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).

3-7 3.2.4 Selected Subwatersheds in Kalu Ganga Basin

Six watersheds from Kalu Ganga basin were selected based on the data availability and the length of the annual flow series.'The data availability varied between 20 - 40 years (Table 3.6). The elevation, drainage area, slopes, shapes and average annual rainfall at selected watersheds are in Taole 3.4.

Table 3.4: Watershed Parameters of Kalu Ganga basin

Watershed at Elevation Area Slope Sti ape Rainfall Station (m) (km2) (%) (m' m) (mm) 1. Putupaula 2 2598 1.5 0.5 3970 2. Milakanda 17 769 1.5 o.: 4110 3. Malawala 18 329 4.0 l.C 4100 4. Nambapana 13 629 3.3 0.3 3745 S. Ellagawa 50 1393 2.1 0.4 3910 5.Dela 29 220 1.8 0.2 2610 Note: Definition of Elevation, Area, Slope, Shape and Rainfall are as in Table 3.1

Average monthly areal precipitation (Table 3.5) and average monthly stream flow values (Table 3.6) at the selected hydrometric stations are shown below. Both rainfall and streamflow show a two-peak behaviour indicating the influence of the two monsoons.

Table 3.S: Average Monthly Areal Precipitation in Kalu Ganga Basin (mm)

Station Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep mean 1. Putupaula 523 363 222 142 139 232 340 540 477 333 302 406 335 2. Millakanda 491 439 289 162 163 273 353 484 404 301 268 372 333 3. Malawala 496 372 248 152 156 245 356 513 481 332 322 409 340 4. Nambapana 397 353 235 132 142 243 289 440 43) 308 284 383 303 5. Ellagawa 491 332 209 119 126 231 309 492 46: 329 297 392 316 6. Dela 341 350 233 109 132 250 254 367 34 252 222 324 265

Table 3.6: Average Monthly Stream Flow in Kelani Ganga Basin (m'/s)

Station Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep mean 1. Putupaula 357 329 198 120 103 125 196 358 370 239 178 239 234 2. Millakanda 110 104 81 34 25 33 62 118 105 74 52 74 71 3. Malawala 47 34 22 12 10 13 22 48 51 35 32 44 31 4. Nambapana 80 77 48 27 22 27 43 78 84 57 47 63 54 5. Ellagawa 183 172 97 52 40 53 106 178 202 133 101 134 121 6. Dela 14 20 15 9 7 9 13 15 18 12 9 10 12

3-8 Figure 3.4: Selected Watersheds in Kalu Ganga Basin 090 XT lU fc!B H U 0.40 ti i B • 0.30 10SO • 030

010 1500 -•- 000 3000 8 1 j| « ! 8 5

Figure 34.1: Watershed at Putupaula Figure 34.6: Watershed at Delii

Figure 34.2: Watershed at MUlakanda

Figure 344: Watershed at Nambapana

Figure 34.9: Watershed at ENagawa

Figure 3.5 Average Monthly Streamflow (cumecs per km2) and and Average Monthly Precipitation (mm) 3.3 Gin Ganga Basin

Gin Ganga is the third major river in the Wet zone of Sri Lanka. It originates from southern slope of the Adam Range local ed in the middle south of Sri Lanka, and finally reaches the sea in the vicinity of Gall harbour. Gin Ganga had severe flood threats and the Government of Sri Lanka with Chinese Government assistant implemented a flood protection project in 197S.

3.3.1 Location and Topography

In the north the Gin Ganga is bounded by Benthota Ganga and Kelani Ganga and in the west Madampe Lake, Telwatta Ganga and Rathgama Ganga. In the south and east the Gin Ganga catchment is bounded by Koggala Lake, Polwatta Ganga, Nilwala Ganga and Walawe Ganga basins. The overall length of the main stream totals to of 113 kilometres. The catchment area of the basin is about 960 square kilometres. Mean gradient of the watershed is 2.4%. The river valley upstream from Udugama is narrow with a nean gradient of 5.9% and the hilly slopes are steep. Down stream plains are low-lying and broadly even. The mean gradient is 0.25%. Paddy fields are predominant on both banks (SDL 1975).

3.3.2 Climate

Gin Ganga basin has a tropical climate ^vith a relatively high temperature Temperature records of Galle station at downstream of basin show that mean annual temperature is around 26.5° C. Average annual rainfall in the Gin Ganga basin is gauged at Agaliya and Thawalama shows to be around 4000mm. There are two rainy seasons due to the influence of the two monsoons usually experienced in the wet zone. Rainfall in these periods is more than 60% of the total in a year. The storm centre is generally occurred at the middle and upper reaches (SDL 1975).

