Dam failures can occur in a number of ways, but most result from overtopping of the dam due to inadequate spillway capacity or flow of water through the dam embankment (commonly referred to as piping) leading to initiation of a breach in the embankment. The first scenario is the most common, as there are many existing dams in the U.S. that do not have spillway capacity adequate to handle the most extreme rainfall events. Piping failures result from pathways through a dam embankment where seepage gradually increases transport of fine materials until a point is reached where pore pressures are high over a relatively large area and a breach initiates. Catastrophic dam failures can also result from causes such as earthquakes, landslides, foundation failure, sabotage or damage to operational equipment.

At the present time, modeling of potential dam failures usually involves two basic scenarios. The first is overtopping of the dam during high flow (Inflow Design Flood) conditions and the second is catastrophic failure of the dam on a day with normal (sunny day) flow conditions. These two scenarios provide a reasonable representation of the range of conditions resulting from the possible failure modes.

D’Appolonia is experienced in determining design basis floods (IDF or PMF) and routing these flows through stream channels considering natural channel and overbank flow conditions, as well as constrictions caused by bridges and other man-made structures. We have performed numerous dam breach analyses and have routed the resulting floods, many times in combination with extreme flows resulting from storms. We experienced in using software for estimating dam breaches and routing flood flows, and we routinely prepare inundation maps for presentation purposes.

Portion of inundation map prepared by D'Appolonia for potential flood resulting from dam breach.