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Orange County moves to update stormwater design after consultants find rising short‑duration rainfall intensity
Summary
County staff and consultants told the Board of County Commissioners that short, intense rainstorms have become more frequent since about 2010 and recommended updating the intensity curves used to size pipes and drainage infrastructure; commissioners directed staff to draft ordinance changes and continue cross‑departmental resiliency work.
Orange County commissioners on April 22 directed staff to begin updating the county's stormwater design guidance after consultants said short, intense rainfall events have increased in frequency since about 2010, producing higher peak intensities that can overwhelm drainage pipes and rights of way.
The county's stormwater manager, Mike Drosik, introduced a study by CDM Smith that compared Orange County rainfall records, Water Management District gauges and NOAA datasets. Mitchell Heinemann, CDM Smith’s rainfall analyst, told the board the county's current guidance — based on a 1994 Orange County table and earlier National Weather Service charts — underestimates the peak 15‑minute and other short‑duration intensities that show up in recent years.
The change matters because drainage design is driven by peak intensity: a pipe sized to an older intensity standard can be overwhelmed by a more intense storm even if the 24‑hour total is similar. Heinemann said much of the recent increase shows up in afternoon convective storms and cited NOAA’s Atlas 14 and national climate projections that indicate higher rainfall intensities through mid‑century.
County staff illustrated the effect with a McKinley Avenue retrofit example: using the higher intensity curve raised modeled peak flows in the same new pipe from roughly 10 cubic feet per second to about 16–17 cfs, which could require upsizing conveyance. The consultants said storage ponds are less sensitive to short‑term intensity changes because they are driven by total volume, but conveyance (pipes, culverts and roadside swales) is more likely to be undersized under the older curves.
Commissioners emphasized that rainfall is only one part of the flood problem. Commissioner Nicole Wilson and others pressed county staff to integrate the rainfall findings with parallel work on wetlands, low‑impact development and the county’s wetland code so that planning preserves natural storage as development continues. County staff said those efforts are already coordinated: the drainage manual update, a vulnerability analysis and wetland studies are being developed together.
After discussion the board directed staff to work with the county attorney to draft ordinance updates replacing the county’s existing intensity curves with the revised distribution recommended by the study, and to return with proposed code language, stakeholder briefings and public hearings. Staff also recommended a 10‑year recheck of the rainfall curves to capture continuing climate trends.
What happens next: staff will begin ordinance drafting, coordinate with environmental and planning divisions and present a proposed ordinance and stakeholder outreach plan for future board review. The board did not adopt a final code change on April 22; it gave direction to prepare the ordinance and to evaluate county projects against the updated intensity curves in the interim.
Ending: Commissioners praised the study and told staff to prioritize public engagement and alignment with ongoing resiliency and wetland protection work as the county moves from analysis to ordinance drafting.
