By Sarah Gross
Matthew Elsasser picked up a piece of equipment that looks like a futuristic yellow sled and dipped it up and down in a slow rhythmic dance while making two full rotations.
“I’m mimicking the pitch and roll of the water to calibrate our equipment,” said Elsasser, environmental technician with the Susquehanna River Basin Commission. “My movement is based on the current conditions of the water. The choppier the water is, the more I move.”
This tool is actually a small boat that carries an Acoustic Doppler Profiler (ADP), which uses a sonar beam to measure the depth to the ground under the water surface.
This is the Commission’s first day on Swatara Creek, a tributary of the Susquehanna River in east central Pennsylvania [Lebanon, Dauphin counties], for a project in collaboration with the U.S. Army Corps of Engineers, Baltimore District, to provide information to the Federal Emergency Management Agency Region III that will help FEMA update their flood risk maps.
These maps will help the communities they serve to better understand their flood risks in order to take the appropriate actions to help lower that risk.
While the Baltimore District will coordinate surveying other waterways within the Mid-Atlantic region as part of a larger project for FEMA, the Swatara Creek study accounts for about 40 percent of this total project.
“Swatara Creek is a high priority for FEMA,” said Craig Thomas, North Atlantic Division regional technical specialist for floodplain management and Baltimore District environmental protection specialist. “An almost 500-year flood completely annihilated Hershey, Pennsylvania, and the surrounding area during Tropical Storm Lee in 2011.”
To complete the Swatara Creek portion of the study, the Corps is conducting on-land surveys, and the Commission is taking on the in-channel survey work.
“The river is so big, and we needed a boat and this specialized equipment to augment our capabilities,” said Thomas. “Luckily, the Commission was able to help, and we were able to figure out a way to provide funding to them, which will only open the door for future collaboration.”
The Corps is documenting surface elevations and bridge dimensions for about 60 bridges within the study area in order to determine potential flow restrictions and conduct modeling to simulate the extent and depth of potential flooding along Swatara Creek. Bridges are a crucial part of survey work the Corps performs because bridges constrict water movement and create potential bottlenecks for water to flow freely during flooding.
The Corps was on hand for the Commission’s first day in the field to provide operational support and to nail down shared project expectations. Luckily for the team, March 9, was an unseasonably warm day, as field work is preferable in the winter, so foliage doesn’t inhibit signals from the equipment.
The Corps team provided the Commission with ideal locations to collect data points from within the creek. These locations are between bridges, so as to not duplicate data the Corps is collecting.
The Commission will collect depth data from the ADP per river mile in the Swatara Creek, as well as one corresponding elevation reading along the bank per river mile using supplemental Real Time Kinematic (RTK) equipment.
The Corps will compile the on-land and in-channel survey data to determine potential flood extents and depths and delineate the floodplain and regulatory floodway boundaries in the watershed under various flood events.
To perform the first channel survey, Elsasser tied a rope anchored atop the survey boat to the calibrated tool, dropped it in the water and guided it back and forth six times across the width of the channel. The depth data was then logged within the ADP collection software and averaged to obtain a more accurate number.
“Multiple crosses allow interpolation between the bottom elevations collected,” said Kimberly Dagen, Commission environmental scientist. “This process helps smooth the data out to ensure a representative cross section of the channel is obtained.”
At the end of this first run, the team had completed data collection for one river mile along the Swatara Creek by lunchtime — leaving 51 more to go.
The Swatara Creek study is anticipated to wrap up in summer 2017, at which time data from the Corps and the Commission will be turned over to FEMA.
“We should get great modeling results from all of our data,” said Thomas. “We have a lot of historical data and high water marks, including data from Lee, to compare our current modeling to.”
This isn’t the first time the Corps has partnered with the Commission.
“This effort represents an extension of a long-standing partnership the Commission has with the Baltimore District to provide flood-hazard reduction strategies to communities in the Susquehanna River Basin, including flood inundation mapping, enhanced flood warning tools, and flood-damage reduction studies," said Benjamin Pratt, P.E., CFM, Commission water resources engineer.
The Commission is currently funding work for the Corps to conduct a Floodplain Management Services study for the Chiques Creek, also a tributary of the Susquehanna River in Lancaster County, as part of an effort funded by the Pennsylvania Department of Environmental Protection to evaluate “green” flood-hazard reduction strategies.
“We have a great collaboration with the Commission,” said Thomas. “They recognized our expertise in hydrologic and hydraulic modeling and mapping, and we performed all of the in-channel survey work for the Chiques study.”
Though not a flood risk management study, the Corps has also worked with the Commission to study stream flow at Bald Eagle Creek, which is a part of Foster Joseph Sayers Reservoir [at Bald Eagle State Park, Centre County] to evaluate the modification of reservoir operations to provide environmental enhancements.
“We have this specialized equipment for channel surveys, and we are always looking for more ways to use it,” said Pratt. “This particular collaboration on the Swatara Creek study started with a conversation between Craig and me while working on the Chiques Creek study. The opportunity just presented itself.”
(Photo: Kimberly Dagen, SRBC Environmental Scientist and Matthew Elsasser SRBC Environmental Technician.)
Related Story:(Reprinted from the U.S. Army Corps of Engineers (Baltimore District) website.)