Final Report

on

Determination of Environmental Flows for Non-lean/

Non monsoon Months for Teesta -IV Hydropower Project,

Sikkim

ICAR-CENTRAL INLAND FISHERIES RESEARCH INSTITUTE

(Indian Council of Agricultural Research)

BARRACKPORE, Kolkata-700120, West Bengal

ICAR-CENTRAL INLAND FISHERIES RESEARCH INSTITUTE

(Indian Council of Agricultural Research)

BARRACKPORE, Kolkata-700120, West Bengal

Foreword

The NHPC Limited vide no. No.NH/ENV.136/2694 dated 22nd December, 2017, requested

for technical expertise from ICAR-Central Inland Fisheries Research Institute (CIFRI),

Barrackpore under a consultancy project for “Determination of environmental flows for

non-lean/non-monsoon months for Teesta- IV Hydropower Project, Sikkim”. The major

focus is to study the estimation of environmental flows during non-lean and non-monsoon

months.

The final report is based on the field investigation conducted in the stretch of approx. 7.3km

in river Teesta from Confluence (RangRang Nala and Teesta river) to Teesta-V Power

Station Reservoir tail-end during the study period between February and April 2018,

representing non-lean/non-monsoon period (March & April). Detailed investigations that

were conducted during the earlier study in the year 2009-10 wherein the total 7.5km stretch

of Teesta River from Teesta-IV dam axis upto TRT was specifically surveyed. The

information provided in this report includes water quality parameters, fish diversity and

plankton diversity based on primary data and hydrological discharge data based on the

secondary information. Several methodologies based on hydrological and hydraulic

simulation were explored to determine the environmental flows favouring the targeted fish

species Schizothorax richardsonii.

All the participant Research Scientists, Technical officers, Administrative and Supporting

staffs and representatives from NHPC are greatly acknowledged for rendering valuable

assistance during sampling period and sharing of secondary data for environmental flows

estimation.

Barrackpore DIRECTOR Date: 15.02.2019

Executive Summary

Teesta IV Hydropower envisages a power generation capacity of 520 MW comprising

construction of power dam on river Teesta with appurtenant headrace tunnels and a

underground powerhouse near Deadkhola viz. Just above reservoir tail end of Teesta -V

Hydropower station. The distance between the Dam to Powerhouse site is approximately 7.5

km which is considered as the ecologically affected stretch. An investigation was carried out

to estimate the environmental flows in the selected stretch from Dam to Powerhouse based on

the approved flows series with 90% dependable year which is considered relevant for the

power potential study. It is established that the daily flow scenario in the river Teesta drives

the life cycle of aquatic biota, particularly predominant fish species of Schizothorax sp.while,

dependency of the local population on the river for other needs like domestic requirement,

irrigation, navigation, commercial fishing is negligible.

Environmental flows (E-flows) was estimated by using both hydrological method

(e.g., Flow-Duration Curve (FDC) method), Hydraulic rating method (e.g., MIKE11-HD that

solves the full dynamic wave equations using the de Saint Venant equations of continuity and

momentum equations), and the fish biology (e.g., adaptable water depth and velocity for its

migration, spawning etc.). Further, the Teesta River is a part of the snow-fed Himalayan river

system. Hence, for recommending the E-flows for this selected stretches of river, there is a

need to consider the snow melt contribution during April-May, which can be identified from

the long-term 10-daily historical time series of stream-flow. Habitat study was carried out for

the short period viz. January to April with the months of March and April representing non-

lean/non-monsoon period. The study indicated that the Rang Rang confluence point which is

approx. 300m and Deadkhola site which is 7500m downstream are suitable for the breeding

and spawning of Schizothorax sp. For habitat requirement of Schizothorax sp. river cross

sections at an interval of 500m were taken into consideration for calculation of E-flows.

Considering the need for a dependable ecological flow scenario for sustenance of aquatic

biota, the following dedicated flow release is recommended.

A dedicated flow release of 20 m3/sec from the dam axis is recommended. This

release will suffice a depth of 0.6m with a flow velocity of 0.4m/s which is estimated as the

minimum habitat for Schizothorax sp. and other indigenous fish species juveniles during non-

lean/non-monsoon season. This release is critical for maintaining the spawn life cycle. In

addition, flows of average 6.2 m3/sec from the nalas /streams will further enhance the aquatic

lives during non-lean/non-monsoon (March to May) months. Hence, it is paramount to ensure

the recommended flow release from the Dam. In addition, it is recommended that the existing

deep pools need to be connected as longitudinal connectivity for the survival and local

migration of brooders or adult fish species in the selected stretches of 7.3 km between dam

axis to power house .Furthermore, in order to augment the indigenous fish germplasm, and

fish population it is recommended to establish a hatchery unit for important fish species like

Schizothorax sp and Tor sp. for ranching of fingerlings in the affected stretches of the

river/streams.

Contents

Index

No.

Subject

Page No.

1.0 Introduction 1-2

2.0 Objectives 2 3.0 Study Area 2 4.0 Methodology 3 4.1 Fish 3 4.2 Plankton& Periphyton 3 5.0 Water Quality Analysis 6 6.0 Sediment Analysis 7 7.0 Biotic Community Analysis 7 7.1 Phytoplankton 7-9 7.2 Periphyton Diversity 10-11 7.3 Fish Diversity 12 7.4 Habitat requirement for Schizothorax sp. 12-13 8.0 Environmental flows and methodologies 13 8.1 Environmental flows and methodologies Worldwide 14-16 8.2 Analysis of Environmental flows 16-17 8.3 E-Flow estimation for the project site in river Teesta 17

8.3.1 Criteria of E-flow estimation 17 8.3.2 FDC Analysis 18 8.3.3 MIKE11-HD Modelling for Different EMCs 20

8.3.4 Flow Depth and Velocity Stimulation at Different River Sections (x=0: Dam Site, x>0: Down stream to Dam) 21-24

8.3.5 Tabular Values of Flow Depth and Velocity at Different River Sections (x=0: Dam Site, x>0: Downstream to Dam) 25-29

8.4 Flow Dependability Analysis 30-32 8.5 River Cross Sections Used 33-41 8.6 Flows from the intermediate tributaries 42 8.7 Conclusions 46 8.8 Recommendations 47 8.9 References 48

List of Tables

Contents

Table 1 Water quality parameters at sampling sites

Table 2 Soil/sediment quality analysis at different sampling Sites

Table 3 Phytoplankton diversity at different sampling sites in River Teesta

Table 4 Periphyton diversity in river Teesta

Table 5 Habitat requirement for Scizothorax sp during non-lean/non-monsoon period

Table 6

90% dependable 10-daily discharge time series (cumec) of the Teesta River using the FDC analysis (For non-lean /non-monsoon season)

Table 7 Flow Depth (in m) for EMC-D Table 8 Flow velocity (m/s) for EMC-D Table 9 Flow Depth (in m) for EMC-C Table 10 Flow Velocity (m/s) for EMC-C

Table 11 Total intermediate catchment as well as the distances from Teesta-IV dam

Table 12

Flow in intermediate catchment (cumec) on the basis of average W.A. between Stage-IV & VI (Common period: 1984 to May 03)

List of Figures

Contents

Fig. 1

Sampling sites in river Teesta (A: Dead Khola confluence B: Deadkhola, C:Rang Rang nala and confluence site D: Dam axis)

Fig. 2

Sampling activities A) Spawn collection at Nalla B) Experimental fishing C) Periphyton collection D) Plankton collection E) Water temperature measurement F) Water quality analysis

Fig. 3

Class wise Phytoplankton Diversity in RangRang Phytoplankton diversity at different sampling sites in River Teesta

Fig. 4 Class wise Phytoplankton Diversity in Deadkhola Fig. 5 Class wise Periphykton Diversity in RangRang

Fig. 6 Class wise Periphyton Diversity in Deadkhola

Fig.7 Juveniles of Schizothorax sp. at Dead kholla Fig. 8 FDC Analysis for non-lean/ non-monsoon season Fig.9 Manning‟s roughness (n) at Sanklang GD site

