FLOODPLAIN MANAGEMENT

Although we live in a desert climate, flooding can and does occur. Unlike some parts of the country with significant rainfall and a network of creeks, streams and rivers; the desert southwest with its arroyos, washes and dry rivers can be overwhelmed during summer monsoon rains. The Town’s floodplain code regulates construction and development in three main areas – FEMA floodplain, locally regulated floodplain, and erosion hazard setback areas.

Areas classified by the Federal Emergency Management Agency (FEMA) as having an annual chance of flooding of 1 percent are classified as Special Flood Hazard Areas by FEMA. FEMA produces Flood Insurance Rate Maps (FIRMs) depicting these areas. The FIRM for Marana was last published on June 16, 2011. The Engineering department maintains copies of the FIRMs, as well as tracking FEMA letters of map corrections to these maps.

Locally regulated floodplains are areas which fall below FEMA’s criteria for federal mapping, usually due to the size of the watershed which generates stormwater runoff. Locally regulated floodplains are also based on the principal of an annual chance of flooding of 1 percent. The Town of Marana maintains maps of these floodplains through drainage reports and studies predominately associated with development.

Due to the sandy nature of the soils in southern Arizona, drainage conduits and washes can be subject to channel migration and avulsions. For this reason, an erosion hazard setback is applied to all natural channels. The erosion hazard setback is a horizontal distance measured from the primary channel bank of an incised channel or from the floodplain limit of a non-incised channel to provide a measure of safety against lateral erosion.

Development in the areas subject to the floodplain code requires an engineering study and a floodplain use permit.

Areas not mapped as floodplain can and do flood. It should be noted that about 25 percent of all insurance claims to FEMA are for areas not classified as SFHA. Flood insurance is available for any structure and is very reasonable for structures outside of the FEMA SFHA. Homeowners may contact their home insurance providers (most of which can broker FEMA flood insurance policies) regarding a “preferred risk policy” for more information.

Pima County Regional Flood Control District Automated Local Evaluation in Real Time (ALERT).

The Pima County Regional Flood Control District operates and maintains a network of real-time sensors used to collect hydrometeorological data. This network provides information to county personnel and other agencies about precipitation, stormwater runoff, and weather conditions affecting our regional watersheds. Using radio telemetry, sensors report data in the National Weather Service Automated Local Evaluation in Real Time (ALERT) format. This type of network is commonly referred to as an ALERT system.ALERT system sensors are event-driven and transmit data in real-time to base station computers at the District's office and the Tucson National Weather Service office.

Event-driven real-time transmission means that transmission of data occurs immediately as data is collected at field sites. Data collected by the ALERT system is used by the National Weather Service to assist in issuing flood watches and warnings and to ground-truth radar-determined rainfall estimates.

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