Fluvial Patterns in the Loktak Lake Sub

Sengupta, M. and Dalwani, R. (Editors). 2008
Proceedings of Taal2007: The 12th World Lake Conference: 717-731
Fluvial Patterns in the
Interlinking Channels
Loktak
Lake
Sub-Basin
Through
Two
Ngangbam Romeji Singh, K. Sonamani Singh and Nayan Sharma
Indian Institute of Technology Roorkee, Roorkee-247667, Uttarakhand, India
Email : ngromezi@gmail.com, nayanfwt@iitr.ernet.in
ABSTRACT
The effects on the fluvial system of Loktak lake sub-basin due to the interexchange of flows between the
lake and Imphal / Manipur River, attributed to as bi-directional flow, is formulated in the study. The flow
transients with respect to direction of flows, referred as ‘inflow’ and ‘outflow’, resulting from rapidly
fluctuating flood spells and barrage regulations, in two interlinking natural channels - Khordak and
Ungamel, is examined with the 1-D numerical HEC-6 program. An integrated multi to auto-regressive
approach is used to check the sensitivity of the observed stage, discharge and sediment discharge. The
observed data is segregated for each of the two flow directions from a base dataset. A built-in procedure
in the HEC-6 program adopting a range of water depths, each separately on a selected set of discharges,
is used to develop a set of Interpretation curves for subjective stage-discharge and sediment discharges
values for respective inflows and outflows in the two channels, with boundaries at the lake and river
confluences. The application and results of the curves are found to be consistent with the prevailing
hydrodynamic transients.
Keywords: bi-directional, hydrodynamic, inflow, outflow, sediment discharge, data, HEC-6,
interpretation curves.
INTRODUCTION
The Manipur river basin though small in entity as
compared to the major river basins in the country and
the globe, finds a phenomenal hydrodynamic
attribute in its existence. Though it seems that the
peculiar conditions of ‘Bi-directional’ flow or
interchange of flow in natural state was observed
long back by the ignoble dwellers around the outlet
point of Loktak Lake, the paradigm has hardly been
necessitated to be delved upon. The present study is
an attempt to outline these hydraulic transient
phenomena with emphasis on the sediment discharge
characteristics in a one-dimensional numerical
model. Numerical modelling has proved to be an
effective tool for the study of morphological
processes in alluvial rivers. Generally, alluvial river
processes evolve over long time periods (Garde and
Ranga Raju, 2000). The fluvial interlink processes of
alarming siltation rate, effect of adverse land-use and
highly alluvial planform, is incorporated through the
Khordak and Ungamel channels in the Loktak subbasin. Though there are a number of streams flowing
into the Loktak lake, these two natural channels are
the only conduits providing outlet of high flood
water and fluvial discharge from the lake as well as
the entire basin. The peculiarity arises when flow
through these channels reverses back or there is
‘inflow’ into the lake. In simpler terms, the
movement of water in both ways along the same
conveyance channels in short time spells, perplexes
the hydrodynamic behaviour of the Loktak lake sub-
basin. These two natural interlinking channels play a
key role in the overall water balance of the lake, and
more profoundly serve as the only functional units of
sediment outflows from the Loktak Lake (the other
major abstraction through the power channel of the
Loktak Multipurpose project is controlled and
therefore not considered in the study from sediment
discharging point of view).This hydraulic transient,
termed here as ‘Bi-directional flow’ prevailing in the
Loktak lake sub-basin of the Manipur River Basin, is
studied with HEC-6 (USACE) program. The study
examines the Loktak lake sub-basin as a fluvial
hydrosystem.
HEC-6 APPLICATION AND COMPUTATIONAL
FRAMEWORK
HEC-6 is a one-dimensional movable boundary open
channel flow numerical model developed by the US
Army Corps of Engineers. It is designed to simulate
and predict changes in river profiles resulting from
scour and/or deposition over moderate time periods
(typically years, although applications to single flood
events are possible). A continuous flow record is
partitioned into a series of steady flows of variable
discharges and duration. For each flow a water
surface profile is calculated thereby providing energy
slope, velocity, depth, etc. at each cross section.
