed, and
initially measured throughout the pit floor by State Parks in the spring of 2005 were used
to measure sediment deposition (C. Walck, personal communication, August 12, 2013).
The rate of annual deposition was determined by comparing sequential measurements
made from the top of the stakes to ground surface (Schumm, 1964). Measurements were
made by State Parks at the time of installation in April 2005, as well as December 2005,
December 2006, and January 2008. Measurements were made during this study at all post
and stake locations in April 2013, and September 2013.
The trail and associated trail markers are within the pit floor are located on the
outer edge of the basin where colluvial and alluvial deposits converge at the base of the
cliffs and meet the ponded floor where vegetation is less dense. The trail is marked by 4
by 4 posts, since active deposition covers the trail each winter. At the time of installation
each post was labeled with a brass number identification plate, and located using GPS
27
(Figure 5; Table 2). Some of the posts were installed prior to 2005 and have been covered
by sediment completely, or enough to be in jeopardy of complete burial. In these
Figure 5. Locations of deposition trail markers (blue and green) and fence posts
(red). Installed and initially measured by California State Parks in 2005.
Subsequent measurement for this study conducted in 2013.
instances, replacement of the post at that location occurred in 2005. Adjacent to the
original, a new post was installed, measured, and demarcated as “new” by State Parks
however, they are fundamentally identical. For example: post 15 became 15N.
Depositional area measurements and volume estimation. The area of the pit
floor where deposition actively occurs was measured using ArcGIS. The depositional
area was used in conjunction with elevation change data from trail markers and stakes to
28
TABLE 2. LOCATION OF DEPOSITION POSTS WITHIN PIT FLOOR
Code #
Type
Latitude
Longitude
1
Old Marker
39 22 4.1274263 N
120 55 29.6677193 W
2
Old Marker
39 22 5.2496049 N
120 55 31.7328409 W
3
Old Marker
39 22 6.0459986 N
120 55 33.9631934 W
4
Old Marker
39 22 6.8948042 N
120 55 34.3933413 W
5
Old Marker
39 22 8.4942084 N
120 55 34.4472626 W
6
Old Marker
39 22 10.2346881 N
120 55 33.6573776 W
FP1
Fence Post
39 22 10.1396269 N
120 55 31.9628758 W
1N
New Marker
39 22 11.1656693 N
120 55 32.3973465 W
7
Old Marker
39 22 12.5688701 N
120 55 31.7716194 W
8
Old Marker
39 22 13.4155462 N
120 55 30.9709315 W
9
Old Marker
39 22 14.7217803 N
120 55 30.4386579 W
2N
New Marker
39 22 16.1377553 N
120 55 28.9322624 W
FP2
Fence Post
39 22 15.4657281 N
120 55 24.2543419 W
3N
New Marker
39 22 16.0280165 N
120 55 23.1242318 W
FP3
Fence Post
39 22 14.3645692 N
120 55 14.3795524 W
4N
New Marker
39 22 14.8660376 N
120 55 14.2036632 W
5N
New Marker
39 22 15.5258189 N
120 55 5.8932371 W
FP4
Fence Post
39 22 15.3183527 N
120 55 5.0399865 W
6N
New Marker
39 22 15.6840688 N
120 55 3.7072785 W
10
Old Marker
39 22 16.9363861 N
120 55 1.765352 W
11
Old Marker
39 22 17.1308157 N
120 55 0.1872851 W
7N
New Marker
39 22 17.3414304 N
120 54 57.3277109 W
12
Old Marker
39 22 17.0618881 N
120 54 56.3883065 W
8N
New Marker
39 22 16.8904998 N
120 54 55.3867235 W
9N
New Marker
39 22 17.2761728 N
120 54 54.4863692 W
FP5
Fence Post
39 22 17.0473119 N
120 54 54.054317 W
10N
New Marker
39 22 17.3398278 N
120 54 53.7898684 W
11N
New Marker
39 22 18.1302629 N
120 54 52.8867419 W
12N
New Marker
39 22 18.7582424 N
120 54 52.36542 W
13N
New Marker
39 22 19.1753429 N
120 54 51.9350359 W
13
Old Marker
39 22 19.8760484 N
120 54 52.3283078 W
14
Old Marker
39 22 20.6594411 N
120 54 51.2228258 W
15
Old Marker
39 22 21.6457266 N
120 54 50.5572282 W
14N
New Marker
39 22 22.5650888 N
120 54 50.3155221 W
16
Old Marker
39 22 22.6009225 N
120 54 50.2338856 W
15N
New Marker
39 22 23.6741842 N
120 54 49.20467 W
29
Table 2 (Continued)
Code #
17
16N
17N
18
18N
19
20
19N
21
20N
22
PP7
FP20
FP16
FP11
FP6
FP15
FP10
FP25
FP24
FP23
FP22
FP21
PP8
FP7
FP8
FP9
FP12
FP13
FP14
FP17
FP18
FP19
Type
Old Marker
New Marker
New Marker
Old Marker
New Marker
Old Marker
Old Marker
New Marker
Old Marker
New Marker
Old Marker
Photo Point
Fence Post
Fence Post
Fence Post
Fence Post
Fence Post
Fence Post
Fence Post
Fence Post
Fence Post
Fence Post
Fence Post
Photo Point
Fence Post
Fence Post
Fence Post
Fence Post
Fence Post
Fence Post
Fence Post
Fence Post
Fence Post
Latitude
39 22 23.9186762 N
39 22 23.9492891 N
39 22 24.6600116 N
39 22 24.3676595 N
39 22 24.3456431 N
39 22 23.4349326 N
39 22 21.8671045 N
39 22 20.9473693 N
39 22 20.0867322 N
39 22 19.4076706 N
39 22 19.0415175 N
39 22 16.4288236 N
39 22 16.7703884 N
39 22 21.6923879 N
39 22 18.4103222 N
39 22 17.5587291 N
39 22 15.0422134 N
39 22 14.2052976 N
39 22 13.2005534 N
39 22 10.7745147 N
39 22 10.0284422 N
39 22 8.35483 N
39 22 8.537618 N
39 22 6.615863 N
Longitude
120 54 49.0945463 W
120 54 48.1498608 W
120 54 47.3148849 W
120 54 45.645783 W
120 54 45.4945338 W
120 54 44.6093595 W
120 54 44.269604 W
120 54 43.5014279 W
120 54 42.8543555 W
120 54 42.2275761 W
120 54 41.8306724 W
120 54 39.5851118 W
120 54 45.8206494 W
120 54 48.36237 W
120 54 51.1210307 W
120 54 52.5757109 W
120 54 48.9224832 W
120 54 49.9496418 W
120 54 50.4940009 W
120 54 55.5512752 W
120 54 58.996299 W
120 55 5.013608 W
120 55 10.9148906 W
120 55 23.216932 W
30
calculate average annual volume deposited. Measurements from the top of the marker to
the soil surface on these stakes were made and compared to previous measurements. The
relative location of posts and stakes used for deposition measurement can be seen in
Figure 5.
