DRAFT FERC Engineering Guidelines Risk-Informed Decision Making: Chapter R21 - Dam Breach Analysis

Dam breach analyses are used to estimate the potential hazards associated with a failure
of a project structure/feature. Dam breach inundation analyses include the following
elements: estimation of the dam breach parameters, estimation of the dam breach outflow
hydrograph; routing of the dam breach hydrograph downstream; and estimation of
downstream inundation extent and severity.
Dam breach prediction models are used to estimate the geometry and formation time of a
dam breach. Typically, dam breach prediction models are based on empirical data
derived from a number of mostly earth and rockfill dam failures case studies. The
available empirical equations relate the dam breach parameters to properties of the dam
and reservoir such as height, dam type and its erodibility, volume impounded, and shape
of the reservoir.
The most common methods of dam breach outflow hydrograph routing are either onedimensional
or two-dimensional with the latter used when higher levels of accuracy are
required or for non-channelized flow situations. For most dam breach analyses, onedimensional
computer software is used. Geographic Information Systems (GIS) are the
current state-of-practice for inundation mapping, especially if the dam breach analysis
involves populated areas and/or other high potential consequences areas.
The methodologies described in these guidelines are intended to highlight the current
state-of-practice tools available to the qualified engineer experienced in hydrology and
hydraulics. It remains incumbent on the engineer to exercise sound engineering
judgment in selecting the appropriate dam breach analysis type and the required level of
detail in modeling and inundation mapping to ensure that they are commensurate with the
anticipated consequences, as well as consider how the study results can best be used to
aid in determining consequences for a risk-informed decision. Sensitivity analyses for
those dam breach analyses with significant impacts are almost always necessary to
evaluate the results over the range of credibly possible input parameters. All studies
submitted to the FERC should contain a summary of the design assumptions, design
analyses, and methodologies used.