RN-22 : Effect of channel processes on flood routing

Abstract

Although a river occupies a small portion of the. catchment, its role in shaping the landscape is enormous. The river and the adjoining plains and as a matter of fact, the whole catchment undergoes changes with time. Some of the changes are man induced. As a result, the same flood volume(caused by similar precipitation) occurred in the past does not behave today in the same manner as it did earlier. The planform might gets changed and severe aggradation/degradation might take place. These in turn change the nature of the flood flow. No systematic study on those aspects of flood movement in natural rivers could be found in the available literature.

in this study, three important aspects have been identified and the literature has been surveyed to review the effect of channel processes on [food flow and routing. They are (1) resistance to flood flow, (2) general aggradation or degradation, causing the flood water level to change, and (3) planform modifying the characteristics.

The stage-discharge relationships are important for discharge computations and some flood routing methods like Kalinin-Milyukov method using rating curves. The shape of the hydrograph also gets affected by channel processes. Both these aspects have been discussed in the report.

The Muskingum method of flood routing is used in many countries. This method uses the past flow records to arrive at values of two parameters. These parameters reflect only the river system existed at time of observation in the past and as such cannot account for the changes that occurred/ would occur to the river due to various channel processes. Certain other hydrologic flood routing methods take the geometry of the river for establishing the parameters. For instance, the Muskingum- Cunge method account the plan area of submergence during floods and slope, the Kalinin-Milyukov method takes care of the volume of river reach involved in flood movement. These methods can be used to route the floods taking the effect of channel geometry into consideration to some extent. Hydraulic flood routing explicitly accounts for the channel geometry and hence may be preferred. However, available methods of computing the changes in geometry due to various channel processes are inadequate.

After studying specific cases of river changes the following main conclusions have been arrived. The river environment undergoes changes at various rates. The change might be seen in 10 years to 30 years. A single flood event could change the river drastically as seen in Rupicon river. In any case flood routing should take into account the effects in the parameters. Alluvial river models could provide a practical solution to this problem.