The volume of sediment deposited in the pit floor was estimated using the
elevation change from the stakes and trail marker posts over the eight-year period of
measurement from 2005 to 2013. The mine floor depositional area is made up of different
source areas and associated depositional plains and colluvial deposits. The pit floor was
divided into four depositional zone polygons representing deposition associated with
these separate sources, each having a different deposition rate (Figure 10 in Chapter V).
The area of each depositional zone was measured using the GIS polygon measure tool on
aerial maps. The volume for each depositional zone was then found by multiplying
average elevation change of the stakes and posts contained in each zone by its area.
Finally, the volume of all depositional zones was added together to find the total volume
deposited in the pit floor (Table 5 in Chapter V).

East: Metal t-posts were installed by State Parks on the east side of the pit
floor in order to measure an area rapidly aggrading with coarse sediment. Stakes 6
through 20 were installed in three transects made up of five stakes each, perpendicular to
flow in the aggrading, alluvial flow in the east lobe of the pit with a slope less than 10
degrees. Trail markers 17 through 22 are parallel to the transects located downslope,
making them functionally the first, or easternmost depositional transect of the four. Tpost18 was unable to be located, and stake 20 was within a stream, having an
31
uncharacteristic measurement of elevation loss associated with localized channel erosion
within the aggrading plain.

North: The northern cliff wall of the pit erodes down onto the plain of the
floor while the braided streams both erode and deposit material at a right angle to the cliff
face and its deposition. The deposition in this zone was measured at posts 4N, 5N, 6N,
7N, 8N, 9N, 10, 11, 12.

West: The westernmost cliffs deposit directly into the pond below. As the
depositional plain at the base of cliffs aggrades and expands, the area of the pond is
encroached upon. Sediment entering the pond from the northwest area is closest to Hiller
Tunnel of all sediment source areas. The deposition in the western quadrant was
measured at posts 1,1N, 2, 2N, 3,3N, 4, 5, 6, 7, 8, 9.

South: The southern depositional zone is the flat plain adjacent to tailings rock
piles and vegetated native soil hillsides along the south rim of the pit. It is downslope
from the east depositional zone. Hillslopes here contribute smaller amounts of sediment
due the fact that they are not as tall or oversteepened, are more protected by vegetation,
and contain a higher fraction of coarse material as mine waste rock, as is evident by the
hummocks and rockpiles. Only a few areas of cliff face exist along the south side of the
mine. Most of the material deposited in the south quad of the pit floor likely originated
from the east, fining downstream. A stream channel runs along the south side of the pit
floor actively depositing in the dense vegetation as it continues toward Hiller Tunnel. The
trail is at the top of the cliffs, and trailmarkers could not be used for measurement
because they were not on the pit floor. The deposition in the southern quadrant was
measured at t- post 21 only. Stakes 22, 23, 24, and 25 in the southern area were not able
32
to be located in 2013 due to dense vegetation. Elevation change data from previous years
was used to complete average annual depositional change.
Sediment Budget (Discharge Calculation)
The simplified sediment budget in this study uses the combination of ArcGIS
estimates of cliff retreat, and direct measurements of surface elevation change using
erosion bridge and deposition stakes to calculate the discharge as the difference between
erosion and deposition. The simplified sediment budget created for this study follows the
processes identified as major contributors by previous studies (NCRCD 1979b; Peterson
1980; Senter, 1987). The quantities of the three primary processes within the system that
include mass wasting, surface wash, and deposition were estimated as annual volumes.
The volume of these processes were estimated using area measurements combined with
measurements of elevation change. Figure 6 shows how the average annual volume
associated with each process were summed within process type and then accounted, to
yield annual sediment discharged from the pit, where: The sum of annual cliff retreat
erosion plus the sum of annual surface wash erosion volume, minus the sum of annual
deposition volume, equals the total annual sediment discharge.
Figure 6. Simplified sediment budget calculation used in this study to quantify
annual sediment discharge.
33
Hydrology
Erosion is caused by precipitation, and therefore, the precipitation regime was
monitored during the present study for analysis. Precipitation data was gathered using the
nearest continous data source at the Camptonville ranger station 19 miles northwest of
Malakoff Diggins. The location was chosen over other nearby rain gages for its historical
availability of continuous data, and its similar topography to Malakoff Diggins and
elevation of 2,755 feet above sea level. Data was accessed using the Department of Water
Resources California Data Exchange Center’s (2014a, 2014b) website. The rain gage at
the ranger station is a tipping bucket type rain gage and daily as well as accumulated and
snow precipitation is recorded.
Surface wash erosion values were compared to precipitation during the winter
of 2013 on January 21, January 29, March 1, March 28, and May 17. The results, while
inconclusive, aid in the understanding the dynamics of precipitation and erosion within
the Malakoff diggings system. The winter of 2013 was within a drought year, and each of
the dates above coincide with a precipitation event prior to measurement with erosion
bridges.
CHAPTER V
RESULTS
Erosion
Cliff Retreat Erosion
Seven hard points such as road crossings and building corners were used to
georectify the aerial photos resulting in a Root Mean Square (RMS) error of 24.8m.
Comparison of the pit edge from 1952 to 2012 by overlaying allowed the area of change
between the two photosets to become its own series of polygon shapes. The total area of
cliff retreat over the 60 years between photos measured at 100,600 ± 25m2 (see Figure 7).
The length of the perimeter of the polygons was measured. Dividing the
perimeter of the long, narrow polygons in half yields the approximate length of the area
eroded or width of the top half of a circular polygon feature. The total volume of material
eroded at the pit rim between 1952 and 2012 was 654,400 ± 110m3. Dividing the total
volume by 60 years, the average annual volume of material lost from the pit rim was
found to be 10,900 ± 99.5 m3/yr (Equation 2).
