Flushing

June 2012

ASSOCIATED TECHNOLOGIES (PVT) LTD 142-D Model Town, Lahore Tel: +92-42-35847194-7 Fax: +92-42-35857637

www.atlpvt.com

REVIEW REPORT ON SEDIMENT FLUSHING THROUGH KAROT RESERVOIR

Ali

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Ali

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Ali

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ANNEXURE-A

Associated Technologies (Pvt) Ltd Sediment Flushing Through Karot Reservoir

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TABLE OF CONTENTS

1.INTRODUCTION ......................................................................................................................... 4

2. SUMMARY OF THE FEASIBILITY STUDY ..................................................................................... 4

3. MODELING SEDIMENT DELTA PROFILE FOR KAROT RESERVOIR USING HEC-RAS .................. 7

3.1 GENERAL ............................................................................................................................. 7

3.2 MODEL DESCRIPTION ......................................................................................................... 8

3.2.1 Cross Section Locations ............................................................................................ 8

3.2.3 Boundary conditions ................................................................................................. 9

3.2.4 Flow series .............................................................................................................. 10

3.2.5 Temperature ........................................................................................................... 10

3.3 Modeling delta profile of Karot reservoir using HEC-RAS: ...................................... 11

3.3.1 Project area ............................................................................................................ 11

3.3.2 Scenario for delta modeling ................................................................................... 11

3.3.3 Input data for The HEC-RAS model ......................................................................... 12

3.3.4 River system schematic .......................................................................................... 12

3.3.5 Geometric Data ...................................................................................................... 12

3.3.6 Cross Sections ......................................................................................................... 13

3.3.7 Dam Data ................................................................................................................ 13

3.3.8 Quasi-Unsteady Flow .............................................................................................. 14

3.3.9 Sediment Data ........................................................................................................ 15

3.3.10 Delta modeling: ....................................................................................................... 17

4. WORLDWIDE EXPERIENCE OF SEDIMENT FLUSHING ............................................................. 19

5. MODELING SEDIMENT FLUSHING THROUGH KAROT RESERVOIR USING HEC-RAS ............... 22

6. STRATEGIES FOR FLUSHING SEDIMENTS THROUGH THE RESERVOIR .................................... 24

6.1 Flushing Strategies ...................................................................................................... 25


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7. FLUSHING INDICATORS FOR THE KAROT RESERVOIR ............................................................. 26

8. CONCLUSIONS ........................................................................................................................ 29

9. RECOMMENDATIONS ............................................................................................................. 29

LIST OF FIGURES

Figure 1: Schematic diagram showing the cross section locations used for the delta modeling ................................................................................................................................... 6

Figure 2: Flow Hydrographs at Karot dam site used as upstream boundary condition………..7

Figure 3: Bed material gradation curve for the Karot reservoir ............................................... 8

Figure 4: Suspended Sediment rating curve for Karot dam site .............................................. 9

Figure 5: Typical result of the HEC-RAS model for water surface profile before delta modeling ................................................................................................................................. 10

Figure 6: Results of Sensitivity Analysis of HEC-RAS Model to various Sediment Transport Functions ................................................................................................................................ 11

Figure 7: Modeled Longitudinal Sediment Delta Profile for Karot Reservoir due to one year sediment deposition………………………………………………………………………………………………………….12

Figure 8: Modeled Longitudinal Sediment Delta Profile for Karot Reservoir due to 12 years sediment deposition ............................................................................................................... 13

Figure 9: Modeled Longitudinal Sediment Delta Profile for Karot Reservoir due to 16 years sediment deposition………………………………………………………………………………………………………….14

Figure 10: Country wise reservoir storage loss in percentages ............................................ 155

Figure 11: Worldwide number of flushed reservoirs of various countries .......................... 166

Figure 12: Bed profile of Karot Reservoir before flushing based on 1 year Sediment deposition ............................................................................................................................... 17

Figure 13: Longitudinal profile of Karot reservoir after flushing the 1 year deposited sediments ............................................................................................................................... 18


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Figure 14: Average daily flows and average flushing discharge ............................................. 19

Figure 15: Surface area of Karot Reservoir as a function of Elevation ................................... 20

Figure 16: Rating curve for discharges through flushing sluices ............................................ 21

Figure 17: Reservoir emptying time and re-filling time.......................................................... 22

Figure 18: Mean Velocities at various sections of the Karot Reservoir……………………….…….23

Figure 19: Critical velocities for various sediment sizes (Yang, 1989) .................................... 24

Figure 20: Flushing durations required to flush one year deposited Sediments ................... 25

Figure 21: Cross section immediately u/s of the dam for simplified reservoir geometry….26

LIST OF TABLES

Table 1 : Twenty nine cross sections used for the delta modeling…………………………………….5

Table 2: Computations for flushing indicators of Karot Reservoir……………………………………28


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1.INTRODUCTION

The Power and Energy Studies carried out by the Consultants and incorporated in Feasibility Study Report (FSR) for the reservoir of Karot Hydropower Project, suggests that a flushing period of 10 days per year will be taken to clear and drain out the accumulated sediments downstream through the bottom sluices provided for in the dam. However, during the review of FSR it was discovered that no in-depth analysis regarding sediment flushing was performed by the Consultants to support the said flushing period.

Hence a need was felt to study the sediment flushing operation in detail to investigate the flushing scenario fully, i.e. when to flush the reservoir, how much amount of water will be utilized in each flushing, how much should be the flushing duration, what should be the suitable flushing discharge and what strategies are needed to enhance the life of the reservoir.

