Indian Journal of Geo-Marine Sciences Vol. 43 (7), July 2014, pp. 1364-1369

Effect of barrages on water level in estuaries

S S Chavan*, M D Sawant, Prabhat Chandra & T Nagendra Central Water and Power Research Station, Khadakwasla, Pune-24, *[E-mail: [email protected]]

Received 12 August 2013; revised 25 November 2013

Barrage is to utilize the fresh water on the u/s side of barrage and also to prevent the mixing of saline water during non- monsoon by fully closing the gates. During the monsoon season, the salinity of the water in Valvanta River becomes very low of about less than 5 ppt due to the predominant effect of River discharges. These types of studies are very vital for the water harnessing projects particularly on where the short estuaries drain into the sea without the water being utilized much. In the present study, is an attempt to predict the changes in the hydrodynamic conditions in the downstream side of barrage at Virdi by using the 1-D model HEC – RAS for different operational and tidal conditions.

[Keywords: Barrage, Tidal reflection, Submergence, Water level, Current]

Introduction Zuari- canal is prominent in state. A The west coast of India is subjected to rainfalls and barrage with gates is under construction at Virdi on fresh water discharges during the southwest monsoon Valvanta River, a tributary of Mandovi estuary in Goa from June to September and the salinity mixing state. This barrage is located at about 6.0 km upstream process in this region show seasonal effects. Unlike from the River confluence with and east flowing Rivers in the Deccan area, the west the confluence is located at about 13 km u/s of the flowing Rivers in the western ghat region are Mandoviestuary mouth (Fig. 1). characterized by very short lengths, say within The proposed barrage is of open type with width of 100 km and are estuarine in most of their reaches. 42.60 m across the Riverand has 4 nos. of openings of Due to the change in the pattern of rain and reduction size 8.50 m × 4.0 m to allow for the flood in the rainfall, the River flow reduces in the lean discharges.The hydraulic conditions in the integrated seasons and almost goes dry. The saline water system of Mandovi – Zuari – have intrusion takes place into the estuarine Rivers which their effects on the hydrodynamics of the Valvanta further restricts the use of water by cultivators during River which is a tributary of Mandovi River. the lean months and thus causing water shortage and In present paper, the case study of barrage result in restriction of development activities.It has construction at Virdi across Valvanta River has been been noticed thatsince the last decade, there has been presented to estimate the likely effects of rise in the consistent increase in the usage of the River water for irrigation and domestic purpose mainly due to the increase in the area under cultivation, increase in the demand of drinking water and increase in industrial demand for water. In order to arrest intrusion of saline water into the River and for storing fresh water in the upstream which would be used for agriculture, industry and domestic purpose, the construction of barrages/bandharas have been proposed in Western Ghats and Goa region.

