TR(BR)-10/96-97 : Effect of channel characteristics on flood wave propagation

Abstract

Describing flood wave propagation in open channel flow with in the frame-work of loop or hysteresis of rating curves has not received much attention though frequent attempts have been made time and again. The presented description in literature has been more intuitive than being rational. The available know-how restricts to that (i) the hysteresis is a manifestation of channel storage and is a phenomenon; (ii) the channel roughness causes unsteadiness in the flow and gives rise to the development of hysteresis in the rating curve at a site; and (iii) the larger hysteresis pertains to the larger flood wave attenuation and vice versa. The shifting control situations also causes hysteresis in the rating curves and it, however, is beyond the scope of the present report.

The present work is an attempt to describe the flood wave propagation in natural and artificial channels within the perspective of site-specific rating curve. This work is a further extension of the earlier works carried out at the institute. In steps, the study on looking at the impacts of channel characteristics-- bed slope, flood plains, channel shape, expansion-contraction combination, expansion-contraction combination, inactive storage, Manning's roughness, expansion, and contraction-- on flood wave propagation deals with: 1) 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;

2) the development of the dimensionless rating curves at all the downstream cross- sections and then computation of hysteresis (n);

3) the analysis of the dam break flood wave propagation characteristics using iv values; and

4) 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 above downstream river reach of Vaigai dam.

Further, the analytically derived relationship between quantified hysteresis (n) and phase difference (ø) is verified using the available numerically derived and observed data. Also, for kinematic wave situation, a unique relationship among n, ø, and logarithmic decrement § is presented.