3.3.3 Hydrology

Occurrence of flood corresponds with the storm and two peaks in average rainfall and average monthly stream flow can 1>e seen as in Kelani and Kalu river basins in wet zone.

3.3.4 Selected Watersheds in Gin Ganga basiin

There are three gauging stations along the main stream, of which the Agaliya station, controlling 71% of the drainage area has a long data series with 44 years of annual peak flow data. Thawalama is the other gauging stations with a short

3-11 annual flow series of 12 years (Table 3.9). The elevation, slope, shape and the rainfall of Gin Ganga at Agaliya and Thawalama are indicated in Table 3.7.

Table 3.7: Watershed Parameters of Gin Ganga Basin

Watershed Elevation Area Slope Shape Rainfall (m) (km2) (%) (m/m) (mm) 1. Agaliya 10 696 1.5 0.5 3970 2. Thawalama 27 361 1.5 0.2 4110 Note: Definitions of Elevation Area, Slope, Shape and Rainfall are as in Table 3.1

3 - 12 J Selected Gauging Stations

River Basin Boundary

Watershed Boundary

'Igare 3.6: Sslected Watersheds in Gin Ganga Basi 3.4 Nilwala Ganga Basin

Nilwala is the other major river basin taken for the study of flood transposition. This watershed with an area of 700 square kilometres drains to sea at Matara. This is located at southern extremity of the island.

3.4.1 Location and Topography

The river flows southwards from the central hills of the island. The maximum length of the basin is 50 km, and its width from east to east to is 30 km. In the north Nilwala Ganga basin is bound by Gin Ganga and in the west by Polwatta Ganga. In the east the Nilwala Ganga catchment is bounded Sinimodara Oya, Kirama Oya, Urubokka Oya and Walawe Ganga basins.

3.4.2 Climate

The southwest and northeast monsoon winds, after covering long distance over the Indian Ocean, maintain a constant temperature over the zone. Annual rainfall ranges from 2000mm along the coast to 4000mm at northestern boundary of the catchment area at a distance of about 50km.

3.4.3 Hydrology

Main tributary of the Nilwala Ganga, which is the Kirama Oya, joins the main stream approximately at the middle. Annual total discharge of Nilwala Ganga has been estimated as 40% of total annual precipitation and it is comparatively low when compared with the other three major rivers in wet zone (ED, 1961).

3.4.4 Selected Watersheds in Nilwala Ganga Basin

Five gauging stations are situated along the river. But none of the stations have more than 12 year of flow data except the station Bopagoda which has 42 years of data (Table 3.6). The elevation, slope, shape and the rainfall of Nilwala Ganga at Boppagoda and Bingimahara are shown in Table 3.8.

3.14 Table 3.8: Watershed Parameters of Nilwala Ganga Basin

Watershed Elevation Area Slope Shape Rainfall (m) (km2) (%) (m/m) (mm) 1. Bopagoda 18 442 20 1.0 3400 2. Bmgimahara 26 333 18 0.6 3240 Note: Definitions of Elevation, Area, Slope, Shape and Rainfall are as in Table 3.1

3.15 i Selected Gauging Stations

River Basin Boundary

— Watershed Boundary

Figure 3.7: Selected Watersheds in Nilwala Ganga Basin 3.5 Data

3.5.1 Data Availability

Watersheds in the wet zone were scanned for data availability. It was identified mat Kelani and Kalu river basins had sufficient gauging stations and the record lengths were long. However Gin and Nilwala basins had two gauging stations each with some reasonable length of records.

Stream flow records, rainfall records and flood peaks at the selected stations for Kelani, Kalu, Gin and Nilwala rivers available from 1940 to 1985 (CEB 1985). Stations having 20 to 43 years of records were considered for this study. The Locations of hydrometric stations and meteorological stations are shown in Figure 3.2, 3.4, 3.6 and 3.8. Selected stations are in Table 3.9.

