County staff outline MS4 stormwater requirements, post-construction costs and low-impact options; staff also highlights beaver-based restoration work

County staff described the scope of Benton County’s municipal separate storm sewer system (MS4) obligations and what landowners and developers must do when construction or new impermeable surfaces exceed regulatory thresholds.

A staff presenter said the county implements phase 2 MS4 controls countywide to avoid mismatched rules along the MS4 boundary and to align stormwater and TMDL (total maximum daily load) programs. The presentation said MS4 implementation is driven by federal EPA requirements and administered by the Oregon Department of Environmental Quality (DEQ).

What triggers permits and key thresholds Staff said the county’s construction-site erosion-and-sediment-control permit is required for ground disturbance of more than a quarter acre (0.25 acre). Post-construction stormwater-management requirements also use a 0.25-acre trigger measured as new or expanded impervious surface (asphalt, concrete, patios, pools). The county fee for an erosion-and-sediment control permit was cited at about $760.

Costs and compliance implications Presenters told commissioners that meeting post-construction requirements often requires an engineered hydrologic analysis and long-term…