Potential sediment transport rates are then computed
at each section. These rates, combined with the
duration of the flow, permit a volumetric accounting
of sediment within each reach. The amount of scour
or deposition at each section is then computed and
the cross section adjusted accordingly. The
computations then proceed to the next flow in the
sequence and the cycle is repeated beginning with
the updated geometry. The sediment calculations are
performed by grain size fraction thereby allowing the
simulation of hydraulic sorting and armoring.
Prediction of sediment behavior in shallow reservoirs
and most rivers, however, requires that the
interactions between the flow hydraulics, sediment
transport, channel roughness and related changes in
boundary geometry be considered. Though the
present program module runs in DOS environment, it
is still a potential tool for sediment-related studies.
Inflowing sediment loads are related to water
discharge by sediment-discharge curves for the
upstream boundaries of the main stem channel,
tributaries and local inflow points. Sediment
gradations are classified by grain size using the
American Geophysical Union scale. The program
computes transport potential for clay (particles less
than 0.004 mm diameter), four classes of silt (0.0040.0625 mm), five classes of sand (from very fine
sand, 0.0625 mm, to very coarse sand, 2.0 mm), five
classes of gravel (from very fine gravel, 2.0 mm, to
very coarse gravel, 64 mm), two class of cobbles
(from small, 64mm, to large cobbles, 256mm) and
three classes of boulders (from small, 256mm, to
large boulders, 2048mm). Transport potential is
calculated at each cross section of the channel using
hydraulic information from the water surface profile
calculation (e.g., width, depth, energy slope, and
flow velocity) and the gradation of bed material.
Sediment is routed downstream after the backwater
computations are made for each successive discharge
(time step).
The amount of sediment in the stream bed, using an
average end area approximation, is:
Vsed =Bo Ys
(1)
For a water depth, D, the volume of fluid in the water
column is:
Vf = Bo D
(2)
Bo and D are which are calculated by averaging over
the same space used in solving the energy equation.
The above description of the processes of scour and
deposition must be converted into numerical
algorithms for computer simulation. The basis for
simulating vertical movement of the bed is the
continuity equation for sediment material (the Exner
equation):
Bo
718
=0
(3)
Equations (4) and (5) represent the Exner Equation
expressed in finite difference form for point
P using the terms as shown in Fig.1:
+
=
-
=0
*
(4)
(5)
Figure 1. Computational Grid of the HEC-6 Program
The initial depth of bed material at point P
defines the initial value of Ysp. The sediment load,
Gu, is the amount of sediment, by grain size, entering
the control volume from the upstream control
volume. The factor of 0.5 is the ‘volume shape factor
which weights the upstream and downstream reach
lengths. For the upstream-most reach, this is the
inflowing load boundary condition defined by the
user. The sediment leaving the control volume, Gd,
becomes the Gu for the next downstream control
volume. The sediment load, Gd, is calculated by
considering the transport capacity at point P, the
sediment inflow, availability of material in the bed,
and armoring. The difference between Gd and Gu is
the amount of material deposited or scoured in the
reach, labelled as "computational region" on Fig.1,
and is converted to a change in bed elevation using
Eqn. (5).
Study Area and Description of ‘Bi-Directional’
Flow
The Manipur River Basin has as irresolute fluvial or
sediment system from the past. Located between 240
to 25025’ N and 93036’ to 94027’ E, it covers an area
of 6,872 sq.km where as many as 12 main-stem
rivers inter-dispersed with tributaries, predominates
the hydrodynamic behavior and morphological
changes since its inception as a valley landform and
bounded by young-folded mountains all around. The
basin is marked with a number of wetlands (locally
known as “pats”) which derive their supply as well as
drainage through minor link channels as well as the
main rivers/streams. The Loktak lake is the most
notable among the wetlands. The indeterminate
expanse of the lake as well as integrity to other
periphery wetlands, is what it can be better described
as the Loktak lake sub-basin. It is located all along
the western periphery of the main drainage channel
of the basin -Imphal or Manipur River. Wetlands,
locally known as Pats, constitute 6.8% of the basin
area and Loktak lake (pat) represents 61% of the total
identified wetlands in the whole state. It is the largest
freshwater lake in the north east of the country and
has been designated as a “Wetland of International
Importance” under the Ramsar convention in 1990.
Besides innate with a unique hydrological regime,
the lake harbors rich bio-diversity and inherits the
socio-economic livelihood of the people.