(2)
Visual examination of the rim retreat in ArcGIS shows that locations where
expansion was greatest are in the far east and west of the pit. (See Figure 7). Mass
wasting expansion is most prevalent along the eastern edge of the pit, where a large
34
35
Figure 7. Digitized outline of pit rim in 1952 and 2012 with areas cliff retreat due to
landslides and gully headcutting represented in yellow. Total area of expansion over
the 60 years was measured at100,600 ± 25 m2.
landslide complex exists. Gullying is more visibly prevalent in the west side of the pit,
where overland flow from outside the pit concentrates. However, the two processes occur
simultaneously in the same location as landslide morphology increases gullying by
concentrating flows within scars, and gulling can lead to failures by oversteepening
hillsides. The expansion of the pit rim in the east and west areas are major source
contributors of material within the Malakoff Diggins.
36
Needle Ice and Dry Ravel
The winter of 2012/2013 saw little precipitation in California. Without
multiple storms of intensity, erosion by rainfall was limited at Malakoff Diggings.
January and February 2013 were predominantly dry and sunny with freezing nighttime
temperatures. Throughout January and February 2013, freezing night time temperatures
caused frost wedging to persist on north facing and other shaded slopes into the day. The
depth of frost measured on January 28 was 38 mm at erosion plot 6, and 25 mm at plot 5.
The small columns of ice making up the frost, known as needle ice held 1-2 mm of
material on top of the needles, and finer sediments were incorporated within the ice. The
needle ice also existed in plot 5 at the same size, holding up material as large as cobbles.
As the ice melted, the dislodged material was either transported downhill as dry ravel, or
remained in place but in a disturbed state. Frost wedging and needle ice are dependent on
the freeze thaw cycle and slope aspect with north facing slope being more susceptible. In
some areas, the freeze thaw cycle caused considerable dry ravel throughout the pit walls.
At erosion plot 4, a cohesive clay layer was broken into sand size chips and transported
as dry ravel down existing rills and filled gullies.
Erosion plots measured the loss and temporary storage from frost wedging.
The process was most evident at plots 6 and 4 where frost wedging had the most impact
due to solar aspect. Measurements made at plots 4 and 6 on March 1 and 28, 2013 show
an increase in surface elevation associated with frost heave and dry ravel stacking midslope. Subsequent rapid erosion from relatively low intensity, low rainfall events in April
and May eroded considerable amounts of loose material.
37
During the study period the average surface elevation change measured by the
7 erosion bridges was – 15.4 mm (± 8.5). See Figure 8. As shown by the standard error, a
wide range of erosion values were measured. Erosion affected individual monitoring
Figure 8. Mean elevation change measured with Erosion Bridge at each plot location
during 2012/2013 water year. Final measurements were subtracted from initial
measurement with change estimated at each of 20 point measurements made at
50mm intervals.
plots quite differently, from extremely high erosion rates to deposition. Some locations
had erosion, midwinter deposition, then erosion again in spring. The lack of correlation
between slope and erosion/deposition supports the argument that short-term
measurements are highly variable. In addition, the contour profile measurements created
38
TABLE 3. EROSION BRIDGE PLOT AVERAGE ELEVATION CHANGE,
LOCATION, SLOPE AND ASPECT
Name 10/12/12 - 9/7/13 10/12/12 - 9/7/13
Lat/Long
Slope
Aspect
Average surface
Average surface
erosion (mm/yr)
erosion(m/yr)
Plot 1
-24.2 +/-6.40
-0.02415
39°22’23.97”N
55°
East
120°54’19.36”
Plot 2
13.1 +/-6.49
0.01313
39°22’25.17”N
53°
West
120°54’19.09”
Plot 3
-29.3 +/-3.74
-0.02925
39°22’23.80”N
7°
WSW
120°54’18.89”
Plot 4
-24.5 +/-5.27
-0.02447
39°22’12.71”N
48°
SSE
120°54’28.85”
Plot 5
7.43 +/-3.81
0.007435
39°22’09.62”N
19°
N
120°55’20.63”
Plot 6
-48.5 +/-11.1
-0.04850
39°22’12.14”N
48°
N
120°54’47.04”
Plot 7
-1.63 +/-8.13
-0.00163
39°22’19.29”N
44°
SSW
120°54’24.27”
Ave.
-15.4 +/-8.41 mm
0.01535 +/0.0084 m
by the multiple rods across the erosion bridge recorded the creation and destruction cycle
of rills at many locations.
The erosion bridge at plot 1 measured -24.2mm (± 6.5) of surface erosion
(Table 1). During the winter, evidence of frost wedging was observed at this location,
hours after melt, and the thickness of the ice went unmeasured. Indications of frost
wedging were observed on March 28 when chips of damp clay falling as dry ravel filled
the adjacent gully roughly half a meter full with unconsolidated material.
Measurements at Plot 2 revealed annual deposition of 13.1 mm ( ± 6.5).
Observations of the slope indicate that the area was aggrading as material from the
landslide area above alluvially accumulated on a bench created by the top of a large
landslide deposit. The slope at the plot 2 location does not exhibit rills like some nearby
39
areas of similar slope, indicating that either rill development and destruction cycles occur
more rapidly, or more likely the slope experiencing sheetwash containing high sediment
loads from above.
At erosion plot 3, measurements show that even though landslides often
contribute material to this alluvial area of relatively low slope (~7 degrees) erosion was
measured as -29.3 mm ( ± 3.7). This fact demonstrates how efficiently sediment is
removed from the base of the landslide area, most likely leading to more landslides
within the complex, recurring landslide area at the east end of the pit.
Erosion plot 4 showed signs of the rill development and destruction cycle both
visually observed, and in the contour data. The effects of dry ravel were observed filling
rills with flows of dry clay chips. Measurements on May 17, 2013 show an average
elevation increase of 1.1 mm associated with dry ravel deposition, from previous
measurements in March. Summer thunderstorms in June and early September eroded the
loose material. Final measurements show average annual erosion at plot 4 was measured
as -24.5 mm ( ± 5.3)
Erosion plot 5 was in an area receiving colluvial deposition from the cliff
immediately above it. The annual deposition measured was 7.4 mm ( ± 3.8), and
consisted of coarse sand and larger material, as fines were carried away by surface wash.
Large rills with natural levees were observed at this location indicating supersaturated
flow.
Erosion Plot 6, located immediately above plot 5 had the highest erosion rate
of all the erosion plots, with -48.5 mm ( ± 11) of elevation change. Severe frost wedging
and needle ice observed at this location was a significant contributing factor to erosional
40
loss. The ~55 degree slope had much less apparent cohesion than other substrates within
the pit, and appeared to be poorly sorted gravel and sand with few fines. The location and
relative lack of clays indicate that it may be an actively eroding placer waste rock pile.