Present study addresses the Sediment flushing operation of the Karot Reservoir in detail including summary of the sedimentation study at feasibility stage, modeling of sediment delta in the reservoir for one year, 12 years and 16 years of sediment depositions using HEC-RAS Model (U.S. Army Corps of Engineers, 2005), worldwide experience of sediment flushing, modeling sediment flushing operation to de-silt the sediment deposits accumulated in one year, strategies for flushing sediments through the Karot reservoir, interpretation of the model results, and finally flushing indicators for the reservoir were determined to explain the feasibility of the flushing for the Karot reservoir.

2. SUMMARY OF THE FEASIBILITY STUDY

Feasibility Report for the Sedimentation study of the Karot Reservoir (SMEC, 2009) describes the sediment characteristics of the watershed of the proposed dam site and reservoir of Karot Hydropower Project on River Jhelum. It covers description of previous sedimentation studies for the dam site, sediment yield of the watershed and sediment transport pattern of the river at the dam site. It also includes computation of suspended sediment inflow to the reservoir from the nearby sediment gauging stations, computations for the bed load, total sediment inflow to the reservoir, density of deposited sediments with time, efficiency of the proposed reservoir in trapping sediments and life of reservoir without any flushing option. Feasibility study also addresses the degradation study on downstream of the dam. The longitudinal sediment delta profile was modeled considering one year of deposited sediments in the reservoir using SHARC Model (Wallingford, 2001).

As per feasibility report, the total reservoir capacity of the Karot reservoir would be 160 MCM (129,760 AF) at Normal Operating Level (NOL) of 461 m. Total height of the dam would be 76 m with a free board of 5 m. The crest level of the dam is proposed as 466 m. The power tunnel’s crown and invert levels are proposed at 440 m and 430 m, respectively. There are four proposed low level outlets of 12 by 18 m cross-section; each with sill level at 410 m, due to which dead storage of the reservoir comes 10.2 MCM (8272 AF) which cannot be sluiced through the dam by any means. Live storage of reservoir then would be 149.8 MCM (121,490 AF). The river bed level at dam site is 390 m. Surface area of the proposed reservoir at Normal Operating Level would be


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5.41 km2 (1337 acres) and length of the reservoir would be about 26 km. Average slope of the reservoir is 0.0026.

Table 2 : Twenty nine cross sections used for the delta modeling

Sr. No. River Station Distance to d/s (m) Remarks

1 27 1000 Reservoir Area 2 26 1000 do 3 25 1000 do 4 24 1000 do 5 23 1000 do 6 22 1000 do 7 21 1000 do 8 20 1000 do 9 19 1000 do 10 18 1000 do 11 17 1000 do 12 16 1000 do 13 15 1000 do 14 14 1000 do 15 13 1000 do 16 12 1000 do 17 11 1000 do 18 10 1000 do 19 9 1000 do 20 8 1000 do 21 7 1000 do 22 6 1000 do 23 5 1000 do 24 4 1000 do 25 3 1000 do 26 2.5 Karot dam site 27 2 1000 D/S of dam side 28 1 1000 do 29 0 0 do

For the sedimentation study of the reservoir at feasibility stage, data of thirteen sediment gauging stations were collected and used. However, to estimate the suspended sediment yield and total sediment inflow to the reservoir, calculations are mainly based on two sediment gauging stations, i.e. Azad Pattan and Karot.

Results of sediment study reveal that average specific suspended sediment yield at Karot dam site is about 1176 tons/km2/year, average suspended sediment load entering into the reservoir


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is about 28.75 MCM (23376 AF), average bed load entering into the reservoir is about 4.31 MCM (3495 AF), average annual total sediment inflow to the reservoir is about 33.07 MCM (26882 AF).

Figure 1: Schematic diagram showing the cross section locations used for the delta modeling

Trap efficiency of reservoir computed on the basis of Churchill curve is 40 % for the first year and finally reduces to 18%. The recommended trap efficiency for any year is 47 % based on total sands and coarser material. Mean annual trapped sediment load in the reservoir is 15.54 MCM (12635 AF).

The life of the un-sluiced Karot reservoir is about 08 years. The recommended density of deposited sediments is 1106 kg/m3 (69 Pcf). The expected degradation on downstream of the dam is 9 m based on the occurrence of rocks.

Sands will start entering into the power tunnel when flow through the reservoir exceeds 6800 m3/s, however a sediment particle of 0.1 mm diameter is expected to enter into the power tunnel when flow exceeds 17500 m3/s. On average basis, the annual sediment load entering into the power tunnels would be 1.22 Mm3.

The pivot point of delta deposits will reach at a distance of 16 km from the dam face as a result of one year deposition.

Flushing discharge required to carry out flushing of the reservoir would be equal to greater than 1600 m3/s, whereas flushing duration required would be around one to two weeks depending on the flow available for the purpose.


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The life of the reservoir can be enhanced to about 48 years with appropriate flushing operations at the rate of one per year in the month of May or June. After 48 years, the life of the reservoir can be further extended by the dredging the sediment deposits closer to the dam outlets.

To carry out efficient flushing, the sediment delta in the reservoir should be monitored every year before flushing. Moreover, operator must be fully vigilant on the coming forecasted flows which may offer suitable flushing opportunity.

Figure 2: Flow Hydrographs at Karot dam site used as upstream boundary condition

It is noted that Feasibility Study does not contain modeling of sediment flushing operation of the reservoir either using SHARC or HEC-RAS Models. Present study models the sediment delta and flushing operation.