Fig.10 Flow Depth analysis for class D Fig.11 Flow velocity analysis for class D

Fig.12 Flow Depth analysis for class C Fig.13 Flow Velocity analysis for class C Fig. 14

Flow dependability curve for the month of March (10 daily)

Fig. 15 Flow dependability curve for the month of April (10 daily)

Fig. 16 Flow dependability curve for the month of May (10 daily)

Fig. 17

River cross sections at interval of 500m used for analysis (A-Q)

1

1.0 Introduction

The river Teesta originates from Teesta - Khangse glacier in Tibet and flows through the

entire Sikkim. It is considered as the lifeline of Sikkim state and one of the most scenic rivers

in the Eastern India. This river drains alpine meadows/pastures, sub-alpine zone, temperate

coniferous forests and sub-tropical dense broad leaved forests. In the downstream, it receives

Lachung Chu, Chakang Chu, Dik Chu, Rongni Chu and Rangpo Chu on the left bank and

Rang Rang Chu, RangpoKhila and Rangit on the right bank. The Himalayan rivers are

mostly exploited through hydel power projects, which affects the river ecosystem majorly on

fish and fisheries. Fish and fisheries being an important component of river deserve utmost

attention in the planned hydroelectric power generation projects with respect to conservation

and livelihood for local fishermen. Hydroelectric projects whether proposed or under

construction in a cascade manner like Teesta river will affect the river ecology by altering

aquatic habitats, that supports normal feeding and breeding grounds for the residential fish

population.

Teesta Stage-IV Hydroelectric Project is proposed in the large meandering zone of

Teesta river in Sikkim. This project is located 695 m above MSL with global position of

88°30'00"-88°32'30" E longitude and 27°25'-27°30' N latitude, in downstream of Sankalang

village, North Sikkim. Riverine flow is a major determinant of physical habitat in rivers,

which in turn influences biotic composition. Flow regime changes lead to habitat alterations,

shifting in species composition, loss of biodiversity and failure in migration and breeding of

residential fishes. The proposed dam construction for Teesta Stage-IV H.E. Project will alter

the natural river flow, which may affect the flora and fauna of the river and associated

livelihoods.

ICAR-CIFRI, Barrackpore undertook a study to determine the environmental flow before

implementation of the Teesta Stage- IV HE Project during 2009-10. The specific terms of

reference for study on environmental flow fixed by the MoEFC were as follows: "an

estimation to be made for environmental flows downstream for sustenance of aquatic

environment and for downstream uses, considering details of streams joining the river below

the proposed dam site with their approximate distance from the dam site, their nature

(whether perennial or seasonal) etc. A detail environmental flow study shall be carried out

through the premier institutions such as Central Inland Fisheries Research Institute (CIFRI),

2

Barrackpore and National Institute of Hydrology (NIH), Roorkee for biological and

hydrological components". Based on the ToR environmental flow for lean season was

estimated and recommended that from the aquatic habitat maintenance particularly the

fisheries point of view, the following recommendations were suggested for the 7.3 km

downstream of proposed Teesta Stage IV Dam axis.

For sustenance of ecological integrity in the 7.3 km stretch of Teesta river, downstream of

Stage IV dam axis:

1) Release of a minimum of 10 cumecs of water from the proposed Teesta Stage IV dam

during lean season is suggested.

2) During the lean months with decline in the water flow there is possibility of the fish

and other aquatic fauna getting isolated in off channel habitats or disconnected pools.

It is suggested that methods should be explored to maintain river bed submergence to

provide shelter to the resident fish species. This may be achieved by creating pool

riffle habitat units at the places near confluence with the seven tributaries within the

7.3 Km stretch by constructing appropriate engineering structures. It is to be ensured

that the connections between different pools created are maintained.

Realizing the seasonal discharge requirement for estimation of environmental flows, NHPC

further approached to conduct a short term investigation (for a period of four months) and

analysis focusing on the determination of E-flow for non-lean / non-monsoon months. With

this backdrop, the assigned study was designed with following objective

2.0 Objective:

Determination of environmental flows for non-lean/ non-monsoon period in river

Teesta only in 7.3 km stretch down stream of the proposed Teesta IV HE Project

3.0 Study Area:

A total of 7.3 km stretch of Teesta river flowing down stream of Stage - IV HE project dam

axis up to TRT (reservoir tail end of Teesta V project) was surveyed and two sites viz., Rang

Rang, and DeadKhola were selected for both biotic and abiotic sample collection during

February and April (keeping in view that, this would be the overlapping month as lean

season ends in February and non-lean/non-monsoon season starts from March)for generating

the current status on river hydrology, river habitat and biological data including fish and fish

food organisms. The survey was conducted in the form of direct site visit, observation from

3

top view, secondary information collection from project officials and other sources on

hydrobiology, diversity of plankton, fish species diversity and their migration pattern which

are likely to be affected during non-lean and non-monsoon period in the Teesta Stage-IV HE

Project. The details of sampling sites and sampling activities are shown in Fig.1& Fig.2.

4.0 Methodology

4.1Fish

Fish sampling was carried out with a wide variety of gears such as gill net, hook and line, cast

net and local traps. Gill nets with length: 100 – 150 m and height/breadth: 8 – 8.5 m were

operated in selected sampling sites majorly in the reservoirs (Teesta-V). While cast net and

hook and lines were operated in the running water and water with shallow depth. The fishes

caught were counted, weighed, fixed in 10 % formalin as per APHA (2010) and further

analysis were carried out at ICAR-CIFRI laboratory. The fishes were identified up to species

level with the help of standard taxonomic literature (Talwar and Kacker, 1984; Talwar and

Jhingran, 1991; Talwar, 1991; Talwar et al., 1992; Jayaram, 1999; Chatterjee et al., 2000;

Raje et al., 2007) and the scientific name of each fish species was ascertained as per updated

and revised scheme provided in the Eschemeyer Catalog of Fishes.

4.2 Plankton and Periphyton

Plankton samples were collected by filtering the 100 Litre water through plankton net of

60µm mesh size and filtrate sample were fixed with 4%formalin.Quallitative analysis was

estimated using a compound microscope. Quantitative study was measured by using drop

counting method(APHA,2012).Derived individual numbers are expressed in terms of cells

per (cells /cm-2).While, Periphyton samples were collected by scraping (10*10sq cm) surface

of rock, large stones and pebbles. The samples were preserved in 4% formalin for further

study. Qualitative analysis was estimated using a compound microscope. Quantity of

periphyton was measured by using drop counting method(APHA,2012).Derived individual

numbers are expressed in terms of cells per 100cm2(cells /cm-2).Phytoplankton and

Periphyton samples were examined under Carl Zeiss Axiostar microscope(63x,100x) and

identified upto genus level with the help of standard books and monographs(Baykal et

al.,2011;Edmondson,1959)

4

(A) (B)

(C) (D)

Fig.1 Sampling sites in river Teesta (A: Dead Khola confluence B:Deadkhola, C:Rang

Rang nala and confluence site D: Dam axis)

5

(A) (B)

(C) (D)

(E) (F)

Fig. 2 Sampling activities A) Spawn collection at Nala B) Experimental fishing C)

Periphyton collection D) Plankton collection E) Water temperature measurement F) Water

quality analysis

6

5.0 Water Quality Analysis:

To understand the existing water quality of the river Teesta especially in the stretch of the

proposed Teesta Stage-IVHE Project, Sikkim, sixteen parameters have been analyzed for

each sampling sites. All the water samples were collected and analyzed following standard

methods(APHA,2012). The value of the water quality parameters are presented in Table 1.

The water temperature in river Teesta was 20 ±0.5°C, this is suitable for growth and survival

of the cold water fishes. The water transparency value was higher in river Teesta which

showed the poor productivity of the aquatic eco system. However, the observed water pH

ranges was found to be optimal for the growth and health of the fishes as well as other aquatic

organisms in the river system. The high Dissolves Oxygen concentration above8.0mg/l is due

to natural aeration of water by surface agitation during flowing through its rocky

embankment courses. The recorded CO2 of the water in the river systems was below the

tolerance limit of the fresh water fishes. The nutrient level of the river system like Phosphate,

Silicate,Calcium and Magnesium were found to be in the suitable range for fish during the

study period indicating the ecosystem as healthy.

