Drainage improvement planning in a low-flat area in Vietnam using unsteady flow model

摘要

The objective of this study is to propose more effective measures to improve the drainage capacity in the Bac Hung Hai irrigation and drainage system, which is in the Red river delta of northern Vietnam. First, Cunge's method developed for a loopednetwork is reviewed, and a more time-saving calculation procedure is proposed. In the proposed procedure, the number of unknowns is reduced in advance, because the CPU time consumed for the matrix calculation is proportional to the 2nd to 3rd power of the number of unknowns. Therefore, this procedure is useful for solving a simultaneous linear equation system having a large number of unknowns. Second, the Bac Hung Hal area is modeled as an open channel network composed of numerous drainage channels, paddy fields, and pumps. Parameters of the model are identified through the calibration and the verification processes using two types of hydrological data for ten days in 1994 and in 1985. Using this model, more effective measures are sought with the aim of reducing damage caused by a heavy rainfall having 1/10 exceedance probability. Four feasible measures are tested to compare their effectiveness: (1) widening outlet sluices, (2) canal dredging, (3) dredging and construction of major pumping stations, and (4) dredging and installation of minor inner pumps. To calculate the area damaged by flooding, the Vietnam standard is employed, in which a flooded paddy field is such that the water depth remains more than 287 mm for more than 24 hours. Through the examination, measure (1) turns out to have no effect, but measures (3) and (4) have remarkable effects if pumping capacities are adequate.