Bastrop Drainage Assessment

Bastrop Drainage Assessment
Bastrop Watershed Facts
Drainage Area
Noncontributing Area
Stream Gradient
Watershed Slopes
Soil Types
Ground Cover
Land Uses
Soil Erodibility
Photo 14: Piney Creek flowing through pastureland
upstream from State Highway 95.
Piney Creek
Gills Branch
Approx. 31.6 sq.mi.
~4.3 sq.mi. (Lake Bastrop)
9.4 ft/mi.
2 to 5% slopes
Axtell fine sandy loam,
Tabor fine sandy loam
Woodlands, grass prairie
Grazing, hay crop
Severe
Approx. 2.6 sq.mi.
~0.5 sq.mi. (stock ponds)
39.6 ft/mi
0 to 10% slopes
Sayers fine sandy loam,
Trinity clay, Crockett soils
Pine forest, floodplain grasses
Residential, urban
Moderate
360
340
360
370
360
360
380
35
350
350
370
360
ek
e
r
C
ey
340
340
Piney Creek drains a large area upstream from the City of Bastrop and flows around the north
edge of town. The creek has a significant sediment load, consisting of fine-medium grained
sand derived from the “Sand Hills” near Phelan. Most of the land surrounding Piney Creek is
pastureland with good grass cover, and the stream channel is stable in most locations. In the
northern part of Bastrop near Mesquite Street, the land is nearly level, and drainage is provided
by an underground storm sewer, which drains to Piney Creek. Span bridges cross Piney Creek
well above flood levels at State Highway 95, Main Street and Reid Bend Road. At River Road,
backwater from the Colorado River extends up the channel of Piney Creek to the downstream
side of the low-water crossing. River Road is susceptible to flooding from Piney Creek and the
Colorado River.
350
36
0
Gills Branch is a relatively small watershed in wooded country upstream from State Highway 95.
All of the road crossings of Gills Branch, with the exception of State Highway 71, consist of
three-chambered concrete box culverts with similar design flow capacity. However, sediment
has been allowed to accumulate at some of the box-culvert crossings (Farm, Chestnut and Pine
streets), thus reducing the available flow area and carrying capacity. Stream bank erosion is
evident at the south entrance to Chestnut Square Theater, an example of potential future impacts.
Along the entrance drive to Emile Elementary School, drainage culverts have caused erosion
problems along the banks of Gills Branch. Downstream from MLK Street, trash and debris in
the channel are more noticeable, including potentially toxic materials (lead-acid batteries,
chemical containers, etc.). Several trash dumps exist on the banks of Gills Branch downstream
from State Highway 71. In the lower reaches of Gills Branch, the channel has incised downward
into the Culvert Bluff formation, exposing layers of shale and lignite in the streambed. Gills
Branch meets the Colorado River just downstream from the Bastrop wastewater treatment plant.
370
11 12
360
35
Mesquite St
360
350
13
350
Maple St
Hill St
Photo 13: Storm drain drop-box inlet south of Mesquite
Street, in area of low gradient where drainage improvements
have been installed.
0
39
0
Pi
n
340
330
370
0
38
390
36
0
350
Magnolia St
Reids Bnd
Magnolia St
Locust St
360
350
360
340
370
380
14
0
The historic downtown district of the City of Bastrop drains to the Colorado River via two
drainages: Gills Branch and Piney Creek. Although the town lies within the alluvial valley of
the Colorado River, most of the downtown buildings are not in the Federal Emegency
Management Agency (FEMA) 100-year floodplain. The mapped FEMA floodplain, shown as
the shaded area on the aerial photomap, extends from the high terrace near the banks of the
Colorado River, upstream along Gills Branch and Piney Creek to State Highway 95, and further
upstream on Piney Creek.
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Photo 1: State Highway 95 crossing of Gills Branch, three
8x8-foot box culverts, stable channel draining woodland
watershed upstream from Bastrop.
95
Vista West Ct
Linden St
Oak St
Jefferson St
0
34
0
Persimmon St
Hickory St
Main St
0
37
Water St
0
!
(
21
!
(
C
o l o rado
95
Photo 5: Poor channel conditions at Pine Street box culvert.
Note how sedimentation and vegetative growth inhibit flow.
Gi
l
320
St
!
(
71
370
Jasper St
350
340
330
320
330
340
350
8
Photo 6: Channel blow-out downstream from culvert drain
near north entrance drive to Emile Elementary School.
Trash
Dump
0
Love
Perkin
s
390
Photo Location and Direction
Major Roads
Minor Roads
Drainage Ditch/Storm Sewer
Contour Line (elevation in feet)
Photo 8: Trash dump in ravine draining to lower Gills
Branch, one of several disposal sites downstream from
State Highway 71 crossing.
FEMA 100-Year Floodplain
Water Crossing
Bastrop City Limit
Known GPS Locations
Photo 7: Sediment-clogged channel of Gills Branch
upstream from railroad crossing. Upper banks are
stabilized by grass cover.
21
71
!
(
Emile St
!
(
City of Bastrop
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(
95
9
32
71
!
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0
Bastrop
WWTP
ranch
B
ls
Photo 10: River Road low-water crossing of Piney Creek,
backwater area of the Colorado River. The road can be
flooded from creek or river.
340
35
0
360
360
35
340
360
0
!
