City Proposes Revisions to Granite Reef Wash 100-year Floodplain

Just like road maps are updated periodically, FEMA’s Flood Insurance Rate Maps are updated periodically to keep them up to date. Updates may be based on improved floodplain modeling techniques, changes within the watershed that result in increased or decreased rainfall runoff, and/or re-grading which affects floodplain hydraulics.

The City of Scottsdale, in partnership with the Flood Control District of Maricopa County and Salt River Pima-Maricopa Indian Community, recently constructed storm drain improvements in the upper end of the Granite Reef Wash watershed, which begins at the Arizona Canal and ends at the Salt River, and which generally lies between Granite Reef Road and the Loop 101 Freeway. These improvements diverted stormwater westward to Indian Bend Wash, which has adequate capacity to handle the additional runoff. Therefore, the 100-year peak flow rate for the lower end of the watershed has been reduced. As a result, the 100-year floodplain for Granite Reef Wash is generally smaller than what is depicted on FEMA’s current Flood Insurance Rate Maps. The City of Scottsdale retained an engineering consultant to remap the floodplain accordingly, using state-of-the-art 2-dimensional floodplain modeling techniques. The new study, if adopted by FEMA, will result in the 100-year floodplain being removed from many properties, which will eliminate the mandatory flood insurance requirement pursuant to federal law for these property owners. While these proposed revisions are good news for many property owners, several dozen properties that are at risk of flooding in a 100-year storm are proposed to be mapped into the new 100-year floodplain, which will make flood insurance mandatory for mortgage holders. These individuals would likely benefit financially from purchasing and maintaining flood insurance before the effective date of the floodplain revisions, which is expected by FEMA in 1-2 years. Yet another group of properties currently within the 100-year floodplain are proposed to remain within the floodplain, although the 100-year flood levels may change. In many locations, the 100-year flood levels will drop roughly one foot. Property owners in the new floodplain will likely benefit financially on their flood insurance premiums by having a new FEMA elevation certificate prepared by a Registered Land Surveyor or Professional Civil Engineer and providing it to their flood insurance agent.

The City of Scottsdale plans to schedule a public open house in late February or early March 2020 to discuss the proposed floodplain revisions and answer questions. Letters showing the proposed 100-year floodplain revisions for each parcel will be mailed to affected property owners, inviting them to this open house. The date, time, and location of this open house will also be posted to this web page once this public meeting has been scheduled.

Application (PDF) to FEMA to revise the Granite Reef Wash 100-year floodplain.

Please direct questions to the City of Scottsdale, Floodplain Administrator Ashley Couch, PE, CFM, at 480-312-4317 or

Contact Information

Report A Problem:
Scottsdale EZ

Stormwater Group
P: 480-312-7740


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