Stormwater Management & Permitting
Challenge & Solution
Rhea redesigned (on a fast-track schedule) the following: 1) a stormwater conveyance system; 2) pre- and post-development hydrologic analysis; 3) infiltration bed design; and, 4) subsurface stormwater storage basins with controlled discharge per McCandless Township and Pennsylvania Department of Environmental Protection (PADEP) regulations.
The proposed church site plan underwent significant changes from initial concept to revised plan. After numerous meetings and discussions with McCandless Township, PADEP, the Client, and Rhea’s construction management consultants, the site was re-designed with multiple iterations using Bentley® InRoads® site and hydraulics modules, and HydroCAD and AES StormTech underground detention software packages.
Project Description & Highlights
Wetlands dominate west to east across the center swale portion of the site. Pre- and post-development discharges for 1-, 5-, 10-, 25- and 100- year storms were analyzed to determine headwater and tailwater impacts to wetlands and to divert all conveyance system flows away from these sensitive locations. Additionally, Rhea analyzed impacts to the controlling stormwater network exiting the site through coordination with PennDOT.
Rhea avoided costly and high maintenance stormwater detention pond installation with green engineering practices including underground infiltration beds for all roof runoff and underground detention for the remaining stormwater. Existing and proposed 3-D surfaces were developed and modeled in Rational and SCS TR-20 hydraulic methods. Upstream culvert and conveyance system design ensured adequate flow capacity for ultimate site build-out.
A comprehensive NPDES Permit application and PADEP Erosion and Sediment Pollution Control Plan, with corresponding Phasing Plan and Post-Construction Manual, were provided. Emphasis was placed on integrating underground storage units and endwall structure with baffles and rock apron for treatment, storage, and discharge from interceptor channels.
Rhea was chosen to perform stormwater design analysis for a proposed site development undergoing changes. Existing and proposed surface modeling was performed. Through value-added engineering review, Rhea redesigned the conveyance system- avoiding potential impacts to existing wetlands and designing an underground stormwater system modeled through five design storm conditions.