La Plata County, CO Environmental Rules: Stormwater Management (2026)

Section 70-9, Grading, Drainage, and Erosion Control, exists to minimize the adverse impacts of development on stormwater quality and quantity for both construction and post-construction runoff, and to protect streams, creeks, rivers and their availability for existing and future use. It applies to all development except development not requiring a land-use permit, wildland fire remediation, plat adjustments and modifications, lot consolidations and boundary adjustments, special events and temporary uses, and additional dwelling units that do not need a major land use permit under section 73-3.

The applicant must demonstrate the project will not adversely impact surface waters by meeting the standards in the Chapter 70 Technical Appendix, Drainage Criteria Standards. The submittal is a set of construction plans, grading plans, erosion control plans and a drainage report, prepared under the direction of and signed and sealed by a professional engineer unless the planning engineer waives that. The planning engineer can waive requirements for minor site improvements, minimal changes in existing drainage patterns or grade, minimal changes in the quantity or quality of stormwater, no need for drainage infrastructure, or a finding that detention is not required. Preliminary documents go in with the project submittal and are revised until approved; final signed and sealed documents are due before construction.

The drainage report carries pre-development and post-development drainage area maps with elevation contours, flow arrows, basin boundaries, labels and areas. Off-site flows must be routed over the detention facility spillway or around the site. The report quantifies pre and post flows, provides on-site detention to limit the rate of runoff unless exempt, describes water quality measures if required, and sizes the interior storm drain system for the ten (10) year storm while directing all drainage from newly created impervious area to the detention facility during the one hundred (100) year storm.

Grading plans must show drainage flowing away from buildings and structures, with contours at two foot intervals up to 20 percent slopes and ten foot intervals on steeper terrain, plus spot elevations, finished floor elevations and culvert inverts. Elevations must come from a survey by a registered surveyor; GIS contour data is not acceptable. For detention projects the engineer's completion letter verifies the constructed pond volume, as-built top of bank and water surface elevations, outlet structure elevations and the orifice size. Where a hazard design alters natural drainage, Section 70-8 requires a drainage easement from impacted downstream owners.