【正文】 8176。40′07″E 19441985 42 323 Medawachchiya 8176。33′33″E 19441991 48 374 Mihintale 8176。53′35″E 19441991 48 445 Padaviya 8176。03′45″E 19441985 42 455 Pangurugaswewa 8176。40′30″E 19441989 46 529 Sigiriya 7176。13′35″N 80176。34′36″N 80176。43′36″N 80176。 (2) Runoff routing (River Flood Routing Method) Flood Frequency Analysis There are the annual maximum discharges at two rivergauge stations, namely Horuwpotana and Pangurugaswewa, which are available and given in table . The length of both records is respectively 32 years from 1952 to 1984 and 34 years from 1947 to 1979. Feasibility Study of Yan Oya Reservoir Project in Sri Lanka Chapter 2 China CAMC Engineering Co., LTD. 14 Table Maximum Observed Flood Peak (m3/s) Year Horowpothana Year Pangurugaswewa Year Horowpothana Year Pangurugaswewa 1946 1946 283 1966 1966 1133 1947 1947 263 1967 1967 1699 1948 1948 2407 1968 107 1968 708 1949 1949 1133 1969 136 1969 1090 1950 1950 736 1970 105 1970 396 1951 165 1951 488 1971 162 1971 991 1952 103 1952 232 1972 1972 320 1953 127 1953 396 1973 560 1973 204 1954 158 1954 200 1974 101 1974 170 1955 8 1955 33 1975 38 1975 62 1956 80 1956 183 1976 91 1976 227 1957 2254 1957 6031 1977 165 1977 1958 158 1958 147 1978 338 1978 85 1959 121 1959 195 1979 89 1979 61 1960 104 1960 566 1980 40 1980 1961 264 1961 1472 1981 63 1981 1962 212 1962 566 1982 141 1982 1963 203 1963 566 1983 310 1983 1964 95 1964 142 1984 64 1984 1965 318 1965 949 Compared with the Horuwpotana station, the flood record at Pangurugaswewa is relatively less reliable in Sri Lanka due to the lack of checking and testing from the related departments. Thus the flood record at Horuwpotana can be used directly in flood frequency analysis with the Gumbel Function. The catchment area of Yan Oya at Pangurugaswewa is 1341km2 while the Horuwpotana one is 955 km2. Compared with peak discharge of 2254 m3/s in 1957, the peak discharge of 6031m3/s in the same time is distinctly too high. Therefore, it is believed that exclude the value of this time when using Pangurugaswewa‘s record to the time frequency analysis. The results of Frequency analysis are shown in Table . Feasibility Study of Yan Oya Reservoir Project in Sri Lanka Chapter 2 China CAMC Engineering Co., LTD. 15 Table Frequency analysis results Recurrence Interval(year) Peak Discharge（ m3/s） Horuwpotana Pangurugaswewa 25 1160 1785 100 1770 2603 1000 2633 3760 2020 3060 3760 10000 3493 4915 Runoff routing Owing to various reasons it is hardly possible to find a continuous, plete hourly flood record at any of these two hydrometric stations. As a result, the two observed hydrographs in December, 1977 and February, 1984 were used for calibration of the model from Huruluwewa tank to Horowpotana. The Kc and m are the principal parameters of the runoff routing model (Kc is an empirical coefficient applicable to the entire catchment and stream work and m is the parameter for nonlinear relationship between storage and discharge for the river reaches which is generally found to be . Each reach can be represented by its own lag parame ter which depends on the ratio of its length to the average flow distance in the river work of the entire catchment. For the two floods, the results are shown in table after the model simulation, which demonstrates that the model has a good accuracy. Table 27 Results of the calibration Flood cK m Simulated Peak(m3/s) Observed Peak（ m3/s） 1977 年 12 月 122 1984 年 2 月 122 301 Feasibility Study of Yan Oya Reservoir Project in Sri Lanka Chapter 2 China CAMC Engineering Co., LTD. 16 The initial loss of the two flood was 10mm while the continuous loss was about 1mm/h. The table above shows that the model has good accuracy in simulation of floods. The achievement in this project feasibility study of 1994 is adopted for the design precipitation and storm patterns, which is given in Table . Table Design storm in different frequencies Recurrence Interval 25 100 1000 10000 24 hours Storm Total ＊ ＊ —— An estima te from freque nc y analysis 。43′25″N 80176。44′55″N 80176。20′10″N 80176。45′01″E 19441974 31 Table Rivergauge Station List Name Basin Location Period of Record Type of Gauge Latitude Longitude Year53′31″E 19571975 19 473 Pelwehera 7176。45′24″E 19641991 28 448 Palampoddar 8176。30′25″E 19441991 48 382 Minnerya Tank 8176。29′14″E 19441988 45 355 Maradankadawala 8176。58′36″E 19441991 48 261 Kebithigollewa 8176。49′16″E 19441970 26 222 Kahatagasdigiliya 8176。57′08″E 19441976 33 162 Habarana 8176。55‘00‖NL, 80176。55‘00‖ ～ 8176。40′23″N 80176。32′44″N 80176。21′12″N 80176。32′28″N 80176。21′16″N 80176。49′22″N 80176。44′12″N 80176。57′05″N 80176。43′40″E Gauging Weir Illukwewa Yan Oya 8176。52′42″E do Pangurugaswewa Yan Oya 8176。51′05″E do Yakawewa Ma Oya 8176。 Table shows the results obtained from the Huruluwewa downstream of the runoff routing model at two areas. Table Design Flood from the Model Recurrence Interval (year) Huruluwewa (m3/s) Horowpotana (m3/s) Pangurugaswewa (m3/s) 25 160 638 1068 100 422 1388 2284 1000 649 2080 3373 2020 700 2180 3500 10000 819 2478 3939 Comparison of results Horowpotana The baseflow of 40 m3/s, 60 m3/s, 80 m3/s and 100 m3/s are separately added to the results from the runoff routing and (river flood routing method). Accordingly, the flood results from frequency analysis should be reduced by 10%， which is given in this project feasibility study of 1994. And table shows the parison of the two different results. Feasibility Study of Yan Oya Reservoir Proj