2 Literature Survey 2.1 Floods and Issues in Gin
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2 LITERATURE SURVEY 2.1 FLOODS AND ISSUES IN GIN GANGA PROJECT 2.1.1 General Gin Ganga is a river situated in the South Western Region of Sri Lanka (Fig. 1). It drains an area of 960sqkm at the sea out fall, which is Ginthota. The river is 112.5km long, with its source reaching Abbey Rock (Elevation 1293m MSL). The river drains part of Southern Province and passes Udugama, Mapalagama, Agaliya, and Baddegama, Dodangoda and flows into the sea at Ginthota (Fig. 2). The last stretch of Mapalagama to Gintota of the river is through flat lands. During rainy season floodwater inundates the Gangaboda Pattuwa and Galle Four Gravets (Bope-Poddala, Akmimana DS Divisions) in Galle District (Fig. 3). The disaster caused by floods menaced 6200ha of paddy lands and 33000 people living in 6000 houses and affected transportation and communication in the lower basin. Nearly 216 000 people lived in the basin and 60000 people lived in the vicinity of floods in lower basin (PDR, GRP, 1975). The Government of Sri Lanka in 1972 invited the Government of People's Republic of China to recommend a plan to control the flooding in the river basin. Chinese Engineers designed the flood control project. The Gin Ganga Regulation Project (GRP) was completed in 1983 by the joint assistance of Chinese and Sri Lanka Governments. Irrigation Department of Sri Lanka commenced operation and maintenance work in 1983. In 1972, about 22000 people living in 4000 houses occupied the project area below Agaliya. Gin Ganga project declared an unprotected area of 2540ha with 830 houses with 4000 inhabitants. Backwater effect inundates 78ha in upstream of Agaliya. Project occupies about 225 ha of paddy lands between flood bunds on both sides of river (PDR, GRP). Project included 6200 ha of paddy lands, which is 45% of the total. Electric pumps were provided to dispose drainage water from 3188ha of paddy lands. Another lOOOha of rainfed paddy lands in the upstream sub basins were unaffected and were taken as free from high scale flooding. 1740ha of Holuwagoda basin was drained by gravity. 800ha of marshy lands were reclaimed and converted as paddy fields in Diviturai. The cost of the project in 1983 was Rs. 430 million. It included the construction of 10 pump houses, 24.2km long flood bunds, 35km long drainage canals, 21 spill structures, 47 culverts and bridges, one ship lock, one switching station with a separate power 14 supply line from Nugaduwa and the office building at Indigasketiya. 2.1.2 HYDROLOGY Gin Ganga basin mainly experiences southwest monsoon rains and floods are usually experienced during May - June. Floods also occur during the inter-monsoon period of October - November but to a lesser degree than the May-June monsoons. Average annual flow into the sea is about 1600 million cubic meters and the mean annual rainfall is 3048mm at Neluwa-Tawalama in mid catchment. Available records show that maximum stagnation of floods was for 30 days in May 1940 with a maximum discharge of 1400cumec at Agaliya (IDGauging, 1940). Rainfall and river stage records are available from year 1928 onwards (PDR, GRP, 1975). The Irrigation Department maintains Agaliya and Tawalama gauging stations. Department of Meteorology maintains Galle meteorology station. Average monthly precipitation records for 1901-1965 period at Beausejour (lower) are given in Fig. 5 (ECIR, 1968) and Monthly precipitation records for 1973 -1994 at Galle are given in Annex F (Met D, 1994), Annex B gives the design flood details (PDR, GRP). Table 1.1 gives the seasonal precipitation data at Galle and Table 1.2 gives the seasonal evaporation data at Ratnapura. Table 1.3 gives average annual values of data at respective locations in the vicinity. 15 Table 1.1 Seasonal Precipitation Data -GALLE YEAR Description TOTAL 1990 NE monsoon total = 887mm 1990 NE monsoon 30 year average = 1090mm 1990 SW monsoon total = 1241mm 1990 SW 30 year average = 1290mm 1990 Annual total = 2118mm 1990 Annual 30 year average = 2380mm 1965 Annual 65 year average = 2500mm Table 1.2: Seasonal Evaporation in Ratnapura (mm) YEAR Description TOTAL 1995/6 Northeast monsoon total = 623mm 1995/6 Southwest monsoon total = 587mm 1995/6 Annual total = 1210mm 1965/6 Annual total = 1140 mm Tawalama Station Catchment area = 26 500ha Coordinates of Tawalama = 0/9 (0.65,2.75) Coordinates of Agaliya = 0/13 (4.90, 0.90) Table 1.3: AVERAGE ANNUAL VALUES OF RAINFALL, EVAPORATION, SPECIFIC YIELD OF FLOW, RUN OFF, PEAK FLOW COMPONENT STATION ANNUAL VALUE REMARKS Rainfall Neluwa-Tawalama 3048mm 1901/65 period average Galle 2380mm 1960/90 period average Pan evaporation Ratnapura 1210mm Pan evaporation 95/96 Ratnapura 1140mm Pan evaporation 65/66 Runoff Tawalama 911MCM Runoff 95/6 Tawalama 3580mm Rain fall 95/96 Specific yield Tawalama 0.68 1995/6 Volume estimated Gin thota 