3. MODELING SEDIMENT DELTA PROFILE FOR KAROT RESERVOIR USING HEC-RAS

3.1 GENERAL

Delta profile modelling and flushing modelling are carried out by using HEC-RAS 4.1 model in addition to SHARC model to become more confident about results in the absence of calibration and validation of data. This study describes the application of HEC-RAS model for the prediction of delta profile and sediment flushing scenario for the Karot reservoir. Delta modelling is carried out using hourly time step for the 16 years of sediment deposition with average flow and sediment discharge conditions, whereas, suitable flushing durations are predicted for various flushing discharges to de-silt the one year deposited sediments.


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The reservoir / river in this study was modelled as the compound channel with 27 cross-sections. The inline structure for the dam is taken in between sections 3 and 2 and named as station 2.5. For Delta profile modeling, the equilibrium load was assigned as upstream boundary condition for the reservoir area. The quasi unsteady equation is used for 16 years inflow data in the reservoir area.

Figure 3: Bed material gradation curve for the Karot reservoir

3.2 MODEL DESCRIPTION The HEC-RAS 4.1 software was developed at the Hydrologic Engineering Center (HEC), which is a division of the Institute for Water Resources (IWR), U.S. Army Corps of Engineers. The software was designed by Mr. Gary W. Brunner, leader of the HEC-RAS development team. HEC-RAS is an integrated system of software, designed for interactive use in a multi-tasking, multi-user network environment. The system is comprised of a graphical user interface (GUI), separate hydraulic analysis and sediment transport analysis components, data storage and management capabilities, graphics and reporting facilities.

HEC-RAS is designed to perform one-dimensional sediment transport calculation for a full network of natural and constructed channels. The following is description of the major capabilities of HEC-RAS (U.S., 2005).

3.2.1 Cross Section Locations The inline weir and gated spillway routines in HEC-RAS require the same cross sections as the bridge and culvert routines. Four cross sections in the vicinity of the hydraulic structure are required for a complete model, two upstream and two downstream. In general, there should


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always be additional cross sections downstream from any structure (bridge, culvert, weir, etc), such that the user entered downstream boundary condition does not affect the hydraulics of flow through the structure. These four cross sections include;

One cross section sufficiently downstream such that the flow is fully expanded, One at the downstream end of the structure (representing the tail water location), One at the upstream end of the structure (representing the headwater location), One cross section located far enough upstream at the point in which the flow begins to contract.

Figure 4: Suspended Sediment rating curve for Karot dam site

3.2.2 Quasi – unsteady flow simulation

Current sediment capabilities in HEC-RAS are based on quasi-unsteady hydraulics. The quasi-unsteady approach approximates a flow hydrograph by a series of steady flow profiles associated with corresponding flow durations. Because this type of analysis requires different information than steady or unsteady flow, so it is necessary to provide different input along with boundary condition.

3.2.3 Boundary conditions Several different boundary conditions are available in HEC-RAS. Each upstream boundary (the most upstream cross section of an open ended upstream reach) must have a Flow Series boundary condition specified. Optional internal boundaries include Lateral Flow Series and Uniform Lateral Flow Series. Each downstream boundary (the downstream most cross section


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of an open ended downstream reach) can be either: Stage Time Series, Rating Curve, or Normal Depth.

3.2.4 Flow series Since Quasi-unsteady flow can have irregular (varying) time steps, each specified flow must also be accompanied by a time duration (over which the flow is constant). Additionally, a computational time step must be entered for each record. Flow Duration: To approximate a flow hydrograph as a series of steady flows, each steady flow profile must have a flow duration. The duration is then broken up into a series of computational increments over which the sediment routing occurs. Due to the non-linear nature of alluvial sediment movement, transport is usually concentrated during large, peak flow events. These events are usually of relatively short duration and are characterized by rapidly changing flow. Because of this non-linearity, an irregular time step is desirable. Low flows, corresponding to small or moderate transport (or bed change), are often approximated with large time steps.

Figure 5: Typical result of the HEC-RAS model for water surface profile before delta modeling

3.2.5 Temperature

Because several aspects of sediment transport mechanics, particularly fall velocity, incipient motion and sediment transport are sensitive to water temperature, hence, HEC-RAS requires temperature information. Only one temperature per time step can be specified for the entire model.


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3.3 Modeling delta profile of Karot reservoir using HEC-RAS: HEC-RAS 4.1 has been used to model the longitudinal delta profile in the reservoir from stating to ending simulation time. The monthly average flows and monthly average sediment flows were given as the input to the reservoir, whereas, sediment calculations are carried out on hourly basis. The results of the modelled delta can be easily seen for all time steps through animation option of the software.

3.3.1 Project area The project area lies along the river Jhelum on Rawalpindi - Kotli (Azad Kashmir) Road nearly 68 km from Rawalpindi. Karot dam site is located on River Jhelum at latitude of 33° 36' and longitude of 73° 37'. At the dam site, there was a stream flow gauging station having wire weight gauge slightly on the upstream of the new bridge; the gauging station was closed by the end of 1979.

3.3.2 Scenario for delta modeling

For the present study, it was assumed that the reservoir is full at NOL of 461 m and the average monthly flow data is used to simulate the delta profile due to deposition of sediments in the reservoir area.

Figure 6: Results of Sensitivity Analysis of HEC-RAS Model to various Sediment Transport Functions


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3.3.3 Input data for The HEC-RAS model

The data needed to perform various computations with HEC-RAS are divided into the following categories: Geometric data; Quasi-unsteady flow data; Sediment data; Bed gradation curve and inline structure data. A brief description of these data formats and requirements in accordance with this study is given in the subsequent sections.