Table 1Water quality parameters at sampling sites

PARAMETERS RANGRANG DEADKHOLA

Air Temp. (°C) 20-24 20-25

Water Temp.(°C) 9.9-14.5 11-13

Alkalinity 12-56 12-50 CO2 (PPM) 2 2-3

DO(ppm) 10.2-17.6 10.4-17.8

Transparency (CM) 66-81 35-61

Conductivity (µS/CM) 90.2 85.1

Salinity 0.04-0.009 0.04-.0235 PH 7.6 7.6 Nitrate (ppm) 0.066 0.071-0.165 Phops-P(ppm) 0.014-0.034 0.007-0.039 Sili-Si (ppm) 2.474-7.371 4.278 - 8.659 Total Hard(ppm) 44.0-48.0 40.0-44.0 Ca++ (ppm) 9.61-11.12 8.01-9.61 Mg++ (ppm) 3.83-5.75 3.83-5.75 Chloride (ppm) 0.005-9.32 0.0129-6.51

7

6.0 Sediments Analysis:

The sediment samples were collected from the selected sites to analyze the physico-chemical

properties at the proposed dam sites and its downstream. Eight soil quality parameters were

analyzed by following standard protocol. The observations of soil/sediment quality

parameters have shown in Table 2

Table 2 Soil/sediment quality analysis at different sampling Sites

7.0 Biotic community Analysis:

7.1 Phytoplankton:

Phytoplankton consists of 14 genus belonging 3 class, 9 orders and 10 families (Table 3).

Class wise phytoplankton distribution shows that Bacillariophyceae are the dominant group

followed by Cyanophyceae and Conjugatophyceae (Fig.3, 4). Abundance of phytoplankton

has been depicted based on presence and absence basis. The group of phytoplankton found

were Navicula sp.,Synedra sp., Nitzschia sp., Gomphonema sp., Encyonema sp., Cymbella sp.

,Cocconeis sp., Closterium sp., Spirogyra sp.,Oscillatoria sp.,Phormidium sp., Lyngbya sp.,

Spirulina sp. out of which Nitzschia sp.,Synedra sp., were predominant followed by

Oscillatoria sp. and Lyngbya sp.

Parameters RANGRANG DEADKHOLA

pH 7.0 7.2 Conductivity (µs/cm)

63.02-68.8 20-27.7

Organic carbon(%)

0.01 0.001

Free Caco3(%) 0 2 Sand(%) 98 98 Silt(%) 1 1 Clay(%) 1 1 Available Phosphate

3.50-4.51 3.02-4.02

8

Table 3 Phytoplankton diversity at different sampling sites in River Teesta

Phytoplankton Deadkhola RangRang

Class: Bacillariophyceae Order:Naviculales Family: Naviculaceae Navicula sp. + +

Family: Pinnulariaceae

Pinnularia sp. + +

Order: Bacillariales

Family: Bacillariaceae

Nitzschia sp. ++ ++

Order:Cocconeidales

Family:Cocconeidaceae

Cocconeis sp. + +

Order :Cymbellales

Family:Gomphonemataceae

Cymbella sp. + +

Encyonema sp. + -

Gomphonema sp. + +

Order: Mastogloiales

Family :Achnanthaceae

Achnanthes sp. - +

Order: Fragilariales

Family: Fragilariaceae

Synedra sp. ++ ++

Fargillaria sp., + +

Order: Licmophorales

Family: Ulnariaceae

Hannaea sp. + +

Order :Surirellales

Family: Ulnariaceae

Surirella sp. + -

Order :Tabellariales

Family: Tabellariaceae

Tabellaria sp. + +

Diatoma sp. + +

Class: Chlorophyceae

Order : Sphaeropleales

Family: Scenedesmaceae

Scenedesmus sp. + +

Family: Microsporaceae

Microspora sp. + +

Class: Cyanophyceae

Order :Oscillatoriales

Family: Oscillatoriaceae

Lyngbya sp. + +

Oscillatoria sp. + +

9

57%

39%

4%

RangRang

Bacillariophyceae Cyanophyceae Conjugatophyceae

54%41%

5%

Deadkhola

Bacillariophyceae Cyanophyceae Conjugatophyceae

Phormidium sp. + +

Class: Xanthophyceae

Order: Tribonematales

Family :Tribonemataceae

Tribonema sp. + +

Class: Conjugatophyceae

Order:Zygnematales

Family :Zygnemataceae

Spirogyra sp. + +

(Symbols++, + and – representing dominant, present and absent respectively)

Fig.3 Class wise Phytoplankton Diversity in RangRang

Fig.4 Class wise Phytoplankton Diversity in Deadkhola

10

7.2 Periphyton Diversity

Periphyton community consisted of 23 genera distributed among14 orders, representing 5

classes such as Bacillariophyceae followed by Chlorophyceae, Cyanophyceae,

Xanthophyceae and Conjugatophyceae (Table 4, Fig.5 & Fig. 6).The group of periphyton

found were Navicula sp., Nitzschia sp., Synedra sp.,Fargillaria sp., Cocconeis sp., Tabellaria

sp., Diatoma sp., Scenedesmus sp. ,Microspora sp., Lyngbya sp., Oscillatoria sp.,

Phormidium sp., Tribonema sp., Spirogyra sp. out of which Nitzschia sp.,Synedra sp.,

Hannaea sp. were predominant followed by Oscillatoria sp.,Phormidium sp., Microspora sp.

and Lyngbya sp.

Table 4 Periphyton diversity in river Teesta

Periphyton RangRang Deadkhola

Class:Bacillariophyceae

Order:Naviculales

Family:Naviculaceae

Navicula sp. + + + + Order: Fragilariales Family: Fragilariaceae Synedra sp. + + ++ Order: Bacillariales Family: Bacillariaceae Nitzschia sp. ++ ++ Hantzschia sp. + ++ Order : Cymbellales Family :Gomphonemataceae Gomphonema sp. + + Encyonema sp. - + Family: Cymbellaceae Cymbella sp. ++ + Order :Cocconeidales Family:Cocconeidaceae Cocconeis sp. + + Class:Conjugatophyceae Order : Desmidiales Family : Closteriaceae Closterium sp. + - Order :Zygnematales Family: Zygnemataceae Spirogyra sp. + - Class:Cyanophyceae Order :Oscillatoriales Family:Oscillatoriaceae

11

Periphyton RangRang Deadkhola

Oscillatoria sp. ++ - Phormidium sp. + + Lyngbya sp. + - Order:Spirulinales Family:Spirulinaceae Spirulina sp. + +

Fig. 5 Class wise Periphykton Diversity in RangRang

Fig. 6 Class wise Periphyton Diversity in Deadkhola

54%34%

2%10%

Deadkhola

Bacillariophyceae Cyanophyceae

Chloro phyceae Xanthophyceae

50%

37%

2%11%

RangRang

Bacillariophyceae Cyanophyceae

Chloro phyceae Xanthophyceae

12

7.3 Fish Diversity Experimental fishing was operated at both the selected sites, through different nets/gears i.e

gill net and cast nets at different times. Though we have not recorded any adult fish, juveniles

of size with an average of 30 mm±0.5 mm were recorded in the nala of DeadKhola indicating

the spawning sites of Schizothorax. Sp. in the upstream of the Deadkhola (Fig. 7)

Fig.7Juveniles of Schizothorax sp. at Deadkhola

From our previous study during 2010, we could record fish species namely Schizothorax sp.,

Tor sp.,Neolissochilus hexagonolepis, Nemacheilus botia and Garra sp. in the same stations

covering four seasons (Lean, pre-monsoon, monsoon, and post-monsoon). While, during our

present sampling we could get only juveniles of Schizothorax sp. This could be due to short

sampling period. Availability of juveniles clearly indicates that the site is suitable for the

spawning/breeding ground for the up-stream migratory fishes.