Erosion plot 7 showed very little change throughout the study period. Erosion
was measured at an average of -1.6 mm ( ± 8) of elevation change. The cohesive sandy
clay at the upper reach of a south facing gully saw little frost and rilling. Field
observations showed the substrate had plastic properties when wet and was very hard
when dry, indicating clay with a high swelling potential. Changes in elevation measured
may have been partially the result of swelling and shrinking processes rather than
deposition.
Surface Wash Erosion Volume Estimate
Erosion plots in the regolith of the pit walls measured erosion during the water
year of 2013 as -15.4 ± 8.41mm/yr, when all measurements were averaged. The mine
wall slopes were divided into twelve erosional zone polygons representing erosion rates
found at the seven erosion plots, each having a different erosion rate. Figure 9 shows the
area, location and extent of each erosional zone, as a GIS polygon on aerial maps. The
volume for each zone was then found by multiplying the average elevation change of the
erosion bridge associated with each zone by its area. This analysis method, separates the
measurements of elevation change for the lithographic sediment type in which they are
located. The volumes associated with areas of surface wash were added together to find a
total volume of surface wash erosion for the 2013 water year (Table 4). The total volume
of surface wash erosion was estimated to be 9,117 ± 51 m3 /yr.
41
Figure 9. Erosional areas morphologically similar to conditions at erosion plot
locations used for volumetric calculations of surface erosion.
The eastern cliff area has expanded since 1952. However, not all of the
material deposited at the base of the cliff by landslides is transported away from the area
immediately. Instead, it is dissected and eroded by rills and gullies. The rate of this
erosion is represented by Erosion Plots 2, 3 and 4. The total area of sheetwash erosion
measured in the east with GIS was 58,148m2. The eastern area contributed an estimated
3,291 ± 396 m3 of sediment to the system by rill and interrill erosion in 2012/2013.
The cliffs in the southeast also contribute sediment to the streams and gullies
originating in the east, as one of the three gullies lies at the base of these slopes. Erosion
within this unit is driven by rainsplash, rills and gullies with comparatively small mass
wasting. The area is represented by erosion plot 4. The area measures as 38,911 m2
42
TABLE 4. ANNUAL VOLUME CALCULATIONS FROM EROSION PLOTS AND
AREA POLYGONS
Erosion Plot
Polygon Area m2 elevation Change (m)
Annual eroded
volume(m3)
Name
Plot 1
51,998
-0.024
-1,256
Plot 2
13,284
0.0131
174
Plot 3
44,884
-0.029
-1,313
Plot 4
38,911
-0.024
-952
Plot 4
73,739
-0.024
-1,804
Plot 5
3,220
0.007
239
Plot 5
9,101
0.007
68
Plot 6
39,041
-0.049
-1,893
Plot 6
26,698
-0.049
-1,295
Plot 6
10,843
-0.049
-526
Plot 6
10,567
-0.049
-513
Plot 7
18,078
-0.0016
-29
Plot 7
7,275
-0.0016
-12
Plot 7
3,030
-0.0016
-5
Total volume---9,117± 51
contributing an estimated 952 ± 209 m3/yr of material by rill and interrill erosion during
2012/2013.
The northern scarp, in contrast to the east is relatively stable with lenses of
ferricrete, and compact, cohesive clays within the gravelly matrix. The area is represented
by erosion plots 1 and 7, which have erosion rates of -24.0 and -1.5 mm/yr respectively.
The area measures as 70,076 m2. The north slope areas contributed an estimated 1,385 ±
147 m3 to the system by rill and interrill erosion in 2012/2013 water year. Near plot 1 the
northern wall wraps to face east, with a small stream at the base of the slope. The east
facing area of the north wall has a relatively high surface erosion rate as evidenced at
erosion plot 1. This area also has multiple mass failures of the uppermost soil strata, of
roughly 5 meters. The failures occur as hypersaturated mudflows that travel down gullies
to fan out on the vegetated steep plain before entering the stream below.
43
The southern slopes of the pit are much different from the rest of the pit. The
pit rim scarp is not as well defined as other places, and slope lengths are shorter. The
natural general slope of the ridge trends from northeast to southwest leaving the south
scarps generally less than 10 meters high. Erosion plots 5 and 6 that had erosion rates of
7.43mm/yr and -48.50 mm/yr. The area contributed an estimated 1,654 ± 293 m3/year of
material to the system, with little to no additional contributions from mass wasting and
gullying.
The western scarp area is dominated by steep gullies which have expanded by
approximately 13,840m2 since 1952. The slopes to the west deliver and estimated 1,825
±119 m3/yr of material directly to the pond.
Deposition
Deposition posts in the alluvial deposits of the pit floor measured aggradation
over the 8 years of monitoring as 47.5 mm/yr ( ± 4.5mm), when all measurements were
averaged over the 8 years of measurement. A more descriptive, analysis method,
separates the measurements of elevation change for the four depositional areas of the pit
floor. Measurements were averaged by individual areas for volume calculation seen in
Figure 10 and Table 5. The findings of the four process based areas are as follows:

East: There are 4 fencepost transects in the alluvial plain where streams
draining gully networks converge in an alluvial plain with a 10 degree slope. The average
deposition rate over the 8-year period in the east side of the pit floor zone was measured
44
Figure 10. Depositional areas by quadrant (North, South, East, West). Area (m2)
values measured using ArcMap which were used in the calculation of volume
deposited.
TABLE 5. ANNUAL DEPOSITION VOLUME CALCULATIONS
Total dep 8 yrs Ave dep rate (m/yr Ave annual volume (m3/yr
Area m2
North
79,560.78
0.43190
0.05399
4,295
South
68,123.69
0.1463
0.01829
1,246
East
83,289.73
0.4228
0.05285
4,402.
West
104,416.80
0.21
0.02647
2,764
3
Total Deposition (m /yr)
12,707+- 889
45
at 0.0528 m/yr. ± 0.0060m. The area of this plain was measured as 83,290 m2 using the
ArcMap 10 area measure tool in the attribute table. Annual volume of deposition in this
area was then calculated as 4,401 ± 27.8 m3/yr.

North: The northern cliff wall of the pit erodes down onto the plain of the
floor while the streams both erode and deposit material along the toe of the slope at a
right angle to the cliff face and its deposition. The area of the north deposition zone was
measured as 77,623-m2 ± 0.3. Using measurements of trail markers over the eight-year
monitoring period, the average deposition in this zone was found to be 0.0539 m/yr ±
0.0101m. Multiplication of the measured values yields a volume of material deposited in
the north quad of the pit floor as 4,295 ± 807 m3/yr.