3.3.4 River system schematic

The schematic defined in the HEC-RAS show the simplified plan view and locations of the cross section data from upstream to downstream of the dam site. The setting up of the model was carried out by considering 26 km river length with 27 cross-sections as shown in Figure 1. The dam site is situated at section number 2.5, whereas three cross sections were taken on downstream side of the dam. i.e. number 2, 1 and 0, respectively. In river system schematic,

river Jhelum was drawn in geometric data editor option of HEC-RAS. In the geometric data editor the name for river reach was specified. Figure 7: Modeled Longitudinal Sediment Delta Profile for Karot Reservoir due to one year sediment deposition

3.3.5 Geometric Data

The basic geometric data consist of establishing the connectivity of the river system (River system schematic); cross section data, reach lengths, and stream junction information. Twenty nine cross sections were incorporated in the model and inline structure was also created in the geometric data in between section numbers 3 and 2.


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3.3.6 Cross Sections

Boundary geometry for the analysis of flow in river was specified in terms of ground surface profiles (cross sections) and the measured distance between these (reach lengths at each cross-section). The cross sectional data of river Jhelum was entered in HEC-RAS by the cross sectional data editor. Cross sections from both the ends of inline structure (dam structure) upstream and downstream were plotted. The data entered into the cross section data editor comprises of River station information, Pairs of station and elevation, Demarcation of main channel bank station. Downstream reach lengths (i.e., the distance up to next downstream cross section.) for main channel, left over bank and right over bank and Manning’s roughness coefficient (both vertical and horizontal variation of n- values were considered). The detailed information about the locations of cross sections is given in Table 1.

Figure 8: Modeled Longitudinal Sediment Delta Profile for Karot Reservoir due to 12 years sediment deposition

3.3.7 Dam Data The data regarding dam was entered in the geometric data editor’s sub-option inline structure data. Karot dam was simulated as an inline structure. The data entered into the In Line Structure data editor comprises of

• River station information

• Weir embankment data

• Weir crest shape

• Information about crest level


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Figure 9: Modeled Longitudinal Sediment Delta Profile for Karot Reservoir due to 16 years sediment deposition

3.3.8 Quasi-Unsteady Flow The flow data which was synthesized from the previous historical data was entered in the Quasi-unsteady flow data editor which comprised of two boundary conditions.

a) Upstream boundary condition

b) Downstream boundary condition

a) Upstream Boundary Condition Monthly inflow hydrograph for 16 years was assigned as the upstream boundary condition. The flow hydrograph was generated using the Karot and Azad Pattan gauging stations data using 37 years data record (1970-2006), 1979-2006 at Azad Pattan which was transformed at Karot using a proposed multiplication factor. The upstream boundary condition flow hydrograph is shown in Figure 2.


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Figure 10: Country wise reservoir storage loss in percentages

b) Downstream Boundary Condition Normal depth was prescribed as downstream boundary condition with friction slope equal to the average river bed slope in the reservoir area at the downstream end (S = 0.0026).

3.3.9 Sediment Data Once the geometric data are entered, the sediment data was entered to develop a delta profile of sediment transport. The sediment data was entered in sediment data editor which comprised of following conditions.

• Initial Conditions and Transport Parameters.

• Boundary Condition

a) Initial Conditions and Transport Parameters: The initial condition and transport parameters specified for HEC-RAS for Karot reservoir are as following at each cross section.

i. Transport Function: A sediment transport function can be selected from the drop down box near the top of the form. For the present study, England-Hansen transport function was selected on the basis of sensitivity analysis of transport functions to model the delta of Karot reservoir. This function gives relatively suitable results closer as computed by the SHARC Model and also determined by empirical approach.


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ii. Sorting Method The sorting method to compute active layer thickness and vertical bed layer tracking assumption. The Exner 5 method is currently used. It is a three layer active bed model that includes the capability of forming a coarse surface layer that will limit erosion of deeper material thereby simulating bed armoring.

Figure 11: Worldwide number of flushed reservoirs of various countries

iii. Fall Velocity Approach

Several methods are available for computing fall velocity. But Report 12 (Default method in HEC-6) was used for delta modeling of the Karot reservoir. It was also selected after carrying out the sensitivity analysis of various fall velocity formulae available in the software.

iv. Maximum Depth In the HEC-RAS sediment framework, a sediment control volume is associated with each cross section. The control volume starts midway from the next cross section upstream and ends midway to the next cross section downstream. The maximum erodible depth used for model is 10 m.

v. Bed Gradation

HEC-RAS first requires the creation of bed material gradation curve. Then the bed gradations curve can be associated with the appropriate range of cross sections using pick and drag functionalities. Bed Gradation curve used for modeling the delta profile is shown in Figure 3, which was generated for the dam site using sediment discharge data of Karot reservoir area. The sediment sizes in the bed material of the dam site ranges in between 0.004 to 32 mm. Figure 4 shows the


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mean gradation curve for the suspended sediments at the Karot and Azad Pattan stream gauging stations of the river Jhelum.

b) Sediment Boundary Conditions

The equilibrium load is used as sediment boundary condition for delta modeling.

3.3.10 Delta modeling: The Karot reservoir area was modeled for 16 years using monthly flow and sediment data. The maximum depth that is distance below the original invert of the channel was assigned as 10 m for all the cross sections and same bed gradation curve generated for Karot was used for the model. The typical result of HEC-RAS for the water surface profile before delta profile modeling is shown in Figure 5.

Figure 12: Bed profile of Karot Reservoir before flushing based on 1 year Sediment deposition

Figure 6 shows the results of the sensitivity analysis for the various sediment transport functions. Results show that Englund-Hansen equation computes the sediment deposition value and pattern very similar to SHARC Model results and as well result of the Empirical Approach which is the basic one.


