Bell County, TX Environmental Rules: Stormwater Management (2026)

Section 307 of the Bell County Subdivision Regulations is the county's core drainage-design standard, requiring every drain, drainage structure, and appurtenance in a new subdivision to be designed by an engineer licensed under the Texas Engineering Registration Act. Runoff has to be calculated using the Rational Method and Manning's Equation for ditch and pipe capacity, or the SCS method for drainage areas over 200 acres. The section sets different storm-return frequencies by road class: arterial streets and roads need a minimum 25-year design storm; collector streets need a 10-year storm, stepping up to a 25-year design at crossroad drainage structures serving larger drainage areas, capped at no more than 12 inches of road overtopping in a 100-year storm; minor residential streets need only a 10-year storm; and driveway culverts need just a 5-year storm, with the designing engineer supplying a pipe summary of sizes and grades.

New roadway culverts 36 inches or larger must be reinforced concrete pipe, while smaller culverts and driveway culverts may use RCP or corrugated metal pipe. 5 percent grade, though the County Engineer can approve shallower or eliminated ditches on a case-by-case basis, including ribbon-curb rural subdivisions with at least 2 percent cross slope. 6's preliminary-plat requirement that post-development peak flows for the 25-year and 100-year storms not exceed pre-development peak flows or exceed erosive velocities.