Study status: Complete
In 2008, in an effort to increase permitting consistency and improve public safety, the District completed a project to map all of the areas known or suspected to be subject to sheet flow flooding in unincorporated Pima County. This study is intended to be the regulatory base until more detailed analyses that provide more accurate floodplain information can be completed. Please read History of Floodplain Management in the Picture Rocks Area for more information about floodplain management history for the area.
On January 14, 2011, the Chief Engineer of the District approved a detailed floodplain analysis for the Picture Rocks alluvial fan area that more accurately reflects actual flood hazards in the area than previous mapping attempts.
The new floodplain mapping was developed using FLO-2D watershed modeling, a method which is appropriate for use in broad, shallow sheet flow areas. The flow characteristics were determined using 50 foot cells and were based on the latest available topographic and aerial photo data for the study area. Additional considerations for the mapping included the geomorphology and land use. The Picture Rocks alluvial fan is characterized by distributary flow with the potential for primary flow paths to change from one flood event to the next. The relatively low surface relief of the alluvial fan means even minor grading or improvements have the potential to modify flood flows, further complicating the task of accurately mapping the floodplain.
Although considerably more accurate that than the approximate mapping previously used, the District determined the direct use of the raw FLO-2D data for regulatory purposes was inappropriate due to the coarseness of the grid cells, the subtle topographic relief of the area and the potential for diversion from even minor grading activities. Instead, the District used the raw data to delineate three different regulatory flood hazard zones. The District also established construction standards for each zone depending on the degree of hazard.
In all of the flood hazard zones, structures of any size shall be located outside the erosion hazard area of regulatory washes and placed in the least hazardous location on the property. Additionally, as property owners consider developing their property they should consider how the proposed improvements will impact neighboring properties so that others aren’t adversely impacted.
The three different flood hazard zones are, from least hazardous to most hazardous; minimal flood hazard area, regulatory sheet flow, and regulatory flow corridor. The zones and their associated construction requirements are described below,