The lake spread provides flood absorbing
capacity to the whole basin of 287sq.kms at the
dictated level of 768.50m above MSL (the valley has
a general elevation of 760.0 m MSL). A total water
spread of 490sq.km was reported during the highest
recorded flood in 1966 so far when Pumlen and
Lamjaokhong pats merged with Loktak lake. The
main water body of the lake is mostly covered by
heterogeneous floating biomasses (locally, known as
phumdis). The lake boundary is difficult to define as
it is surrounded by shallow water stagnating over
marshes or swamps on all its sides. The depth of the
lake varies intermittently from 0.5m to 4.6m. Since
the formation of the valley, the Loktak lake sub-basin
(total area of 1062.96sq. km) has been acting as the
sediment trap sub-basin for the Manipur river basin
which is under acute erosion from the mountains,
composing mainly of erodible shales, much due
anthropogenic land-use adversities. The alluvial
planform changes in the sub-basin as well as in the
lake body due to the accumulating phumdis, is
depicted with False colour composite images
(source: IRS 1C-LISS III) in Figs. 2(a) and (b)
The focus of the study is the striking and unique
hydrodynamic attribute with respect to the flow
through the two natural channels- Khordak and
Ungamel, which interlinks the Loktak lake with the
Imphal or Manipur River, which serves as the artery
of the drainage network of the entire basin (the river
does not fall directly into the lake). These two
channels are the key natural regulators of water
balance in Loktak lake besides other hydrological
processes as evaporation, infiltration, etc. Table.1
highlights the water balance in the lake.
Figure 2(a). FCC Image of Loktak lake sub-basin 1998. Figure 2(b). FCC Image of Loktak lake sub-basin 2003
719
Table 1. Water balance between Imphal/Manipur
River and Loktak Lake (note that the period 1958
and 1959 are before the commissioning of the Ithai
barrage).
Year/period
1958
1959
2000-01
Inflow
(from
Imphal river to
Loktak) Mcum
102
104
388.01
Outflow (vice versa)
Mcum
375
256
170.38
These two natural channels have rapidly
varying hydraulic transients in its direction of flows.
Depending on the relative water levels in the lake
and the Manipur river, flow ‘averses or reverses’ its
direction as from ‘lake to the river’ or vice versa in
short durations. This phenomenal hydraulic condition
had its natural control (known as the Sugnu hump)
located about 27 kms downstream, in the form of a
rock impediment about 7m high and stretching for
more than 2kms all along the longitudinal bed of the
Manipur river acting as a barrier to the flow which is
conveying most of the outflows of the entire basin.
Though now, control is regulated at Ithai barrage
Figure 3(a). Loktak lake sub-basin and drainage
structure (the two interlink channels are shown
with arrows)
720
(just after the Khuga river joins the Manipur river)
which is the main outflow point of the basin,
intricacy in unprecedented water levels and flow
direction is still observed at this point (where the
Ungamel link channel connects the Lake and the
Manipur River). It is to be noted on this regard that
Khordak channel confluence with the Manipur river
is located 5,100m upstream of the Ithai barrage
control. Figures 3(a) and (b) gives an overview of the
two interlink channels along with the drainage
structure.
Geological studies have suggested that
transgression and regression of Loktak lake can be
inferred from related lacustrine deposits and from
alluvial and colluvial formations (G.S.I. report,
1988). It may be inferred in this regard that the
recessive or recursive flooding patterns mostly in the
lower reaches of the Imphal/Manipur River (where
exchange of flow takes place) is the result of largescale degradation / scour in the upper and middle
reaches due to alluvial landforms and negligible
erosion in the bottom reaches due to the presence of
bedrock and lateral rock shales.
Figure 3(b) Overview of the two interlink channels
OBJECTIVES OF THE STUDY
Data and Pre-Processing
The study is directed to explore the links between
channel/floodplain
morphometry,
streamflow
variability and sediment transport in a ‘bidirectional’ hydrodynamic boundary in the two
interlink channels of Loktak lake sub-basin. The
fluvial regime transition from net-erosional to netdepositional, and vice-versa, is examined with the
numerical model. Observation and multi-scaling of
datasets is directed to identify the frequency
distribution of daily and maximum discharges /water
levels and the quantile at which transition of change
in flow direction takes place within the flow
parameters. The numerical model framework is
applied to arrive at the critical points at which the
variability of floods with scale changes from
increasing to decreasing associations with scale, such
that the floodplain gets well established due to its
increased frequency of occupation by the flow. The
main objective of the study is to arrive at ‘calibration
to interpretation curves’ for sediment assessments in
the lake entity.
