Thornton, CO Environmental Rules: Flood Zones (2026)

Section 22-312(a) requires a floodplain development permit before construction or development begins anywhere within an area of special flood hazard established under Section 22-303, and any development permit application touching a parcel that shows special flood hazard area must be evaluated for that overlap and, if present, reviewed under Article VI. Section 22-312(b) folds this review into the standard development-activity and permit process of Chapter 18. Applicants must also submit, under Section 22-312(c), the elevation of the lowest floor (including the basement) of new and substantially improved structures relative to the 1988 North American Vertical Datum, the elevation to which any nonresidential structure has been floodproofed, the elevation of electrical and mechanical equipment, engineer certification of floodproofing on FEMA forms, a description of any watercourse alteration or relocation, and evidence of approval from other federal, state, and regional agencies.

Section 22-303 adopts the governing flood studies by reference, led by the Flood Insurance Study for the City of Thornton, Adams County, Colorado dated December 2, 2021, with its accompanying Flood Insurance Rate Map, along with a series of Mile High Flood District major drainageway plans for basins including Grange Hall Creek, the South Platte River, Niver Creek, and Brantner Gulch. The floodplain administrator, per Section 22-305, reviews and approves or denies every floodplain development permit, checks proposed building sites for flood safety, records lowest-floor elevations before a certificate of occupancy issues, and notifies FEMA and neighboring jurisdictions before any watercourse is altered or relocated.