3. SELECTED RIVER BASINS 3.1 Kelani Ganga Basin The Kelani Ganga is one of the four river basins taken for the study of flood transposition in the wet zone of Sri Lanka. This river takes the sixth place in the island with respect to its catchment area of approximately 2280 square kilometres. 3.1.1 Location and Topography j The Kelani Ganga flows through southwestern parts of the island, in the wet zone of Sri Lanka. The river flows westwards, maximum length of the basin is 90km, the width of the basin from north to south being is_45km. In the north, the Kelani Ganga basin is bounded by Attanagalla Oya and Maha Oya basins and in the east by the Mahaweli Ganga. In the south the Kelani Ganga catchment is bounded by the Kalu Ganga basin. The watershed boundaries of Kelani Ganga basin J from east, northeast and southeast are high and clearly defined. In the north, the boundary with the Maha Oya is comparatively low (about 100m M.S.L.). The Kelani Ganga basin can be divided in to two sections, the mountainous country which is approximately 2/3 of the catchment and the flat plain. The flat plain is located mainly below 30m M.S.L. and slightly intersected by low hills. The boundary between the mountainous and flat countries approximately lies through Glencourse hydrometric station. Both flat and mountainous countries are covered with thick vegetation. In the mountainous country the greater part of the land is under rubber and tea plantations. Paddy and coconut are the main cultivation in the flat plain. 3.1.2 Climate ! i Hot humid and rainy climate of the wet zone is characteristic of the Kelani Ganga basin. Rainfall and temperature changes with the altitude above M.S.L. Mean annual air temperature is 80° F at Colombo and 60°-70° F in the mountainous country. The southwest monsoon occurs from May to August-September. The northeast monsoon blows from October to March. Mean daily wind velocity averages 200 km/day, varying 145 km/day in March to 250 km/day in June August. Annual precipitation in the Kelani Ganga basin varies from 2100mm to 5600mm. The highest precipitation of over 5000mm is observed in the upper reaches of the Sithawaka Ganga, the lower reaches of the Kehelgamu Oya and Maskeliya Oya and We Oya. On the western and eastern part of the basin precipitation gradually decreases to 2500mm and even less. 3.1.3 Hydrology The basin of (he Kelani Ganga has a well-developed drainage network. The Kelani Ganga starts at the confluence of the Kehelgamu Oya and Maskeliya Oya originating from mountainous range of Kirigalpotta at the elevation of 2400m M.S.L. In the mountainous country the river takes in a number of large tributaries abundant in water such as Kehelgamu Oya, Maskeliya Oya, We Oya, Gurugoda Oya, Sithawaka Ganga etc. Figure 3.1 indicated that there are 2 peaks in average rainfall at October and June and in average monthly stream flow has significant rise in the same months. Though the rainfall and streamflow values show some differences from catchment to catchment the general pattern of the monthly distribution is similar each year. No flow interventions in Kelani Ganga has taken place before construction of Castlreigh(1959) and Maussakelle(1969) reservoirs (ID, 1961) 3.1.4 Selected watersheds in Kelani Ganga Basin Five watersheds in the Kelani Ganga basin have been selected for this study. Data availability and the length of the annual flow series were based on the selection. In Kelani Ganga subcatchments, the data availability varied from 26 to 38 years (Table 3.9). The elevation, drainage area, slope, shape and average rainfall are shown in Table 3.1. Table 3.1: Watershed Parameters of Kelani Ganga Basin Watershed Elevation Area Slope Shape Rainfall 2 at station (m) (km ) (%) (m/m) (mm) 1. Glencourse 18 1463 4.0 0.3 4060 2. Metiyadola 20 606 4.6 0.1 4100 3. Deraniyagala 82 152 2.2 0.4 5000 4. Kitulgala 56 388 5.1 0.2 3620 5. Imbulana 26 329 1.5 0.6 3525 Note: Elevation: Elevation at the hydrometric station Slope: Total fall between the highest and lowest points divided by the stream length Shape: Ratio of width and length of the watershed Rainfall: Mean Annual rainfall Average monthly areal precipitation in the subcatchments and average monthly streamflow values at the selected hydrometric stations were obtained from the Master Plan for Electricity Supply (CEB 1985) and are shown in Tables 3.2 and 3.3. Both rainfall and streamflow show a two-peak behaviour indicating the influence of me two monsoons. In General the streamflow peaks coincides with the rainfall. 