Table 3.9: Data Availability of Selected Watersheds

No Stream gauging station River Data Duration Length of recordstyrs 1 Glencourse Kelani Ganga 1948- 1984 37 2 Metiyadola Kelani Ganga 1948- 1981 34 3 D era niya gala Sithawaka Ganga 1948- 1976 29 4 Kithulgala Kelani Ganga 1947- 1984 38 5 Imbulana Gurugoda Oya 1948- 1973 26 6 Putupaula Kalu Ganaga 1943- 1981 . 39 7 Mlllakanda Kalu Ganaga 1950- 1977 28 8 Malawala Kalu Ganaga 1954- 1978 25 9 Nambapana Kalu Ganga 1956- 1978 23 10 Ellagawa Kalu Ganga 1956- 1980 25 11 Dela Wey Ganga 1956- 1984 29 12 Agallya Gin Ganga 1941 - 1984 43 13 Thawalama Gin Ganga 1973- 1984 12 14 Boppagoda Nilawala Ganga 1940- 1982 42 15 Binglmahara Nilwala Ganga 1973-1984 12

It should be noted that the number of years mentioned always do not have continuous records. Some gaps in the records were found and upon inquiry it was found that mainly washing away of gauge posts due to floods had taken place. Annual Peak Flows at Kelani, Kalu, Gin and Nilwala Basins are in Table 3.10.1, 3.10.2 and 3.10.3 respectively.

3.5.2 Data Checking.

Data obtained from the CEB Master Plan for the Electricity Supply of Sri Lanka were plotted and compared with upstream or adjacent watersheds. Missing periods were eliminated as far as possible but no data filling was carried out.

3-17 Table 3.10.1: Annual Maximum Peakflows in Kelani Ganga Basin

Hyd. year Oleancourse Metiyadola DeranSyagala Kitulgala Imbulana Date Value Dale Value Date Valuo Date Value Date Value 1940 1941 1942 1943 1944 1945 1943 1947 12.05.48 527 1948 27.08.49 1770 27.08.49 892 27.08.49 249 29.07.49 535 27.08.49 299 1949 14.09.80 • 1019 28.07.50 767 13.09.50 113 27.07.60 756 27.07.60 200 1950 31.05.51 892 31.05.51 743 07.06.51 183 31.05.61 705 31.05.61 153 1951 05.05.52 1667 24.05.52 963 24.05.52 766 24.05.62 1034 24.05.52 652 1952 04.10.52 1331 03.10.52 936 03.10.62 687 03.10.52 847 03.10.62 566 1053 02.05.54 722 01.05.64 391 02.05.64 224 28.05.54 386 01.05.64 204 1954 16.05.55 1303 16.05.65 912 16.05.65 340 15.05.55 1497 17.05.65 268 1055 19.06.56 1104 23.06.66 793 19.06.66 446 23.06.66 1188 20.10.65 991 1958 31.05.57 1246 31.05.57 750 31.05.57 439 16.06.57 663 31.05.67 235 1957 25.12.57 1232 25.12.67 766 10.05.68 266 10.05.68 440 26.12.57 276 1958 22.06.59 1700 21.06.59 871 19.06.58 366 21.06.69 1526 21.06.69 402 1959 17.06.60 895 28.10.69 432 16.06.60 654 28.09.60 772 16.06.60 656 1960 23.05.61 736 28.05.61 532 12.08.61 239 12.08.61 413 12.08.61 187 1961 20.05.62 844 17.06.62 666 27.05.62 348 04.06.62 496 08.09.62 361 19C2 25.10.62 957 25.10.62 513 11.10.62 292 04.08.63 496 10.11.62 272 1933 22.10.63 1954 22.10.63 1076 21.10.63 289 22.10.63 973 22.10.63 396 1964 07.05.65 1730 22.05.65 948 07.05.65 320 23.05.65 1526 07.05.65 661 1935 - 25.09.66 986 30.09.66 329 25.09.66 903 25.09.66 312 1966 15.06.67 3795 14.06.67 664 . 14.06.67 488 15.06.67 297 1987 21.10.67 3076 20.10.67 1060 20.10.67 323 20.10.67 909 20.10.67 328 1988 29.05.69 1897 29.05.69 1215 29.05.69 249 04.06.69 359 29.05.69 300 1969 20.10.69 983 09.08.70 331 01.08.70 246 20.10.69 297 21.02.70 148 107Q 23.09.71 2039 20.08.71 821 16.05.71 231 22.09.71 1200 20.10.70 232 1971 13.05.72 1400 13.05.72 722 13.05.72 280 16.07.72 712 13.05.72 312 1972 01.08.73 826 01.08.73 446 24.07.73 266 27.08.73 235 01.08.73 206 1973 27.07.74 3121 27.07.74 1185 28.04.74 306 27.07.74 1517 29.07.74 297 1974 23.05.75 2982 25.09.75 980 01.10.74 321 25.09.75 1058 1975 03.11.75 1323 03.11.75 544 23.10.75 210 23.07.76 487 1973 25.05.74 1380 02.06.77 623 02.06.77 329 15.07.77 795 1977 14.05.78 1681 21.10.77 510 14.05.78 529 1978 25.11.78 4220 24.11.78 2344 24.11.78 2259 1979 11.11.79 573 18.10.79 416 17.10.79 430 1080 17.09.81 2695 17.09.81 936 17.09.81 6jlo 1981 09.06.82 1974 07.08.82 654 07.08.82 416 1982 20.08.83 1060 11.08.83 323 1083 12.07.84 4286 12.07.84 1342 1084 06.06.85 2096 15.07.85 925 1985

