New Inundation Mapping Technology Helps
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 and it includes an 'inundation map' that shows the hazard area. Fortunately owners of most dams in California, especially HHP dams, are required to provide an inundation map as part of the dam permitting process.
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. 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.
Inundation Mapping Resources
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.
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 California, 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.
Inundation maps are available for most large California dams. These maps and EAPs are under the jurisdiction of the California Office of Emergency Services (CalOES) Dam Safety Program. California Government Code Section 8589.5 requires dam owners to submit copies of inundation maps developed by civil engineers to the CalOES. Owners of some small and/or rural dams may be allowed a waiver from the inundation mapping requirement by CalOES, or allowed to submit less detailed maps.
This is based on whether the effects of potential inundation in terms of death or personal injury, as determined through onsite inspection by CalOES in consultation with the affected local jurisdictions, can be ascertained without an inundation map, or where adequate evacuation procedures can be developed without benefit of an inundation map. If development occurs in any exempted area after a waiver has been granted, the local jurisdiction must notify CalOES of that development. All waivers must be reevaluated every two years by CalOES.
The CalOES Dam Safety Program coordinates with other state and federal agencies in activities to assure effective dam incident emergency response procedures and planning. The CalOES Dam Safety Program is also the designated repository of the official dam failure inundation maps used in California's Natural Hazard Disclosure Statement as specified in Civil Code § 1103 for real estate transactions. Copies of dam inundation maps are also provided to the appropriate public safety agency of any city and/or county likely to be affected. Most local agencies overlay these inundation maps onto their parcel map systems. CalOES provides a directory of local emergency managers to contact to determine what parcels are within inundation zones.
Inundation Map vs. Evacuation Area
Inundation maps are developed from the best available information using reasonable assumptions and standardized methods. They are approximations of the maximum water surface extents resulting from a complete dam breach and draining of the full reservoir. Inundation maps are empirical hydrologic and hydraulic simulations which can only be field verified in the event of an actual breach.
Evacuation areas and emergency call lists based on these maps should take into consideration the anticipated local impacts of flooding, knowledge of local infrastructure, both occupancy and ownership of property, and potential interrupted services or cut-off access caused by a dam failure. Depending upon actual circumstances, appropriate alert and evacuation areas could be more or less extensive than the simulated inundation zones shown on the maps.