Materials and Methods

The integrated estuarine system of the Mandovi– Fig.1—Index plan

CHAVAN et al.: EFFECT OF BARRAGES ON WATER LEVEL IN ESTUARIES 1365

water levels and change in flow conditions by the the Rivers Madei and Candepar (by the southern analysis of hydraulic, hydrographic and hydrologic gauging division of CWC). The data on other data of prototype and using suitable 1-D mathematical tributaries of the Mandovi River are not readily modeling technique. available. The variation of the River discharge is spasmodic and it is found that the River discharge Physical and hydraulic features of Mandovi estuary rises to a very large magnitude and falls again within Mandovi – Zuari– Cumbarjuais major estuarine a couple of days. The hydrograph of Mandovi River system in Goa on the west coast of India (Fig. 1) based on the available data are shown in Fig. 3. The which provided excellent sites for not only major port spasmodic variation of River discharge in the development at but also many sites for Mandovi River is evident from the nature of fisheries harbour development and ship yards so also hydrograph. The maximum discharge in Mandovi for inland navigation for iron ore export. At the 3 River during the observation period was 4463 m /s confluence of the Mandovi estuary with the Arabian which occurred on 16.6.1978. Sea, an embayment of Aguada bay exists with a width of 3.2 km at the entrance of estuary and further Hydrographic Data upstream, a shallow sandy bar exists. The width of the Water Resources Department, Goa furnished plan, bay reduces to about 600 m near Tejo point with L-profile and cross-sections of Valvanta River on depths of about 7 m to 8 m. Further upstream, the 4.5 km downstream of Virdi Barrage. The width of the River channel is about 400 to 500 m hydrographic survey charts (scale 1:2500) of Mandovi and the depths are about 3 to 4 m for a reach of about River, and Cumbarjua canal were 50 km. The total length of Mandovi estuary is about available at CWPRS. These hydrographic surveys 70 km. The tidal currents are 0.6-0.8 m/s in most of were carried out by Minor Ports Survey Organization the reaches. The salinity mixing in the Mandovi (Government of India) from 2000 to 2003. The estuary is influenced by the action of tidal flow and cross-section data used in model studies is prepared fresh water discharges while the influence of waves by digitizing these hydrographic charts and numbers on the mixing process is marginal.The tidal prism in of cross-sections are decided based on depth the Mandovi estuary is estimated to be about 30 mM3. variations. About 94 cross-sections were taken in Prototype data in respect of currents, salinity, tides Mandovi River, 57 cross-sections in Zuari River, were collected by CWPRS in July and September of 1981 in the Mandovi River, Goa so as to cover two different rates of upland River discharges. The available prototype data on velocity and water level were used for calibration of 1D mathematical model in the present study.

Tides Tides in the estuary are semidiurnal with pronounced diurnal inequality having considerable difference in the tidal ranges and the elevation of low Fig. 2—Typical tide as observed at Mormugao waters and high waters in the successive tidal cycles1. The spring tidal range at Mandovi is about 2.4 m. ………… Hydrograph of Year 1971 ______Hydrograph of Year 1977 typical tide observed at Mormugao is shown in Fig. 2. ------Hydrograph of Year 1978 Hydrologic Data The estuarine system is situated in a coastal region which receives most of the rainfall during southwest monsoon season only. The fresh water discharges in the Rivers are significant during southwest monsoon period (June to September) only. The fresh water discharge data for the Mandovi River pertains to year 1971, 1977 and 1978 which have been collected on Fig. 3—Hydrograph in Mandovi River during 1971, 1977 and 1978

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Table 1—Surface water resources in Goa Sr. No. Name of the River basin Length within the state Length within salinity zone Basin AreaSq.kms Average runoff (MCM) 1 Terekhol 26 26 71 164.25 2 Chapora 32 32 255 588.35 3 Baga 10 10 50 116.42 4 Mandovi 52 36 1580 3580.04 5 Zuari 145 42 973 2247.4 6 Sal 40 14 301 694.39 7 Saleri 11 5 149 343.04 8 Talpona 32 7 233 515.39 9 Galgibag 14 4 90 187.11 Total 362 176 3702 8436.59 30 cross-sections in Cumbarjua canal and 45 separate hydraulic analysis components, data storage cross-sections in Valvanta River. and management capabilities, graphics and reporting Surface water resources in Goa facilities. The Goa state has nine Rivers, of which six Rivers The HEC-RAS system contains three dimensional originate and flow exclusively within the state hydraulic analysis components for: boundaries and do not have any interstate 1 Steady flow water surface profile computations implications. However, Terekhol and Chapora Rivers 2 Unsteady flow simulation , and originate in Maharashtra while Mandovi River 3 Moveable boundary sediment transport originates in state. All the Rivers except computation Sal in the south originates on western slopes and A key element is that all three components use a subsequently meanders over falls and rapids into the common geometric data representation and common coastal plains (during young/initial) stage, from where geometric and hydraulic computation routines. In they tend to become sluggish (during mature stage) addition to the three hydraulic analysis components, and then ultimately joins estuary mouth and then into the system contains several hydraulic design features the sea (old stage). Out of the nine Rivers in the state, that can be involved once the basic water surface rough estimation reveals that basin-wise average profiles are computed. runoff is of the tune of about 8436 Mcum. The entire water resource should not be mistaken as Unsteady flow simulation available for harnessing. As assessed by Central HEC-RAS model simulates gradually varied flow Water Commission (CWC), the water resource of Goa 3 and subcritical unsteady flow by continuity and is 8570 mM . Many of these rivers are tidal prone up momentum equations. The continuity equation to a distance of 20-40 km and utilizable part of the describes conservation of mass for the one- yield should necessarily be identified basin wise. The dimensional system and the momentum equation Valvanta River is originating from the Western Ghats states that the rate of change of momentum is equal to in the border state of Maharashtra and Karnataka. The the external forces acting on the system, and these Valvanta River is one of the tributaries of Mandovi River and finally joins the Arabian Sea. equations are the basis for the unsteady flow solution within the software are described below. 1- D mathematical model studies Continuity Equation About the Model Conservation of mass for a control volume states The 1D HEC-RAS software was developed by U.S. that the net rate of flow into the volume be equal to Army corps of Engineers, Hydrologic Engineering the rate of change of storage inside the volume. The Centre (HEC), California, and U.S.A. This software final form of continuity equation is as follows:

