RN-21 : Effect of flood plain on flood routing

Abstract

Because of ever increasing human activities in the flood plains, the river environment gets affected. The movement of flood along the flood plains is quite complex. The flood plains act as conveying as well as storage medium for passage of floods. The valley storage naturally regulates the flood flow as it moves towards the downstream end of the reach. The flow over the flood plains is relatively slower than the main channel. This is mainly due to relatively low depth of flow and also due to the increased frictional resistance.

Civil constructions affect the flood plain accessibility, i.e. the relative ability of the water to leave or enter the flood plains. There are a few studies, made on the effect of encroachments on floods. Some studies suggest that flood height would not be substantially changed by urban development; since actual development is such that significant floodplain storage is still available for the passage of floods.

Routing of floods can be achieved either through black box models or through mathematical models. The data requirements for these modelling approaches differ significantly. Accuracy-wise both the methods are comparable. If channel processes have not altered the river environment very much from the past and if the available record of floods of this period covers the movement on flood plain also, then black box method can be prefered. The Muskingum Cunge method appropriately takes into account the geometry of flood plains and channel while establishing the routing parameters, and hence it can be prefered over conventional Muskingum method. The Kalinin-Milyukov method is much simpler to use than other methods and at the same time it involves a physically meaningful establishment of parameters making it somewhat better approach for dealing with movement of floods in flood plains.

Sometimes, one dimensional mathematical models are used to route the floods in rivers with flood plains. The data requirement is largely on cross-section and roughness coefficients. There are three different approaches used to treat floodplains, viz.(i) Off channel storage, (ii)Composite channel, (iii) Channel-plain model. The off channel-storage model attenuates the hydrograph much more than the other two models.

The complexities involved in a meandering river with flood plain create problems in modelling. Experimental studies on the floodplain effects have been carried out at Hydraulic Research Laboratory in Belgrade. Certain empirical relationships on the channel flood plain interactions have been established by these studies. These, however, have limited scope in their application only to the river systems for which they have been developed.