There has however been an inadequacy in the
temporal data which has affected the studies
conducted to consider the multiplicity of controls on
the resolute signatures of the Loktak lake
hydrodynamic regime. After carrying out
reconnaissance survey of the Loktak Lake sub-basin,
“It has been recognized that the data base is grossly
inadequate and not of appropriate standard in
quantity and quality” (WAPCOS, 1993).However,
after the interest taken up under the Indo-Canadian
Environment Friendly relation (ICEF) programme,
refine hydrographic observations and data collection
were conducted. Datasets for the period from March
2000 to April 2002 thereby, has been adopted as the
base time scale for the framing inputs to the 1-D
numerical model. The dataset is segregated for the
respective flow directions (outflow and inflow) for
the two channels, as the original dataset records the
flow on daily time-scale and irrespective of the flow
direction.
Table 2. Sediment distribution of Loktak lake sub-basin and factors for calculation of their properties.
Sl.
Classification
Sediment
ID (HEC-6)
1
Clay
CLAY
(mm)
Less
0.002
2
2.1
SILT :
Very
Fine Silt
Fine silt
Medium
silt
Coarse silt
SILT 1
0.002-0.008
SILT 2
SILT 3
SILT 4
2.2
2.3
2.4
Size
Col(6)/100
0.082
(5)x(6)
0.0082
(6)x(8)
15.498
26.9
0.269
7.56
0.1076
203.364
0.011
0.022
17.6
16.5
0.176
0.165
10.235
10.620
0.1936
0.363
180.136
175.230
0.044
14.0
0.140
11.39
0.616
159.460
0.077
12.36
0.682
95.79
%(Pi)
Distribution
(%)
8.20
0.004
0.008-0.016
0.016-0.031
0.0310.0625
than
(mm)
0.001
(assigned)
In
Fractions
∑ d Δp ∑ r
Unit
(γ0)
weight
respective
KN/cum
1.89
Geometric
mean (di)
i
i
oi
Δp i
(Percenta
ge
Silt
=75.0%)
3
3.1
3.2
3.3
3.4
SAND :
Very fine
sand
Fine sand
Medium
sand
Coarse
Sand
VFS
0.06250.125
0.088
7.75
∑ d Δp
50
i
i
1.9704
0
FS
MS
0.125-0.250
0.250-0.500
0.177
0.354
4.75
2.30
0.048
0.023
13.32
14.42
0.841
0.814
63.27
33.166
CS
0.500-1.00
0.707
2.00
0.020
15.21
1.414
30.420
(Percenta
ge Sand
=16.8 %)
Total :
(50%) finer
demarcatio
n)
100
∑ d Δp
i
i
3.069
50
956.334
721
Figure 4. Monthly streamflow hydrograph of Khordak channel
The grain sizes of sediment particles commonly
transported by rivers may range over several orders
of magnitude. Small sizes behave much differently
from large sizes. Therefore, it is necessary to classify
sediment material into groups for application of
different sediment transport theories. The three basic
classes considered by HEC-6 are clay, silt, and
sands-boulders. The groups are identified and
subdivided based on the American Geophysical
Union (AGU) classification scale. HEC-6 accounts
for 20 different sizes of material including one size
for clay, four silt sizes, five sand sizes, five gravel,
two cobble sizes, and three boulder sizes. The
representative size of each class is the geometric
mean size, which is the square root of the class
ranges multiplied together.
Referring to the sediment distribution (Table.2),
the bulk sediment properties worked out are:
Arithmetic Standard Deviation , σ = 0.0117mm ;
Geometric Standard Deviation,
σ g = 4.545
(lying between the normal range of 2.0 to 4.80) ;
S 0 = 2.9664 ; Kramer’s
Sorting coefficient,
Uniformity coefficient, Μ = 0.642 (as M is less
than 1.0, the sediment sample is non-uniform); Unit
or Specific Weight,
γ 0 = 9.563 KN / m 3 .
