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About EAPS

About EAPs

New Inundation Mapping Technology Helps EAP Development

NEW FROM FEMA

Federal Guidelines for Inundation Mapping of Flood Risks Associated with Dam Incidents and Failures

Federal Guidelines for Inundation Mapping of Flood Risks Associated with Dam Incidents and Failures - PDF DownloadThis 145-page document is informative for dam owners, emergency managers, and the public regarding the importance of inundation mapping as part of a comprehensive Emergency Action Plan. It provides dam safety professionals with guidance on how to prepare dam breach inundation modeling studies and conduct mapping that can be used for multiple purposes, including dam safety, hazard mitigation, consequence evaluation, and emergency management including developing EAPs. This guidance is intended to provide a consistent approach that can be applied across the nation.

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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. A thorough EAP will include an 'inundation map' that shows the hazard area.

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 Missouri Highway and Transportation Department. 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.

Global Positioning System (GPS) static session surveying.

The Missouri Department of Natural Resources has the authority to prepare inundation maps on HHP dams, although it is an unfunded mandate. In 1990, a provision was added to the state's geologic hazards law requiring inundation maps on all state-regulated dams. Responsibility was assigned to the DNR. This is found in Missouri Code's Title XVI – Conservation, Resources and Development, Chapter 256 – Geology, Water Resources and Geodetic Survey, paragraph 173. It says:

  1. The department shall provide each county in the state and a city not within a county, as the information becomes available, a geologic hazard assessment, such assessment to be prepared by the division of geology and land survey. The department shall provide to each county assistance in the use and application of the geologic hazard assessments in the county to which the assessment pertains. The geologic hazard assessment shall be made available to the general public.
  2. The department shall provide each recorder of deeds of each county in the state a map showing the downstream area that would be affected in the event of a dam failure.

The intent of this statute extends beyond dam safety because inundation maps serve many other purposes, including public safety, transportation, and land conservation. The legislature did not fund additional staff or other resources needed to implement the law. DNR typically has only four persons assigned to dam safety inspections and EAP compliance. The DNR Water Resources Center (WRC) staff in the Dam and Reservoir Safety Program spend most of their time on inspections and engineering work. While a state can include inundation mapping as part of an EAP or waive that portion, DNR would prefer that EAPs include inundation maps. As DNR staff recently noted:

"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 a means to implement the inundation mapping directive. DNR staff in the Surface Water Section (WRC) recently began developing a prototype procedure for creating inundation maps using high-resolution LiDAR (Light Detection and Ranging) data to enhance breach inundation mapping. WRC is using a combination of LIDAR and field survey, in conjunction with HEC-RAS and GEO-RAS, to perform these analyses. DNR notes: "This inundation mapping is very different than the 100-year flood event. A dam breach and subsequent flood wave develop over time and flow dynamically through the downstream environment rather than a steady-state backwater calculation. One of the deliverables is a map of the downstream environment zone displaying the structures that are inundated by at least two feet of water during the time of maximum water surface elevation."

As these analyses and maps are completed, the inundation mapping will be transferred to the dam owner and county Emergency Management Directors for additional development of the EAP. After several of these maps are completed for a particular county, public meetings or workshops will bring together dam owners and other stakeholders for discussion and education on how to use the maps to complete EAPs. The state then will begin to enforce the dam safety provisions requiring EAPs. DNR staff note:

"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 department, dam owners, emergency responders, and input from the public is necessary." DNR plans to develop a case study involving an HHP dam to demonstrate the dam breach and inundation mapping methodology.

DNR is starting with counties in Missouri that have numerous HHP dams and higher populations at risk. For example, rapidly growing Warren County just west of St. Louis has 125 dams, 28 of which are HHP and 46 are classified as Significant-Hazard Potential (SHP). Missouri has 114 counties. Many counties (35) have fewer than 20 dams, and 30 have no HHP dams. As of early 2011, the DNR mapping team had completed breach studies and inundation mapping for dams in Warren County, counties north of St. Louis, metropolitan Kansas City, and areas north of Kansas City. The inundation mapping process will take about two more years to complete. Citizens will need to be patient as this process begins to result in more EAPs on HHP dams. Public support can aid DNR in its inundation mapping initiative.