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Task force examines FEMA map implications and cautions on USGS StreamStats output
Summary
Members discussed whether the town would need a FEMA Letter of Map Revision (LOMR) after dam removal and reviewed limitations of the USGS StreamStats product; staff will seek written confirmation and estimate whether a LOMR would add about $50,000 to the budget.
Town of Concord task force members raised two technical questions: whether removing the impoundment will require a formal FEMA Letter of Map Revision (LOMR) and whether an online USGS product called StreamStats can reliably predict the post-removal channel path.
Why it matters: a required LOMR could add measurable cost and schedule implications; misunderstanding StreamStats output could mislead residents about likely channel location after removal.
FEMA mapping: Task force members said initial guidance from the Division of Ecological Restoration suggested a LOMR is typically not required for dam removals where the project is "run of river" and does not change the 100-year flood extent materially. Jeff reported that Gilead/VA (project contacts) will look further into the requirement and that a LOMR — if required — could cost on the order of $50,000 to complete because it requires hydrologic calculations and supporting documentation.
USGS StreamStats: The group reviewed a screen grab from USGS StreamStats that appeared to route the channel south of Scout Island. Jeff explained that StreamStats uses a digital-elevation-based "stream grid" and, for impounded (flat-water) areas, creates an artificial shortest path from inflow to outflow. He said that StreamStats is not intended to predict the natural, post-removal stream route inside an impoundment because the model does not incorporate bathymetry; instead it uses a shortest-path algorithm over a gridded elevation surface.
EA Engineering’s hydrology and previous alternatives analyses were cited as the more reliable sources for where the channel is likely to form; those reports indicate the post-removal flow path is unlikely to follow the StreamStats shortest-path output and, at least initially, will follow the channel path identified in the consultants’ reports.
Next steps: staff will seek written confirmation about the likelihood a LOMR is required and a cost estimate for that work. The task force will use EA Engineering’s bathymetry and hydrology findings — not StreamStats screen grabs alone — when preparing public-facing graphics.
Ending: Members agreed that StreamStats output should be treated cautiously and that any LOMR question needs a definitive, written answer from the appropriate technical or regulatory authority before the public presentation.
