Effect of barrages on water level in estuaries

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, India *[E-mail: sudheer_chavan@rediff.com] 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 nonmonsoon 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 Western Ghats 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 The west coast of India is subjected to rainfalls and fresh water discharges during the southwest monsoon from June to September and the salinity mixing process in this region show seasonal effects. Unlike east flowing Rivers in the Deccan area, the west flowing Rivers in the western ghat region are characterized by very short lengths, say within 100 km and are estuarine in most of their reaches. Due to the change in the pattern of rain and reduction in the rainfall, the River flow reduces in the lean seasons and almost goes dry. The saline water intrusion takes place into the estuarine Rivers which further restricts the use of water by cultivators during the lean months and thus causing water shortage and result in restriction of development activities.it has been noticed thatsince the last decade, there has been 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 Zuari-Cumbarjua canal is prominent in Goa state. A barrage with gates is under construction at Virdi on Valvanta River, a tributary of Mandovi estuary in Goa state. This barrage is located at about 6.0 km upstream from the River confluence with Mandovi River and the confluence is located at about 13 km u/s of the Mandoviestuary mouth (Fig. 1). The proposed barrage is of open type with width of 42.60 m across the Riverand has 4 nos. of openings of size 8.50 m 4.0 m to allow for the flood discharges.the hydraulic conditions in the integrated system of Mandovi Zuari Cumbarjua canal have their effects on the hydrodynamics of the Valvanta River which is a tributary of Mandovi River. In present paper, the case study of barrage construction at Virdi across Valvanta River has been presented to estimate the likely effects of rise in the 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 analysis of hydraulic, hydrographic and hydrologic data of prototype and using suitable 1-D mathematical modeling technique. Physical and hydraulic features of Mandovi estuary Mandovi Zuari Cumbarjuais major estuarine system in Goa on the west coast of India (Fig. 1) which provided excellent sites for not only major port development at Mormugao but also many sites for fisheries harbour development and ship yards so also for inland navigation for iron ore export. At the confluence of the Mandovi estuary with the Arabian Sea, an embayment of Aguada bay exists with a width of 3.2 km at the entrance of estuary and further upstream, a shallow sandy bar exists. The width of the bay reduces to about 600 m near Tejo point with depths of about 7 m to 8 m. Further upstream, the width of the River channel is about 400 to 500 m and the depths are about 3 to 4 m for a reach of about 50 km. The total length of Mandovi estuary is about 70 km. The tidal currents are 0.6-0.8 m/s in most of the reaches. The salinity mixing in the Mandovi estuary is influenced by the action of tidal flow and fresh water discharges while the influence of waves on the mixing process is marginal.the tidal prism in the Mandovi estuary is estimated to be about 30 mm 3. Prototype data in respect of currents, salinity, tides 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 waters and high waters in the successive tidal cycles 1. The spring tidal range at Mandovi is about 2.4 m. typical tide observed at Mormugao is shown in Fig. 2. 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 the Rivers Madei and Candepar (by the southern gauging division of CWC). The data on other tributaries of the Mandovi River are not readily available. The variation of the River discharge is spasmodic and it is found that the River discharge rises to a very large magnitude and falls again within a couple of days. The hydrograph of Mandovi River based on the available data are shown in Fig. 3. The spasmodic variation of River discharge in the Mandovi River is evident from the nature of hydrograph. The maximum discharge in Mandovi River during the observation period was 4463 m 3 /s which occurred on 16.6.1978. Hydrographic Data Water Resources Department, Goa furnished plan, L-profile and cross-sections of Valvanta River on 4.5 km downstream of Virdi Barrage. The hydrographic survey charts (scale 1:2500) of Mandovi River, Zuari River and Cumbarjua canal were available at CWPRS. These hydrographic surveys were carried out by Minor Ports Survey Organization (Government of India) from 2000 to 2003. The cross-section data used in model studies is prepared by digitizing these hydrographic charts and numbers of cross-sections are decided based on depth variations. About 94 cross-sections were taken in Mandovi River, 57 cross-sections in Zuari River, Fig. 2 Typical tide as observed at Mormugao Hydrograph of Year 1971 Hydrograph of Year 1977 ------------- Hydrograph of Year 1978 Fig. 3 Hydrograph in Mandovi River during 1971, 1977 and 1978

