Review of Hydrological Potential in Combined Gin and Nilwala River Basins of Sri Lanka

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1 Review of Hydrological Potential in Combined Gin and Nilwala River Basins of Sri Lanka A.V.D.Fernando 1, L.S.Sooriyabandara 2 and G.H.A.C.Silva 1 1 Department of Civil and Environmental Engineering, Faculty of Engineering, University of Ruhuna Galle, SRI LANKA 2 Irrigation Department, Galle, SRI LANKA amila@cee.ruh.ac.lk Abstract: Gin and Nilwala rivers are located within the southern wet-zone of Sri Lanka and discharge considerable quantities of water to the sea annually, without being utilized. Lower basins of the rivers are subjected to frequent floods causing severe damage while the adjacent south-eastern areas are experiencing severe shortage of water. The primary aim of this study is to identify the feasible hydrological potential of combined Gin-Nilwala river basins. Flood controlling, water management and water diversion are demanding hydrological processes that require more reliable identification of hydrological potential. The research procedure comprised with setting up, calibration and validation of a hydrological model, which is a prerequisite to evaluate the hydrological potential. HEC-HMS hydrological modelling system with Muskingum routing method was used to setup the model. Model has been calibrated and validated using observed rainfall and discharge data. Hydrological potential of river basins were evaluated using design storms and the validated HEC-HMS models. Ultimately, water demand and topographical features have been incorporated to identify the feasibility, duration, and potential of trans-basin diversion of combined Gin-Nilwala river basins. Keywords: Hydrological potential, Diversion potential, Hydrological model. 1. INTRODUCTION 1.1. Background Gin river, situated in the South-Western region of Sri Lanka, and drains an area of 958 km 2. It is approximately 113 km long with its source at Abbey Rock. The river passes Udugama, Baddegama and flows into the sea at Gintota. The last stretch of 24 km is through flat plains (Figure 1). The entire basin is essentially at the South-West monsoonal area and floods are experienced during the monsoonal period. The mean annual precipitation and mean annual discharge are about 3,300 mm and 1,850 MCM respectively. River Nilwala is also located at southern part of the Sri Lanka and flows into the sea at Matara. It is approximately 72 km long and it consist a total drainage area of 971 km 2. The entire basin is in the South monsoonal zone and experiences an average rainfall of 2,920 mm. Flood occurrences are frequent during the monsoon but Nilwala is relatively dryer comparing to Gin at times. Annually about 1,233 MCM of water reaches to the sea without being utilized (Arumugam 1969). The main objective of this study is to identify the feasible hydrological potential of combined Gin- Nilwala river basins using more reliable hydrological modelling approach. Feasible hydrological potential, locations, duration, and seasons of the year at each river basins (both independently and combined) are the expected outcomes of the research. Flood controlling and river basin water management can be implemented with identification of characteristics/hydrological potential of each river basin. Excess water can be transferred from both river basins to adjacent dry zone areas to supplement their increasing water demand. It is evident that part of Nilwala is located at relatively dry area where it will remain as dry with the rainfall pattern variation along the river basin in different time period. Firstly, water shortage of Nilwala river basin need to be considered. It can be compensated from excess water availability in Gin basin. Subsequently, water demands in adjacent dry areas are to 323

2 be considered. Area around the Gin and Nilwala basins are flooding often due to the excess water availability within each basin. The water quantity that goes to the sea without being utilized is considerable. Figure 1 Study area 1.2. Why Diversion of Combined Gin and Nilwala? Studies have been made in the recent past to find the possibility of diverting the upper reaches of Nilwala to the Muruthawela tank (located eastward to Nilwala basin) to augment the commanding area of the Muruthawela reservoir. Since it is a medium scale reservoir, Nilwala alone is unable to meet the shortages in the southern dry zone. Hence diversion of Gin to Nilwala should also be considered to increase the reliability of flow during the dry weather periods (Atukorala 2006). The project proposed to divert water of Gin river basin to existing Muruthawela tank through Nilwala river basin through a series of reservoirs and transfer tunnels. 2. METHODOLOGY Flow chart in Figure 2 illustrates the adapted methodology to achieve the research objectives. Figure 2 Methodology to review of hydrological potential 324