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Figure 13: Longitudinal profile of Karot reservoir after flushing the 1 year deposited sediments

Figure 7 shows the aggradations on the upstream side of the dam and degradation on the downstream of the dam after one year flow and sediment transport processes. The pivot point of the delta approaches at 16 km from the dam face whereas level of pivot point reaches at 451.7 m. The total sediment deposited in the reservoir was estimated as 15.5 MCM. The maximum degradation appears at just downstream from the dam site with a degradation depth of about 4 m.

Figure 8 shows aggradations in the reservoir area and degradation on downstream of the dam site after 12 years. The result shows that the he pivot point of the delta approaches at 8 km from the dam face whereas level of pivot point reaches at 446.8 m. The total sediment deposited in the reservoir was estimated as 66.63 MCM. The maximum degradation appears at just downstream from the dam site with a degradation depth of about 4.1 m.

Figure 9 shows aggradation in the reservoir area and degradation on the downstream of the dam after 16 years of fluvial processes. The total sediment deposited in 16 years in the reservoir was estimated as 76.7 MCM. The pivot point of the delta will reach as close as about 7.9 km from the dam face.

The overall sediment deposition patterns in the reservoir for several years up to 16 years, show that flushing of the sediment becomes essential at the end of each year to flush the deposited sediments accumulated in one year. After a long deposition period, it would become difficult to flush sediments in a shorter period which can be sacrificed for the flushing process.


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Moreover, the high pivot point location of the delta deposits after 16 years would be such that it will cause entry of heavy sediment load with bigger sediment diameters into the power tunnel in case of high flows. This is highly objectionable as far as the safety of the hydraulic machines is concerned.

4. WORLDWIDE EXPERIENCE OF SEDIMENT FLUSHING

Globally there are about 25,500 storage reservoirs with the total storage volume of about 6,464 Bcm (ICOLD 1998, White 2000). The maximum number of reservoirs are in North America, i.e., 7,205, with the storage volume of about 1,845 Bcm., whereas the minimum numbers of reservoirs are in Central Asia, i.e., 44, with the storage volume of 148 Bcm. The number of storage reservoirs with storage volumes (in Bcm) in other regions are as: South Asia 4131(1039), South Europe 3220(938), Pacific Rim 2278(277), North Europe 2277(938), China 1851(649), South America 1498(1039), Africa 967(575), Middle East 895(224), North Africa 289(188), and South East Asia 277(117).

World annual reservoir storage loss due to sedimentation varies from 0.1 to 2.3 percent, with average annual world storage loss of about 1.0 percent (Liu, 2002). As shown in Figure 10 the maximum storage loss is in China, i.e., 2.3%, whereas the minimum storage loss is in U.K., i.e., 0.1%. The storage loss in other regions is: Turkey 1.5, India 0.46, South Africa 0.34, South East Asia 0.30, U.S.A. 0.22, and Japan 0.15. As a result of reservoir sedimentation rate, about 300 to 400 new dams need to be constructed annually just to maintain current total storage. This Storage loss is mainly due to different vegetation cover, topographic and geological conditions of the watershed areas (Liu, 2002).


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The 20th Century was concerned with the development of reservoir storage, more emphasis will be required in the 21st century on the conservation of storage. Sediment management will become crucial. The goal will be to convert today’s inventory of non-sustainable reservoirs into sustainable assets for future generations (White W.R. (2005))

Several methods by which the life enhancement of storage Reservoir can be made are: Watershed Management, dredging (conventional dredging, hydrosuction and dry excavation), flushing of sediments from Reservoir, sediment routing/sluicing, sediment bypassing and Density current venting, used independently or in combination (Palmieri A.,2003).


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Figure 16: Rating curve for discharges through flushing sluices Flushing sediments through a Reservoir has been practiced successfully and found to be inexpensive in many cases. However, the great amount of water consumed in the flushing operation might affect it (Fi-John Chang, 2003). Every Reservoir of the world cannot be flushed successfully due to the number of parameters affecting it like flatter bed slope, wider section, higher height of the dam and availability of water for flushing. Flushing also causes sediments to be released from the Reservoir at a much higher concentration than occurs in the natural fluvial system which may create unacceptable environmental impacts downstream, however, these impacts are less severe as compared to absence of flushing at all. Two approaches to flushing exist; complete drawdown flushing, partial drawdown flushing. In complete drawdown flushing the Reservoir is emptied before the flood season, resulting in the creation of river-like flow conditions in the Reservoir. Low level outlets for flushing operation are provided close to the original riverbed level and sufficient hydraulic capacity to achieve full drawdown White (2000). Some irrigation Reservoirs in China are emptied for flushing during the first part of the flood season, passing early season floods through the impoundment without significant detention. The Reservoir is refilled during the latter part of flood season Chaudhry, M.A (2007). Flushing is most effective in preserving Reservoir storage when outlets are placed near the original streambed level and Reservoir is completely emptied. However constraints may limit either the allowable drawdown or the invert elevation of flushing outlet, requiring the flushing be undertaken with only partial drawdown. Flushing with partial drawdown may be used to clear more live storage space and locate the sediments in a more favorable position for future complete drawdown flushing Morris and Fan (1997).


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Emptying timeRefilling time

Figure 17: Reservoir emptying time and re-filling time

Flushing is not a new technique and has been experienced for the last 6 decades and probably attempted for thousands of the reservoirs of the world. The study reveals that there are about 50 reservoirs which are flushed, out of which flushing data is available for about 22 reservoirs. As shown in Figure 11, the maximum numbers of reservoirs are flushed in China which are 21. The number of flushed reservoirs in different countries are as: Switzerland 5, Former USSR 4, India 3, USA 3, Puerto Rico 2, Algeria 1, Austria 1, Costa Rica 1, Guatemala 1 Iran, Japan 1 New Zealand 1 Pakistan 1, Sudan 1, Taiwan 1, Tunisia 1, Venezuela 1.