1366 INDIAN J MAR SCI VOL 43 NO 7, JULY 2014 30 cross-sections in Cumbarjua canal and 45 cross-sections in Valvanta River. Surface water resources in Goa The Goa state has nine Rivers, of which six Rivers originate and flow exclusively within the state boundaries and do not have any interstate implications. However, Terekhol and Chapora Rivers originate in Maharashtra while Mandovi River originates in Karnataka state. All the Rivers except Sal in the south originates on western slopes and subsequently meanders over falls and rapids into the coastal plains (during young/initial) stage, from where they tend to become sluggish (during mature stage) and then ultimately joins estuary mouth and then into the sea (old stage). Out of the nine Rivers in the state, rough estimation reveals that basin-wise average runoff is of the tune of about 8436 Mcum. The entire water resource should not be mistaken as available for harnessing. As assessed by Central Water Commission (CWC), the water resource of Goa is 8570 mm 3. Many of these rivers are tidal prone up to a distance of 20-40 km and utilizable part of the yield should necessarily be identified basin wise. The Valvanta River is originating from the Western Ghats in the border state of Maharashtra and Karnataka. The Valvanta River is one of the tributaries of Mandovi River and finally joins the Arabian Sea. 1- D mathematical model studies About the Model The 1D HEC-RAS software was developed by U.S. Army corps of Engineers, Hydrologic Engineering Centre (HEC), California, and U.S.A. This software allows one to perform one-dimensional steady flow, unsteady flow calculations. 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 graphic user interface (GUI), 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 separate hydraulic analysis components, data storage and management capabilities, graphics and reporting facilities. The HEC-RAS system contains three dimensional hydraulic analysis components for: 1 Steady flow water surface profile computations 2 Unsteady flow simulation, and 3 Moveable boundary sediment transport computation A key element is that all three components use a common geometric data representation and common geometric and hydraulic computation routines. In addition to the three hydraulic analysis components, the system contains several hydraulic design features that can be involved once the basic water surface profiles are computed. Unsteady flow simulation HEC-RAS model simulates gradually varied flow and subcritical unsteady flow by continuity and momentum equations. The continuity equation describes conservation of mass for the onedimensional system and the momentum equation states that the rate of change of momentum is equal to the external forces acting on the system, and these equations are the basis for the unsteady flow solution within the software are described below. Continuity Equation Conservation of mass for a control volume states that the net rate of flow into the volume be equal to the rate of change of storage inside the volume. The final form of continuity equation is as follows: A Q q1 0 (1) t where, Q = Discharge q = lateral inflow per unit length 1

CHAVAN et al.: EFFECT OF BARRAGES ON WATER LEVEL IN ESTUARIES 1367 A = total cross sectional flow area x = distance along the channel t = time Momentum Equation Conservation of momentum for a control volume states that net rate of momentum entering the volume (momentum flux) plus the sum of all external forces acting on the volume is equal to the rate of accumulation of momentum. The final form of momentum equation is as follows: Q Q z v ga gas f t 0 (2) where, S f = slope of the energy grade line (friction slope) A i = wetted cross sectional area v = velocity through area z = water surface slope The most successful and accepted procedure for solving these one dimensional unsteady flow equations is the four-point implicit scheme, also known as box scheme. Under this scheme, space derivatives