3 Rainfall, discharge, irrigation and water supply demand and topographical data were collected for last 10 years in order to characterize the catchment. Study area and hydrological data extraction locations are as illustrated in Figure 1. Seasonal variation and long-term variation patterns of both rainfall and discharge were obtained by analysing the rainfall and discharge data. Rainfall and discharge relationship patterns were used to identify hydrological characterize of both Gin and Nilwala river basins. Best possible water diversion locations were selected according to the maximum hydrological potential throughout the basins. Hydrological models were used to evaluate the hydrological potential along the Gin and Nilwala river basins. HEC-HMS (Feldman 2000) models were calibrated for Gin and Nilwala river basins and validated models were used to evaluate the hydrological potential at selected river junctions. Design storms were selected based on the minimum of maximum storm technique. Irrigation and water supply demands were collected from the relevant authorities. Total water demand for the entire Gin and Nilwala river basin areas have been used to compute the water demand in selected river junctions. River basins and sub-basins were generated using GIS technique. The total water demand was distributed based on the sub-basin areas corresponding to each river junction. The accurate water requirement at different areas has been identified and the decisions were taken according to the availability of water. Excess water availability in both Gin and Nilwala river basins were estimated from already setup hydrological model. Further diversion possibilities were identified after matching with the current and future water demand within each river basin. 3. RESULTS AND DISCUSSION 3.1. Hydrological Data Analysis Figure 3 Mean monthly rainfall of Gin and Nilwala rivers 325

4 Hydrological (rainfall and discharge) data were investigated from period 2000 to Figure 3 illustrates the long-term monthly mean rainfall variations along Gin and Nilwala river basins from upstream to downstream. All rain gauging stations have almost similar variation throughout the year. In all cases, peak precipitation has been recorded in April-May and October-November. The mean annual rainfall is reduced from upstream to downstream. It is evident that river Gin receives higher rainfall and generates more discharge than river Nilwala Design Storms Design storms can be generated using both IDF curves and observed rainfall data. Galle principal meteorological station IDF curves are the closest available for the study area. It is revealed that the IDF curves were derived from high resolution rainfall data until year Hence they do not capture recent rainfall variations due to various scenarios. On the other hand, exact storms through observations at all rain gage locations would give more reliable and realistic representation in determining hydrological potential for the basins. Hence, observed rainfall data were preferred against the IDF curve predictions in determining design storms which are the main forcing data for the HEC- HMS model. A properly calibrated and validated model would generate discharge at un-gauged upstream locations. Minimum of maximum method was used to identify the design storms of each river basin. Storms were separated by minimum of two consecutive dry days within rainfall data. Storms with maximum cumulative discharges were selected for each year and ranked them in ascending order. Finally, the storm with minimum cumulative discharge was selected as the design storm. Rainfall and discharge data from year 2000 to 2011 and 2003 to 2008 were used to generate a design storm for Gin and Nilwala rivers respectively. Considering the minimum of maximum cumulative daily discharge in each storm, year 2001 was selected as the design storm for Gin river and year 2007 storm was selected as the design storm in Nilwala river. Table 1 illustrates the selection of design storms for both Gin and Nilwala river basin. Figure 4 and Figure 5 illustrate 2001 design storm for Gin river basin and 2007 design storm for Nilwala river basin respectively. Table 1 Selection of Design storm Rank (a) Gin river Maximum cumulative Year discharge (m 3 /s) Rank (a) Nilwala river Maximum Cumulative Year Discharge (m 3 /s)

5 Figure 4 Gin river design storm Figure 5 Nilwala river design storm 3.3. Setting up of HEC-HMS Model HEC-HMS hydrological model was setup to evaluate water availability at the un-gauged upstream locations of Gin and Nilwala river basins (Fernando, Silva and Sooriyabandara 2016). The model was calibrated using Thawalama and Pitabaddara discharge data for Gin and Nilwala rivers respectively using precipitation data in several rainfall stations. Figure 6 and Figure 7 illustrate calibration results for Gin and Nilwala rivers respectively. Figure 6 Calibration results for Gin river 327