7.4 Habitat requirement for Schizothorax sp.

From our field observation, it is recorded that the Deadkhola site is more suitable for the

breeding of Schizothorax sp. Based on the habitat suitability following important habitat

parameters were fixed (Table 5)

Juveniles of Schizothorax sp.

13

Table 5 Habitat requirement for Scizothorax sp. during non-lean/non-monsoon period

Habitat suitability for Schizothorax sp. during non-lean/non-monsoon

Elevation 690m River bed Rocky and silt Stream Depth(m) 0.6-0.8m Stream Velocity(m/sec) 0.4-0.6m/s Substrate Composition (Relative %)

Bolders-70% +

Stones-20% +

Sand- 5% Silt-5

Recruitment/ Fish Seeds Fish seeds(fry)/

fingerlings

obtained

8.0 Environmental flows and methodologies:

Environmental flows refer to the water considered sufficient for protecting the structure and

function of an ecosystem and its dependent species. Environmental flows are required to be

maintained through a river reach for sustaining its ecosystem and dependent species. It means

enough water is to be released in the downstream of the river system after utilizing the water

for the development projects in order to ensure downstream environmental, social and

economic benefits. Realizing its importance, several countries have adopted environmental

flows as mandatory for river health protection. For example, The Mekong River Agreement,

1995; South Africa‟s National Water Act, 1998 and the Swiss Water Protection Act, 108.

These legislations attempt to ensure required minimum flow in the river system to sustain

ecosystem services. A wide range of environmental flow methodologies (EFMs) have been

developed to determine flow thresholds for various objectives such as the preservation of

natural conditions, the maintenance or restoration of ecological integrity and cultural and

recreational values. Most of these methods were developed primarily to protect endangered

fish species and to maintain fisheries resources in human-modified rivers. Until now, these

methodologies have been mostly applied to small upland rivers and headwater streams.

Although a growing body of literature summarizes the status of available EFMs the question

remains whether or not these EFMs are suitable to protect fish diversity and fisheries

resources in regulated large lowland rivers.

14

8.1 Environmental flows methodologies worldwide:

Most currently available EFMs can be grouped in four main categories and as given below

1. Hydrological methods

2. Hydraulic rating

3. Habitat simulation methods

4. Holistic methodologies

In European countries, hydrological and habitat simulation are the prevailing methods, while

some developing countries and countries with newer environmental legislation have focused

on holistic methods. Hydrological methods rely primarily on flow measures and indices,

which are drawn from historical time series data on annual or daily mean flow. Still widely

used is the Tennant method (also known as Montana method), which relates the ratio between

river discharge and fish habitat availability to certain percentages of annual flows to meet

predefined requirements (Tennant, 1976). The Tennant method assumes similarity of aquatic

habitats when carrying the same proportion of average flow but rarely considered the

effective habitat quality at varying flows. The method lists eight categories of in-stream flow

that range from maximum to severe degradation. Below the threshold of 10% mean flow, the

environmental conditions for fish are judged to be degraded, whereas 50% provides for

excellent conditions in terms of stream width, water depth and velocity. While the Tennant

method does not explicitly consider duration or the timing of flow events, some extensions

integrate flow duration and frequencies. Other hydrological methods include duration

percentiles or single flow indices that are usually generated from historical stream flow

databases. One example is the widely used 7Q10, which is defined as the „seven-day,

consecutive low flow with a ten-year return frequency; the lowest stream flow for seven

consecutive days that would be expected to occur once in ten years‟ (United States

Environmental Protection Agency) or similar discharge indices.

Hydraulic rating considers the channel morphology of a given river and calculates

acceptable flows by relating river discharge with a variety of hydraulic characteristics such as

water depth, velocity or wetted perimeter. These methods rely on transects measured across a

river section comprising habitat factors that are assumed to be limiting factors for target

biota. The wetted perimeter method considers the variation in wetted perimeter or river width

with water discharge. Plotting the wetted perimeter against discharge shows a breakpoint

where a comparable small decrease in discharge results in a comparably larger decrease in

wetted perimeter. This breakpoint is used as a minimum in-stream flow recommendation. The

15

widely used habitat simulation methods are sophisticated extensions of hydraulic rating

methods within a framework addressing many ecological components of riverine ecosystems

(IFIM, In-stream Flow Incremental Methodology, Bovee, 1982). Within the context of IFIM,

a broad range of modelling tools such as PHABSIM aim to predict how physical habitat

conditions (i.e. water depth, velocity, cover, substrata) change with discharge. Typically,

detailed hydrological and hydraulic data for a grid of cells in a river stretch are compared

with the habitat suitability of a target species. The habitat suitability is expressed as a habitat

suitability curve (e.g. suitability index curve, probability of use or preference curve), often

seasonally defined, which specifies the assumed seasonal requirements of different species,

life stages or habitat guilds. The curves depict the relationship of target organism‟s response

to a gradual changing habitat variable scaling from unsuitable to suitable, which are usually

obtained from existing data or field measurements. By comparing the curves with the

predicted habitat area at various flows, the minimum flow thresholds can be defined in a way

so that the discharge provides optimal habitat conditions, retains a percentage of habitats at

average flow or provides a minimum amount of habitat area. Most commonly, the flow

threshold is set at the breakpoint in the habitat/flow curve where proportionally more habitat

is lost with decreasing flow than is gained with increasing flow. The commonly used output

of, for example PHABSIM quantifies the suitability of a location for a target species in terms

of a weighted usable area (WUA; expressed as, e.g. m2 1000m stream length1). More

advanced software tools such as 2D and 3D models achieve greater hydraulic representation.

Other models allow for inclusion of water quality, temperature and other biological factors

such as prey densities, energy allocation and behavioural components. The complexity of

current models is growing and there are many approaches to establish statistical techniques

that improve the predictability of species abundance on the basis of biotic and abiotic

variables. The growing recognition that rivers are closely connected to their watersheds has

led to the realization that protecting and rehabilitating riverine ecosystems requires sensitivity

not only to the key hydrological, biological and ecological, but also to the economic and

social aspects of a riverine ecosystem. Assuming that a natural flow system will maintain the

ecological function of a riverine ecosystem, so-called holistic methodologies will define the

critical environmental flows of an entire riverine ecosystem rather than focusing on the needs

of a single species.

Holistic methods rely less on modelling and more on multidisciplinary panels

covering biophysical disciplines such as hydrology, geomorphology, sedimentology, water

16

chemistry, botany and zoology. Advanced methodologies such as DRIFT (Downstream

Response to Intended Flow Transformations Methodology, King et al., 2003) consist of

different modules that integrate biophysical, and economic and social factors and aim at

participating stakeholder groups. Within the biophysical module, various EFMs such as

habitat-modelling tools can also be implemented. The growing recognition that rivers are

closely connected to their watersheds has led to the realization that protecting and

rehabilitating riverine ecosystems requires sensitivity not only to the key hydrological,

biological and ecological, but also to the economic and social aspects of a riverine ecosystem.

Assuming that a natural flow system will maintain the ecological function of a riverine

ecosystem called holistic methodologies will define the critical environmental flows of an

entire riverine ecosystem rather than focusing on the needs of a single species. Holistic

methods rely less on modelling and more on multidisciplinary panels covering biophysical

disciplines such as hydrology, geomorphology, sedimentology, water chemistry, botany and

zoology. Advanced methodologies such as DRIFT consist of different modules that integrate

biophysical, and economic and social factors and aim at participating stakeholder groups.

Within the biophysical module, various EFMs such as habitat-modelling tools can also be

implemented.

8.2 Analysis of Environmental flows

Mostly, to maintain specific river classes, E-flows are estimated world-wide at monthly time-

scale using the Flow-Duration Curve (FDC) analysis. For this, generally, a minimum of 25

years river discharge time series data is used. A river may be classified into six environmental

management classes of A, B, C, D, E, and F depending on the following flow modification

category (Kleynhans, 1996). The more pristine is the desired management class, the higher is

the E-flow requirement.