and function values are evaluated at an interior point. The Mandovi Zuari Cumbarjua canal system with tributaries like Madei, Candepar, Narora, and rogar, Mapuca, Sanguem and Valvanta are modelled with the help of 1-D mathematical model HEC RAS (version 4.0). The plan of Mandovi Zuari Cumbarjua canal estuarine system simulated1d HEC- RAS model is shown in Fig. 4. The prediction of water levels, velocities and discharges on upstream and downstream of a Barrage at Virdi with vertical lift gates (sluice gates) for different tidal phase s viz. 22.0645* 20.1290* 18.1935* 16.2580* 13.8387* 12.3871* 10.4516* 9 2.16470* 6.67 0.01 4.51529* 8.00111* 7.00222* 5.20422* 3.80577* 61.2216* 2.40733* 52.6706* 57.0455* 1.40844* 48.4945* 44.9151* 32.835* 35.908 40.3413* 28.4733* 30.303 25.93* 21.332 2.37 23.7312* 43.717 5.698* 7.598 18.6416* 1.38666* 38.0118* 40.519* 10 4.3575* 17.1747* 0.01 34.829 16.631 8.4016* 13.0542* 3.318* 0.01 32.3056* 15.531 6.6034* 8.52300* 2.285 26.0204* 28.814 31.175* 4.8052* 14.311* 5.39133* 23.109 1.225 3.4066* 13.0397* 2.918* 21.1754*.183571* 2.008* 10.383 11.273* 0.4 18.7068* 15.729*.409600* 9.02675* 14.554 7.63475* 11.766 5.45333* 3.1225* 10.5866* 2.085* 8.81766* 0.39 7.34942* 5.34425* 3.42175*.707142* Fig. 4 Plan of Mandovi Zuari Cumbarjua canal estuarine system simulated1d HEC-RAS mode 24 65 47.3277* 50.777 49.8457* spring and neap for non-monsoon season were determined under existing condition as well as for fully close gate condition. HEC-RAS 4.0 was selected for studies because it has the ability to model inline weirs and gated structures with radial gates or sluice gates and bridges. The available hydraulic data such as discharges, velocities, tide levels etc. collected in the field studies program by CWPRS during 1978-79 was used for simulation and calibration of the model. The various conditions studied are as below: Input parameters and boundary conditions Both upstream and downstream boundary conditions were given since the model was to perform a mixed flow regime calculation. Unsteady flow in terms of stage hydrograph (tide level in the sea) was given as downstream boundary condition and flow hydrographs were given as an upstream boundary condition. For non-monsoon existing condition, a steady flow of 10 cumecs was given as an upstream boundary condition for all rivers along with their tributaries. For monsoon July average discharge existing condition as well as with sluice gates, corresponding steady discharges of different rivers were given as upstream boundary condition. Similarly peak hydrographs were given as an upstream boundary condition for monsoon peak discharge existing condition as well with sluice gates condition. The model was simulated for 4 days with computational interval of 30 seconds with output interval of 15 minutes. Calibration of model The water levels and their respective time lags were observed at different locations were matching with prototype data observed at locations in Mandovi river, Zuari river and Cumbarjua canal. The comparison of observed water levels in meters and time lag in minutes in HEC-RAS model and the same in the prototype for existing condition (without barrage) is given in Table 2. The model results are validated with the data collected in Mandovi estuary during field studies conducted by CWPRS in 1978-1979. Spring tide non-monsoon under existing condition The model was run for 48 hours for non-monsoon season under existing condition with spring tide. The bed level of 100 m in the model corresponds to chart datum. The plot of water level vs. channel distance

1368 INDIAN J MAR SCI VOL 43 NO 7, JULY 2014 Table 2 Calibration of Hec-Ras model Location LW Prototype LW Hec-Ras HW Prototype HW Hec-Ras Lag(m) Lag Lag(m) Lag(m) 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 Cumbarjua canal 0.05 45 to 60-0.02 15 D/s of confluence of Tonca and Cumbarjua canal -0.1 to 0.15 30 to 45 0.01 45 0.05 30 to 45-0.02 60 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 Elevation (m) 104 103 102 101 100 99 mandov i riv er Plan: v irdi-report-nm-spring-existing 03-07-2012 valvanti 1 Legend EG Max WS WS Max WS Crit Max WS Ground Elevation (m) 112 110 108 106 104 102 100 mandov i riv er Plan: n-m spring with barrage at Virdi 04-07-2012 cross-sectiion at Virdi barrage.02.03.02 98-10 0 10 20 30 40 50 Station (m) Legend EG Max WS WS Max WS Ground Bank Sta 98 97 2000 4000 6000 8000 10000 Main Channel Distance (m) Fig. 5 Longitudinal profile of Valvanta River for spring tide non-monsoon (existing condition) has been shown in