Will removal of the dam cause increased flooding to downstream areas, including downtown Westerly?

An explanation of why downstream flood risks will not increase following removal of Potter Hill Dam was provided during the June 10 public information meeting.

In summary, the Potter Hill Dam is a “run-of-the-river” dam that provides negligible flood storage as the river’s normal water surface elevation upstream of the dam is at or slightly greater than the elevation of the dam’s spillway crest (depending on the time of year). Unlike “flood control” dams that have storage capacity to hold additional water during a flood event, for “run-of-the-river” dams, the additional water from precipitation is not held back and continues to spill over the dam (and adjacent land areas depending on the height of water during the flood).

As a result, peak flood flows discharged downstream of the dam subsequent to its removal will be equivalent to or less than those currently (with the dam in place) discharged to downstream areas. In fact, lowering normal flow elevations upstream of the dam will increase available temporary storage within the river channel and restored floodplain upstream of the dam that is no longer impounded during normal flows. As a result, there will not be any increased flood risks to downstream properties, and an incremental decrease in flood risk.

The above findings have been confirmed by detailed hydraulic (HEC-RAS) modeling for this project (and other dam removal projects on the Pawcatuck River, including at White Rock Dam). These analyses use methods that have been approved, and are required by, the Federal Emergency Management Agency (FEMA), the federal agency responsible for developing and issuing flood maps for all regions of the United States. Following removal of the White Rock Dam, HEC-RAS modeling files and as-built survey information were provided to the FEMA team (which was led by USGS) that was at the time completing a study to update Flood Insurance Rate Maps (FIRMs) for southern Rhode Island.

In addition to the above consideration of downstream flood risks during storm events, removal of the dam avoids any scenario where a breach would release a flood wave from the impoundment downstream, causing inundation of downstream properties and infrastructure.

Show All Answers

1. Why is the dam being removed?
2. Could the dam be repaired or partially removed?
3. Could a design similar to that of the Bradford fish passage project be used for the Potter Hill project?
4. What alternative designs have been considered and why was the proposed design selected?
5. What safety hazards are associated with the mill and dam structures?
6. How will removing the dam affect water levels in the river?
7. Will my household well be affected?
8. Many properties along the Pawcatuck River have private drinking water wells, especially on the Hopkinton side of the river. How many homeowners’ wells could be affected by the predicted drop
9. Will Westerly’s municipal water wells adjacent to the Pawcatuck River be affected?
10. How will removing the dam reduce flood risk to upstream and downstream properties? Will larger flood events (i.e., the 500-year recurrence flood) be worse upstream or downstream following dam removal?
11. Will removal of the dam cause increased flooding to downstream areas, including downtown Westerly?
12. How will removing the defunct dam and mill affect public access to and uses of the river?
13. How will removing the dam affect fish and wildlife?
14. How will removing the dam affect wetland systems upstream of the dam?
15. Is the proposed design accounting for climate change projections and worst-case future hydrologic scenarios?
16. Who is the project team?
17. What are the next steps?
18. Where can I get more information on the project?