Flooded Roadway and Bridge Crossing Mapping and Early Warning System for Selected Communities in Hunterdon County, New Jersey
This project seeks to develop methods to identify when a flooded roadway or bridge crossing condition may exist and to provide a tool for users to select rainfall scenarios in addition to the selected river condition to estimate if road flooding may occur with the selected combination. These tools will provide officials in Hunterdon County and at the state level of management better early warning information about potential road flooding locations. Identifying when and where road flooding may be occurring will provide early warning information and hopefully improve road closures for the protection of human life and property.
Background
Remnants of Hurricane Ida produced extremely heavy rainfall rates across central and north-central New Jersey on September 1-2, 2021. The resulting flooding caused severe damage to roads, bridges and infrastructure. Statewide, 30 deaths were connected to flooding with 6 fatalities in Hunterdon County alone. In addition to the deaths, hundreds of water rescues were performed for people and vehicles caught in torrential flooding. Many of the roadway locations are not connected to a National Weather Service (NWS) flood forecast, particularly in rural Hunterdon County.
Several stream reaches of concern were identified by New Jersey Office of Emergency Management and Hunterdon County officials. Most were small basins that do not have an active streamgage. 4 reaches were ultimately selected: Neshanic River, Pleasant Run, Swan Creek, and Hakihokake Creek. Neshanic River has an active U.S. Geological Survey gaging station in the vicinity of Reaville, NJ. The other 3 reaches do not have streamgages. The Neshanic and potentially other nearby gaged river reaches will be evaluated as an “index” reach for the remaining 3 study reaches.
Study Area
Four study reaches within Hunterdon County, New Jersey were selected for the project. These sites were chosen primarily based on impacts reported from the Hurricane Ida flooding event, as well as other recent flooding events. Neshanic River has an active U.S. Geological Survey gaging station in the vicinity of Reaville, NJ to the southeast of Flemington, NJ. Neshanic River passes into Somerset County before draining into the South Branch Raritan River. Pleasant Run is a small reach further to the northeast that also drains directly into the South Branch Raritan River. Hakihokake and Swan Creeks both drain towards the Delaware River through the municipalities of Milford and Lambertville, NJ, respectively.
Data collection and synthesis
Evaluation of each study reach will be accomplished by construction and calibration of hydraulic models and rainfall runoff models. Construction of the hydraulic models necessary for the project involves collection and evaluation of existing data. Some of these data include pre-existing legacy models, digital elevation data, road and bridge information, dimensions, and elevations, historical information on road flooding and closures, and precipitation data. Once assembled, the pre-existing data can be reviewed, and gaps identified.
Field data collection to fill gaps includes collection of ground-truth structure dimensions, channel bathymetry, ground-truth bank elevations to quality-assure digital elevation data as well as discrete discharge measurements for aid in calibration of hydraulic models. Recording of peak elevations within the 3 ungaged study reaches will be accomplished using crest-stage gages installed at key locations in each reach.
These data will be used to construct 2-dimensional hydraulic models of each study reach using the U.S Army Corps of Engineers (USACE) Hydraulic Engineering Center’s River Analysis System software, or HEC-RAS. The project is somewhat unique because it will attempt to utilize data from a continuous record streamgage like the Neshanic River at Reaville to develop a mathematical correlation with the ungaged study reaches along Pleasant Run, Swan Creek, and Hakihokake Creek. Using hydrographs synthesized from relationships developed between gaged and ungaged reaches, the hydraulic models will be calibrated with high-water marks recorded from crest-stage gages installed in these reaches, or in the case of the gaged basin, the recorded gage values for flow and gage-height.
Rainfall runoff models will be constructed using USACE Hydrologic Modeling Simulation (HMS) software. Several rainfall scenarios will be employed within each reach. These scenarios will be a series of “localized” rainfall intensity curves that will be used within the HMS models to generate hydrographs to be linked to the calibrated hydraulic models.
Evaluation of impact locations and display of inundation and impact layers.
Model performance will be evaluated and compared to existing road and bridge closure/impact data obtained from state and local officials. This will assist in refining the index to ungaged reach relationship. The completed flood inundation maps will be hosted using the New Jersey Flood Decision Support Toolbox (FDST) mapping application employing a custom interface for display of the rainfall scenarios “on top” of the selected river condition. Within the application, areas of increased critical hazards from both inundation depth and velocity on roadways will be highlighted to support early warning information and improve road closures for the protection of human life and property.
Planned Project Outcomes
In addition to the mapping application available on the New Jersey FDST, the inundation layers for each reach will be available via a USGS Sciencebase data release. A summary of findings as well as a full description of model development and calibration will be available within a USGS Scientific Investigations Report (SIR).