Class A (Natural): Pristine river condition or minor modification of in-stream and

riparian habitat.

Class B (Slightly modified): Largely intact biodiversity and habitats in the river despite

water resources development and/or basin modifications

Class C (Moderately modified): The river habitats and dynamics of the biota have been

disturbed, but the basic ecosystem functions are still intact. Some sensitive species are

lost/or reduced in extent; however, the alien species are present.

17

Class D (Largely modified): There are large changes in the river natural habitats, biota

and basic ecosystem functions with a clearly lower than expected species richness. A

clearly lower than expected species richness.

Class E (Seriously modified): Habitat diversity and availability have declined. A

strikingly lower than expected species richness. Alien species invaded the ecosystem.

Class F (Critically modified): Modifications have reached a critical level and

ecosystem has been completely modified with almost total loss of natural habitat and

biota. This river status is not acceptable at all.

Hence, the E-flow should be released from the dam so that it can maintain the EMC-C/D

condition.

8.3 E-Flows estimation for the project site in river Teesta

Generally, for Hydropower Projects, it is advised to carry out E-flow estimation using the 10-

daily flow values. There are several E-flow estimation methods existing world-wide.

However, due to the complexity of the target fish species, its aquatic habitat, and temporal

variability of flow depth and velocity characterized by the irregular river cross-section, there

is a need to integrate the hydrological method (e.g., Flow-Duration Curve (FDC) method),

Hydraulic rating method (e.g., MIKE11-HD that solves the full dynamic wave equations

using the de Saint Venant equations of continuity and momentum equations), and the fish

biology (e.g., adaptable water depth and velocity for its migration, spawning etc.). Further,

the Teesta River is a part of the snow-fed Himalayan river system. Hence, while

recommending the E-flows for this river, there is a need to consider the snow melt

contribution during April-May, that can be identified from the long-term 10-daily historical

time series of stream flow

8.3.1 Criteria of E-flow estimation

Target fish species: Schizothorax sp.

Water depth and flow velocity along the selected river stretch (≈7300 m below the

proposed dam section):

Season Water depth (m) Flow velocity (m/s)

Non-lean/Non-monsoon 0.6-0.8 0.4-0.6

Preferred time-scale of E-flows prescription for Hydropower projects: 10-daily

Based on the above criteria, E-flows was estimated through FDC analysis as indicated in

Fig. 8& Table 6

18

8.3.2. FDC Analysis

Fig. 8FDC Analysis for non-lean/ non-monsoon season

0

20

40

60

80

100

120

140

160

180

200

220

240

260

I II III I II III I II III I II III I II III I II III I II III I II III I II III I II III I II III I II III

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Dis

ch

arg

eQ

90

(c

um

ec)

----

--->

Pristine

Class A

Class B

Class C

Class D

Non-lean non-monsoon study periodNL -NM study

period

Not covered under scope of this work

Not covered under scope of this work

Not covered under scope of this work

19

Table690% dependable 10-daily discharge time series (cumec) of the Teesta River using the

FDC analysis (For non-lean /non-monsoon season)

Flow to be

released (cumec) Pristine Class A Class B Class C Class D

Mar-I 58.1 53.8 51.2 49.7 49.5

Mar-II 70.5 62.1 55.8 51.9 51.0

Mar-III 105.3 102.7 100.1 98.4 98.0

Apr-I 113.8 111.9 110.5 109.6 109.4

Apr-II 84.4 70.1 60.1 54.2 52.9

Apr-III 100.3 78.8 62.6 52.6 50.5

May-I 116.7 101.1 89.2 81.9 80.3

May-II 168.6 147.6 128.8 116.2 113.5

May-III 214.3 199.2 186.4 178.0 176.1

20

8.3.3 MIKE11-HD Modeling for Different EMCs

The following Manning‟s roughness (n) has been used in all the simulations (Fig.9). This data

on velocity and cross sectional area at the upstream Sanklang Gauge-Discharge site were

taken for analysis. The value of n varies with respective water flow depth, and hence, with

respect to the water elevation. Since the downstream reach is ungauged in terms of n, the

same flow-depth specific n values have been used for all the downstream cross sections.

Fig.9 Manning‟s roughness (n) at Sanklang GD site

21

8.3.4 Flow Depth and Velocity Stimulation at Different River Sections (x=0: Dam Site, x>0: Downstream to Dam) (Fig. 10, 11,12,13)

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.09.5

10.010.511.011.512.012.513.013.514.014.5

1-Jan 31-Jan 1-Mar 31-Mar 30-Apr 30-May 29-Jun 29-Jul 28-Aug 27-Sep 27-Oct 26-Nov 26-Dec

Flo

w d

epth

(m

)

Julian days (from Jan 1-Dec 31)

x=0m

x=500m

x=1000m

x=1500m

x=2000m

x=2500m

x=3000m

x=3500m

x=4000m

x=4500m

x=5000m

x=5500m

x=6000m

x=6500m

x=7000m

x=7500m

x=8000m

EMC - D

Fig.10 Flow Depth analysis for class D

22

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

2.50

2.75

3.00

3.25

3.50

3.75

4.00

4.25

4.50

4.75

5.00

5.25

5.50

5.75

6.00

1-Jan 31-Jan 1-Mar 31-Mar 30-Apr 30-May 29-Jun 29-Jul 28-Aug 27-Sep 27-Oct 26-Nov 26-Dec

Flo

w v

elo

city

(m

)

Julian days (from Jan 1-Dec 31)

x=0m

x=500m

x=1000m

x=1500m

x=2000m

x=2500m

x=3000m

x=3500m

x=4000m

x=4500m

x=5000m

x=5500m

x=6000m

x=6500m

x=7000m

x=7500m

x=8000m

EMC - D

Fig.11Flow velocity analysis for class D

23

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

5.50

6.00

6.50

7.00

7.50

8.00

8.50

9.00

9.50

10.00

10.50

11.00

11.50

12.00

12.50

13.00

13.50

14.00

14.50

1-Jan 31-Jan 1-Mar 31-Mar 30-Apr 30-May 29-Jun 29-Jul 28-Aug 27-Sep 27-Oct 26-Nov 26-Dec

Flo

w d

epth

(m

)

Julian days (from Jan 1-Dec 31)

x=0m

x=500m

x=1000m

x=1500m

x=2000m

x=2500m

x=3000m

x=3500m

x=4000m

x=4500m

x=5000m

x=5500m

x=6000m

x=6500m

x=7000m

x=7500m

x=8000m

EMC - C

Fig.12Flow Depth analysis for class C

24

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

2.50

2.75

3.00

3.25

3.50

3.75

4.00

4.25

4.50

4.75

5.00

5.25

5.50

5.75

6.00

1-Jan 31-Jan 1-Mar 31-Mar 30-Apr 30-May 29-Jun 29-Jul 28-Aug 27-Sep 27-Oct 26-Nov 26-Dec

Flo

w v

elo

city

(m

)

Julian days (from Jan 1-Dec 31)

x=0m

x=500m

x=1000m

x=1500m

x=2000m

x=2500m

x=3000m

x=3500m

x=4000m

x=4500m

x=5000m

x=5500m

x=6000m

x=6500m

x=7000m

x=7500m

x=8000m

EMC - C

Fig.13Flow Velocity analysis for class C

25

8.3.5 Flow Depth and Velocity at Different River Sections (x=0: Dam Site, x>0: Downstream to Dam) (Table 7,8,9,10)

Table 7. Flow Depth (in m) for EMC-D

10-

dail

y

X

=0

m

X

=500

m

X

=100

0m

X

=150

0m

X

=200

0m

X

=250

0m

X

=300

0m

X

=350

0m

X

=400

0m

X

=450

0m

X

=500

0m

X

=550

0m

X

=600

0m

X

=650

0m

X

=700

0m

X

=750

0m

X

=800

0m

Ma

r-I

3.0

2 2.85 2.15 1.69 1.57 2.98 2.67 2.33 2.34 2.10 4.08 1.60 2.15 1.60 1.91 2.61 6.54

