Envireau Water has recently carried out a number of dam breach analyses for reservoir planning applications.
A breach analysis is now a standard requirement of a flood risk assessment requested by the Environment Agency and the Local Planning Authority. The deliverable is a hazard map which shows the level of hazard a breach in the retaining dam poses to the population downstream.
The complexity of the breach analysis required is dependant upon the topography of the downstream catchment. For a reservoir located on a flat plain away from a population, a simple analysis can be sufficient. This involves calculating the relevant depth for each hazard class on a site specific basis, and spreading the spill volume to these depths using a concentric circle method.
For a reservoir located in a valley which would channel the flood water stemming from a breach, a much more detailed analysis is required. Elevation data analysis is required to construct detailed cross sections along the catchment. The speed, depth and attenuation of the flood volume are then calculated for each of the cross sections. This data is used to calculate the relevant depth for each hazard class and used to plot a hazard map for the channel. Due to the complexity of the topography and channel a mapping package is required to plot the hazard map.