Fig. 5. It is observed from Fig. 5 that the maximum water level is well below average bank level (+6.0 m) of Valvanta River. The maximum velocity in Valvanta River was observed to be about 0.8 m/s.the systematic variations in water levels were found in Valvanta River as per tidal phase due to insignificant discharges in non- monsoon. Neap tide non-monsoonunder existing condition The model was run for 48 hours simulation period of prototype under non-monsoon and existing condition with neap tide. It is observed that the maximum water surface level in Valvanta River was not exceeding 2.0 m above C.D. which is well below average bank level (+6.0 m). The maximum velocity was found to be about 0.58 m/s in Valvanta River. The systematic variation in water levels is found as per tidal phase due to insignificant discharge in non-monsoon. Spring tide non-monsoon with a Barrage at Virdi Fig. 6 Cross-section of Valvanta River for spring at Virdi Barrage (Gates fully closed) (Gates fully closed) The HEC-RAS model 2 was run for 48 hours simulation period of prototype under non-monsoon spring tide condition with a Barrage at Virdi (Gates fully closed). The cross section of the Valvanta River with fully closed gate condition is shown in Fig. 6. The longitudinal profile of Valvanta River for maximum water surface is given in Fig. 7. It is observed that the maximum water surface in the downstream of the barrage is not exceeding 2.2 m above C.D and the maximum water surface in the upstream of the barrage is not exceeding 3.0 m above C.D. The maximum water levels in Valvanta River was not exceeding average bank level (+6.0 m) both on upstream and downstream of the barrage. The maximum velocity in Valvanta Riveris found to be 1.0 m/s at about 3 km downstream of Virdi barrage and maximum velocity is about 0.9 m/s in the upstream reaches of the barrage. Results and Discussions The HEC RAS 1 D Mathematical model was utilized to assess the water levels and velocity conditions as a result of construction of gated barrage at Virdi in Valvanta River. It was observed during the studies that there is no significant effect on the water 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 Elevation (m) 106 104 102 100 98 96 94 mandov i riv er Plan: n-m spring with barrage at Virdi 04-07-2012 valvanti 1 2000 4000 6000 8000 10000 Main Channel Distance (m) Legend EG Max WS WS Max WS Crit Max WS Ground Fig. 7 Longitudinal profile of Valvanta River for spring tide non-monsoon with barrage nonmonsoon season. The water levels would remain below the existing bank levels in the downstream region of Virdi barrage after full closure of the gates. It is observed that the water levels with proposed barrage at Virdi will be marginally higher than the existing condition and the same trend was observed for the velocities. The tidal range also reduces considerably from the mouth of the Mandovi estuary upto the location of Virdi barrage in Valvanta River due to steeper gradients in upstream reaches of Valvanta River. The tidal excursion from inlet of Mandovi estuary is estimated to be about 27 km. The salinity values are also not likely to be modified in the downstream region of the barrage due to full closure of the gates. In the upstream region, the salinity conditions will be modified gradually after the closure of the gates during nonmonsoon season. Conclusions Abundant fresh water flows available in estuaries on Western Ghats which presently drained into Arabian Sea, can be harnessed effectively by constructing barrages with controlling gates. The 1D mathematical model is a very effective tool in the analysis of the tidal regime and change in water levels in estuaries in order to assess the primary viability. The detailed hydraulic studies are required to be supported adequately with prototype data on bathymetry, rainfall, velocity, salinity and sediment etc. The proposed construction of barrageat Virdi across the Valvanta River would not adverselyaffectthe hydraulic conditions in respect of water levels and velocities in upstream and downstream regions for the fully closed gate condition during the nonmonsoon season. Acknowledgements The authors are thankful to Dr. I. D. Gupta, Director, CWPRS, for his kind permission to publish this paper. References 1 Sundar, D. and Shetye, S. R., Tides in the Mandovi and Zuari estuaries, Goa, west coast of India. Journal of Earth System Sciences, 2005.114(5): 493 503. 2 Brunner, G. W., 2010, HEC-RAS River Anaylsis System - Hydraulic reference manual, U.S.Army Corps of Engineers, 411 p.