1600MCM 1901-1965 average Peak flow Agaliya 1400 cumec Peak flow May, 1940 River water quality Wakwella Salinity, Iron River water quality 16 2.1.3 LAND USE PATTERN OF THE PROJECT AREA Gin Ganga basin is fairly well developed in agricultural pursuits excepting Singharaja forest in the mid-basin. The upper reaches are planted with tea, rubber, coconut and cinnamon. (Fig. 3, Fig. 4, Fig. 6, These data maps were prepared before 1972) Rubber predominates in the lower areas. Paddy lands are in lowland pockets, all along the river amounting to about 14,133ha (PDR, GRP, 1975). About half of paddy lands, situated in the low lands, suffer from periodical floods (ECIR, 1968). The tea plantations are said to contribute to the sediment in water currents. Paddy fields lose finer sediments. Mountains in mid basin also said to have low vegetation due to erosion. The new short rooted bud tea plantations with high foliage reduced the erosion after 1970 (Soil, 1994). Land use map for 1972 in Topographical map of Ambalangoda, sheet number 67, -scale 1:63360- is shown in figures 3, 4 and 6. Low- lands in the basin are about 15% and most are paddy lands, which were concentrated in the lower basin. The cash crop plantations in high lands are about 34 % cultivated with tea, cinnamon and rubber. Domestic areas are cultivated with coconuts and vegetables. About 25 % of the basin is under jungles. Project area had 40% paddy, 20 % rubber, 10 % tea and others in the lowest section. In the upper project area 31% paddy, 30% tea, 30% rubber, 5% gardens and 4% scrubland are available (ECER, 1968). The soil map of Sri Lanka shows red yellow podzolic soils from Udugama to Kimbi ela and red yellow podzolic soils with well-developed laterites and their drainage associates from Kimbi ela to Galle (Panabokke, 1967). 2.1.4 HISTORY OF GINGANGA BASIN Flooding of Gin basin has a long history. The abundance of aquatic palm known as Gin at Ginthota had given its name to the river. Lower basin had experienced heavy flooding but the effect was well taken for seasonal needs. Though crocodiles were living in the river but the oru paru boats were used for traveling along the river. Ginthota, Halpathota, Nugethota were reputed landing stations. Water dependent agriculture had been the main living of the population and after 13* century international trade activities had taken its place in lower basin. Galle became a busy harbour to exchange goods from Arabian and Indian vessels. Spices and goods were 17 brought along the river from main land. In 1505 Portuguese ships landed in Galle and fighting for trade rights were intensified. Many foreigners settled in the coastal area and they found the floods as a disaster. In 1648 Dutch took the control of Galle harbour and made a fortress. They traveled up to Mapalagama and created Thanayam rest houses. Cinnamon was newly introduced and bonded labour immigrants were introduced in colonies. Galle became the capital of the low country, when the Kandyan King lost the control over the Galle area. Dutch officers cut new waterways for easy transport. Tax collectors gathered high income using the waterways as checkpoints. Effort had been made during Dutch period to protect certain pockets of areas from floods by means of earth embankments along river and canals but without satisfaction. The Dutch government was protecting banks across Heen Ela in Majuwana and cut a canal to divert water to Hikkaduwa across Gonapinuwala. Dutch canal, which is known as Kepu Ela provided another bypass to take excess water of the river from Ukwatta to Mahamodara. This has provided a water transports service from Galle town to upstream villages using the "Angula" boats. The people to drive these "Angula", upstream from Mahamodera, Ukwatte to Mapalagama, harnessed high tidal drift. Sediments and materials carried by floods are deposited on the Gin Ganga flood plains. Gin Ganga flood plains possess clay deposits, which are used for pottery, brick and tile making while using the same land for paddy cultivation. Sugarcane was also cultivated in the low lands. Vegetables and green leaves are cultivated mostly near Labuduwa, Ukwatte villages. Sand mining from the riverbed was a long time practice among the villagers. This was accelerated due to cement mortar brick production. Though bamboo plants are found along banks the erosion can be observed during high flows. 2.1.5 THE FLOODING ISSUE Problem of Gin Ganga flooding was considered as the first and foremost environmental hazard of the district. Government budget allocations for flood relief activities included food and lodging services for the displaced persons. Irrigation Department physically controlled annual floods through the operation of lock gates provided at Dodangoda and Ukwatte across flood bunds (Brohier, 1954). Daily river flow gauging at Agaliya was commenced in 1940 by the Hydrology Division of Irrigation Department.