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County advances flood-management planning; Sligo Creek pilot map to be published this summer
Summary
Montgomery County DEP described progress on a countywide flood-management plan that models 11 watersheds, with Sligo Creek as a pilot. The plan updates rainfall and impervious-area assumptions, will produce public web maps, and raises potential changes to planning and building-code approaches to reduce future flood risk.
Montgomery County Department of Environmental Protection officials told the Transportation and Environment Committee on March 17 that phase 2 of a comprehensive flood-management plan is underway and that watershed modeling for 11 watersheds should be complete by the end of 2027, starting with a Sligo Creek pilot.
The planning work will use updated precipitation and development assumptions, generate publicly accessible maps, and inform a later round of policy and capital decisions. DEP staff emphasized that the planning work is intended to identify where future flooding risk will rise with climate change and to develop project and policy responses that consider racial equity and social vulnerability.
Stan Edwards, chief of the division of energy, climate compliance and DEP’s lead on the plan, said the county updated precipitation assumptions to reflect recent gauge data and climate projections. He summarized the projected change for a 24-hour, 100-year event: “Right now, it's 8.2 inches in 24 hours. ... going up to 9.7 in 02/1950 and 11.9 in February,” Edwards said while reviewing the work’s long-range scenarios. Edwards noted the plan will model events from 10-year up to 500-year storms and that the county is projecting out several future time horizons to reflect long-lived infrastructure lifecycles.
DEP staff also described the methodology for estimating future impervious area, including a “reasonable worst case” based on maximum development under current zoning and additional increases within transit-focused growth corridors such as Silver Spring, Wheaton and along the Purple Line and BRT corridors.
Staff previewed a pilot visualization for Sligo Creek that shows modeled inundation depths in different colors and said they plan to publish an interactive web tool that allows residents to zoom to neighborhoods. “We think it's important to share what the model shows, but we don't want to cause anybody to panic about what that model shows,” Edwards said; DEP staff said the Sligo Creek map should be publicly available this summer and that maps for additional watersheds will follow as models are completed.
The committee and DEP discussed policy options the county might consider once modeling is complete. DEP listed potential regulatory or code approaches—such as base-floor elevations, protections for below-grade mechanical systems, and limits or mitigation on new impervious surface—that could be tied to the plan’s findings. Edwards and staff cautioned several of those topics would be controversial and require sustained public outreach and interdepartmental work.
Committee members asked about federal technical inputs; DEP staff said the county uses NOAA precipitation analysis methods (the Atlas product series) and FEMA riverine floodplain data as reference sources but is projecting beyond currently published federal updates to create county-specific time- and climate-forward models.
The committee supported the planning-focused CIP amendments to increase watershed modeling work and directed DEP to continue preparing outreach materials, including multilingual information and careful messaging, ahead of public map releases.
