Granite Reef Watershed Drainage and Flood Control Improvement
The primary objective of the Granite Reef Watershed Improvement Project is to eliminate the risk of structural flooding, eliminate the Federal Emergency Management Agency (FEMA) floodplain along Granite Reef Wash, and reduce street and yard flooding. The project will also include a new storm drain in Pima Road that will help reduce floodwater flows in Granite Reef Wash and provide street drainage for the future Pima Road widening project. The existing FEMA floodplain is approximately 2 miles long and encompasses over 600 homes; extending from Thomas Road downstream to the City’s southern boundary at McKellips Road, generally between Granite Reef Road and Pima Road.
The project area is bounded by the Arizona Canal on the north, Loop 101 on the east, the Salt River on the south and the drainage divide that runs between Hayden Road and Granite Reef Road on the east.
NEW HYDROLOGIC AND HYDRAULIC ANALYSIS
The project includes the development of a two-dimensional hydrologic and hydraulic computer model, utilizing new technology, to analyze storm water runoff in the project area. The preliminary results from the model indicate that the 100-year peak discharges on Granite Reef Wash are significantly less than the effective FEMA flows. Therefore, even without any drainage improvements in the project area, it is likely possible to reduce the size of the floodplain and 100-year flood elevations based on the new hydrologic and hydraulic model.
APPLICATION TO FEMA FOR FLOODPLAIN REDUCTION
Once the hydrologic and hydraulic model is complete, the City plans to submit an application to FEMA to reduce the size of the current, effective floodplain, as well as the 100-year flood elevations. The application is called a request for Letter of Map Revision (LOMR). The plan is to submit the LOMR application to FEMA in the early part of 2017. Assuming FEMA is in agreement with the new hydrologic and hydraulic modeling results, it usually takes about 12 to 18 months to obtain the official map revision.
PLANNED DRAINAGE IMPROVEMENTS
Drainage improvements are being planned for the project area to control the runoff from the 100-year storm in order to eliminate the FEMA-designated floodplain on Granite Reef Wash. The improvements will be implemented in two phases. Phase 1 includes the planned storm drain improvements north of Indian School Road and Phase 2 covers the planned improvements south of Indian School Road.
Phase 1 Improvements - Phase 1 of the implementation plan includes new storm drain laterals that will connect to the existing storm sewers on Jackrabbit Road, Chaparral Road, Camelback Road and Indian School Road. The new laterals will discharge more flow to the existing storm drains allowing more runoff to be diverted to Indian Bend Wash, so it doesn’t flow south, resulting in flooding problems downstream. The proposed storm drain laterals will control all the 100-year runoff north of Indian School Road. The City plans to begin construction on the storm drain laterals by the summer of 2017.
Phase 2 Improvements - Phase 2 of the implementation plan includes new drainage improvements south of Indian School Road that, together with the Phase 1 improvements, will hopefully eliminate the FEMA floodplain on Granite Reef Wash. These improvements are still in the planning stages so a detailed plan has not yet been developed, but potential improvements include: 1) storm drain in Osborn Road from 87th Street to Indian Bend Wash, 2) storm drain in Thomas Road between 82nd Street and Pima Road, 3) stormwater detention basins in both Pima Park and Apache Park, 4) storm drain in Pima Road from Thomas Road to McKellips Road, 5) Granite Reef Channel improvements both upstream and downstream of McDowell Road, 6) storm drain in McKellips Road between 82nd Street and Pima Road, and 7) new channel or storm drain in the Granite Reef Wash alignment from McKellips Road to the Salt River. The City plans to complete the master plan for the Phase 2 improvements by the Spring of 2017 with construction starting by spring of 2019.
APPLICATION TO FEMA TO ELEIMINATE FLOODPLAIN
Once the construction is completed on all the Phase 1 and Phase 2 drainage improvements, the City will submit an application to FEMA to eliminate the floodplain along Granite Reef Wash.