Ma

r-II

3.0

7 2.89 2.18 1.71 1.60 3.01 2.69 2.35 2.40 2.14 4.14 1.61 2.18 1.62 1.94 2.66 6.62

Ma

r-

III

4.0

4 3.69 2.59 2.27 2.23 3.86 3.38 2.97 4.38 3.05 5.44 2.08 2.88 2.35 2.76 3.64 8.96

Ap

r-I

4.1

4 3.83 2.66 2.38 2.36 4.03 3.52 3.09 4.57 3.24 5.65 2.18 3.03 2.50 2.93 3.87 9.50

Ap

r-II

3.1

3 2.94 2.20 1.74 1.63 3.05 2.72 2.37 2.48 2.18 4.21 1.63 2.21 1.66 1.98 2.72 6.79

Ap

r-

III

3.0

5 2.88 2.17 1.71 1.59 3.00 2.68 2.34 2.38 2.13 4.12 1.61 2.17 1.61 1.93 2.64 6.60

Ma

y-I

3.8

7 3.46 2.47 2.07 2.01 3.57 3.14 2.76 4.03 2.74 5.06 1.92 2.64 2.10 2.47 3.29 8.08

26

Ma

y-II

4.1

7 3.88 2.68 2.42 2.41 4.08 3.57 3.13 4.64 3.30 5.72 2.22 3.08 2.56 2.99 3.94 9.63

Ma

y-

III

4.7

0 4.54 3.01 2.98 3.04 4.81 4.25 3.72 5.34 4.12 6.37 2.70 3.79 3.28 3.75 4.83 11.25

Table 8 Flow velocity (m/s) for EMC-D

10-

dail

y

X

=0

m

X

=500

m

X

=100

0m

X

=150

0m

X

=200

0m

X

=250

0m

X

=300

0m

X

=350

0m

X

=400

0m

X

=450

0m

X

=500

0m

X

=550

0m

X

=600

0m

X

=650

0m

X

=700

0m

X

=750

0m

X

=800

0m

Ma

r-I

1.1

2 0.60 0.92 1.61 1.37 1.10 0.87 1.05 2.03 1.36 1.06 0.88 0.97 1.00 2.04 0.94 0.19

Ma

r-II

1.1

3 0.60 0.91 1.62 1.37 1.11 0.89 1.07 2.01 1.36 1.06 0.90 0.97 1.02 2.03 0.94 0.19

Ma

r-

III

1.1

8 0.75 1.15 1.88 1.62 1.37 1.13 1.31 1.41 1.56 1.29 1.08 1.19 1.21 1.88 1.07 0.23

Ap

r-I

1.2

3 0.78 1.20 1.93 1.67 1.42 1.18 1.36 1.45 1.59 1.33 1.11 1.22 1.25 1.87 1.07 0.23

Ap

r-II

1.1

3 0.60 0.93 1.63 1.38 1.13 0.90 1.09 1.98 1.38 1.08 0.91 0.99 1.02 2.02 0.94 0.19

27

Ap

r-

III

1.1

3 0.60 0.91 1.61 1.37 1.11 0.88 1.06 2.02 1.36 1.06 0.89 0.97 1.01 2.03 0.94 0.19

Ma

y-I

1.0

9 0.69 1.05 1.81 1.54 1.29 1.06 1.23 1.36 1.49 1.22 1.02 1.12 1.14 1.93 1.04 0.22

Ma

y-II

1.2

5 0.80 1.22 1.95 1.68 1.44 1.19 1.38 1.46 1.61 1.34 1.11 1.24 1.26 1.86 1.08 0.24

Ma

y-

III

1.3

5 0.93 1.44 2.13 1.87 1.70 1.38 1.58 1.69 1.82 1.57 1.24 1.37 1.43 1.89 1.17 0.31

Table 9 Flow Depth (in m) for EMC-C

10-

dail

y

X

=0

m

X

=500

m

X

=100

0m

X

=150

0m

X

=200

0m

X

=250

0m

X

=300

0m

X

=350

0m

X

=400

0m

X

=450

0m

X

=500

0m

X

=550

0m

X

=600

0m

X

=650

0m

X

=700

0m

X

=750

0m

X

=800

0m

Ma

r-I

3.0

2 2.86 2.16 1.70 1.58 2.98 2.67 2.33 2.35 2.11 4.09 1.60 2.16 1.60 1.91 2.61 6.55

Ma

r-II

3.1

0 2.91 2.19 1.73 1.61 3.03 2.71 2.36 2.44 2.16 4.17 1.62 2.20 1.64 1.96 2.69 6.67

Ma

r-

4.0

5 3.69 2.59 2.27 2.23 3.87 3.38 2.97 4.38 3.06 5.45 2.09 2.89 2.35 2.76 3.65 8.98

28

III

Ap

r-I

4.1

4 3.83 2.66 2.39 2.36 4.03 3.52 3.09 4.57 3.24 5.66 2.18 3.03 2.50 2.93 3.87 9.51

Ap

r-II

3.1

7 2.98 2.22 1.76 1.65 3.08 2.74 2.39 2.54 2.21 4.26 1.65 2.23 1.68 2.00 2.76 6.86

Ap

r-

III

3.1

2 2.93 2.20 1.74 1.62 3.04 2.72 2.37 2.47 2.17 4.20 1.63 2.21 1.65 1.97 2.71 6.71

Ma

y-I

3.8

9 3.48 2.48 2.09 2.03 3.59 3.16 2.78 4.06 2.77 5.10 1.93 2.66 2.12 2.50 3.32 8.14

Ma

y-II

4.2

0 3.91 2.69 2.45 2.44 4.12 3.60 3.16 4.68 3.34 5.77 2.24 3.12 2.59 3.03 3.99 9.71

Ma

y-

III

4.7

2 4.56 3.02 2.99 3.06 4.82 4.27 3.74 5.36 4.15 6.38 2.72 3.81 3.30 3.77 4.85 11.29

Table 10 Flow Velocity (m/s) for EMC-C

10-

dail

y

X

=0

m

X

=500

m

X

=100

0m

X

=150

0m

X

=200

0m

X

=250

0m

X

=300

0m

X

=350

0m

X

=400

0m

X

=450

0m

X

=500

0m

X

=550

0m

X

=600

0m

X

=650

0m

X

=700

0m

X

=750

0m

X

=800

0m

Ma 1.1 0.60 0.91 1.60 1.36 1.11 0.87 1.06 2.03 1.36 1.06 0.88 0.96 1.01 2.05 0.94 0.19

29

r-I 3

Ma

r-II

1.1

3 0.60 0.92 1.62 1.38 1.12 0.89 1.08 1.99 1.37 1.07 0.90 0.98 1.02 2.02 0.94 0.19

Ma

r-

III

1.1

7 0.75 1.15 1.88 1.63 1.37 1.14 1.32 1.42 1.56 1.29 1.07 1.19 1.21 1.89 1.07 0.23

Ap

r-I

1.2

3 0.79 1.20 1.92 1.67 1.42 1.18 1.37 1.45 1.60 1.33 1.11 1.23 1.25 1.87 1.08 0.23

Ap

r-II

1.1

4 0.60 0.93 1.63 1.39 1.13 0.91 1.10 1.96 1.38 1.08 0.91 1.00 1.03 2.02 0.94 0.19

Ap

r-

III

1.1

3 0.60 0.92 1.62 1.39 1.13 0.90 1.08 1.98 1.38 1.08 0.90 0.98 1.02 2.03 0.94 0.19

Ma

y-I

1.0

9 0.71 1.06 1.81 1.55 1.30 1.06 1.24 1.36 1.50 1.22 1.03 1.13 1.15 1.92 1.04 0.22

Ma

y-II

1.2

5 0.80 1.24 1.95 1.69 1.45 1.20 1.39 1.47 1.62 1.35 1.12 1.24 1.27 1.86 1.08 0.24