allows one to perform one-dimensional steady flow, unsteady flow calculations. HEC-RAS is an integrated A Q  q  0 … (1) system of software designed for interactive use in a t x 1 multi-tasking, multi-user network environment. The system is comprised of a graphic user interface (GUI), where, Q = Discharge q1 = lateral inflow per unit length

CHAVAN et al.: EFFECT OF BARRAGES ON WATER LEVEL IN ESTUARIES 1367

A = total cross sectional flow area spring and neap for non-monsoon season were x = distance along the channel determined under existing condition as well as for t = time fully close gate condition. HEC-RAS 4.0 was selected Momentum Equation for studies because it has the ability to model inline Conservation of momentum for a control volume weirs and gated structures with radial gates or sluice states that net rate of momentum entering the volume gates and bridges. The available hydraulic data such (momentum flux) plus the sum of all external forces as discharges, velocities, tide levels etc. collected in acting on the volume is equal to the rate of the field studies program by CWPRS during 1978-79 accumulation of momentum. The final form of was used for simulation and calibration of the model. momentum equation is as follows: The various conditions studied are as below:

Q Q z Input parameters and boundary conditions  v  gA gAS f  0 t x x … (2) Both upstream and downstream boundary conditions were given since the model was to perform where, S f = slope of the energy grade line a mixed flow regime calculation. Unsteady flow in (friction slope) terms of stage hydrograph (tide level in the sea) was given as downstream boundary condition and flow Ai = wetted cross sectional area hydrographs were given as an upstream boundary v = velocity through area condition. For non-monsoon existing condition, a z steady flow of 10 cumecs was given as an upstream = water surface slope boundary condition for all rivers along with their x tributaries. For monsoon July average discharge The most successful and accepted procedure for existing condition as well as with sluice gates, solving these one dimensional unsteady flow corresponding steady discharges of different rivers equations is the four-point implicit scheme, also were given as upstream boundary condition. Similarly known as box scheme. Under this scheme, space peak hydrographs were given as an upstream derivatives and function values are evaluated at an boundary condition for monsoon peak discharge interior point. existing condition as well with sluice gates condition. The Mandovi– Zuari – Cumbarjua canal system The model was simulated for 4 days with with tributaries like Madei, Candepar, Narora, and computational interval of 30 seconds with output rogar, Mapuca, Sanguem and Valvanta are modelled interval of 15 minutes. with the help of 1-D mathematical model HEC RAS (version 4.0). The plan of Mandovi – Zuari – Calibration of model Cumbarjua canal estuarine system simulated1D HEC- The water levels and their respective time lags RAS model is shown in Fig. 4. The prediction of were observed at different locations were matching water levels, velocities and discharges on upstream with prototype data observed at locations in Mandovi and downstream of a Barrage at Virdi with vertical lift river, Zuari river and Cumbarjua canal. The gates (sluice gates) for different tidal phase’s viz. comparison of observed water levels in meters and

