New Inundation Mapping Technology Helps EAP Development
Knowing where a HHP dam is located that may impact your home, business, or favorite recreational area is important. But knowing the boundaries of the 'hazard area' also is important. This information may not be clearly defined unless there is an EAP for that dam. This information may not be clearly defined unless there is an EAP for that dam. FEMA Guidelines for EAPs state that an inundation map is one of the six basic elements needed in an EAP.
An inundation map will vary in detail and content according to the characteristics of the hazard area. For unincorporated areas, the map shows county, state, and federal roads, and houses, buildings, and other features. Usually the map for the affected counties is taken from the county highway maps published by the Texas Department of Transportation. If the map includes incorporated areas, it will show streets bounding the inundation zone. In other cases, the inundated area will be sparsely populated so that a narrative describing the areas flooded may be sufficient. Flooding of key points can be established and interpolation can be used to determine if a feature between these points will be flooded. Rate of travel of flooding can be described so that timing can be estimated.
Accurate dam breach analyses and inundation mapping are critical components for constructing useful Emergency Action Plans (EAPs) within downstream environment zones. Recent national guidance suggests EAP development as a top priority for all high-hazard dams.
An inundation map is used to depict areas that will flood if a dam fails. Inundation maps may be in one piece for a small inundation zone or may consist of several sequential maps if the inundation zone is lengthy. The maps usually should denote various segments of the inundation zone with detailed notation as to the time to flood (the elapsed time from the breach to the when critical structures are flooded) and the time until the flow peaks. The detail and complexity of inundation maps depends on the level of development located downstream from a dam.
The Texas dam safety regulations that took effect in 2009 do not strictly require inundation maps in EAPs except for new construction. Maps also may be required when a dam undergoes renovation or repairs. Dam Safety Program engineers strongly urge owners of all SHP and HHP dams to provide maps, whether they are highly detailed or merely “vicinity” and “downstream” maps from public sources. The Texas EAP guidelines suggest that:
“Detailed information on dam failure and inundation analyses can be found in the Hydrologic and Hydraulic Guidelines for Dams in Texas. The guidelines outline a simplified procedure for estimating dam-break inundation for small and intermediate-size dams and dams with limited downstream development. Prepare inundation maps which clearly depict the flooded areas from a dam break. For dams with limited downstream development, a narrative description and/or generalized map may suffice.”
Some dam owners and local emergency managers have been able to create a rudimentary inundation map by overlaying a downstream U.S. Geological Survey topographical map and a county assessor’s map. This helps identify properties within the potential inundation zone. Dam owners should ask themselves a simple question: Am I confident that everyone in harm’s way can be notified if emergency responders work from these inundation maps?
New Technologies Can Help
New technology is now providing ways to create inundation
maps that are highly accurate and yet do not require weeks
of field time by survey crews. High-resolution LiDAR (Light
Detection and Ranging) data is being used to enhance breach
inundation mapping. A combination of LIDAR and field survey, in conjunction
with U.S. Army Corps of Engineers software such as HEC-RAS
and GEO-RAS, enable detailed breach analyses.
Inundation mapping of a dam breach is very different from what one may see on the 100-year flood maps for the same area. A dam breach and subsequent flood wave develop over time and flow dynamically through the downstream environment rather than a steady-state backwater calculation. These flood waves may travel very swiftly whereas a flood from heavy rains may rise more slowly. Inundation mapping shows the downstream environment zone structures that are inundated by at least two feet of water during the time of maximum water surface elevation.
It is critical that emergency managers focus their limited resources where they are needed most in the event of a dam failure. The objective of accurate inundation mapping is to facilitate this focus. For this work to be effective, cooperation of the Dam Safety Program, dam owners, emergency responders, and input from the public is necessary.
In some states, inundation mapping is provided by the state dam safety program. In other states, including Texas, the cost of inundation mapping is more likely to be the responsibility of the dam owner unless grant funds become available from the state or federal agencies such as USDA or FEMA. Public support can be helpful in convincing governmental units of the value that inundation mapping provides in creating high quality EAPs.