Effect of channel characteristics on flood wave propagation - A research report by National Institute of Hydrology

The study attempts to explain flood wave propagation in natural and artificial channels within the perspective of site-specific rating curve. The study looks at the impacts of channel characteristics – bed slope, flood plains, channel shape, expansion-contraction combination, inactive storage, Manning’s roughness, expansion and contraction on flood wave propagation.

Describing flood wave propagation in open channel flow within the framework of loop or hysteresis of rating curves has not received much attention though frequent attempts have been made time and again. The literature on the subject has been more intuitive than rational. The available knowledge is limited to these aspects –

• Channel hysteresis is a manifestation of channel storage and is a phenomenon
• Channel roughness causes unsteadiness in the flow and gives rise to the development of hysteresis in the rating curve at a site, and
• The larger hysteresis pertains to the larger flood wave attenuation and vice-versa.

The study attempts to do the following  -

• The dam break flood computation for Vaigai dam and routing through the downstream channel using National Weather Service’s Dam Break Flood Forecasting (NWS DAMBRK) model;
• The development of dimensionless rating curves at all the downstream cross sections and the computation of hysteresis;
• The analysis of the dam break flood wave propagation characteristics using hysteresis values and;
• The analysis for the impacts of above channel characteristics on the above dam break flood wave propagation in a  hypothetical channel of approximately the same length as of the above downstream river reach of Vaigai dam

The analytically derived relationship between quantified hysteresis and phase difference is verified using the available numerically derived and observed data. Also for kinematic wave situation, a unique relationship among hysteresis, phase difference and logarithmic decrement is presented. The shifting control situations also causes hysteresis in the rating curves but is however, beyond the scope of the present report.

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