CGA served as the design engineer for the Grand Crossing Parking Lot in Waterloo, Iowa, a project distinguished by its full-site use of permeable interlocking concrete pavement (PICP). Working in close coordination with the City of Waterloo Engineering Department and the Iowa Economic Development Authority (IEDA) through the Iowa Green Streets Initiative, CGA prepared the design plans and specifications and provided construction-phase coordination with the prime contractor and subcontractors.
The project presented several engineering challenges, including high developed runoff rates compared to low existing runoff, shallow municipal storm infrastructure, and the presence of potentially contaminated soils. CGA addressed these constraints through an integrated stormwater strategy that combined bioswales, a permeable paver detention system, and impermeable liners to isolate contaminated areas. This approach allowed effective stormwater management while protecting environmental and regulatory concerns.
Site grading posed an additional challenge due to the inclusion of an underground parking garage, which raised the main building entrance significantly above existing grades. This condition caused portions of the parking lot to exceed the preferred slopes for PICP systems. Rather than increasing aggregate depth, CGA implemented internal fabric berms within the stone storage layer to reduce lateral stone movement and maintain infiltration performance into the subgrade.
Subsurface exploration and construction excavation confirmed the presence of contaminated soils in select areas. CGA responded with real-time design modifications, assisting in delineating the affected limits and incorporating liners to prevent infiltration in those zones, ensuring both environmental protection and system functionality.
At the time of design, CGA utilized a draft version of the SUDAS (Statewide Urban Design and Specifications) standards for interlocking pavers, as the standards had not yet been finalized. CGA collaborated closely with the IEDA’s design review representatives, refining the application of these evolving standards and positioning the project at the forefront of PICP implementation in Iowa.
The scale of the PICP system is a defining feature of the project, as permeable pavers were used across the entire parking area—an approach made possible through IEDA grant funding. The project exceeded typical Iowa Green Streets criteria, achieving near-complete on-site treatment and detention of stormwater through the combined use of PICP, bioswales, and bioretention ponds. The parking layout further enhanced durability and aesthetics by using contrasting paver colors instead of painted stall and accessibility markings.
Despite higher costs typically associated with PICP systems, CGA successfully designed the project within budget constraints through collaboration with the owner and IEDA. The result is a sustainable, visually appealing development that significantly reduces pollutants, peak runoff, and runoff volume. CGA is proud to have contributed to a project that exemplifies innovative stormwater management and supports the core principles of the Iowa Green Streets Initiative, with potential to serve as a model for zero-to-low runoff site design.
Download a PDF copy of the Grand Crossing Parking Lot project overview.