It can be further seen that the streamflow
hydrograph in the two interlink channels do not
follow a frequency as based on simple regression
plots. The particular form of a Fourier series
translation is observed in the case of Khordak
channel where both the inflow (assigned +ve) and
outflow (assigned -ve) discharges are plotted for a
monthly time periods (Fig.4). As sensitive inputs to
the model boundary values, the stage-discharge (SQ) relations need to sufficiently dictate the hydraulic
transient behaviour. In the course it has been found
that simple curve-fitting is not satisfactory. This may
be indebted to the hysteresis effects (Maha et. al.,
722
1997). Sometimes the relationship between stage and
discharge cannot be represented by a single
regression equation, because it exhibits hysteresis.
Multi-variate to auto-regressive procedures are
extensively used to find the best (S-Q) relation, as
well as the sediment discharge rating relations (Qt Q) to properly quantify the non-equilibrium sediment
transport (where the outflowing sediment discharge
from a river reach does not equal the inflowing
sediment discharge to that reach). Outliers or ‘noisy’
data is removed from the dataset wherever necessary.
The average sediment concentrations are not
used to plot sediment rating curves because of
invariability and lack of concise record data. Instead,
the sediment discharge quantity ( in terms of kgs per
day ) is used as the function to develop the sediment
discharge rating curves (Qt -Q) for the two channels
in the Loktak lake sub-basin. (Qt -Q) curves have
been developed on the premise that a stable
relationship between the concentration and
discharge, although exhibiting scatter, will allocate
the mean sediment yield to be determined on the
discharge annals (Sarkar et.al, 2004).
MODEL FORMULATION
Bi-directional flow structure
A special framework to model the ‘Backflow’
(Inflow) and Outflow, in the frame as bi-directional
flow within the same channel is devised. In both the
cases of the two interlink channels – Khordak and
Ungamel , the “base or starting station is the Imphal
river confluence in the outflow (-ve) convention and
Loktak confluence in the backflow or inflow (+ve)
convention”. The cross-sections remain the same,
accept for the order depending on the base station
(the cross section data input is reversed when
simulating for outflow to that used in the inflow
along the same channel). Station No.1 (each for
Khordak and Ungamel channels) is assigned at the
section upstream of the meeting point of each of the
two interlink channels with Imphal / Manipur River
and Station No.2 is assigned at the section
downstream with Loktak lake confluence of each of
the channels (Figs. 5a and 5b). Station-wise stagedischarge and sediment discharge rating boundary
values are applied under the same sediment
distribution. It is noted that though the Khordak
channel has larger conveyance, Ungamel channel
carries more discharge.
Sensitivity analysis
It is usually desirable during the course of an HEC-6
application to perform a sensitivity test. Quite often
certain input data (such as inflowing sediment load)
are not available, or subject to substantial
measurement error. The impacts of these
uncertainties on model results are studied by
modifying the suspected input data by ± x% and rerunning the simulation. If there is little change in the
simulation results, the uncertainty in the data is of no
.
consequence. If large changes occur, however, the
input data needs to be refined. The flow parameters
as stage/gauge, discharge and sediment discharge are
extensively examined of their correlations and
determination coefficients. Multi-regression and
auto-regression data transformations are done by
assigning one parameter with respect to the other two
flow parameters. Wherever a correlation less than
80% is found, then the data is checked for
inconsistency or errors.
Based on the analyses of inflows and outflows
at the assigned stations (Figs. 6 to 7), the points
where less correlation is found are excluded from the
database. Further check and calibration is carried out
in the database set to arrive at the ‘most responsive’
hydrodynamic parameters. Note that best correlation
is represented wherever found during the multi- or
auto-regression transformation of the boundary
values: discharge, sediment discharge and water
surface elevation (stage) at the respective stations of
consideration.
