County staff outline flood-management modeling, propose public watershed maps and targeted upstream projects

Montgomery County Department of Environmental Protection staff updated the Transportation and Environment Committee on the countywide comprehensive flood management plan and related CIP amendments on March 17, saying the project will expand watershed modeling, update precipitation assumptions to reflect climate projections, and move toward a public web map of modeled flood depths.

Stan Edwards, chief of the Division of Energy, Climate Compliance and DEP, said the plans current phase is modeling detailed watersheds, beginning with Sligo Creek as a pilot. "We're doing 11 watersheds. They should be modeled by the end of 2027," Edwards said, and staff will create models for design storm events from 10-year to 500-year events and project conditions through 2100.

Edwards described updates to precipitation assumptions using techniques similar to NOAAs methods but extending projections to 2100. The countys preliminary analysis shows higher projected rainfall for the 24-hour 100-year event than the historic baseline; staff summarized that projections could increase the 100-year, 24-hour rainfall from roughly 8.2 inches historically to higher values in mid- and late-century scenarios.

Amy Stevens, division chief for the watershed restoration division, described several small upstream "bioretention" projects that the executive recommended adding to the stormwater CIP, including a partnership program called "Congregations for Clean Water" (projects at Temple Emanuel and Saint Stephen Lutheran Church were cited) and a reforestation site identified by a pay-for-performance contractor. Stevens said those projects treat runoff upstream of stream restoration work.

Staff showed an excerpt of the Sligo Creek model that displays shallow to deeper inundation levels by color; Edwards said the county will prepare explanatory materials so residents understand modeled depths before maps are published. "We want to make sure we're explaining what we're showing here," he said, and staff indicated a web-based tool for public access is anticipated "probably this summer" as models for additional watersheds are completed.

Committee members asked how modeling inputs are coordinated with NOAA updates and how the county will address increased impervious area from growth. Edwards said NOAA (Atlas 14) provides national intensity-duration-frequency data but that the county is using updated local analysis and longer-term climate projections specific to Montgomery County. He also said the county modeled a "reasonable worst case" for future imperviousness based on current zoning and separately considered focused growth around transit corridors.

On policy implications, Edwards told the committee the work will lead to recommendations on planning and building-code changes and on how long-range development should mitigate flood risk. He warned some recommendations will be controversial because they raise tradeoffs about who pays for protection and how to treat redevelopment in higher-risk areas. "Those are controversial subjects, as you can imagine," he said.

The committee approved the technical CIP correction for storm drains and supported advancing targeted, upstream stormwater projects; members asked DEP to return with more detailed planning and public-engagement timelines.