West: The westernmost cliffs of the mine deposit directly into the remaining
small pond. As the depositional plain builds below the cliffs, the area of the pond is
encroached upon with alluvium. Using ArcGIS, the area of this depositional zone was
measured at 104,417m2 ± 0.3. Using trail markers on the alluvial slope, the average
annual deposition was measured at 0.0265m/yr ± 0.0070m. The average annual volume
of deposition in the west zone was calculated as 2,763 ± 20 m3/yr.

South: The southern depositional zone is the flat plain adjacent to tailings rock
piles and vegetated native soil hillsides along the south rim of the pit. It is downslope
from the east depositional zone. Hillslopes here contribute smaller amounts of sediment
due the fact that they are not as tall and are more protected by vegetation, and contain a
higher fraction of coarse material as waste rock evidenced by hummocks and rock piles.
As a result, only a few areas of cliff face exist along the south side of the mine. Most of
the material deposited in the south quad of the pit floor likely originated from the east, as
46
the main channel runs along, and actively deposits in the vegetation at the south side of
the pit floor. The area of the south zone was measured using ArcMap as 68,124m2 ± 0.3.
The annual deposition rate in the southern quadrant was found to be 0.01829 m/yr ±
0.0005 m. The average annual volume of deposition in the quadrant was calculated as
1,245.98 ± 35m3/yr.
Sediment Budget
Annual sediment yield from the mine was calculated by adding the volume
eroded by pit rim recession of 10,900 ± 99.5m3 to the volume contributed by surface
wash measured with erosion bridges of 9,100 ± 50.5 m3 , and subtracting the 12,700 ±
889 m3 volume of deposition in the pit floor measured with posts and stakes. The
estimated total annual sediment yield discharged from Malakoff Diggins mining pit was
7,300 ± 1,039 m3/yr. The sediment budget is an important gross estimation of surficial
processes within the pit, and the impact to downstream waterways.
Hydrology
The precipitation regime for 2012/2013 was, characterized by large high
intensity storms in December and smaller lower intensity storms throughout spring as is
shown in Figure 11. Rain and snow events were unseasonably small and short in January
February and March. The lack of large winter storm events in 2013 marked the beginning
of a drought period in California. The storm regime resulted in increased amounts of
erosion associated with December storms. Multiple storms of 2 inches or greater in
November and December delivered 32 inches of the years 45.76 total inches of
precipitation. December 2012 only saw eleven days without precipitation. January
47
Figure 11. Precipitation events by incremental magnitude measured at
Camptonville Ranger Station.
through May 2013 consisted of cold, dry periods punctuated by small storms which left
eroded material mid slope in gullies and rill channels. Precipitation data is from the
nearest USFS weather station in Camptonville. Data was gathered by downloading the
dataset of incremental precipitation from the Department of Water Resources California
Data Exchange Center’s (2014a, 2014b) website.
The precipitation from mid winter and early spring storms did not have the
intensity or duration to cause the widespread erosion events which occur during normal
and above normal precipitation years. In 2013, small storms with dry periods of a week
or more caused the antecedent moisture conditions to be dry. With dry antecedent
moisture the “wetting up” period at the beginning of each storm was longer. Dry soils
combined with short low intensity storms produced little primary rainsplash erosion and
48
secondary washload erosion with runoff. Nonetheless, erosion by needle ice and frost
wedging dominated during January, February, and March 2013.
Frost wedging and dry ravel associated with freeze thaw cycles between snow
events, and the re-creation of rills and re-mobilization of recently deposited loose
material in gullies. Low intensity storms and frost wedging erode upper slopes which are
steeper than 50 degrees and deposit this material within the slope’s rills or gully bottom
where slope is 30 deg or less. Periodic measurements highlight this process where
deposition was recorded at erosion bridges 4 and 6, leading to a small average of erosion
measured on February and March, with a sharp reduction in average height measured
following large April storms (Figure 12). This translocational deposition is effectively
mid-slope storage, leaving the loose material in channels where it is easily moved by the
flowing water of a storm of sufficient intensity and duration.
Figure 12. Detail of spring 2013 precipitation intensity and duration with sampling
events at erosion plots. Data shows buildup of material associated with small storms
and frost of February and March, and subsequent erosion of the material during
warmer, higher intensity storms in April and May.
49
Observations of precipitation and frost highlight the mechanisms of the
sediment pulses associated with storm events. Small storms, and frost create conditions of
loose material stored mid slope, allowing subsequent large storms to mobilize large
volumes of this material in short period of time as washload. This could explain
extremely high sediment concentration of waters within the pit (NCRCD, 1979a),
occurring when a large storm follows a period with small weather events and or freeze/
thaw cycles common at this altitude. During March 2012, a relatively small intensity
storm produced runoff that had a significant sediment load (Nepal, 2013).
CHAPTER VI
DISCUSSION
The study area has multiple factors contributing to relatively high rates of
erosion, and subsequent effects on turbidity downstream. Native substrates contain large
percentages of fine, highly weathered clays such as kaolinite, large areas void of
vegetation, and oversteepened slopes prone to gullying, hypersaturated flows, and
landslides that deliver sediment to the basin by separate mechanisms within similar space
and time scales. The cumulative effect of surficial processes in time and space makes
quantifying the amount of sediment attributed to each individual process difficult.
Deposition at the base of actively eroding slopes, and re-mobilization of these deposits
complicates calculations of sediment volume delivered to Hiller Tunnel and Humbug
Creek.
The data may not fully represent a typical water year’s erosion, 2012/2013
was a relatively dry year with only 45 inches of precipitation, compared to an average of
52 inches. The volume of erosion estimated using data from the erosion bridges in
2012/2013 remains less than the volume of deposition. How can this be possible? This
may be due to the fact that erosion data were only collected for one year, while
deposition data were averaged over an eight year period coinciding with a wetter than
average period in California. Furthermore, most of the posts and stakes used for
measurement were located in areas around the edge of the pit floor where the influence of
50
51
alluvial fans may make depositional rates higher than areas that are more central within
the pit floor. This “edge effect” may be falsely representing deposition.