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rs Ln
36
360
Lake
Bastrop
Maynard St
Newton St
340
Martin Luther King Dr
290
95
St
0
Gutier36rez
Debris
Pile
380
380
360
£
¤
!
(
0
µ
Elgin
!
(
!
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Hasler St
36
370
360
71
0
35
Area of Detail
City of Bastrop
Bastrop County, Texas
College St
!
(
360
Piney Creek is an intermittent stream with a large drainage area upstream from Bastrop and
a relatively wide floodplain (photo 11). Fortunately, there has been little encroachment on
the floodplain by urban development, except for a mobile home community at the north end
of Main Street. The channel of Piney Creek is stable despite the fact that it contains
potentially erodible sandy bed and banks. Piney Creek is in good hydrologic condition with
stable, well-vegetated banks and efficient sediment transport in the channel.
6
380
34
Gills Branch receives most drainage from the historical district, where impervious cover is
estimated to be 30 to 50 percent. Rainfall runoff, when it reaches the urban drainage of Gills
Branch, has caused erosion in some locations and sedimentation in others. Examples are
erosion of stream banks and deposition of sediment in culvert crossings (see photos 3-7).
Downstream of State Highway 71, there are several trash dumps (see photo 8), and Gills
Branch has eroded downward into the bedrock underlying the terrace alluvium of the
Colorado River, exposing the Calvert Bluff formation (see photo 9). This small ephemeral
stream is changing its channel to adjust to variations in flow and sediment content caused by
urbanization. For comparison purposes, small creeks in urbanized areas of Travis County
have become 5 to 10 times larger in size due to unmanaged stormwater runoff. In Gills
Branch, management of erosion and sedimentation may be a priority goal.
Emile St
Broken
Concrete
7 Apron
5
St
Higgins St
360
0
Austin St
370
0
kins
36
s St
Jone
36
g Dr
360
Martin Luther Kin
Walnut St
Per
The historical downtown district of Bastrop is on a terrace above the Colorado River.
Drainage is directed toward Piney Creek to the north, Gills Branch to the south and the
Colorado River to the west. For the most part, the historical district is above the 100-year
floodplain. However, there are local drainage issues due to nearly level ground, so drainage
canals and underground storm sewers are used in some parts of the city.
Haysel St
0
Hill St
340
350
360
36
21
4
0
370
360
0
32 0
33
0
Photo 4: Sedimentation in box culvert, 8x8-foot chamber
one-half blocked by sediment build-up.
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3
Pine St
32
360
36
Bastrop Watershed Summary
Chestnut St
0
36 0
35 0
34 0
33
360
21
390
370
!
(
400
Spring St
Fayette St
r
Jefferson St
ive
Farm St
370
0
Photo 11: Piney Creek and floodplain. Note deposits of sand
in overbank area from recent floods.
ad
oR
Photo 3: Erosion of rip-rap channel from culvert drainage,
Chestnut Square Theater entrance. There is a potential for
headwall failure as channels erode.
2
Beech St
370
34
0
0
32
lo r
Co
36
380
0
Buttonwood St
Fayette St
33
Catalpa St
Fayette St
Church St
350
Jefferson St
0
36 0
35 40
3 30
3
Buttonwood St
1
Gi
lls
Cedar St
390
Rosanky St
370
0
Delano St
Cypress St
Sereni3ty
40 Ln
32
Photo 2: Tree-lined channel of Gills Branch downstream
from State Highway 95 crossing, banks supported by roots
of pecan and sycamore trees.
Linda St
Wilson St
0
Cypress St
Elm St
370
Li n
Elm St
32
Elm St
Pecan St
ln St
co
Garfield St
t
Roosevelt S
Photo 12: Main Street bridge crossing of Piney Creek, near
Bastrop city limits.
Main St
10
360
Buchanan St
River Rd
Carter St
0
Chambers St
0
33
35
Hawthorne St
410
Juniper St
37
h
Branc
0
Laurel St
380
36
Laurel St
360
Laurel St
R
!
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CORRELATION OF PHOTOGRAPHS TO THE MAP
The red arrows on the watershed map are at the location and direction of a photograph.
For example, the red arrow with 9 on the map relates to photo 9. The arrow orientation indicates
the direction the photograph was taken. For example, photo 9 is looking downstream (west). The
photographs were taken in February 2005.
0
1,000
Richard Winkelbauer ‘05
2,000
Feet
Photo 9: Mouth of Gills Branch near Colorado River. Incised
channel eroded into shale and lignite of the Calvert Bluff
formation.
Data Source: Aerial Photography - Capital Area Planning Council (CAPCO), 2002;
Watersheds derived from U.S. Geological Survey digital elevation model;
Roads - CAPCO; Floodplain boundaries - Federal Emergency Management Agency;
Landowners/boundaries - Travis Central Appraisal District, 2003
This map has been produced by LCRA for its own use. Accordingly, certain
information, features or details may have been emphasized over others. LCRA
does not warrant the accuracy of this map as to scale, accuracy or completeness.
The FEMA 100-year floodplain was represented by electronically using the preliminary
Digital Flood Insurance Rate Map (DFIRM) for Bastrop County. This preliminary data is
dated September 27, 2004, and is not the final data for this area. Every effort was made to
geographically orient creeks and floodplains to known locations. There are instances
on this map where creeks and associated floodplains are not consistent. LCRA
cannot correct nor take responsibility for these inconsistencies.