Innovative Stormwater Management

Challenge & Solution

This project focused on a cell tower site within an extremely sensitive watershed in the City of Lancaster, Pennsylvania. City regulations require the post-development peak runoff be no more than 50 percent of the pre-development peak runoff, unless it can be shown that the volume of post-development flow is no greater than the volume of pre-development flow for all design storms.

In addition, a concern with this site is its karst subsurface, i.e., its potential for sinkhole development. With karst bedrock relatively close to the surface, infiltration water could not be concentrated, a situation our design avoided. For the cell tower site, Rhea, with the support of a local engineer, designed an innovative grass permeable pavement that improved stormwater infiltration over a large majority of the site. Both stormwater runoff peaks and volumes were improved from pre-development values.

Project Description & Highlights

The planned increase in impermeable area for the site was minimal, i.e., less than 1,000 square feet. After reviewing the City of Lancaster Stormwater Ordinance, Rhea determined that the best course of action for this site would be to seek an exemption from the requirements of a formal Drainage Plan. In order to receive an exemption, the total impermeable area for the site cannot exceed 1,000 square feet, and the development stormwater discharge must not negatively impact surrounding properties or the City of Lancaster stormwater management infrastructure. The developer’s planned construction and our design met the exemption criteria.

Rhea prepared a letter report describing our Basis of Design and requesting an exemption from the more rigorous Drainage Plan submittal. Accompanying our report were the stormwater calculations performed, AutoCAD construction drawings, and a performance specification for permeable grass pavement installation. Approval of the permeable grass pavement design and the requested exemption were granted, and construction of the Cell Tower is complete.

Rhea’s creative use of permeable grass pavement saved over $50,000 in stormwater design and construction costs for the cell tower site developer. 


North Carolina