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DEP outlines phased countywide flood management plan and updated rainfall projections
Summary
DEP told the Transportation and Environment Committee that Phase 1 of the comprehensive flood management plan is complete and Phase 2 is underway, using updated IDF rainfall data and hydrologic modeling for 11 priority watersheds; staff said mapping results and pilot studies should be available by late fall.
Montgomery County Department of Environmental Protection staff on Monday briefed the Transportation and Environment Committee on the county’s comprehensive flood management plan, describing a phased program of technical modeling, policy work and capital projects intended to address increasing urban and riverine flooding.
"We looked at which watershed should we do the detailed engineering studies in," said Stan Edwards, chief of the Division of Energy, Climate Compliance and DEP, describing Phase 1 results and the start of Phase 2 modeling. Edwards said the team identified 11 priority watersheds for detailed study that together represent about 101,000 acres — roughly 32 percent of county land area and about 64 percent of the population.
Staff emphasized updated rainfall and climate projections: DEP said it used rainfall observations through 2023 to update NOAA Atlas 14 baseline totals and then projected conditions to 2050 and 2100 using climate models. "A current 10‑year, 24‑hour storm in Rockville is 4.8 inches. In 2100 we expect it to be 7.3 inches," Edwards said, and noted increases across most recurrence intervals influence mapping and infrastructure sizing.
DEP described three mitigation categories it will evaluate — conveyance (moving water), storage (large detention or underground vaults) and property acquisition where no feasible structural fix exists. Edwards showed an Arlington example of a 4‑million‑gallon detention vault (cost cited at about $18 million) as a potential analog for heavily developed areas.
Helen Waller, a DEP planning specialist for flooding programs, discussed lot‑to‑lot (localized) surface runoff and acknowledged that many tools that address lot‑to‑lot flooding are limited under current rules; DEP plans clearer public communication, real‑time information and a homeowner portal describing mitigation options and potential funding.
Council members praised the equity focus in watershed selection and urged careful public communication to avoid undue alarm while offering homeowners practical mitigation steps. DEP said it will return with the Sligo Creek pilot results and Army Corps cost‑share studies later in the calendar year and produce neighborhood‑level maps and tools in the fall.
"Phase 2 is the detailed engineering assessments and should last through FY26," Edwards said. "Phase 3 is implementation of improvements and programmatic changes that will require additional budget and likely legislative action for some policy changes."
