In the Santa Clara Valley, development projects must comply with the Program's NPDES Permit. Unless the project is exempted, hydromodification controls are required in accordance with Permit Provision C.3.f. The requirements include the following:

• Increases in runoff peak flow, volume, and duration shall be managed for all Group 1 Projects (projects that add or replace greater than or equal to 1 acre of impervious surface).
  
• Post-project runoff shall not exceed estimated pre-project rates and durations.
  
• These conditions apply to areas where such increases in runoff flow or volume can cause increased erosion of creek beds and banks.

Projects That May Be Exempt

Projects that meet the following criteria are exempt from HMP requirements.

• Projects that do not meet Group 1 definition;
  
• Individual single family homes that are not part of a larger plan of development;
  
• Projects that drain into tidally influenced areas, directly to the Bay, or to channels continuously hardened to the Bay or the Sunnyvale East or West Channels;
  
• Infill projects in highly developed subwatersheds (i.e. those that are 90 percent or more built out)

Plan for Santa Clara Valley

To help local agencies and developers meet the requirements, the Program prepared a Hydromodification Management Plan (HMP) which was adopted by the Water Board in 2005. The HMP delineates areas where increases in runoff are most likely to impact channel health and water quality and provides management options for maintaining pre-project runoff patterns. The HMP is not designed to correct existing erosion problems, but to prevent worsening of creek channel erosion problems from new or redevelopment projects.