Worldwide flushing has been successfully implemented at Baira-India, Gebidem-Switzerland, Gmund-Austria, Hengshan-China, Palagnedra-switzerland, Santo-Domingo-Venezuela Reservoirs, while the unsuccessfully flushed reservoirs are: Chinese reservoirs, Gaunting, Heisonglin, sanmenxia, Shuicaozi, Naodehai, Nanqin, Guernsey-USA, Ichari-India, ouchi-Kurgan and Zemo-Afchar of former USSR, sufid-Rud-Iran, Warsak-Pakistan, Jensanpei-Taiwan, KHASHM EL GIBRA-Sudan, Mangahao-Newzealand, and Cachi of Costa Rica (White, 2000).

5. MODELING SEDIMENT FLUSHING THROUGH KAROT RESERVOIR USING HEC-RAS

As the reservoir is small and the sediment deposition rate is high, the project can only be considered feasible with successful flushing of the deposited sediments.

To investigate the flushing performance of the reservoir, flushing scenario is modeled using HEC-RAS 4.1 Model. For modeling, the same 27 basic locations of cross sections were used as the geometric data, except that these were modified as obtained after one year delta modeling. It was assumed that the flushing becomes essential after one year sediment deposition in the


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0.0

0.5

1.0

1.5

2.0

2.5

3.0

0

2,00

0

4,00

0

6,00

0

8,00

0

10,0

00

12,0

00

14,0

00

16,0

00

18,0

00

20,0

00

22,0

00

24,0

00

Distance from dam face (m)

Mea

n Ve

loci

ty, F

roud

e Nu

m

0

200

400

600

800

1000

1200

1400

1600

1800

Flow

Velocity (m/s)Froude NumFlow (cumecs)

reservoir. The longitudinal profile of the delta which was used as input for the flushing scenario is shown in Figure 12.

Figure 18: Mean Velocities at various sections of the Karot Reservoir

For flushing modeling, similarly a quasi unsteady file was prepared. As boundary condition, daily flows of 800, 1200, 1600, 2000 and 2400 m3/s were tried as the constant flushing discharges for the entire flushing duration of 120 days. Flushing durations required to flush the deposited sediments in one year were determined. The temperature of the water was assigned for each day as it affects the sediment transport processes. The normal depth was given by assigning a value of friction slope as 0.0026.

For initial condition, bed material gradation curve at the dam site was used. Sediment transport function used for the analysis was the same England-Hansen, sorting method used was of Exener 5 and fall velocity method of Report 12 was used for the estimation of fall velocities.

For the sediment boundary condition, sediment rating curve derived for the dam site based on long past data record was used. Moreover, fraction of the clay, silt, sand and gravel was assigned on the basis of the bed material gradation characteristics and amount of the bed load transport rate.

Figure 13 shows the reservoir bed profile after flushing the deposited sediments in the reservoir which were accumulated in one year. The flushing duration required to flush the deposited sediments was 20 days for a flushing discharge of 1600 m3/s. Due to flushing operation, slight aggradations have been obtained on the upstream of the dam site. It is due to the fact that the


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0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.001 0.01 0.1 1 10 100

Average Sediment dia (mm)

Criti

cal V

eloc

ity (m

/s)

sill level of the flushing sluices is sufficiently higher than the bed level and hence it has to be filled with sediments. However, there is an increase in the degradation of bed profile on the downstream of the dam site. It is due to the fact that the height of the dam up to the sill level of flushing sluices acted as a barrier in the transport of bed load and much of the suspended load, and hence relatively silt free water has caused scouring on down stream side. This scouring on the downstream of the dam site will automatically check once the bed level on the upstream of the dam site will raise up to the flushing sluices sill level. Then instead of degradation on downstream side of the dam, aggradations will start and at a later stage an equilibrium condition will be maintained by the river.

Figure 19: Critical velocities for various sediment sizes (Yang, 1989)

6. STRATEGIES FOR FLUSHING SEDIMENTS THROUGH THE RESERVOIR

Literature on flushing the reservoirs reveals that the flows required for flushing should be of the order of double of mean annual flow. As the mean annual flow is 816 m3/s, hence a recommended flushing discharge would be about 1632 or 1600 m3/s. A mean daily flow hydrograph for the Karot dam site is plotted in Figure 14 along with the flushing discharge of 1600 m3/s constant line. This Figure show that the suitable flushing time would be somewhere in May or June, when the flows are highest. However, for every year it depends on the availability of the flows closer to 1600 m3/s.


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0.02.04.06.08.0

10.012.014.016.018.020.022.024.0

800 1000 1200 1400 1600 1800 2000 2200 2400Flushing discharge (m3/s)

Flus

hing

Dur

atio

n (d

ays)

6.1 Flushing Strategies To carryout flushing of the sediments through the reservoir, it should be emptied somewhere at the end of April on the basis of average flows. Practically it would depend on the actual temporal distribution of the flows. After emptying, certain days are required for continuous flushing at riverine flow conditions, after flushing operation, it should be re-filled back. To study this whole process, time for emptying the reservoir, flushing the reservoir and refilling the reservoir must be estimated. To do so graphical relationship between surface area of the reservoir and elevation was established and is shown in Figure 15. Rating curve for total outflows from the dam is shown in Figure 16.

