The following guidance information helps ensure that coastal development permit (CDP) projects are planned, designed, and managed to minimize both the transport of pollutants in runoff, and any adverse post-development changes in a site’s runoff flows.
CDP projects should incorporate appropriate stormwater Best Management Practices (BMPs), both during construction and post-development, to minimize potential adverse impacts to coastal resources from changes in the site’s runoff quality and runoff flow regime (i.e., volume, flow rate, timing, and duration). Stormwater BMPs include structural devices or systems, operational and maintenance procedures, and activities such as training.
Certain types of development – due to the development’s size, type of land use, or proximity to coastal waters – have a greater potential for generating polluted runoff or changes in runoff flows that may adversely impact coastal resources. The categories of “Developments of Water Quality Concern” will be identified by the permitting agency (i.e., local government or Coastal Commission). For Developments of Water Quality Concern, structural BMPs used to retain runoff on-site and/or to remove pollutants (i.e., “treat” runoff) should be designed using, at a minimum, the “85th percentile design storm” sizing standard, as follows:
BMPs shall be sized, designed, and managed to infiltrate, retain, or treat, at a minimum, the runoff produced by the 85th percentile 24-hour storm event for volume-based BMPs, or two times the 85th percentile 1-hour storm event for flow-based BMPs.The 85th percentile storm event for a particular location is the amount of rainfall equal to or greater than that produced by 85 percent of storms that have occurred in that locale over a specified duration (i.e., 24-hours or 1-hour), based upon long-term historical records of local storm events. This rainfall data is provided below.
The following two tables can be used to determine the rainfall amount for 85th percentile storm events at specific locations in California coastal counties. Use the data from a rain station nearest the project location, at a similar elevation.
Low Impact Development (LID) is an approach to stormwater management that aims to replicate the site’s natural hydrologic balance. LID emphasizes site design strategies that protect the site’s natural capacity to retain stormwater, integrated with small-scale distributed Best Management Practices (BMPs) that rely on infiltration, evapotranspiration, harvesting, detention, or retention of stormwater runoff. Coastal Commission Water Quality Program staff recommends giving precedence to an LID approach to stormwater management in all development.
As an alternative to the conventional approach of conveying a site’s runoff to the storm drain system, designing a project so that stormwater runoff infiltrates into the ground preserves the natural hydrologic conditions. Infiltrating runoff on-site helps to minimize the adverse impacts of increased runoff flows, minimize transport of pollutants, maintain stream base-flows, and recharge groundwater. Coastal Commission Water Quality Program staff recommends designing development to maintain or enhance on-site infiltration of runoff, where appropriate and feasible. Examples of design techniques include: