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Picture Rocks Alluvial Fan Floodplain Study

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,

Minimal Flood Hazard Zone

Minimal Flood Hazard Zones are areas with anticipated base flood depths of several inches or less, but that may have low spots or small washes that may cause more concentrated and deeper flows. Due to the low level of ground relief, even minor improvements such as fences and driveways have the potential to divert flow and change the characteristics of flooding, however, the level of hazard in these areas does not meet the regulatory threshold outlined in the Floodplain and Erosion Hazard Management Ordinance (Ordinance).  With this in mind, the District does not require Floodplain Use Permits (FPUPs) within this area, but does require all new habitable structures to be elevated one foot above highest adjacent natural grade (HANG). Additionally, all structures are required to be placed outside of the erosion hazard setback of any regulatory wash, regardless of the size or use of the structure.

Regulatory Sheet Flow Zone

Regulatory Sheet Flow Zones are areas of moderate flood hazard in which the flood depths during the base flood is expected to be approximately 6 inches deep on average.  Within this area most development activities will require an FPUP including, but not limited to, manufactured homes, any site-built structures 200 square feet or larger, block walls, some types of perimeter fencing/walls, and grading.  The lowest structural member of a manufactured home and the lowest finished floor of a habitable site-built structure must be elevated to the Regulatory Flood Elevation (RFE) of 1.5 feet above highest adjacent natural grade (HANG). Non-habitable site built structures such as detached garages and storage buildings must either have the lowest floor of the structure elevated at or above the RFE or be wet floodproofed, meaning that they are constructed of floodproof materials up to the RFE and the walls of the flood vented in order to reduce hydrostatic pressure. Flood vents shall be installed in accordance with Technical Policy 022.

The State of Arizona and the District have standard foundation details that individuals may opt to use when constructing buildings within the regulatory sheet flow zone. A property owner may also obtain the services of an Arizona registered civil engineer to design a site-specific foundation that is designed to withstand flood and erosion hazards.

Flow Corridor

Flow Corridors are areas of high flood hazard and development in Flow Corridors must be avoided, if possible.  This zone contains flows that are more highly concentrated and thus generally deeper and faster flowing.  Within this area most development activities will require an FPUP including, but not limited to, manufactured homes, any site-built structures 200 square feet or larger, block walls, some types of perimeter fencing/walls, and grading. 

Most development within these areas will also require the owner/applicant to obtain the services of an Arizona registered civil engineer to determine whether or not the development will adversely impact any neighboring property and/or to design a site-specific foundation that is safe from flooding and erosion hazards.  It may also be necessary for the engineer to determine the Regulatory Flood Elevation (RFE) to which structures must be elevated.  Whenever an engineering analysis is required, the permitting process usually takes longer than normal. Prior to placing improvements in the Flow Corridor, both the hazards and the time necessary to obtain a permit should be accounted for.

Washes and Erosion Hazard Setbacks

As stated above, in all of the flood hazard zones, structures of any size must be placed outside of the erosion hazard setback of any regulatory wash and/or placed on higher ground on the property when available.  This requirement is designed to put the structure in the least hazardous location on the property.  Other improvements such as walls and certain types of fences must also avoid washes or be elevated over them.  While some improvements may be placed near a wash, the District recommends that all improvements avoid the area around washes to the greatest extent possible.

Maps

The following maps are available: