At the Shandaken town hall on January 25, Mark Carabetta, senior project manager for engineering firm Milone and MacBroom, reported on his company’s findings regarding the Local Flood Analysis (LFA) performed for Phoenicia and Mount Tremper. The project, which began in the fall of 2014, was funded by New York City to evaluate the feasibility and effectiveness of various flood control measures, including dredging, floodplain modification, removal of sediment bars, replacing bridges, and relocation of buildings in the floodplain.
Using FEMA flood data for the Esopus and Stony Clove Creeks, feedback from local residents, and computer modeling, Milone and MacBroom made a cost-benefit analysis for a series of proposed projects. In both locations, the most cost-effective measures involved floodplain modification — that is, sculpting the areas alongside the streams to make more space for floodwaters. Replacing adjacent bridges would greatly increase the effectiveness of such changes but would be prohibitively costly, unless the replacement occurred as part of some other government program.
Floodplain enhancement has the additional benefit of providing a parklike area or walkway that can be used when there’s no flooding. In the case of the stretch of the Esopus Creek behind Phoenicia’s Main Street, it would be necessary to relocate a few buildings that are close to the stream, but the action would preserve other houses. However, the Bridge Street bridge would have to be replaced with a wider structure in order to make the streamside modifications effective, since the bridge causes constriction of flow and buildup of floodwater upstream.
Tom Rinaldo, one of two to three dozen audience members, observed that the Route 28 designation as a Scenic Byway makes more funding accessible for roadside and recreation enhancements, which could possibly contribute to paying for the floodplain changes. Town supervisor Rob Stanley said the LFA study was undertaken because it’s required by the Catskill Watershed Corporation to open up funding sources through them. He also pointed out that the New York State Department of Transportation is preparing to replace two bridges in Shandaken this year, and the Mount Tremper bridge may be due to be replaced soon, which would improve the cost-benefit ratio for flood mitigation.
In Mount Tremper, where damage to homes during Hurricane Irene was severe, floodplain enhancement would require extensive excavation, removal of the disused Mount Pleasant bridge, and relocation of 14 homes. Modeled with and without replacement of the Route 28 bridge, the study predicted a significant reduction in water height for a ten-year flood and even more for a 100-year flood.
Dredging: Effective or not?
Also modeled at the location were various ways of reconfiguring the levee along the creek, as well as construction of a diversionary flood chute, but these measures all proved more harmful than helpful. Creating a floodplain bench (a small-scale streamside excavation) along the Beaverkill and replacing the Plank Road bridge both were predicted to be not cost-effective.
Dredging the creek bottom at both Phoenicia and Mount Tremper, said Carabetta, would produce a relatively small effect, which decreases as the size of the flood increases, unlike floodplain enhancement, which provides more space for high floodwaters. Drawbacks to dredging include the need to repeat the action every ten years, as well as the potential to cause unstable channel conditions, which could create problems upstream and downstream of the dredging areas. Removing sediment deposition bars would have a negligible effect, according to computer models.
Several audience members reacted to the news that dredging was not recommended, insisting that historically, dredging has been effective as a flood prevention measure. Carabetta said he’d canoed the creek to identify gravel bars, and Rick Ricciardella asked how many times he’d had to take the boat out of the water to get around shallow spots. Carabetta agreed there had been several portages. “There’s a lot of aggregation in the channel,” said Ricciardella. “Across from Uncle Pete’s, now there are trees growing there.”
Stanley said the removal of gravel bars actually increases the velocity of floodwaters and worsens damage downstream. “The benefits are localized right next to the bar,” he said. “For the overall flood reduction, there’s some, but compared to the cost, it’s not an overall positive benefit.”
Several people observed that dredging along the Stony Clove at Phoenicia’s Main Street appeared to be effective, preventing flooding when Tropical Storm Lee hit, ten days after Hurricane Irene. Stanley said the dredging there had been combined with a carefully engineered reconfiguration of the channel, and the New York State Department of Environmental Conservation (DEC) had given in to the modifications because of the emergency nature of the situation. “It’s being monitored,” he added. “We haven’t had much of a flood since Irene. So far it’s maintaining its cross-section without dropping out sediment. It shows promise, but until it proves itself, we can’t move forward. No one is giving us funding to do repetitive action in the stream bed. The DEC will talk about degradation of the fishery.”
Town board members were given copies of the Milone and MacBroom report. Stanley said their comments would be solicited in the next couple weeks so the report can be finalized by March. Once it’s accepted by the town board, funding sources can be sought. He will bring the report to the Shandaken Area Flood Assessment and Remediation Initiative (SAFARI) for its recommendations, and the community will also have a chance to give further input.