Moodna Creek Watershed Flood Mitigation Assessment

Princeton Hydro performed a flood assessment and flood mitigation resiliency analysis for the communities within the Lower Moodna Creek Watershed (Cornwall, New Windsor, Cornwall-on-Hudson, and Woodbury) using funds from a 2016 grant program sponsored by the New England Interstate Waters Pollution Control Commission (NEIWPCC) and the NYSDEC-Hudson River Estuary Program (HREP).

Watershed Scope and Regional Significance

The greater Moodna Creek Watershed covers 180 square miles of eastern Orange County. To provide context, the watershed includes 22 municipalities and hundreds of smaller streams that flow into Moodna Creek and ultimately the Hudson River.

Flood Vulnerability and Historical Context

Many of the areas within both the Upper and Lower Moodna watersheds are highly prone to flooding during rainfall events. This vulnerability to high-water events was most clearly exposed between August 2011 and October 2012, when this region experienced three extreme weather events. In one year, Hurricane Irene, Tropical Storm Lee, and Hurricane Sandy caused significant flooding throughout the Moodna Creek Watershed, damaging several public facilities, roadways, and private properties. The Upper Moodna watershed was studied previously as part of the 2014 New York Rising Community Reconstruction Program.

Project Goals and Site Investigations

Our primary goal was to identify areas within the study area that have reoccurring flood issues, may be susceptible to damage from large rain events, and additional problems may surface as climate and watershed conditions change over time. Part of this goal was also to develop structural and non-structural strategies to mitigate flooding. In pursuit of this, Princeton Hydro staff conducted site investigations and reach assessments at sites with critical infrastructure and photo-documented existing structures, infrastructure, and channel and floodplain conditions. The team recorded signs of recent disturbances, man-made impacts, flooding, and floodplain constrictions. Furthermore, they conducted visual assessments to collect data on flow constrictions, areas of floodplain development, and areas of floodplain connection.

Hydrologic and Hydraulic (H&H) Modeling

Princeton Hydro modeled flooding within the watershed during normal rain events, extreme rain events, and future rain events due to climate change. The project assessed the facilities, infrastructure, and urban development that are at risk from flooding along Moodna Creek and its tributaries and developed a series of hydrologic and hydraulic (H&H) models to assess the extent of potential flooding from the 10-year, 100-year, and 500-year storm recurrence intervals. Specifically, the modeling included flows for these storm events under existing conditions and hypothetical scenarios with predicted increases in precipitation and population growth.

Mitigation Strategies and Implementation Roadmap

The project team proposed and evaluated a series of design measures to help reduce and mitigate existing and anticipated flood risk. The proposed solutions prioritized approaches that protect and/or mirror natural flood protection mechanisms within the watershed such as floodplain reconnection and wetland establishment. Overall, the analyses and recommendations, in conjunction with the modeling and conclusions developed for the Upper Moodna, resulted in a fully developed flood assessment master plan and flood mitigation plan that can serve as a roadmap for reducing flooding issues within the entire Moodna Creek Watershed.

Watershed Analysis Key Data

• Study Region: 180 square miles of eastern Orange County.
• Impacted Communities: Cornwall, New Windsor, Cornwall-on-Hudson, and Woodbury.
• Historical Storms: Hurricane Irene, Tropical Storm Lee, and Hurricane Sandy.
• Recurrence Intervals Modeled: 10-year, 100-year, and 500-year storms.
• Core Services: Floodplain & Watershed Modeling and H&H Analysis.