Ma

y-

III

1.3

4 0.93 1.44 2.14 1.88 1.72 1.38 1.58 1.69 1.82 1.58 1.23 1.37 1.44 1.90 1.18 0.31

30

8.4 Flow Dependability Analysis for non-lean/non-monsoon months (Fig. 14,15,16)

y = -0.5817x + 107.59R² = 0.9607

0

10

20

30

40

50

60

70

80

90

100

110

120

130

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

10

-dai

ly a

vera

ge d

isch

arge

(cu

me

c)

Exceedence probability, P (%)

Mar-I

y = -0.0002x3 + 0.0309x2 - 1.875x + 129.45R² = 0.987

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

10

-dai

ly a

vera

ge d

isch

arge

(cu

me

c)

Exceedence probability, P (%)

Mar-II

y = -0.0002x3 + 0.0371x2 - 2.205x + 147.48R² = 0.9853

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

10

-dai

ly a

vera

ge d

isch

arge

(cu

me

c)

Exceedence probability, P (%)

Mar-III

Fig. 14 Flow dependability curve for the month of March (10 daily)

31

y = -0.0001x3 + 0.0227x2 - 1.9204x + 174.45R² = 0.9839

0

20

40

60

80

100

120

140

160

180

200

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

10

-dai

ly a

vera

ge d

isch

arge

(cu

me

c)

Exceedence probability, P (%)

Apr-I

y = -0.0002x3 + 0.0282x2 - 2.1776x + 188.55R² = 0.976

0

20

40

60

80

100

120

140

160

180

200

220

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

10

-dai

ly a

vera

ge d

isch

arge

(cu

me

c)

Exceedence probability, P (%)

Apr-II

Fig. 15Flow dependability curve for the month of April (10 daily)

32

Fig. 16Flow dependability curve for the month of May (10 daily)

33

694

696

698

700

702

704

706

708

710

712

714

-50 -30 -10 10 30 50 70 90 110 130 150

Re

du

ced

Le

vel,

RL

(m)

Reference Distance, RD (m)

x = 0 m

8. 5. River Cross Sections used for flow depth and velocity simulation (Fig.17(A-Q))

A

B

34

C

D

35

E

F

36

G

H

37

I

J

38

K

L

39

M

N

40

585

590

595

600

605

610

615

-50 -30 -10 10 30 50 70 90 110 130 150

Re

du

ced

Le

vel,

RL

(m)

Reference Distance, RD (m)

x = 7000 m

O

P

41

Fig. 17River cross sections at interval of 500m used for analysis (A-Q)

Q

42

8.6 Flows from the intermediate tributaries

Though there is no observed discharge data on any tributary in the intermediate catchment

between Teesta-IV dam and TRT outlet to exactly quantify the discharge in cumec. As

tributaries are normally steep and turbulent the exact measurement of discharge is not

practical. In absence of observed data, it is a universal practice to estimate the discharge on

basis of catchment area proportion using observed data at nearby G&D site. The same has

been done for estimating the discharge of intermediate tributaries. The catchment area of all

intermediate tributaries along with their percentage (%) catchment area w.r.t. total

intermediate catchment as well as the distances from Teesta-IV dam is shown in Table 11.

The average 10-daily discharge at all the these tributaries (intermediate catchment) has been

computed on the basis of difference of average 10-daily discharge between Teesta-IV &

Teesta-VI using CWC approved water availability series of both projects. The 10-daily

discharge series of intermediate discharges thus worked out at all locations where tributaries

meet the main river is mentioned (Table 12). The intermediate catchment area of 108.4 sq.km

between Teesta-IV dam and TRT outlet would reasonably contribute during the months of

non-lean/non-monsoon period. In Teesta basin generally flow starts increasing in the month

of May and recedes by the end of October. There would be substantial flow in the

intermediate catchment during this period thereby augmenting the E-flows released from

Teesta-IV dam.

43

Table 11Total intermediate catchment as well as the distances from Teesta-IV dam

Sl.No. Tributary

position

Approx distance

from Teesta-IV

dam (Km)

Catchment

Area(Sq.km.)

% C.A. of tributary

w.r.t. total

intermediate

catchment

1 Left Bank 0.60 2.8 3%

2 Left Bank 0.72 34.7 32%

3 Right 3.27 13.2 12%

4 Left 4.13 2.4 2%

5 Left 4.75 2.9 3%

6 Right 5.26 31.3 29%

7 Left 7.22 9.4 9%

8 Right 7.27 2.5 2%

9 Right 8.56 9.3 9%

Total =108.4

44

Table 12Flow in intermediate catchment (cumec) on the basis of average W.A. between Stage-IV & VI (Common period: 1984 to May 03)

Month

Discharge

at Teesta-

IV

Discharge at

Teesta-VI

Intermediate

Contribution

Contribution

at 600m d/s of

Teesta-IV

Contribution

at 720m d/s of

Teesta-IV

Contribution

at 3.27km d/s

of Teesta-IV

Contribution

at 4.1 km d/s

of Teesta-IV

Contribution

at 4.7 km d/s

of Teesta-IV

Contribution

at 5.26km d/s

of Teesta-IV

Contribution

at 7.22km d/s

of Teesta-IV

Contribution

at 7.3km d/s

of Teesta-IV

CA=3910

sq.km.

CA=4500

sq.km.

CA=590

sq.km.

CA=2.8 sq.km

(trib 1L)

CA=34.7

sq.km (trib

2L)

CA=13.2

sq.km (trib

1R)

CA=2.4 sq.km

(trib 3L)

CA=2.9

sq.km (trib

4L)

CA=31.3

sq.km (trib

2R)

CA=9.4

sq.km (trib

5L)

CA=2.5

sq.km (trib

3R)

