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Commission signals support for larger detention basins: staff proposes holding 500‑year runoff, not street capacity
Summary
Public Works recommended changing county standards so on-site basins retain runoff equivalent to a 500‑year event (about 6.3 inches in 24 hours), while streets remain sized to the 100‑year event; commissioners and the public pressed staff to return with additional water‑quality and percolation‑well guidance.
San Benito County Public Works presented a technical briefing on April 16 proposing that the county revise its subdivision and drainage standards to require detention/retention basins sized to hold the runoff from a 500‑year storm event rather than the current 100‑year standard.
Steve Loop, county engineer, said the typical local design uses underground pipes sized to carry 5‑ or 10‑year storms and streets that accommodate a 100‑year event (roughly four inches of rain in 24 hours in most of the county). He told commissioners the draft code would continue to design streets for the 100‑year event but would require larger basins—deeper and/or wider—to retain the additional runoff so it does not leave the development site during a 500‑year event (about 6.3 inches in 24 hours). Loop noted that basins have practical spill points and cannot hold infinite volume: beyond the 500‑year magnitude heavier runoff will overflow.
Commissioners and the lone in‑chamber speaker, Annette Perez, endorsed stronger on‑site retention. Perez said, “I am really glad that you guys are looking at this drainage issue... I see so many homes that are flooded in lots of places,” and asked the county to require developers to do a better job with drainage. Commissioners asked staff to bring a separate, agendized discussion on percolation wells and water‑quality treatments; Loop agreed that percolation‑well and water‑quality standards required more detailed review and should be handled in a follow‑up meeting.
Public Works presented a list of code sections that currently reference the 100‑year requirement and proposed striking “100” and replacing it with “500” in those places for basins and channels; staff noted streets and pipes would remain sized to standard practice. The commission voiced support and asked staff to return with draft code language and the planned separate presentation on percolation wells and water‑quality measures.