This project seeks to develop methods to identify when a flooded roadway or bridge crossing condition may exist and to provide a tool for users to select rainfall scenarios in addition to the selected river condition to estimate if road flooding may occur with the selected combination. These tools will provide officials in Hunterdon County and at the state level of management better early warning information about potential road flooding locations. Identifying when and where road flooding may be occurring will provide early warning information and hopefully improve road closures for the protection of human life and property.
Background
Remnants of Hurricane Ida produced extremely heavy rainfall rates across central and north-central New Jersey on September 1-2, 2021. The resulting flooding caused severe damage to roads, bridges and infrastructure. Statewide, 30 deaths were connected to flooding with 6 fatalities in Hunterdon County alone. In addition to the deaths, hundreds of water rescues were performed for people and vehicles caught in torrential flooding. Many of the roadway locations are not connected to a National Weather Service (NWS) flood forecast, particularly in rural Hunterdon County.
Several stream reaches of concern were identified by New Jersey Office of Emergency Management and Hunterdon County officials. Most were small basins that do not have an active streamgage. 4 reaches were ultimately selected: Neshanic River, Pleasant Run, Swan Creek, and Hakihokake Creek. Neshanic River has an active U.S. Geological Survey gaging station in the vicinity of Reaville, NJ. The other 3 reaches do not have streamgages. The Neshanic and potentially other nearby gaged river reaches will be evaluated as an “index” reach for the remaining 3 study reaches.
Study Area
Four study reaches within Hunterdon County, New Jersey were selected for the project. These sites were chosen primarily based on impacts reported from the Hurricane Ida flooding event, as well as other recent flooding events. Neshanic River has an active U.S. Geological Survey gaging station in the vicinity of Reaville, NJ to the southeast of Flemington, NJ. Neshanic River passes into Somerset County before draining into the South Branch Raritan River. Pleasant Run is a small reach further to the northeast that also drains directly into the South Branch Raritan River. Hakihokake and Swan Creeks both drain towards the Delaware River through the municipalities of Milford and Lambertville, NJ, respectively.
Data collection and synthesis
Evaluation of each study reach will be accomplished by construction and calibration of hydraulic models and rainfall runoff models. Construction of the hydraulic models necessary for the project involves collection and evaluation of existing data. Some of these data include pre-existing legacy models, digital elevation data, road and bridge information, dimensions, and elevations, historical information on road flooding and closures, and precipitation data. Once assembled, the pre-existing data can be reviewed, and gaps identified.
Field data collection to fill gaps includes collection of ground-truth structure dimensions, channel bathymetry, ground-truth bank elevations to quality-assure digital elevation data as well as discrete discharge measurements for aid in calibration of hydraulic models. Recording of peak elevations within the 3 ungaged study reaches will be accomplished using crest-stage gages installed at key locations in each reach.
These data will be used to construct 2-dimensional hydraulic models of each study reach using the U.S Army Corps of Engineers (USACE) Hydraulic Engineering Center’s River Analysis System software, or HEC-RAS. The project is somewhat unique because it will attempt to utilize data from a continuous record streamgage like the Neshanic River at Reaville to develop a mathematical correlation with the ungaged study reaches along Pleasant Run, Swan Creek, and Hakihokake Creek. Using hydrographs synthesized from relationships developed between gaged and ungaged reaches, the hydraulic models will be calibrated with high-water marks recorded from crest-stage gages installed in these reaches, or in the case of the gaged basin, the recorded gage values for flow and gage-height.
Rainfall runoff models will be constructed using USACE Hydrologic Modeling Simulation (HMS) software. Several rainfall scenarios will be employed within each reach. These scenarios will be a series of “localized” rainfall intensity curves that will be used within the HMS models to generate hydrographs to be linked to the calibrated hydraulic models.
Evaluation of impact locations and display of inundation and impact layers.
Model performance will be evaluated and compared to existing road and bridge closure/impact data obtained from state and local officials. This will assist in refining the index to ungaged reach relationship. The completed flood inundation maps will be hosted using the New Jersey Flood Decision Support Toolbox (FDST) mapping application employing a custom interface for display of the rainfall scenarios “on top” of the selected river condition. Within the application, areas of increased critical hazards from both inundation depth and velocity on roadways will be highlighted to support early warning information and improve road closures for the protection of human life and property.
Planned Project Outcomes
In addition to the mapping application available on the New Jersey FDST, the inundation layers for each reach will be available via a USGS Sciencebase data release. A summary of findings as well as a full description of model development and calibration will be available within a USGS Scientific Investigations Report (SIR).