A modified Muskingum routing approach for floodplain flows: theory and practice

Hydrological or hydraulic flood routing methods can be used to predict the floodplain influences on a flood wave as it passes along a river reach. While hydraulic routing uses both the equation of continuity and the equation of momentum to describe the dynamics of river flows, the simpler data requirements of hydrological routing makes it useful for preliminary estimates of the time and shape of a flood wave at successive points along a river. This paper presents a modified linear Muskingum hydrological routing method where the floodplain effects on flood peak attenuation and flood wave travel time are included in routing parameters. Developing the routing parameters initially involved routing hydrographs of different flood peak and duration through a 1-dimensional model of a generalised river reach in which a range of geometrical and resistance properties were varied. Comparison of upstream and simulated downstream hydrographs for each condition investigated, allowed the attenuation and travel time (storage constant, K, in standard Muskingum routing) of the flood wave to be estimated. Standard Muskingum routing was then used to develop downstream hydrographs for each K value together with assumed storage weighting factors (x) ranging from 0 to 0.5. Flood peak attenuations were again determined through comparison of the upstream and routed downstream hydrographs and with these, linear relationships between x and these attenuations were developed. Actual weighting factors, corresponding to storage constants, were subsequently determined using these relationships for all attenuations determined from the 1-dimensional model simulations. Using multi-variate regression analysis, the computed values of K and x were correlated to catchment and hydrograph properties and expressions for determining both K and x in terms of these properties were developed. The modified Muskingum routing method based on these regressed expressions for K and x was applied to a case study of the River Suir in Ireland where good agreement between measured and routed hydrographs was observed.