Brandywine Flood Study Technical Compendium
Section 8
Analysis of Potentially Obstructive Infrastructure
April 2025
PREPARED BY Brandywine Conservancy Chester County Water Resources Authority University of Delaware Water Resources Center
Section 8 Analysis of Potentially Obstructive Infrastructure
8.1 Structural Evaluation Summary
A combination of hydrologic and hydraulic (H&H) analyses and FEMA Flood Insurance Study flood profiles were used to evaluate 269 bridges, culverts, and dams throughout the Brandywine watershed. The purpose of this evaluation was to determine which structures may be acting as obstructions during floods in ways that create negative impacts to surrounding communities and/or critical infrastructure. Structures that act as obstructions can cause upstream increases in water surface elevation, leading to backwater flooding in areas upstream of the structure itself. In these cases, removing or upgrading the structure may be necessary to reduce the localized flood risk.
Based on the available data, each of the analyzed structures was grouped into one of the following categories:
• Potentially obstructive with significant community impact potential (retrofit recommended)
• Potentially obstructive with minimal community impact potential (retrofit not recommended)
• Potentially obstructive with insufficient information to determine community impact potential (retrofit not recommended – pending additional data gathering)
• Not meaningfully obstructive (retrofit not recommended)
Figures 8.1 to 8.3 illustrate the locations of all analyzed structural locations, during the Brandywine Flood Study. Ultimately, the structural recommendations included in the report are for those bridges, culverts, and low head dams that were determined to be potentially obstructive with significant community impact potential. These determinations were made using the H&H modeling outputs, along with water surface elevation and inundation mapping results to assess the risk posed by backwater flooding to nearby residences, structures, and community infrastructure.
Left Photo: Low head dam and U. S. Route 1 Bridge in Chadds Ford Township and Pennsbury Township.
Right Photo: Bridge on U. S. Route 30/Lancaster Avenue over the East Branch Brandywine Creek in Downingtown Borough.
Figure 8.1 West Branch Brandywine Structural Analysis Results
Figure 8.1 West Branch Brandywine Structural Analysis Results
Figure 8.2 East Branch Brandywine Structural Analysis Results
Figure 8.2 East Branch Brandywine Structural Analysis Results
Figure 8.3 Main Stem Brandywine Creek Structural Analysis Results
Figure 8.3 Main Stem Brandywine Creek Structural Analysis Results
It is important to note that while a structure may act as an obstruction to flow during flood events it may not pose or exacerbate flood risks. In some cases, a hydraulically undersized bridge or culvert may back water up into undeveloped areas, offering an opportunity for flood storage and reduced discharge rates downstream. Removing or upgrading such structures would provide minimal flood mitigation benefits and may worsen downstream risks. Each structure should be evaluated further on an individual basis to determine the benefits and risks associated with any modifications. All results of this analysis are to be considered preliminary, and more detailed investigations should be conducted prior to any implementation.
8.2 H&H Modeling Results
This study utilized HEC-RAS models for the main stem, East Branch, and West Branch Brandywine Creek, as well as Beaver Creek. The models allowed for the analyses of all documented structures along these waterways to assess their impact on peak flood discharge and water surface elevations. These structures included bridges, culverts, and dams. With the exception of the watershed’s major flood control structures, the dams in question were predominantly legacy low head dams associated with historic mill activities in the region. Table 8.1 summarizes the results for structures analyzed using available H&H computer models, based on the difference in water surface elevation (WSE) upstream of the structure for the 50-yr, 100-yr, and 500-yr flood.
Bridge replacement on Route 322 in West Bradford Township and East Bradford Township. Photo courtesy of Chester County Planning Commission
Table 8.1 Results of H&H Model Analysis of Structures in the Brandywine
Watershed
Table 8.1 Results of H&H Model Analysis of Structures in the Brandywine Watershed
Table 8.1 (continued)
190
Table 8.1 (continued)
8.3 FEMA Flood Insurance Study (FIS) Flood Profile Review
The project team applied FEMA FIS flood profiles to evaluate potential obstructions to flood flows caused by existing bridges, culverts, and dams where HEC-RAS models were not readily available. As the flood profiles do not contain comprehensive information on potential obstructions (for example, bridge abutments), the Flood Study team did not make any determinations regarding community impact related to differences in water surface elevation. Table 8.2 summarizes the results for structures analyzed using FIS flood profiles, based on the difference in water surface elevation upstream of the structure for the 50-yr, 100-yr, and 500-yr flood.
Union Street Bridge in Modena along the West Branch Brandywine Creek
Table 8.2 Results of FIS Flood Profile Analysis of Structures in the Brandywine Watershed