Figure 5(a). Longitudinal bed profile of Khordak channel (stns-1 & 2 for the model are shown)
Figure 5(b). Longitudinal bed profile of Ungamel channel (stns-1 & 2 for the model are shown)
723
Figure 6.1(a). Auto-regression of inflow sediment
discharge w.r.t.stage and discharge) of Khordak
channel at stn-1
Figure 6.1(b). Auto-regression of outflow sediment
discharge (w.r.t. stage and discharge) of Khordak
channel at stn-1
Figure 6.2(a). Auto-regression of inflow discharge
(w.r.t. stage and sediment discharge) of Khordak
channel at stn-2
Figure 6.2(b). Multi-regression of outflow
discharge (w.r.t. stage and sediment discharge) of
Khordak channel at stn-2
Figure 7.1(a). Multi-regression of inflow discharge
(w.r.t. stage and discharge) of Ungamel channel at
stn-1
Figure 7.2(a). Multi-regression of inflow discharge
(w.r.t. stage and sediment discharge) of Ungamel
channel at stn-2
Figure 7.1(b). Multi-regression of outflow
discharge (w.r.t. stage and sediment discharge) of
Ungamel channel at stn-1
Figure 7.2(b). Multi-regression of outflow
discharge (w.r.t. stage and sediment discharge) of
Ungamel channel at stn-2
724
Boundary Conditions
There are three boundary conditions that can be
prescribed by HEC-6 program: water discharge,
sediment discharge, and water surface elevation
(stage). The water and sediment discharges are
defined at each upstream boundary and at each local
inflow point, stn-1 and stn-2 respectively for the each
case of flow direction as outflow or inflow. The stage
is specified at the downstream boundary of the
stream segment coupled with stage-discharge rating
curves for the respective outflow or inflow in each of
the two interlink channels. For example, for outflow
case in Khordak channel, stn-2 becomes the U/S
boundary section where the flow hydrograph is
prescribed and stn-1 becomes the D/S boundary
section where the rating curve is specified. The
simulation is carried out for different arbitrary water
depth values (effective stages in RL) in the range of
observed data (Ha values of 2.04m, 1.80m, 1.60m,
1.40m, 1.00m, 0.70m, 0.40m as in Figs. 8.2a and b),
which is assigned at the D/S boundary. Within an
equivalent stage, arbitrary discharge values (again in
the observed data range) are used to frame boundary
for ‘computed stages’ and sediment discharges. In
the inflow case, the respective stations are reversed
for the U/S and D/S boundaries.
Summary and discussion of results
The HEC-6 numerical program gives outputs of the
status of the stream bed profile viz., bed change,
water surface elevation, and sediment transport rates
of silt, clay and sand in respect of the two
interlinking channels – Khordak and Ungamel, of the
Loktak lake sub-basin. Extensive application of the
HEC-6 numerical program under the boundary
conditions derived from multi-regression and autoregression of flow data show that there is little
deposition in the stream bed at the cross-sections of
consideration. The computed water depths (in RL),
are tabulated under each arbitrary discharge value for
each assigned stage. Similarly, the sediment
discharges (in tons per day) are tabulated under each
arbitrary discharge value for each assigned stage.
The plots of “Calculated stage vs discharge” and
“Calculated sediment discharge vs discharge” for
respective flow directions are presented in Figs.8 and
Figs.9, respectively.
The “Combined stage-discharge curves” for
Khordak channel shows an association that suggests
it is not regulated by the flow boundaries for both
inflow and outflow. The limitation of 1-D HEC-6
program in the fixation of the boundary between
consecutive cross sections remaining fixed for the
study may be the reason. However, the “Combined
sediment discharge rating curves” reflect a patent
sediment transport identity for the respective
computed stages. Notably, at the inflow boundary the
sediment discharge decreases with relatively
increasing flow depth (Fig.8.1b). The condition
reverses in the outflow boundary (Fig.8.2b). It may
be recalled on this regard that Khordak channel
confluence is located quite upstream of the Ithai
barrage control.
In the case of Ungamel channel, the “Combined
stage-discharge curves” are well-delineated for the
assigned computed stages for both inflow and
outflow. Higher water depth shows relatively less
fluctuations in discharge suggesting a no flow (due to
full gate closure) or high flood condition. An
interesting derivation from the “Combined sediment
discharge rating curves” for Ungamel channel is that
there is a transformation in the curves from
‘concavity to convexity’ (with decreasing depths)
between water stage of 2.50m to 2.60m in both
inflow and outflow. The shift in sediment discharge
in this depth range may be considered as critical
points in establishing the non-equilibrium fluvial
regime as well as serve guidance to barrage gate
control regulations in the Loktak lake sub-basin.
