St. Martin Parish, LA Environmental Rules: Stormwater Management (2026)

Section 14-102 sits in Article IV of the flood chapter, Development Drainage Detention, which Ord. No. 19-12-1282-OR enacted on December 3, 2019. Subsection (a)(1) puts the burden on the developer's design engineer: provide in the drainage improvements for runoff from the entire upstream drainage area, whether inside or outside the development, and study the effect on existing downstream drainage facilities or roadside ditches. If the runoff will overload one, the engineer must say so in the drainage impact analysis and make provision to prevent the overload. Existing natural and artificial channels shall not be incorporated into or function as a proposed storm water management facility.

Subsection (b) picks the runoff method by drainage area: the rational method for areas under 200 acres, the SCS method as modified by DOTD for areas between 200 and 2,000 acres, and the USGS regression procedure above 2,000 acres. Subsection (c) sets the storms: a minimum ten-year storm for subsurface storm sewers and outfall channel sewers, a ten year occurrence interval for open ditch systems, a ten-year storm for cross drains, and a 25-year storm for channel crossings in excess of 100 square feet. Rainfall intensity and duration come from the latest DOTD hydraulics manual under subsection (d), and the rational method coefficients are printed in tables 4.1 and 4.2, such as 0.30 to 0.50 for single-family detached and 0.40 to 0.60 for two-family (duplex).

Plan content is also fixed. Section 14-102(a)(2) requires drainage plans with benchmarks, NGVD elevations, annotated culverts, hydraulic calculations and drainage area maps, and puts a lateral drainage conduit from the street to an outfall channel across lots in a minimum 15-foot permanent drainage servitude, or 20 feet for approved open ditch drainage. Under Sec. 14-101(5)(a), a comprehensive drainage impact analysis goes to the parish after preliminary plat approval, construction plans are not approved until the parish approves it, and a developer can ask in writing for a waiver. Separately, Sec. 8-31 prohibits discharging anything other than stormwater into the MS4, with listed exemptions such as water line flushing and landscape irrigation. Chapter 8 article III (2014) still prints its own drainage criteria at §§ 8-100 to 8-104, including a five year storm for subsurface storm sewers, so ask the parish Planning and Development office which text controls a given plat.