Carroll County closes public hearing on Piney Run watershed study after engineers outline potential $11 million fixes to auxiliary spillway

Carroll County commissioners closed a public hearing on April 3 after county staff presented the final Piney Run watershed study, which recommends widening and armoring the dam's auxiliary (emergency) spillway and installing a cutoff wall to reduce the risk that a rare, extreme storm could erode the spillway and cause dam failure.

The study, funded by the U.S. Natural Resources Conservation Service (NRCS) and performed by consultant AECOM, examined hydrology, hydraulics, geotechnical borings, sedimentation and cultural resources at the Piney Run Reservoir and dam. County staff said the dam is classified as high hazard and must safely pass a probable maximum flood (PMF) — modeled in the study as roughly 39 inches of rain in 72 hours — without water flowing over the dam embankment itself.

"If that were to catastrophically let loose, you can see just downstream is Maryland 32, and then also, Springfield Hospital and the residences and so on down there," a county staff member said while reviewing aerial photos and cross-sections. The presentation noted the spillway currently could allow overflow to reach the dam crest by 3 to 4 feet during a PMF event.

Why it matters: Piney Run Dam is the county's largest dam. The structure is roughly 73 feet tall and 600 feet long; the reservoir is about 290 acres and holds about 1.7 billion gallons of water. A failure that sent a large volume of water downstream could threaten roadways, critical facilities and homes, county staff said — a rationale for the high-hazard classification and strict regulatory standards.

The study found two principal concerns raised earlier by the Maryland Department of the Environment (MDE): (1) whether the auxiliary spillway has the hydraulic capacity to pass a PMF and (2) whether the earthen spillway and underlying materials are readily erodible. Geotechnical borings in the spillway found materials that could be susceptible to erosion, the consultant reported.

Recommended mitigation measures are conceptually straightforward but costly. County staff summarized them as widening the auxiliary spillway so that modelled PMF flows are routed through the spillway rather than over the dam, armoring the downstream end of the existing spillway to resist erosion, and constructing a buried concrete cutoff wall across the spillway to stop any headward erosion before it reaches the reservoir. The county estimated design, permitting and construction at about $11,000,000.

Funding and schedule: The study and initial steps are funded through an NRCS grant; county staff said NRCS programs could potentially cover about 65% of the cost for design and construction if the county is successful in future NRCS funding rounds. County staff estimated design and permitting would take at least two years and placed potential construction in fiscal 2028 at the earliest. MDE had asked for mitigation by 2027 but county staff said MDE is allowing more time provided the county continues to make progress.

There were no public comments at the hearing. The board voted to close the hearing by voice vote. County staff said the full watershed study (about 650 pages) is posted on the county website and that public comments will be accepted through April 13 before the county completes the NRCS grant application for engineering design and permitting.

Next steps: County staff will finalize the public-comment period, close out the watershed study, and apply to NRCS for funding to design and permit the spillway and erosion mitigation measures. If NRCS funding is approved, the county expects multi-year design and construction steps to follow.

Ending: Commissioners did not make any immediate construction commitments at the hearing; the county will return with grant applications, design work and permitting documents as the NRCS process proceeds.