How Is Site Runoff Controlled at Composting Facilities?

The engineer draws the watersheds before sizing anything: the contact-water watershed (pads, stockpiles, tipping areas) draining inward to leachate collection, the clean watershed (roofs, clean roads, undisturbed areas) draining to the stormwater system, and the off-site watershed intercepted at the perimeter. Berms, curbs, and diversion ditches make the separation physical, and the engineer sizes each diversion for the design storm on its tributary area — a berm that overtops in the 25-year storm is not separation. The grading plan shows spot elevations and flow arrows dense enough that a contractor can build it and an inspector can verify it. The pad edge is the most critical transition on the site. The engineer details the curb, thickened edge, or berm that keeps leachate on the pad and stormwater off it, with the grades on both sides falling away from the line. Equipment crossings — where loaders and trucks cross from clean areas onto the pad — get wheel-wash or stabilized entrances where the permit requires them, and the engineer grades the crossing so wash water and tracked material drain back to the pad side, not out to the road.

Open conveyance is designed for capacity, velocity, and maintenance together. The engineer sizes ditches for the design-storm peak with freeboard, checks the velocity against the erosion threshold for the lining — grass, riprap, or concrete depending on the velocity and the maintenance plan — and provides access for mowing and sediment removal. Culverts are sized for the storm plus debris, with headwalls that do not catch sediment and outlet protection that prevents scour. The engineer avoids flat grades that silt up and steep grades that erode; the workable window is narrow and the design lives in it. Detention is designed to the jurisdiction's standard — typically matching pre-development peak flows for the design storms — with the outlet structure as the precision element: orifices and weirs sized for the release rates, trash racks that do not blind, and a drawdown time that empties the pond between storms. The engineer details the pond with a forebay for sediment, a maintenance access ramp, and a vegetated or armored embankment, and locates the sampling points for the stormwater permit where they capture the true discharge. On county sites the engineer confirms whether the county's grading ordinance adds requirements beyond the state permit.

Use this checklist before the runoff design goes to the agencies: • Watersheds delineated: contact, clean, and off-site, each with its defined outlet • Perimeter interception sized for the design storm on the upgradient watershed • Pad-edge transitions detailed to keep leachate in and stormwater out at every crossing • Ditches and swales sized for peak flow with velocity-checked erosion protection • Culverts sized for storm plus debris with headwalls and outlet scour protection • Detention sized to the jurisdiction's flow-control standard with maintainable outlets • Stormwater sampling points located for representative discharge monitoring • Maintenance access provided to every ditch, culvert, pond, and structure