The measurements made in 2013 were not adequate to cover the large area
and multitude of terrains and substrates which erode at different rates. The 7 small
erosion plots were not able to measure all of the slopes or even fully characterize erosion
on the slopes where they were located. Also, data of erosion was only gathered for one
year, far too short to prove definitive trends relating the processes observed and measured
regarding erosion rates to deposition below. Nonetheless, measurements of this study
provide a first order estimate associated with rill and interrill erosion.
Wide variation in erosion across space and time created large uncertainty in
results. Each erosion bridge plot was impacted by rilling, and deposition from above. The
use of the contour profile of the erosion bridge showed that rills deeply cut through
measurement plots, causing large erosion related elevation change at 2-4 points of the 20
along the 1m transect of measurement. However, many of the locations where this
occurred also measured deposition at other points along the 1m transect, which
sometimes resulted in an average net increase in elevation. Slopes greater than 50̊ were
expected to have the highest erosion rates. Measurements at Malakoff diggings showed
that the highest erosion rates were located not only on steep slopes, but more precisely at
a location on steep slopes just above the concave break in slope (mid to ¾ height to top).
Slopes greater than 70̊ were unable to be measured due to their inaccessibility, although
observations suggest that the near vertical slopes are prone to mass wasting failure as
their primary erosion mode.
52
The most surprising results were from plot 3, where deposition was expected
on an upper alluvial plain in the far east side. Erosion of 29.25 mm at his site identifies
that the alluvial plain is actually a colluvial bench where landslide deposits are reworked
and transported into the gully below. The reworking and transport of large amounts of
disturbed landslide colluvium at this location sustains the landslide complex as material is
constantly transported away from the toe of the slope. Two clay layers within the cliff
wall act as aquicludes to infiltration from forested lands above, resulting in seeps within
the scarp. The seeps at these layers lubricate a slip layer for recurring landslides, while
the water flowing from them constantly reworks material on the bench plain near erosion
plot 3 even during dry months.
Sediment Budget Discussion
Remedial actions at Malakoff Diggins are dependent upon the quantification
of the volume of sediment discharged from the mine through Hiller tunnel.
Approximation of the annual sediment volume could aid in the design and lifespan
calculation of a new sediment settling basin to control sediment delivery. In addition, the
annual volume of sediment discharge could serve as a baseline upon which the success of
remedial actions can be compared to. Measurements of sediment sources and sinks in this
study were used to calculate an approximate annual volumetric sediment budget.
However, the values can only be approximate estimations because the calculation of
erosion associated with mass wasting is imprecise and relies on assumptions which
cannot be verified. Furthermore, the volume of each source and sink were measured over
different time scales. The mixing of values taken as averages of many years and
53
compared to values from single year measurements introduces and propagates a large
degree of error into the sediment budget calculation. Nonetheless, the approximate
volume of sediment annually discharged from Malakoff Diggins is still of value to
stakeholders.
Comparison of Discharge Estimate to
Suspended Sediment Load
In a parallel effort at Malakoff Diggins, Harihar Nepal a Chico State graduate
student (2013) measured suspended sediment load in Humbug Creek downstream of the
Hiller Tunnel discharge. The suspended sediment in grab samples collected during the
2011 and 2012 water years was compared to continuous turbidity measurements collected
in Humbug Creek. The annual suspended sediment load from Hiller Tunnel was
estimated to be 1,021,490 kg/yr ± 75,000kg of suspended sediment (Nepal, 2013). The
bed load was not measured as a part of this estimate. The annual sediment load in 2011
and in 2012 was dominated by a single storm event that contributed nearly half of the
annual load, ~45,000 kg in December 2nd storm 2012 and 500 tons in March 17th storm
in 2012, (Nepal, 2013; Monohan 2014).
Comparison of the findings of Harihar Nepal and the findings of this study are
helpful to identify the accuracy of estimations made. To compare the two, unit
conversion from kg to m3 was necessary. Using the density of the dominant clay within
Malakoff Diggins, kaolinite, (2.65 g/cc) conversion of the units yielded a mass of
1,021,490 kg /2600kg/m3 = 393m3 in Nepal’s study. This number seems quite low,
particularly in the face of the 7,316m3 proposed in this study. Both of these studies rely
heavily on assumptions for calculation which may not hold true during all conditions. For
54
instance: the relationship of turbidity and discharge may become exponential outside of
measured points like during extreme discharge. Or, material lost to mass wasting may not
become mobilized into overland flow during re-working, but instead stays as colluvium
at the base of slope.
The assumption of kaolinite as the suspended material greatly influences the
volume calculated during unit conversion. For instance, if the material were bentonite,
the volume linked to Nepal’s study would be 1,722m3. The presence of water bound
within clays can alter their density, shifting it from 2.6g/cc to 1.7g/cc when wet or
saturated (Grim, 1968). Nonetheless, the difference between the two studies is too large
to ignore, and more study is needed.
Potential Bias and Uncertainty in Erosion
and Deposition Measurements
Potential bias and sources of error of erosion bridge measurements come from
many sources. One of the sources of error is measurement rods sinking into soft soil
giving a false measurement of increased erosion. Another potential source of error is
caused by the installation and measurement influencing the erosion and deposition. Rebar
pins that are the foundations of the erosion bridges gather sediment behind them on the
upslope side, potentially reducing the measured erosion at rods on the edge of the
apparatus, namely rods 1, and 20. Approaching erosion plots to measure them could alter
the erosional rate below the plot through footstep erosion and compaction. It is unknown
exactly how disturbance downslope could affect measurements, but altering the flowpath
of natural erosion could lead to accelerated erosion or mid slope deposition. Other
55
sources of potential error include the movement of the rebar pins due to clay shrink/swell,
frost heave, soil creep, or disturbance from animals or park visitors.
Potential error of deposition post measurements include confusion of trail
marker posts due to the duplicate numbers of new and old, and loss of some of the posts
from burial. A more serious introduction of error is scour in the laterally shifting
channels, and the trailside effects of on trail markers and t-posts like preferential
channeling, footprints, and the fact that many people use the tops of the shorter posts to
scrape the clay from their boots. A more inherent source of error is the lack of stakes in
the center of the pit floor.
Potential bias associated with ArcGIS measurements are primarily associated
with the georeferencing of the 1952 image to the orthorectified image of 2012. As stated
in results, the RMS error was 24.8 m. Only measurements of pit rim expansion rely on
the georeference, and georectification processing. Area measurements of surface erosion
and deposition are much more precise, at ±0.3m as they rely on the 2014 ESRI imagery
(ESRI, 2014).
