In recent time high and low flow computation in rivers and canals is good enough for practical purpose with overflow by explicit procedure. Therefore sometimes the oscillation unstability of polution can not be overcome. | Vietnam Journal of Mechanics, NCST of Vietnam Vol. 21, 1999, No 3 (129 - 136) AN IM P LI CIT ALGORITHM FOR COMPUTATION OF OVERFLOW IN FLOOD PLAINS NGUYEN TAT DAC Institute of Applied Mechanics, NCST of Vietnam ·1. Introduction In the Mekong Delta, annually, flood is one of the three natural factors stronq;ly influenced on agriculture activities and living condition. For a very dense syste:m. of rivers , canals and flood plains, mathematical model is the main tool for p laJ!lning and for computation of flood control scenarios. In recent time high and lo~ flQw computation in rivers and canals· is good enough for practical purpose, btJ,t all existing ·computer programs in Vietnam, such as VRSAJ>, KOD, SAL, have dealt with ·overflow by explicit procedure. Therefore sometimes the oscillation ~r unstability of ~olution can not be overcome. This difficulty can be reduced by an implicit algorithm given in this study. The effectiveness of the proposed meth~d can be seen when one has to handle a big problem like the flood situation in t1 e Mekong Delta during 6 months. · ·• 2. Hydraulic computation for rivers . Governing equations For gradually varied unsteady flow in rivers and canals it is customary to u1e the following one-dimensional Saint-Venant system of equations, which consist s ?f a continuity equation and an equation of motion [1]: · BaH + aQ = q at ax . aQ A aH at + ax A + g ax !._'(Q2) () gQjQI :_ BT = o + AC 2 R o () where: H = water level above a datum, Q = discharge, B = width at the water surface of river cross section including storage averaged for each segment, A t= cross section area, C = Chezy number, g = acceleration due to gravity, R = hydraulic radius, q = q 1 + P: lateral in/ out flow to unit length where P is the exchange flow with adjacent plains and q 1 is pumping or discharging flows , T = 129 ) wind stress over water surface which will be neglected in computation, density, t = time and x = distance along the river. {J =