No of years 37 34 29 38 I 26 Missing data 1 0 1 o ! 0

Nolo ;— Hydrologlcal year (1940) Is token as from 1st of October (01.10.1940) to 31st of September (31.09.1941)

(CEB, 1985)

3-18 Table 3.10.2: Annual Maximum Peakfiows in Kaiu Ganga Basin

Hyd. year Putupaula Millakanda Malawala Nambapana' Bllagawa Dela Dato Value Date Value Date Value Date Value Dcte Value Dato Value 1940 1941 1942 1943 26.05.44 2011 1944 19.10.44 1430 1945 31.10.45 1529 1946 16.08.47 2549 1947 26.10.47 1713 1948 . 1949 25.10.49 • 1184 1950 13.06.51 1436 01.06.61 382 1951 29.05.62 1459 12.11.51 439 1052 05.10.52 1352 04.10.52 348 1953 31.05.54 1314 27.10.63 538 1954 19.05.65 1671 17.05.55 413 15.05.55 646 1955 23.10.55 1467 20.06.66 396 19.06.66 430 1956 03.06.57 1518 02.06.57 357 31.05.57 552 03.10.66 453| 02.06.67 448 24.11.66 108 1957 12.05.68 1218 11.06.68 340 10.05.68 327 07.08.58 363 08.08.58 333 30.11.67 135 1958 24.06.69 793 31.08.69 363 21.08.69 609 21.06.59 589 23.06.69 448 21.06.69 126 1059 21.07.60 578 29.11.59 272 13.09.60 406 28.09.60 268 20.07.60 340 21.12.69 140 1960 03.11.60 708 02.11.60 244 27.05.61 360 22.07.61 292 14.08.61 339 12.08.61 61 1961 19.05.62 1048 16.05.62 496 20.05.62 984 16.05.62 408I 20.05.62 464 16.05.62 135 1962 26.05.63 864 18.09.63 411 04.08.63 892 04.08.63 462 19.09.63 366 07.06.63 135 1963 24.05.64 1028 23.05.64 532 22.05.64 1067 22.05.64 481 23.05.64 481 28.11.63 123 1964 30.09.65 947 28.09.65 411 22.05.65 803 23.05.65 399 27.05.65 384 23.05.65 104 1965 30.09.66 1226 29.09.66 533 29.09.66 1403 29.09.66 579 ; 30.09.66 685 29.09.66 127 1966 - 01.10.66 464 14.06.67 427 15.06.67 316 01.10.66 662 18.10.66 99 1067 23.10.67 1283 21.10.67 654 20.10.67 1066 20.10.67 729 22.10.67 766 21.10.67 144 1968 25.05.69 1047 24.05.69 564 29.05.69 1288 29.05.69 736 30.05.69 598 23.05.69 147 1960 22.10.69 797 31.12.69 297 29.03.70 1062 30.03.70 473 31.03.70 496 18.12.69 133 1970. 25.09.71 1029 31.12.69 462 02.09.71 869 22.09.71 633 23.09.71 931 21.09.71 117 1971 29.10.71 978 24.10.71 469 13.05.72 807 08.05.72 714 15.05.72 862 08.12.71 118 1972 07.10.72 779 07.06.73 311 04.10.72 600 27.08.73 376 06.10.72 578 06.11.72 153 1973 26.07.74 857 11.05.74 453 19.08.74 521 28.07.74 638 29.07.74 770 27.07.74 121 1074 09.05.75 1055 08.05.75 773 01.10.74 950 07.05.75 609 08.05.75 1113 25.09.75 153 1975 30.10.75 864 03.11.75 446 23.10.76 1070 24.10.75 438 26.10.78 679 1076 16.05.77 807 11.10.76 447 14.06.74 253 10.10.76 274 05.06.77 529 19.03.77 104 1977 16.05.78 1444 15.05.78 773 - - 15.05.78 1337 1978 29.09.79 975 24.10.78 920 24.11.78 841 26.11.78 670 24.11.78 190 1979 - 03.06.80 663 27.11.79 140 1980 19.06.81 876 19.09.81 810 04.01.81 165 1981 09.06.82 1155 02.06.82 162 1082 16.11.82 130 1983 12.07.84 148 1984 23.05.85 153 1985