One year of data for surface erosion measured with erosion bridges, and only
seven plots representing a large area is not enough to be precise. Many areas of the pit
were not directly measured, and data from only one erosion bridge was applied to large
morphologically similar areas nearby which may not have the exact same erosional
characteristics as those measured, as assumed.
The auriferous gravels of the pit are highly heterogeneous, with lenses of
clays and gravels of various types throughout. The area was created in a laterally shifting
depositional floodplain, and buried channels from different time periods are evident in
56
the cut face of the exposed mine walls. There is a general fining upward within the
terrace gravel, with larger gravels at the bottom, and clayey sand at the top, capped by
soils derived from parent material (Yeend, 1974). Multiple soil types exist as the upper
unit at different locations around the pit with aiken loam to the south horseshoe series at
the east and west and Cohasset cobbly loam to the north. Representative units were
chosen in this study based on their general grain size, and matrix characteristics
indicating the terrace formation they belong to.
CHAPTER VII
CONCLUSIONS
The major sediment sources within the pit were the eastern and western cliffs
which were identified using GIS measurements of rim expansion over the past 60 years.
The eastern and western cliff walls were considered significant source contributors of
sediment within the system because expansion was more prevalent from the stratigraphic
units in these areas which contain a higher proportion of fines, and have a lower cohesion
when compared to cliff units in the north where expansion was less prevalent (Peterson,
1980). In addition, the west and east cliffs have larger contributing slopes area than the
north slopes, and are introducing surface water runoff which is aiding in the
destabilization, mobilization, and erosion of the east and west slopes. Oversaturation of
soils in the east and west cliffs during rain events can lead to mass wasting failures
initiated by the continued oversteepening of the translocational and depletion zones of the
concave slopes by gullies as they incise and headcut. Pit rim expansion was driven by
factors of strength, location, and shape such as: matrix cohesion and stratigraphy,
saturation from contributing slopes above, cliff height and angle.
The annual erosion volume in the pit was measured using representative point
measurements with erosion bridges. Stratigraphic units associated with plots 1, 4, and 6
were identified as major source contributors of sediment from surface wash. Areas where
these units are exposed exhibit attributes which increase their sediment delivery such as:
57
58
low cohesion due to a homogeneous matrix of sand gravel and clay which can be easily
eroded (unlike compact clay layers or cemented gravels). Contributions of sediment from
these units can be exacerbated by close proximity to a channel in a well-defined rill and
gully network that can rework material. Large areas of exposed material meeting these
criteria exist in the southeast and west sides of the pit and have the potential to deliver
large amounts of sediment to the water column downstream during rain events.
The annual erosion volume from surface wash erosion was 9,117 ± 51 m3/yr
of sediment to the system during the 2013 water year. Erosion results show a wide
variation between plots, and throughout the year (Figure 9). Observations and results
reveal the episodic nature of erosion at Malakoff Diggins. Measurements at erosion plots
identified slopes within gully complexes such as plots 1, 4, and 6 as having the highest
erosion rates. Erosion plots located on colluvial deposits such as 2, and 7 measured much
lower erosion rates and even deposition as above slopes contributed material by
slumping, creep, and dry ravel. Furthermore, stratigraphy appeared to play an important
role in cohesion. Sites with large sand, gravel and cobble lost the most elevation to
erosion, where sites dominated by clay such as plot 7 eroded very little comparatively.
The annual deposition volume in the pit was measured using representative
point measurements with posts and stakes on the pit floor. Deposition is dependent on the
velocity of overland flow within the pit floor. Higher velocity in channelized flow
reworks previously deposited material and keeps particulates in suspension with
turbulence of flow. Deposition on the pit floor is aided by the vegetation growing within
the alluvial plains around the pond, by reducing the velocity of flow, allowing sand size
and greater to settle out. This is evident in observations within the pit floor where soft
59
clay and silt mud exists nearest to Hiller Tunnel, where vegetation is very dense, and
coarse sand and gravels become more prevalent upgradient where vegetation is less
dense.
The annual deposition volume measured on the pit floor was 12,700 ± 889
m3/yr. The deposition rate was created from an average of elevation change over an eight
year period from 2005 to 2013. Erosion rates were measured within individual quadrants
of the pit floor related with different source areas above (Figure 10, Table 5).
Additionally, areas with high deposition rates may have contributing areas upstream
which could be major source areas of material. Observations and measurements in the pit
floor indicate a potential to reduce sediment load in the discharge from the mine, by
increasing deposition rates in quadrants with higher deposition rates, such as the east
quadrant by reducing flow velocity and creating conditions for finer grain sizes settle out.
Using a simplified sediment budget to calculate annual sediment budget at
Malakoff Diggins the two erosional components were added together and the deposition
subtracted from this sum to estimate the annual discharge volume. Pit rim expansion by
mass wasting and gullying delivered 10,900 ±99.5 m3/yr, and surface wash erosion
delivered 9,100± 51 m3/yr., the deposition on the pit floor trapped 12,700 ± 889 m3/yr
which discharges an estimated total of 7,300 ± 1,039m3 of sediment annually from the
mine to the Humbug Creek watershed.
The estimated annual sediment discharge was significantly lower than
previous studies which estimated 23,000-35,000 m3 of sediment per year discharged from
Hiller Tunnel (Yuan, 1979; Peterson, 1980). The reduced discharge volume in this study
may be in part due to a lack of rainfall in the 2013 water year for which surface wash
60
erosion was measured. Also, this study used a more comprehensive estimate of
deposition within the pit floor, which Peterson and Yuan did not account for (Yuan,
1979; Peterson, 1980). The large array of deposition posts and t-stakes around the entire
pit floor enabled deposition to be measured at locations accepting runoff from nearly
every slope. However, vegetation growth and slope stabilization, as slopes approach the
angle of repose, have occurred in the 30 years since the previous studies were conducted
(Yuan, 1979), and may have also reduced the total volume of sediment delivered to the
outlet at Hiller Tunnel.
Sediment volume estimates for erosion and deposition made for any water
year at Malakoff Diggins are dependent on the magnitude, duration and frequency of
precipitation events. Direct field measurements of surface erosion made in 2012/13 for
this study occurred during a relatively low water year with minimal precipitation,
2012/13 only had two storms lasting more than one day of constant rainfall. The reduced
amount of precipitation during the 2013 water year may have led to a reduced amount of
sheetwash erosion during this study period. However, estimates of pit rim erosion were
based on a 60-year period of measurement, and deposition was measured over an eight
year period, allowing for changes associated with episodic wet or dry periods to have less
effect on average results. Sheetwash erosion measured in this study may be
underrepresented due to the reduced rainfall, and therefore, a longer period of sheetwash
erosion is needed for increased precision of the sediment budget. Nonetheless, this study
utilized more measurements of erosion by surficial processes and a larger area of
deposition than previous studies.
