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It is important for the Flood Control District to have accurate up-to-date information that identifies flood hazards throughout Maricopa County. The District accomplishes this goal by doing Floodplain Delineation Studies that identify land areas (floodplains) subject to inundation by a flood that has a one-percent probability of being equaled or exceeded in any given year, also known as a 100-year flood. The results of a Floodplain Delineation Study are submitted to the Federal Emergency Management Agency (FEMA) in order for the study to be incorporated onto the Flood Insurance Rate Maps (FIRM). Any development in an area that is determined to be a floodplain must meet the requirements of local, state and federal regulations.
Floodplain Delineation Studies provide the community with more accurate and up-to-date information on flood hazards and allow residents to prepare for the flood risks they may face. These studies maximize public safety, and minimize the later need to remediate flooding problems.
There are many areas of the County that haven't been studied yet and the flooding hazards for these areas aren't known. It is important to understand that floodplains and flood hazards exist in these areas even though they haven't been documented yet by a Floodplain Delineation Study. Although your property may not have been in a floodplain when you moved in, that could change in the future. Your area may not have been studied yet, or the floodplain limits might be modified because of changes that occurred during flooding events, or development in the area caused some change. Also, the District may use new technology that it believes will result in a more accurate identification of the floodplain limits.
Detailed Delineations
Detailed Delineations are conducted in developed areas and identify the floodplain limits using detailed technical information. Base flood elevations within the floodplain are determined. Detailed studies may also involve the identification of a floodway, which is the area within the floodplain that must be kept clear in order to carry the floodwater without causing more than a one-foot rise in the water surface elevation.
Approximate Delineations
Approximate Delineations are conducted in areas with limited or no development. When these studies are published on the FIRMs no base flood elevations are provided. These studies are typically done in order to establish initial floodplain delineation before development occurs.
LOMR and CLOMR
- The Letter of Map Revision (LOMR) is an official revision of a current Flood Insurance Rate Map (FIRM) accepted by FEMA, which reflects changes in mapped areas for flood zones, floodplain areas, floodways and flood elevations.
- The Conditional Letter of Map Revision (CLOMR) allows for approval of anticipated map revisions based on proposed modifications or conditions that are expected to exist in the future. Data may be submitted for a proposed project or future condition with a request that FEMA review the data and issue a CLOMR describing the revision(s) that may be made upon completion of the proposed work.
Alluvial Fans
An alluvial fan is a special, fan-shaped type of flooding hazard that can occur where a watercourse exits a canyon or mountain pass onto a flatter plain, usually along a mountain front. The flooding hazards associated with alluvial fans are high-velocity flow, debris-laden flow, rapid changes to channel paths and uncertainty in the flow path, and abrupt deposition and erosion of sediment.
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