NTM Engineering, in conjunction with SKELLY and LOY, Inc., modeled the hydrology and hydraulics (H&H) of the Borough of Carlisle’s storm sewer system to determine the cause of the drainage and flooding problems that disrupted traffic and caused economic damage. NTM performed data collection of the problem areas, used LiDAR data, surveyed storm sewers, provided GIS analyses, and modeled the watershed and storm sewer’s H&H. In addition to the flooding issue, water quality for NPDES compliance was a secondary concern. The project’s established procedures are customizable to any municipality that wants to create a solution for flooding, stormwater, and water quality problems.
The Letort Watershed research and study had several objectives:
- Determine the cause of the flooding problems and whether they were from storm sewer backwater from Letort Spring Run, inadequate storm sewer or inlet capacity, or surface runoff volume (or a combination thereof)
- Obtain detailed survey information of the Borough’s storm sewers
- Identify areas within the Borough or surrounding watersheds where stormwater management measures may be implemented to reduce flood flows to the problem areas
- Evaluate proposed drainage/stormwater management solutions, including alternative concepts, and provide recommendations for improvements where applicable
- Establish a working model that could also be used to analyze the runoff’s water quality
- Develop low impact development (LID) practices to improve water quality.
In order to evaluate the complex situation, data was imported into GIS and both the Watershed Modeling System (WMS) and the EPA stormwater management model (SWMM) to generate flows and compute storm sewer backwater conditions. This aided in the analysis of whether flooding was caused by excess runoff, inadequate inlet capacity, inadequate storm sewer capacity, or backwater from the Letort Spring Run and/or the storm sewers themselves. This also allowed for siting and analysis of potential LID scenarios and stormwater management facilities’ impact to reducing flood flows.