Stormwater Utility Commission


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  AGENDA ITEM: 9.       
Meeting Date: 09/17/2020  
Submitted By: Justin Turner, Public Works

In 2017, the Pima County Regional Flood Control District (District) finished hydrologic modeling and floodplain mapping for the Carmack Wash and tributaries (Technical Data Notebook, [Draft] October 12, 2017). This earlier study examined most of Carmack Wash, starting downstream at its confluence with CDO Wash near Interstate 10, and continued upstream until Linda Vista Boulevard.  Analyses were made using HEC-HMS for hydrology and HEC-RAS and FLO-2D for floodplain mapping.  Upon completion of the 2017 study, the Town requested that the District continue the investigation, and this time focusing the mapping effort on the six major tributaries to Carmack Wash located within the Town, especially those located in areas comprised of pre-FIRM homes (i.e., those built before December 1974).
The updated 2020 study is nearly finished.  This new study focuses on the watershed areas within the Town, and more or less from Hardy Road on the south, Calle Buena Vista on the west, Linda Vista Boulevard/ Valle Del Oro on the north, and the boundary with Coronado National Forest on the east. Computer models and floodplain maps were received by the Town in early May, and the draft Technical Data Notebook (TDN) was received in late July.  Review comments were returned, and a final copy of the computer models, TDN and work maps are expected to be received by mid- to late-September 2020.
In addition to receiving HEC-HMS and FLO-2D models of the study area, work products included a detailed floodplain map (with north and south halves) at a scale of 1”=300’ and it showed the FLO-2D depths, floodplain boundaries, and calculated 100-year discharges at the recording cross sections.  The 100-year discharges were calculated at numerous locations within the study area, and summarized in the report.  Computed discharged were favorably compared with those calculated using USGS Regional Regression Equation 5, as well as those reported in the District's earlier 2017 report.  Regulatory floodplain limits (i.e., those with 100-year peak discharges larger than 50 cfs) were delineated for the main stem of Carmack Wash, and it six principal tributaries and sub-tributaries.
When finished, the Carmack Wash Floodplain Study will be used for floodplain management purposes, including (1) evaluating whether new development activities are reasonably safe from flooding, and (2) informing homeowners of their true floodplain risks and mitigation opportunities.
This study revealed unknown areas of possible uncontrolled flooding, and the Stormwater Utility is currently talking to the District about having them continue with this investigation to include "mini-basin management plans" to better identify flood risks to residential and commercial structures, and to recommend remedial structural or non-structural flood-control methods.
At present, there is no plan to submit the report to FEMA. The Town will use this information for floodplain management as a Local Floodplain Study as mentioned above.
See Executive Summary above.


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