What is Post-Construction Runoff?

Post-construction runoff is runoff from areas that have just undergone construction and the new development has disturbed one acre or more of land within a regulated SM4 or SMS4. Regulation of runoff from post-construction is important for two reasons:

1. Runoff from roofs, parking lots, streets and driveways can pick up oil and grease, metals, pesticides and nutrients.
2. With the increase in impervious surfaces, the quantity of runoff and the peak discharges will measurably increase as the times to peak flow are reduced. This can lead to increased erosion on the parcels as well as within the receiving channels, loss of aquatic life, and increased flooding downstream.

The County addresses this second concern by requiring the construction of detention ponds to lower the peak discharges to pre-construction rates. Detention ponds will alleviate some of the sediment transport and associated chemical load, and will attenuate the peak discharge, but they won’t treat the dissolved chemical load, oil and grease and that portion of the metals load that does get through on the sediment that isn’t trapped in the pond.

Best Management Practices

The County endorses best management practices (BMPs) that combine both structural and non-structural controls:

• Structural controls include:
  • Storage or detention ponds such as wet ponds, dry basin, and multi-chamber basins
  • Infiltration practices such as infiltration basins and trenches, dry wells, and porous pavement
  • Vegetative practices such as grassy swales, vegetative filter strips, artificial wetlands, and rain gardens.
• Non-Structural controls include:
  • Master plan and zoning ordinance controls that guide community growth away from sensitive areas
  • Site based controls such as buffer strips, riparian zones, minimizing disturbance and impervious areas and maximizing open space.

Post-construction runoff and pollution control is one of the 3 program elements that require enforcement on the part of Coconino County.