3-2 Table 3.2: Average Montl.Iy Areal Precipitation in Kelani Ganga Basin (mm) WS atStation Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep imean 1. Glencourse 232.5 200.1 104.2 58.3 46.6 61.0 107.0 190.7 228.4 189.4 145.3 176.o{ 145.0 2. Metiyadola 90.6 66.7 37.6 21.3 155 20.7 31.0 71.9 101.7 62.5 63.0 76.6 56.6 3. Deraniyagala 34.9 21.3 9.6 4.6 3.0 4.3 10.9 33.3J 39.2 31.4 24.9 32.2 20.8 4. Kitulgala 57.3 42.0 23.4 15.9 13.4 14.0 17.9 49.21 71.4 56.4 47.8 51.3 38.3 5. Imbulana 35.2 36.2 18.0 7.6 8.0J 8.1 1S.5 2S.7| 29.0 25.1 17.2 22.4| 20.7 Table 3.3: Average Monthly Streamftow in KeluvA Ganga Basis* (m3/s) WS at Station Oct Nov Dec Uan Feb jMar Apr May Jun Jul Aug ,'Sep jrnean 1.Glencourse 552 445 211 105 128 260 409 517 478 370 310J 425} 351 2. Metiyadola 499 378 201 99 111 216 346 481 525 436 3671 431 i 341 3. Deraniyagala 635 424 222 117 127 250 427 659 657 509 44S! 532j 417 4. Kitulgala 416 318 187 34 99j 187 302 405 433 412 339j 377| 302 5. Imbulana 482 415 1G2 31 106{ 222 3a 395 362 316 2551 336j 294 4 A 3-3 Watershed Boundary Figure 3.2: Selected Watersheds in Kelani Ganga Basin I3> g M 0 1.1 H fed 1 0 40 ii 1 *| 500 ] 030 1000 ' 1 0 10 0 10 r-t ' 1500 0 no MM 8 I & $ i S 5 5 5 * ? Figure 3.3.1: Watershed at Gieancourse g go 0 0 £ 040 T 500 I I ID 1000 | f o m | 010 1500 I g .in MO Figure 3.3.2: Watershed at Metiyadola g gg E oto on 030 010 I fill Figure 3.3.3: Watershed at Deranlyagala ii-.n . J v.! 0 40 u Li I. )0 500 I 030 1000 f 010 1500 | onri MOO S I * * * i 5 5 s ? I f Figure 3.3.4: Watershed at Kitulgala n M 0 0 40 0 500 030 1000 !) 10 8 °"> 1500 | OM moo Month Figure 3.3.3: Watershed at Imbulana Figure 3.3 Average Monthly Streamflow (cumecs per km2) and and Average Monthly Precipitation (mm) 3-6 3.2 Kalu Gangs Basin Kalu Ganga watershed takes the third place in trie island with respect to its catchment area (2700 km2) and the first place when the runoff ratio is concerned. (77%). 3.2.1 Location and Topography The Kalu Ganga flows through southwestern part of the island in the wet zone of Sri Lanka. The river flows westward, maximum lejngth of the river basin is 85 km and its width from north to south is 60 km. In the north the Kalu Ganga basin is bounded by Bolgoda Lake and Kelani Ganga basin while in the east by the Walawe Ganga basin. In the south Kalu Ganga catchment is bounded Benthota Ganga and Gin Ganga basins. The Kalu Ganga basin can be broadly divided in to two sections. The mountainous region of the catchment and the flat plain. Both flat and mountainous regions are covered with thick vegetation. In the mountainous region the greater part of the land is under rubber and tea plantations. Paddy and coconut are cultivated mainly in flat plain. The land use in Kalu Ganga basin has no significant difference when compared with Kelani Ganga basin. 3.2.2 Climate Hot humid and rainy climate of wet zone is characteristic of the Kalu Ganga basin as well as the Kelani Ganga basin. Rainfall anl temperature changes with the altitude. Mean annual temperature is 75° F at Kiilutara in the flat plain and around 65° F in the mountainous region. The southeasi monsoon and the northeast monsoon winds come in May to September and October to March respectively. Annual precipitaion in the Kalu Ganga basin varies from 6000mm to 2500mm. The highest precipitation over 5000mm in observed in the northern upper reaches of the Kalu Ganga and it is quiet lover at upper reaches of the Denawak Oya, Wey Ganga and Hangamuwa Oya. On (he western and eastern part of the basin precipitation gradually decreases to 2500mm 3.2.3 Hydrology The basin of the Kalu Ganga has a well-developed drainage network. The Kalu Ganga originates from the mountainous range of the ^vestem part of central hills at an elevation of about 2000m above M.S.L. Variation of average rainfall and average streamflow are similar to Kelani Ganga basin (Figure 3.2).