Jun

I 352.1 526.7 174.6 0.8 10.3 3.9 0.7 0.9 9.3 2.8 0.7

II 466.4 647.5 181.1 0.9 10.7 4.1 0.7 0.9 9.6 2.9 0.8

III 545.0 741.1 196.0 0.9 11.5 4.4 0.8 1.0 10.4 3.1 0.8

Jul

I 590.7 845.5 254.8 1.2 15.0 5.7 1.0 1.3 13.5 4.1 1.1

II 595.1 840.7 245.5 1.2 14.4 5.5 1.0 1.2 13.0 3.9 1.0

III 586.1 836.0 249.9 1.2 14.7 5.6 1.0 1.2 13.3 4.0 1.1

Aug

I 540.0 786.9 246.9 1.2 14.5 5.5 1.0 1.2 13.1 3.9 1.0

II 552.1 805.7 253.6 1.2 14.9 5.7 1.0 1.2 13.5 4.0 1.1

III 543.4 803.9 260.5 1.2 15.3 5.8 1.1 1.3 13.8 4.2 1.1

Sep

I 484.0 732.1 248.0 1.2 14.6 5.5 1.0 1.2 13.2 4.0 1.1

II 410.6 622.3 211.7 1.0 12.4 4.7 0.9 1.0 11.2 3.4 0.9

III 358.4 567.8 209.4 1.0 12.3 4.7 0.9 1.0 11.1 3.3 0.9

Oct

I 288.9 462.5 173.6 0.8 10.2 3.9 0.7 0.9 9.2 2.8 0.7

II 245.8 393.2 147.4 0.7 8.7 3.3 0.6 0.7 7.8 2.3 0.6

III 178.8 299.5 120.7 0.6 7.1 2.7 0.5 0.6 6.4 1.9 0.5

Nov

I 142.2 213.8 71.6 0.3 4.2 1.6 0.3 0.4 3.8 1.1 0.3

II 126.1 182.5 56.3 0.3 3.3 1.3 0.2 0.3 3.0 0.9 0.2

III 110.1 152.4 42.2 0.2 2.5 0.9 0.2 0.2 2.2 0.7 0.2

Dec I 94.7 138.1 43.4 0.2 2.6 1.0 0.2 0.2 2.3 0.7 0.2

II 84.2 125.8 41.6 0.2 2.4 0.9 0.2 0.2 2.2 0.7 0.2

45

III 74.9 115.2 40.3 0.2 2.4 0.9 0.2 0.2 2.1 0.6 0.2

Jan

I 70.0 104.0 34.0 0.2 2.0 0.8 0.1 0.2 1.8 0.5 0.1

II 67.7 98.8 31.1 0.1 1.8 0.7 0.1 0.2 1.7 0.5 0.1

III 67.0 97.4 30.4 0.1 1.8 0.7 0.1 0.1 1.6 0.5 0.1

Feb

I 68.6 99.7 31.1 0.1 1.8 0.7 0.1 0.2 1.7 0.5 0.1

II 72.1 102.5 30.4 0.1 1.8 0.7 0.1 0.1 1.6 0.5 0.1

III 77.9 112.3 34.4 0.2 2.0 0.8 0.1 0.2 1.8 0.5 0.1

Mar

I 85.0 122.6 37.6 0.2 2.2 0.8 0.2 0.2 2.0 0.6 0.2

II 94.6 140.7 46.2 0.2 2.7 1.0 0.2 0.2 2.4 0.7 0.2

III 106.7 156.0 49.3 0.2 2.9 1.1 0.2 0.2 2.6 0.8 0.2

Apr

I 118.8 179.8 61.0 0.3 3.6 1.4 0.2 0.3 3.2 1.0 0.3

II 130.4 200.2 69.7 0.3 4.1 1.6 0.3 0.3 3.7 1.1 0.3

III 155.8 241.7 86.0 0.4 5.1 1.9 0.3 0.4 4.6 1.4 0.4

May

I 179.2 277.9 98.7 0.5 5.8 2.2 0.4 0.5 5.2 1.6 0.4

II 210.0 324.0 114.0 0.5 6.7 2.5 0.5 0.6 6.0 1.8 0.5

III 268.1 420.4 152.3 0.7 9.0 3.4 0.6 0.7 8.1 2.4 0.6

Avg. Of

NL-NM

months

96.4 0.5 5.7 2.2 0.4 0.5 5.1 1.5 0.4

46

8.7Conclusions

A critical examination of the daily flow pattern at the proposed dam site to the powerhouse

was made. The environmental flows for this critical stretch was already been accessed by the

ICAR-Central Inland Fisheries Research Institute for the lean period during 2009-2010 and

observed that the Teesta IV dam axis 7.3 Km downstream is the critical stretch for aquatic

habitat alteration. Analysis of the four sites cross sectional flow area in terms of depth, width

of flow under five different flow levels viz., 5, 7, 10, 15 and 20 cumecs indicate an

approximate submergence range of 15-18 %; 21-25%; 30-36.2%; 45-48.3% and 60-72.5%

respectively in the low flow months. Hence, a minimum discharge of 10 cumecs from dam

during lean season appears to satisfy the habitat requirements for fish.

Fish and other aquatic flora and fauna like plankton and other benthic invertebrates

require seasonal water flows/discharge for their physiological processes. Any alteration in the

natural flow regime results in habitat alteration. Consequent upon fluctuation of water level in

post Dam condition, following impacts are expected to be seen in fish and associated biota in

the ecosystems during non-lean/non-monsoon period;

a) Food and feeding habitat and availability of food items

b) Migration of juveniles in upstream/ downstream for food

The short term study during January to April 2018 indicated that Phytoplankton consists of

14 Genra belonging to 3 classes, 9 orders and 10 families. Class wise phytoplankton

distribution shows that Bacillariophyceae are the dominant group followed by Cyanophyceae

and Conjugatophyceae. While, periphyton community consisted of 23 genera distributed

among14 orders, representing 5 classes such as Bacillariophyceae followed by

Chlorophyceae, Cyanophyceae, Xanthophyceae and Conjugatophyceae. It is well established

that Schizothorax sp. is a grazing feeder. Though impact analysis has not been made, once

these food organisms get altered in their diversity and composition, availability and

abundance of these indigenous fish species may be altered.

Based on the field surveys and secondary sources, the Snow Trout is the most

dominant fish species at the selected sampling sites in Teesta river.Although none of the

previously available fish species such as Tor spp., Neolissochilus hexagonolepis,

Nemacheilus botia and Garra sp. have been reported during the short period of study time,

their occurrence in the selected study strech could not be ruled out. It has been observed that

Schizothorax richardsonii of Cyprinidae family fall under the vulnerable category in the

47

streams. Realising the importance, a Habitat study was carried out during February and April

representing the overlapping period of lean season and non-lean/non-monsoon season

respectively and observed that the RangRang confluence point which is approx. 300m and

Deadkhola site which is 7500m down stream are more suitable for the breeding and spawning

of Schizothorax sp. Hence, protecting the entire life cycle of Schizothorax sp. through

adequate environmental flows and through a seed production unit, of Schizothorax sp.

adjacent to the reservoir or downstream area would be an ideal component in order to

conserve the fish species through ranching as well as to develop sustainable fishery in the

project is need of the hour from the conservation point of view.

8.8 Recommendations

Considering the need for a dependable ecological flow scenario for sustenance of aquatic

biota, the following dedicated flow release is recommended

A dedicated flow release of 20 m3/sec from the proposed Teesta IV dam is

recommended. This release will suffice a minimum depth of 0.6m with a flow

velocity of 0.4m/s which is essential for maintaining the juveniles habitat for

Schizothorax sp. and other indigenous fish species. In addition, the discharges from

the natural streams/nalas between the proposed dam and power house will further

enhance the aquatic lives during non-lean/non-monsoon months. Hence, it is

paramount to ensure the recommended flow release from the proposed dam.

It is further recommended that methods should be explored to maintain longitudinal

connectivity through connecting or joining different deep pools for the sustenance of

adult fishes and their migration for feeding during Non-lean/ Non-monsoon period.

In order to augment the indigenous fish germplasm, and fish population it is

recommended to establish a hatchery unit for important fish species like Schizothorax

sp and Torsp. for ranching in the affected stretches of the river/streams. The location of

the hatchery can be identified somewhere in an area in the near vicinity of the Dams from

where the fish seed can easily be transported upstream and downstream of dam without

much stress to the fish.

48

8.9References

1. APHA, 2012. Standard methods for the examination of water and wastewater.22nd

edn of the American Public Health Association, 15th Street, NW, Washington DC,

1422 pp.

2. Baykal, Tülay, İlkayAçikgöz, Abel U. Udoh and KazımYildiz, 2011. Seasonal

variations in phytoplankton composition and biomass in a small lowland river-lake

system (Melen River, Turkey). Turk J Biol., 35, 485-501p

3. Chatterjee, T. K., Ramakrishna, T. S and Mukherjee, A. K., 2000. Fish and fisheries

of Digha coast of West Bengal. Records of the Zoological Survey of India, Occasional

Paper No. 188: i – iv, 1 – 87p

4. Edmondson W.T. 1959. Freshwater Biology. John Wiley and Sons, Inc. 1248p

5. Jayaram, K. C., 1999. The freshwater fishes of the Indian region. Narendra Publishing

House, New Delhi, 571 p

6. Raje, S. G., Sivakami, S., Mohanraj, G., Manoj Kumar, P. P., Raju, A and Joshi, K.

K., 2007. An Atlas on the Elasmobranch Fishery Resources of India. CMFRI (Central

Marine Fisheries Research Institute), Special Publication No. 95: 253 p.

7. Talwar, P. K., 1991. Pisces: In: Faunal resources of Ganga, Part 1. Zoological Survey

of India, Calcutta: 59 – 145p

8. Talwar, P. K and Jhingran, A. G.,1991. Inland fishes of India and adjacent countries.

Oxford IBH Publishing Co Private Limited, New Delhi.

9. Talwar, P. K and Kacker, R. K., 1984. Commercial Sea Fishes of India. Zoological

Survey of India, Calcutta, 997 p.

10. Talwar, P. K., Mukherjee, P., Saha, D., Pal, S. N and Kar, S., 1992. Marine and

Estuarine Fishes. In: State Fauna Series 3: Fauna of West Bengal, Part 2. Zoological

Survey of India, Calcutta, 243 – 342p