61
The previous study by Yuan utilized photogrammetry over a 69 year time
period between photos, beginning at the time of the mine closure. Erosion and deposition
rates were most likely much higher during the first 25 years within the pit because the
area was completely void of vegetation, the walls were near vertical, and the pond was
larger, making it a more effective sediment trap. Measurements of surface wash erosion
using photogrammetry were based on the elevation change of the top of knife-edge
ridges, and did not measure surface wash on lower slopes where erosion rates may be
higher or lower depending on rill networks. Furthermore, pit rim recession, and
deposition on the pit floor was not measured as part of the discharge estimate.
The previous study by Peterson estimated mass wasting contributions in the
east end of the pit only, and was not able to account for the gully expansion of the
northern and western pit rim. Furthermore, the deposition rate used was based on
measurements using ground penetrating radar to estimate the total volume deposited
since the closure of the mine, and averaging this over 100 years. This may have
overestimated deposition as the basin likely had historically higher erosion and
deposition rates in the early years after closure. Surface wash erosion was measured using
dowels in four 1-meter plots. Originally, six plots were installed, but two were lost during
a landslide, and Peterson concluded that four plots were not enough to fully represent
erosion within the pit.
In many ways, this study mirrors the Peterson study of erosion at Malakoff
Diggins, in that mass wasting, surface wash, and deposition were all measured in order to
estimate annual discharge. This study updates the erosion rates estimated by Peterson
62
thirty-three years later using technology of GIS for area measurements, and a modern
direct measurement of surface wash and deposition.
This sediment budget is a slightly more complete description of surficial
processes than previous estimates made at Malakoff Diggins primarily because pit rim
erosion was more comprehensively measured using GIS technology and tools.
Furthermore, sheetwash erosion was measured at seven locations within the pit which
were chosen to represent dominant substrates, and at the upper, middle and base of slopes
to capture the differing erosion rates. Even so, the seven erosion plots monitored for one
year were less than enough to fully characterize surface wash erosion. The deposition
measurements were made with a large array of 69 posts on the pit floor monitored over 8
years. Due to the time period, the measurements of deposition made in this study are
representative of modern deposition rates within the pit floor. All studies have
limitations; this study was limited by and the small number of surface wash erosion plots,
and their one year of data collected during a below normal rainfall year.
CHAPTER VIII
RECOMMENDED FUTURE WORKS
Future studies should use modeling techniques such as modified RUSLE for
disturbed lands with a focus on stratigraphy and slope geometry/morphology. Future
empirical measurements could use more intensive types of erosion measurements such as
catchment basins on representative slopes which can be scaled up to encompass the entire
pit. However, catchment basins are invasive and require digging to create. DPR may not
be able to allow alterations to the historic mine in this way, as it could destroy part of the
historic cliffs with drainage alterations. Additionally, leaving plastic tarps and equipment
in place for more than a year has a visual impact.
Light Detection And Ranging (LiDAR) would be the most accurate and
comprehensive alternative to labor intensive and potentially destructive physical
measurements. Ground based LiDAR scanning of erosive cliffs has been used by USGS
at the confluence of Humbug Creek and the South Yuba River where mine tailings are
actively eroding (Fleck et al., 2011). To apply the same technology to the entire pit at
Malakoff Diggins could be cost and labor prohibitive due to the size and complexity of
the pit walls. Terrestrial LiDAR is proposed by USGS to occur in Malakoff Diggins at at
least 3 locations in the pit, in addition to an aerial flyover using LiDAR. The areal map
will provide much needed detail of the highly heterogeneous landscape and could be used
to improve the accuracy of calculations based on representative areas.
63
64
Monitoring of surface wash erosion over a longer (multi-year to decadal) scale
would improve the understanding of surficial processes within the pit. Correlation of
surface wash erosion with precipitation and frost events across a longer time period is
needed to understand the relationship between these events and temporary storage.
Temporary storage within the pit may be partly responsible for episodes of high sediment
discharge as colluvium is reworked.
Previous studies measuring suspended load at ephemeral streams within the
pit and Humbug Creek are useful in tracking future changes to the watershed either by
natural processes of vegetation recovery or alteration of the basin as a result of
remediation activities. Measurements of suspended load and bedload in discharge may
prove the most, effective method of quantifying the success of future management
attempts to control sediment discharge from Malakoff Diggins.
Any additional monitoring should consider that erosion in particular is driven
by the magnitude, duration, frequency and timing of precipitation events and that at this
elevation (2,900 feet) can also be temperature dependent. Rain is much more erosive than
snow due to the force impact of raindrops. Surface erosion rates are increased when
precipitation as rain is intense over a long duration such as multiple days in a row.
Precipitation in California is somewhat cyclical based on El Nino/La Nina 3-7
year cycle where droughts are punctuated by flood. Northern California can receive
moisture from the northern edge of El Nino effects, and the southern edge of La Nina
effects. However, within the central Sierra, all but one of the biggest floods have
occurred during La Nina winters, as La Nina is more closely associated with rain on snow
events (Redmond, 1998). Therefore, it is not infeasible that a very wet warm series of
65
winter storms known as a “pineapple express” such as occurred in 1986, and 1997 could
deliver more than 10 -20 inches of rain in a 24 hour period at Malakoff diggings, a
location where orographic rainfall is prevalent due to topography.
Rainfall of this intensity could trigger mass wasting failures particularly in the
eastern landslide complex, and have the duration to carry much of the landslide
colluvium downslope toward the pond in overland flow. During an event of magnitude,
surface wash erosion could be more than double the amount estimated in this study.
Although the Malakoff Diggins has become more vegetated since the mine was created,
and the cliff walls have expanded closer to their angle of repose, a heavy winter could
still discharge much more than the 7,300 m3 reported in this study during an extreme
flood year.
During this study many storms simply delivered less precipitation than
expected, and the intervals between were completely dry, where a normal year would
have cloudy, high humidity days and light rain between major storms which increase
overall rainfall totals, but do little to increase surface erosion directly. Therefore,
additional monitoring to determine the annual rate of erosion and deposition should be
conducted within the context of the hydrologic regime and can only represent the time
period during which measurements are collected.
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QUANTIFYING SURFICIAL PROCESSES IN MALAKOFF DIGGINS

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