24 22.0645* time lag in minutes in HEC-RAS model and the same 20.1290* 18.1935* 16.2580* in the prototype for existing condition (without 13.8387* 12.3871* 10.4516* 2.16470* 6.67 0.019 4.51529* barrage) is given in Table 2. The model results 8.00111* 7.00222* 5.20422* are validated with the data collected in Mandovi 65 3.80577* 61.2216* 2.40733* 52.6706* 57.0455* 1.40844* 48.4945* 44.9151* estuary during field studies conducted by CWPRS in 32.835* 35.908 40.3413* 28.4733* 30.303 50.777 49.8457* 25.93* 47.3277* 21.3322.37 23.7312* 43.717 1978-1979. 5.698* 7.59818.6416* 1.38666* 38.0118* 40.519* 10 4.3575* 17.1747*0.01 34.829 16.631 13.0542* 8.4016* 3.318* 0.01 32.3056* 15.531 8.52300* 28.814 6.6034* 2.285 26.0204* 31.175* 14.311* 5.39133* 23.109 4.8052* 1.225 13.0397* 2.918* 21.1754* Spring tide non-monsoon under existing condition 3.4066* 18.7068* .183571*10.38311.273* 0.4 2.008* 15.729* .409600*9.02675* 14.554 7.63475* 11.766 5.45333* The model was run for 48 hours for non-monsoon 3.1225* 10.5866* 2.085* 8.81766* 0.39 7.34942* season under existing condition with spring tide. The 5.34425* 3.42175* .707142* bed level of 100 m in the model corresponds to chart

Fig. 4—Plan of Mandovi – Zuari –Cumbarjua canal estuarine datum. The plot of water level vs. channel distance system simulated1D HEC-RAS mode

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Table 2—Calibration of Hec-Ras model Location LW Prototype LW Hec-Ras HW Prototype HW Hec-Ras WSL Time Lag WSL Time Lag WSL Time Lag WSL Time Lag

Lag(m) (min) Lag (min) Lag(m) (min) Lag(m) (min) Panjim Jetty 0.00000015 15 0.01 15 0.05 15 -0.01 15 Confluence of Mandovi and -0.02 to – -0.15 to 0.0 45 to 60 0.0 15 45 to 60 -0.02 15 Cumbarjua canal 0.05 D/s of confluence of Tonca and -0.1 to –0.15 30 to 45 0.01 45 0.05 30 to 45 -0.02 60 Cumbarjua canal Near d/s end of Cumbarjual canal -0.15 to –0.2 15 to 30 0.0 15 -0.05 15 to 30 -0.03 45

mandov i riv er Plan: v irdi-report-nm-spring-existing 03-07-2012 mandov i riv er Plan: n-m spring with barrage at Virdi 04-07-2012 valvanti 1 cross-sectiion at Virdi barrage 104 Legend .02 .03 .02 112 Legend EG Max WS 103 110 WS Max WS EG Max WS Crit Max WS 108 WS Max WS 102 Ground 106 Ground

104 Bank Sta

101 Elevation (m) 102

100 100 Elevation(m) 98 -10 0 10 20 30 40 50 99 Station (m)