In practical applications of the interpretation
curves formulated in the study, for a water level /
depth value, the mean discharge (Qmean) can be found
by averaging the respective discharge values for each
representative stage (using the Combined stagedischarge curves). Then for this mean discharge, the
sediment discharge can be interpolated for that flow
stage (using the Combined sediment discharge rating
curves). As an example, for an inflow stage of 2.40m
in Ungamel channel, the mean discharge is
= 15 cumecs using the plot in Fig.9.1 (a). Then, for
Qmean = 15 cumecs, the interpolated sediment
discharge using plot in Fig.9.1 (b) is 3000 tons/day.
725
Figure 8.1(a) Computed ‘Inflow Stage-Discharge’curves for Khordak Channel
Figure 8.1(b) Computed ‘Inflow Sediment-Discharge’curves for Khordak Channel.
726
Figure 8.2(a) Computed ‘Outflow Stage-Discharge’curves for Khordak Channel
Figure 8.2(b) Computed ‘Outflow Sediment-Discharge’curves for Khordak Channel
727
Figure 9.1(a) Computed ‘Inflow Stage-Discharge’curves for Ungamel Channel
Figure 9.1(b) Computed ‘Inflow Sediment-Discharge’curves for Ungamel Channel
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Figure 9.2(a) Computed ‘Outflow Stage-Discharge’curves for Ungamel Channel
Figure 9.2(b) Computed ‘Outflow Sediment-Discharge’curves for Ungamel Channel
CONCLUSIONS
The interpretation charts developed to model the
transient ‘bi-directional’ hydrodynamic conditions in
the two interlinking channels – Khordak and
Ungamel, depicts the non-equilibrium fluvial pattern
at the outlet of the Loktak lake sub-basin. The
perplexity in fitting a ‘responsive’ stage-discharge to
sediment discharge rating curves up to a certain
extent is resolved with the formulation of the
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interpretation charts. As considerations of alternate
‘downstream’ and ‘upstream’ boundary values with
respect to its direction of flow as ‘inflow / backflow’
and ‘outflow’ in the 1-D numerical model, indicate
that the scour in particular sections and deposition
in other subsequent sections of consideration in
inflow boundary formulation is reverted during the
outflow boundary formulation of the same sections
and channel (i.e., deposition in the particular sections
and scour in the other subsequent sections with
respect to that stated earlier). This could be the
reason why the longitudinal sections of these two
interlink channels follow a transition ripple or dunes
bed form.
Silt transport is found to dominate the fluvial
entity of Loktak lake by more than 80 percent
(relative to clay and sand) in exchange of flows with
the main drainage river in the basin. Inverse
variations in sediment discharge with respect to stage
in inflow and outflow, as the case with Khordak, are
prominent when more channel morphometry are
defined in the HEC-6 numerical model. The
benchmarking of critical point in flow stage for
Khordak channel should be identified with further
investigations with more observed data, as was
earmarked for Ungamel channel. On the whole, the
study emphasizes that the two interlink natural
channels deserves a germane attention to authorities
concerned with Loktak lake conservation and
hydrologists meriting uncharacteristic hydrodynamic
occurrence.
NOTATION
The following symbols are used in this paper:
Bo = width of the movable bed
Bsp = width of movable bed at point P
di, d15.9, d25, d50, d75, d84.1 = sediment particle size at
respective percentage distribution
D = hydraulic depth
G
= average sediment discharge (ft3/sec) rate
during time step ∆t
Gu = sediment load at the upstream cross sections,
Gd = sediment loads at the downstream cross
sections
Lu = length of the upstream reach used in control
volume computation
Ld = length of the downstream reach used in control
volume computation
∇pi = percentage weight corresponding to sediment
size
di
Q = inflow or outflow discharge
Qt = sediment inflow or outflow discharge
S = stage of flow
t
= time
∆t = computational time step
Vsed = volume of sediment in control volume
x = distance along the channel
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Ys = depth of sediment in control volume
Ysp = depth of sediment before time step at point P
Y'sp = depth of sediment after time step at point P
ACKNOWLEDGEMENTS
The present study was part of the M.Tech
dissertation work (2006) of the writer at the Indian
Institute of Technology Roorkee, India. The data
used in this study were provided by the Loktak
Development Authority (LDA). Some data were
provided by the Manipur Wetlands Society
(MAWETS). The writer wishes to thank the
organizations and the staff of the LDA who were
associated with data observation, processing, and
management of database. The writer also
acknowledges the support of his friends in ground
record assessments.
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