As it is necessary to empty the reservoir before flushing, time required to empty the reservoir at various levels have been computed and shown in Figure 17. The time required to empty fully the reservoir is 11 hours with the provided facility of under sluices. The same Figure illustrates the time required to refill the reservoir with a standard flow of 1600 m3/s.

Figure 20: Flushing durations required to flush one year deposited Sediments

To achieve draw-downs, the flushing sluices will be opened and a riverine flow will be obtained. Corresponding to any discharge value what will be the depth of flows, Froude number and mean velocities, can be obtained from the output of the HEC-RAS model in tabular form. For discharge of 1600 cumecs, the velocity of flows at various sections are given in Figure 18. The velocity of flows during flushing varies from 0.5 to 2.6 m/s. Figure 19 depicts the findings of Yang et al. about the critical velocities required to move various particle sizes. This Figure shows that even a 50 mm diameter sediment particle can be flushed with 1600 cumecs discharge value.


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The one year delta deposit was given as input to the flushing model HEC-RAS. The model was run for various discharges and flushing durations were determined. These are plotted and shown in Figure 20. Flushing was modelled for range of flushing discharges from 800 to 2400 m3/s. It is learnt from the Figure that suitable flushing discharges for the Karot reservoir are 1600 to 2000 m3/s, for which flushing durations required would be 20 to 17 days. At low flushing discharge (800 m3/s), the flushing duration required would be 22 days.

Figure 21: Cross section immediately u/s of the dam for simplified reservoir geometry

The above calculations show that around 18 days flushing is required to desilt the about 90% of the annual trapped sediment load in the Karot reservoir.

Total time required for the flushing process would be time for emptying the reservoir (11 hours), time for flushing (18 days) and time for refilling the reservoir (30 hours). In total 20 days would be required for complete flushing operation of the reservoir. This much duration of flushing is expected for average flow and sediment years.

7. FLUSHING INDICATORS FOR THE KAROT RESERVOIR

The various indicators for successful flushing of sediments from Reservoir are Sediments Balance Ratio (SBR), Long Term Capacity Ratio (LTCR), Draw Down Ratio (DDR), Sediment Balance Ratio during full draw down ( SBRd ), Flushing Width Ratio (FWR) and Top Width Ratio (TWR). For the successful flushing the limits of these indicators are: - SBR >1, LTCR approaching to unity, DDR = 0.7, SBRd >1, FWR > 1 and TWR = 1-2 (Atkinson, 1996). But the main indicators that are to be

Scoured channel cross section

Reservoir cross section


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fulfilled to assess the successful flushing of the reservoir, are the SBR and LTCR. Successfully flushed Reservoirs fulfill the above two indicators whereas unsuccessfully flushed reservoirs do not fulfill the LTCR criteria at all (Habib et al., 2009). The detailed discussion about two parameters is given below:

Sediment Balance Ratio (SBR) is defined as the ratio between sediments mass flushed annually and sediments mass deposited annually.

dep

f

MM

SBR = (1)

Where Mf is sediments mass flushed annually and Mdep is the sediments mass deposited annually. ( )min2 ElElSSWW fresbotres −+= (2)

5.08.12 fQWf = (3)

LElEl

S f−= max

(4)

6.0

2.16.1

WSQ

Q fS ψ=

(5)

Sff QTM 86400= (6)

100TEM

M indep =

(7)

LTCR is defined as the ratio between sustainable capacity to the original capacity of the reservoir; whereas sustainable capacity is the total volume of the reservoir which can be maintained due to the flushing of the reservoir (Atkinson, 1996).

r

f

AA

LTCR = (8)

( )fStf ElElSSWW −+= max2 (9)

( )minmax ElElSSWW resbott −+= (10)


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S.No Parameter Symbol Value Units1 Original Storage Capacity Co 160 MCM2 Reservoir Length L 26,000 m3 Elevation of top water level at dam Elmax 461 m4 River Bed Level at dam site Elmin 390 m5 Water surface elevation at dam during flushing Elf 412 m6 Representative bottom width Wbot 90 m7 Representative Side Slope SSres 1.58 Representative Side Slope for Sediment SSs 1.59 Mean Annual Water Inflow Vin 25,840 MCM10 Mean Annual Sediment Inflow Min 36,070,000 Tonnes11 Tsinghua University Factor for Sediment Type Ψ 30012 Sediment Load factor (if different China) 313 Capacity inflow ratio Co/Vin 0.006214 Trap Efficiency TE 47 %15 Flushing Discharge Qf 1600 cumecs16 Flushing Duration Tf 20 days

S.No Parameter Symbol Value Criteria1 Sediment Balance Ratio SBR 35.42 >12 Long Term Capacity Ratio LTCR 0.81 13 Drawdown Ratio DDR 0.69 > 0.74 Flushing Width Ratio FWR 1.73 >15 Top Width Ratio TWR 1.00 1 to 2

Input data for flushing Analysis

Output Flushing Parameters

Table 2: Computations for flushing indicators of Karot Reservoir

( )minmax2ElEl

WWA tf

f −+

= (11)

( )resS

resm SSSS

WWh

−−

=2

(12)

mfl hElElh −−= max (13)

ff ElElh −= max (14)

Where Af is the cross sectional area of valley scoured out by flushing (m2), Ar is the cross sectional area of reservoir in reach immediately upstream from dam (m2), Elf is the water surface elevation at the dam during flushing (m), Elmax is the elevation of top water level (m), Elmin is the minimum river bed elevation immediately upstream from the dam (m), hf is a height defined in Figure 21 (m), hl is a height defined in Figure 21 (m), L is the reservoir length (m), Mdep is the mass of sediments which deposits annually in the reservoir (Tons), Mf is the mass of sediments flushed annually from the reservoir (Tons), Min is the mean annual sediments inflow (Tons), Qf is the discharge passing through reservoir during flushing (m3/s), Qs is the sediment load during flushing (Tons/s), S is the longitudinal slope during flushing, SSres is the


