Effect of Barrages on Water Level in Estuaries
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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: [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 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 Zuari-Cumbarjua canal is prominent in Goa 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 Mandovi River 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 – Cumbarjua canal 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 Mormugao 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, Zuari 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 1366 INDIAN J MAR SCI VOL 43 NO 7, JULY 2014 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 Karnataka 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 .