6 Figure 7 Calibration results for Nilwala river 3.4. Hydrological Potential Hydrological potential at un-gauged river upstream points were obtained from HEC-HMS hydrological model using design storms and incorporating downstream water demand. Figure 8 illustrates the generated resultant discharges through HEC-HMS model which is the hydrological potential at extraction points G-00, G-01, G-02 and G-03. Hydrological potential at five junction points (N-00, N-01, N-02, N-03 and N-04) were obtained for the Nilwala river model. Figure 9 illustrates the generated resultant discharges through HEC-HMS model at Nilwala river. Figure 8 Hydrological potential of Gin river Figure 9 Hydrological potential of Nilwala river 328

7 3.5. Diversion Potential and Duration According to the feasibility studies from Irrigation Department, it was recommended that proposed tunnel capacity as 20 m 3 /s to water diversions (Sooriyabandara n.d.). Figure 10 and 11 illustrate the graphical representation of possible diversion duration considering without-storage and with-storage at 20 m 3 /s capacity. From the results it is evident that with proposed diversion capacity it is possible to divert water continuously for several days even without storage. Figure 10 Gin river diversions Figure 11 Nilwala river diversions 3.6. Elevations and Topography Figure 12 illustrates the combine Gin and Nilwala river profiles where it is clear that profile of Gin is having high elevation at upstream compared to Nilwala which is an essential parameter for diversion. Figure 13 illustrates the surface profile and diversion path variation along a feasible diversion path and the exact diversion points shall be selected according to the hydraulic head requirement, geological features (if a reservoir to be impounded) and land use of the area including sociological aspects. Figure 12 Gin-Nilwala River profile variation 329

8 4. CONCLUSIONS Figure 13 Elevation variations along feasible diversion root Gin and Nilwala river basin data analysis illustrate that monthly mean rainfall is gradually reduced from upstream to downstream. Two rainfall peaks can be observed in April to May and October to November. The results illustrate that observed discharge is increased from upstream to downstream. The rainfall and discharge data analysis, illustrate that Gin river basin receives greater precipitation and releases greater discharge than the Nilwala river basin. Design storms were used to generate hydrological potential of Gin and Nilwala rivers. Minimum of maximum storm technique was used to identify the design storms in each river basin that results zero failure probability for any given year. It is evident that Gin river basin has greater hydrological potential and diversion potential than the Nilwala river even after releasing downstream water demand. Due to the availability of greater hydrological potential in Gin river, eventually it causes downstream flood damage. Evaluation of hydrological potential of Nilwala river reveals that it will not be able to manage downstream water demand at times. Trans-basin water diversion between Gin and Nilwala would be a feasible solution to control floods in Gin river and supplement the water shortage in Nilwala river. Further diversion is also identified as a possible alternative during monsoonal period where both Gin and Nilwala river basin get considerable rainfall and even other periods if small-scale storage reservoirs are built. The outcome of this research would solve the water scarcity issues in adjacent South-Eastern parts of Sri Lanka. 5. ACKNOWLEDGMENTS Authors wish to express their sincere gratitude to Dr. T.N. Wickramaarachchi and Dr. Cyril Kariyawasam, senior lecturers at the faculty of Engineering, University of Ruhuna, Sri Lanka for the encouragement and useful comments. Thanks and gratitude is also extended to Eng. N.D.P. Ransara, and Eng. K.L.A. Rangana for their supports. 6. REFERENCES Arumugam, S., 1969, Water Resources of Ceylon, A Water Resources Board Publication, Colombo, Sri Lanka. -East Dry Zone in Sri Lanka: Sustainable development of water resources, water supply and environmental nal Conference, Colombo, Sri Lanka. Consultancy services for review of feasibility study of Gin Nilwala Ganga diversion Project (CSRFS), 2014, Mahaweli Consultancy Bureau (Pvt) Ltd, Ministry of Irrigation and Water Resources Management, Colombo, Sri Lanka. Feldman, A.D., 2000, Hydrological Modelling System HEC-HMS, Technical Reference Manual, US Army Corps of Engineers, Hydrological Engineering Center, Davis, California. Sooriyabandara, L.S., n.d., Nilwala Ganga Flood Protection Scheme, Department of Irrigation, Matara, Sri Lanka. 330