No or yean 39 28 25 23 25 29 Missing data 3 0 1 1 0 2

Note Hydrological year (1940) Is takon as from 1st of October (01.10.1940) lo 31st of September (31.09.1941)

(CEB. 1985)

4

3-19 Table 3.10.3: Annual Maximum Peakfiows in Gin and Niiwaia Ganga Basins

Hyd. year Agaliya Thawaiama Sappageda Binglmahafa — Date Value Date Value Date Value Dale Veiue_ 1040 28.05.41 319 1041 07.04.42 602 03.06.42 638 1042 07.08.43 390 06.10.42 878 1043 19.06.44 971 18.05.44 16S2 1044 17.10.44 436 - 1B4S 07.11.45 420 '•8.10.45 140 1946 17.08.47 591 15.08.48 566 1947 23.1047 426 21.11.47 253 1948 29.08.49 295 27.CS.49 340 1949 23.10.49 •210 07.12.49 253 1850 02.06.51 326 31.05.61 453 19S1 13.11.61 227 12.VI.6if 453 1932 31.07.53 218 03.10.62 245 1953 28.05.64 323 28.04.64 374 1954 17.05.55 404 16.05.65 863 1055 - 18.08.66 278 1956 - 21.02.67 164 1657 12.05.58 269 03.05.68 292 1958 06.10.58 283 17.07.59 278 1059 - 12.02.60 232 1660 02.1&60 _224 26.09.51 127 1961 - 04.09.62 133 1962 - 04.06.63 453 1963 24.05.64 326 23.05.64 363 1964 11.10.64 266 16.C8.65 221 1965 28.09.66 329 28.08.66 340 1966 01.10.66 306 24.07.G7 207 1967 22.10.67 396 21.10.87 477 1968 26.05.69 363 23.05.G9 729 1969 31.12.69 346 29.12.63 1008 1070. 23.09.71 265 10.10.70 283 1971 16.05.72 268 09.05.72 748 1972 29.07.73 215 16.10.72 130 1673 25.07.74 350 10.05.74 924 24.07.74 263 23.07.74 269 1974 09.05.75 319 07.05.75 872 07.05.75 216 19.06.76 177 1975 29.10.75 516 02.11.75 483 24.11.75 154 16.12.75 252 1876 10.10.76 397 10.10.75 1067 14.05.77 134 10.10.73 276 1977 15.05.78 404 14.05.78 983 16.05.78 178 14.05.78 326 1078 05.11.78 350 27.09.79 931 03.11.73 270 10.05.79 256 1979 13.11.79 224 01.12.79 340 30.05.80 146 1080 31.05JM 337 29.05.81 29.05.81 180 20.11.60 146 1981 - - 30.11.81 172 10.06.82 268 1982 04.11.82 303 - 29.11.82 89 29.11.82 139 1983 24.05.84 327 12.07.84 504 12.07.34 189 1984 26.06.85 278 19.08.85 1020 19.06.85 178 1985

No of yean 43 12 42 12 Misting data 6 2 2 0

Note Hydrological year (1940) Is taken as from 1st of October (31.10.1540) to 31st of September (31.09.1941)

(CEB. 1985)

3-20 Table 3.1 ii: Annual Average Area! Rainfall at Selected Watersheds

Watershed Average Annual Rainfall (mm)

Kelani Ganga Basin 1 Glencourse 4060 2 Metiyadola 4100 3 Deraniyagala 5000 4 Kitulgala 3620 5 Imbulana 3525

Kalu Ganga Basin 6 Putupaula 3970 7 Millakanda 4110 8 Malawala 4100 9 Nambapana 3745 10 Ellagawa 3910 11 Dela 2610

Gin Ganga Basin 12 Agaliya 3970 13 Thwalama 4110

Nilwala Ganga Basin 14 Boppagoda 3400 15 Bingimahara 3240

(CEB, 1985)

3-21