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.

Some descriptions of D'Appolonia projects involving dam breach analysis and flood routing are provided in the following:


Safety Inspection and EAP Preparation for Six Dams

D'Appolonia was retained by the Mercer County Conservation District to prepare Emergency Action Plans (EAPs) for six dams. The scope of work included determination of the Probable Maximum Flood (PMF) for each dam and analysis of a dam breach occurring simultaneously with the peak flow. A dam breach was also calculated for non-flooding ("sunny day") conditions. The resulting flows were routed downstream and maps of the inundated areas were prepared.

Part 12 Dam Safety Inspections and Remedial Studies

To fulfill Federal Energy Regulatory Commission (FERC) Part 12 requirements, D'Appolonia has conducted 5-year comprehensive inspections and performed stability, PMF and dam breach analyses for the Hawks Nest Dam in West Virginia.

Engineering for Low-Head Hydroelectric Power Installations

D'Appolonia was retained to assess geotechnical and hydrologic/hydraulic conditions related to low-head hydropower development at two lock and dam installations on the Allegheny River near Pittsburgh, PA. The work at one site included flood routing studies to evaluate the effects of river flow restrictions resulting from construction of temporary cofferdams and the permanent facility.