98 Fig. 6—Cross-section of Valvanta River for spring at Virdi Barrage (Gates fully closed) 97 2000 4000 6000 8000 10000 (Gates fully closed) Main Channel Distance (m) 2 The HEC-RAS model was run for 48 hours Fig. 5—Longitudinal profile of Valvanta River for spring tide simulation period of prototype under non-monsoon non-monsoon (existing condition) spring tide condition with a Barrage at Virdi (Gates fully closed). The cross section of the Valvanta River has been shown in Fig. 5. It is observed from Fig. 5 with fully closed gate condition is shown in Fig. 6. that the maximum water level is well below average The longitudinal profile of Valvanta River for bank level (+6.0 m) of Valvanta River. The maximum maximum water surface is given in Fig. 7. It is velocity in Valvanta River was observed to be about observed that the maximum water surface in the 0.8 m/s.The systematic variations in water levels were downstream of the barrage is not exceeding 2.2 m found in Valvanta River as per tidal phase due to above C.D and the maximum water surface in the insignificant discharges in non- monsoon. upstream of the barrage is not exceeding 3.0 m above C.D. The maximum water levels in Valvanta River Neap tide non-monsoonunder existing condition was not exceeding average bank level (+6.0 m) both The model was run for 48 hours simulation on upstream and downstream of the barrage. The period of prototype under non-monsoon and maximum velocity in Valvanta Riveris found to be existing condition with neap tide. It is observed 1.0 m/s at about 3 km downstream of Virdi barrage that the maximum water surface level in and maximum velocity is about 0.9 m/s in the Valvanta River was not exceeding 2.0 m above C.D. upstream reaches of the barrage. which is well below average bank level (+6.0 m). Results and Discussions The maximum velocity was found to be The HEC – RAS 1 D Mathematical model was about 0.58 m/s in Valvanta River. The systematic utilized to assess the water levels and velocity variation in water levels is found as per tidal conditions as a result of construction of gated barrage phase due to insignificant discharge in at Virdi in Valvanta River. It was observed during the non-monsoon. studies that there is no significant effect on the water Spring tide non-monsoon with a Barrage at Virdi level and velocities in the downstream region of the barrage due to fully closure of the gates in

CHAVAN et al.: EFFECT OF BARRAGES ON WATER LEVEL IN ESTUARIES 1369

mandov i riv er Plan: n-m spring with barrage at Virdi 04-07-2012 upstream region, the salinity conditions will be valvanti 1 106 Legend modified gradually after the closure of the gates EG Max WS during nonmonsoon season. 104 WS Max WS Conclusions Crit Max WS Ground  Abundant fresh water flows available in estuaries 102 on Western Ghats which presently drained into Arabian Sea, can be harnessed effectively by 100 constructing barrages with controlling gates. Elevation(m)

98  The 1D mathematical model is a very effective tool in the analysis of the tidal regime and change 96 in water levels in estuaries in order to assess the primary viability. The detailed hydraulic studies 94 2000 4000 6000 8000 10000 are required to be supported adequately with Main Channel Distance (m) prototype data on bathymetry, rainfall, velocity,

Fig. 7—Longitudinal profile of Valvanta River for spring tide salinity and sediment etc. non-monsoon with barrage  The proposed construction of barrageat Virdi nonmonsoon season. The water levels would remain across the Valvanta River would not below the existing bank levels in the downstream adverselyaffectthe hydraulic conditions in respect region of Virdi barrage after full closure of the gates. of water levels and velocities in upstream and It is observed that the water levels with proposed downstream regions for the fully closed gate barrage at Virdi will be marginally higher than condition during the nonmonsoon season. the existing condition and the same trend was observed for the velocities. The tidal range also Acknowledgements reduces considerably from the mouth of the The authors are thankful to Dr. I. D. Gupta, Director, Mandovi estuary upto the location of Virdi barrage in CWPRS, for his kind permission to publish this paper. Valvanta River due to steeper gradients in upstream reaches of Valvanta River. The tidal References excursion from inlet of Mandovi estuary is estimated 1 Sundar, D. and Shetye, S. R., Tides in the Mandovi and Zuari to be about 27 km. The salinity values are also not estuaries, Goa, west coast of India. Journal of Earth System Sciences, 2005.114(5): 493–503. likely to be modified in the downstream region of the 2 Brunner, G. W., 2010, HEC-RAS River Anaylsis System - barrage due to full closure of the gates. In the Hydraulic reference manual, U.S.Army Corps of Engineers, 411 p.