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representative side slope for the reservoir, SSs is side slope for the deposits exposed by flushing, TE is the trapping efficiency of reservoir (%), Tf is the duration of flushing (days), W is the width of flow for flushing conditions (m), Wbot is the bottom width for the reservoir (m), Wf is the width of flow at the bed of the flushing channel (m), Wres is the reservoir width in the reach upstream from the dam at flushing water surface elevation (m), Wtf is the top width of the scoured valley at the top water level (m), ψ is the multiplier in the Tsinghua University method for sediment load prediction during flushing.

The values of 05 flushing indicators have been computed for the Karot reservoir and are reported in Table 2. The input data required for these calculations are also given in the same Table. The parameters are computed for a flushing discharge of 1600 cumecs with a flushing duration of 20 days. The computed value of Sediment Balance Ratio (SBR) is over satisfied. The output values show that Karot reservoir fulfills all the criteria except for Long Term Storage Capacity (LTCR), its value is 0.81 except 1. The value of Drawdown Ratio (DDR) is very close to the criteria i.e. 0.7, whereas Flushing Width Ratio (FWR) and Top Width Ratio (TWR) are also fully satisfying.

As the Karot reservoir does not satisfy the LTCR criteria fully, hence the flushing efficiency of the reservoir wouldn’t be 100%. It would be around 80 to 90%.

8. CONCLUSIONS

Feasibility report does not contain any study relating to sediment flushing modeling through the reservoir using either SHARC or HEC-RAS Model. The mentioned one to two weeks flushing duration was just based on the experience of the Consultant’s expert.

Results of the HEC-RAS Model for longitudinal sediment delta profile of the Karot reservoir reveal that in one year deposited sediments would be 15.5 MCM and the pivot point will reach at 8 km from the dam face. After 12 and 16 years, sediment deposited would be 66.63 and 76.7 MCM, whereas, pivot point would be expected to reach at 8 and 7.9 km, respectively.

Appropriate flushing discharge determined for the Karot reservoir flushing is 1600 m3/s and the appropriate month to flush the reservoir should be either in May or June, whereas, total flushing duration required to flush the deposited sediments of one year including the emptying and refilling time would be around 20 days.

Numerical values obtained for 05 flushing indicators for the Karot reservoir are well satisfied except for LTCR, hence flushing efficiency of the reservoir to flush the sediments wouldn’t be 100 %, hence a longer duration of the flushing is expected.

9. RECOMMENDATIONS

Flushing discharge of 1600 m3/s with a total flushing duration of 20 days is recommended in the month of either May or June to flush the deposited sediments of one year in the Karot reservoir.


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10. REFERENCES

Atkinson E., “Feasibility of flushing sediment from reservoir”, Report OD137, HR

Wallingford, Wallingford, x+21 pp, 1996.

Chaudhry, M.A., and Habib-ur-Rehman. (2007), Approaches to enhance the life of reservoirs. Int. Conf. and Expo of Water Technologies, January 9-10, Lahore Pakistan, pp.3-13.

Emamgholizadeh S., Bina M., Fathi-Moghadam M.,Ghomeyshi,M, “Investigation and evaluation of the pressure flushing through storage reservoir”. ARPN Journal of Engineering and Applied Sciences Vol. 1 No. 4., pp.7-16, 2006

Fi-John Chang, Jihn-Sung Lai, Li-shan Kao. "Optimization of operation rule curves and flushing schedule in a reservoir", Hydrological Processes, Volume 17 Issue 8, pp.1623-1640, 2003

Habib-ur-Rehman, Chaudhry, M. A, and Akhtar, N., (2009), Assessment of sediment flushing efficiency of Reservoirs. Pakistan Journal of Science, 61(3): 181-187.

ICOLD World Register of Large Dams. CD ROM, 1998

Liu J., Liu B-Y., Ashida K. “Reservoir sedimentation management in Asia.” Advances in Hydro-Sciences and engineering, 5th int. Conf. on Hydro-science and Engineering, Warsaw, Poland, 2002.

Morris G.L., Fan J. (1997), Reservoir Sedimentation Handbook: Design and Management of Dams, Reservoirs and watershed for sustainable use. McGraw Hill, New York, xxiv+805 pp. , 1997

Palmieri A., Shah F., Annandale G. W., Dinar, A., Reservoir Conservation: Volume 1 The RESCON Approach, Volume 2 RESCON Model and User Manual. The International Bank for Reconstruction and Development / The World Bank, Washington D.C. ,2003

SMEC, (2009), Karot Hydropower Project, Feasibility Study Report, Volume 4, Sedimentation Report, April, 2009, Snowy Mountain Engineering Corporation Ltd, Australia, MAES & EGC, Lahore, pp. 40.

U.S. Army Corps of Engineers, (2005), HEC-RAS (Hydrologic Engineering Center, River Analysis System) Manual, 609 second street, Davis CA 95616.

Wallingford HR, (2001), SHARC-Procedure for the Selection and Outline Design of Sediment Control Structures- Technical Manual, March 2001.

White W.R “A review of Current Knowledge World Water Storage in Man- Made Reservoirs”, FR/R0012, April 2005, Foundation for Water Research Allen House, The Listons, Liston Road, Marlow, 40 pp., .2005

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