Port Angeles, Washington, transformed its East City Hall parking lot with low-impact design strategies, proving that sustainable stormwater solutions are possible even in challenging soil conditions.
By Vince McIntyre
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Municipalities today face the dual challenge of maintaining and replacing aging hardscape while meeting current stormwater management and sustainability goals. To find a sustainable design for the re-development of its East City Hall parking lot, the city of Port Angeles, Washington, looked beyond common roadblocks to low-impact solutions and devised a workable solution that responsibly manages stormwater runoff from the facility and serves as an example for local developers faced with similar challenges.
Setting the stage
Known for its breathtaking natural beauty, the Olympic Peninsula stands out as one of the most desirable places to live and visit in the continental United States. Every year, millions of people flock to Washington State’s far northwest corner to enjoy its stunning landscapes, abundant wildlife, and unmatched recreational opportunities. Against this backdrop, local regulators are focused on preserving the region’s unique environment by requiring low-impact development (LID) as the preferred approach to site development.
LID is a land development strategy that minimizes disturbance and incorporates onsite stormwater management into the overall design of a project. Stormwater is the leading cause of urban water pollution in Washington State, and in response, the state has long promoted onsite stormwater management and developed LID strategies to manage runoff. Common LID techniques include permeable pavements, vegetative roofs, and bioretention ponds or rain gardens. The goal of LID is to reduce negative effects downstream by incorporating ways to collect, control, and treat runoff in a project so that the developed site hydraulically resembles its pre-developed condition.
Most often, LID relies heavily on the ability to infiltrate stormwater runoff back into the ground. In doing so, native soils provide treatment of the runoff, ground water is recharged, and landscaped depressions provide flow attenuation and detention. Additionally, plantings can be aesthetically pleasing while offering shade and providing habitat. For these reasons, LID is the preferred approach to development in Western Washington. However, it is not uncommon for projects in Port Angeles to be completed without the installation of any LID features.
A major factor controlling the feasibility of infiltration is native soil infiltration rates. In Port Angeles, the predominant soil types across the region — compacted glacial till and clay — do not readily facilitate infiltration. As a result, LID is often deemed categorically infeasible for new and re-development projects in the area. In such cases, developers have little recourse and often default back to traditional designs of simply collecting and conveying runoff downstream to the nearest waterbody. If this waterbody is a tributary to a salmon-bearing stream, some developers are required to install expensive engineered treatment and detention systems that can make a project cost prohibitive.
Project origination
In 2015, the city of Port Angeles set out to overcome common LID barriers and began developing a plan to showcase a range of LID techniques in a parking lot retrofit project located at Port Angeles City Hall. Because of the project's location at a community gathering place, the city made it a key goal to help change the perception of LID infeasibility within the community and among local developers. The heavily used, 1-acre parking lot is located just upstream of Peabody Creek, a local salmon-bearing stream. Earmarked by the community for preservation and restoration, Peabody Creek would directly benefit from upstream stormwater management improvements.
Another project goal involved sourcing local tradesmen and materials without compromising on performance. During the design phase, specifications for material and certifications were tailored to the region, giving contractors options at a local level. These outcomes expanded local job opportunities, reduced material trucking costs, increased investment in the local economy, and improved repeatability by other local developers. This project also provided local contractors and subcontractors with valuable training and education regarding LID techniques.
To help fund the effort, the city sought and received a water quality grant for the project from the Washington State Department of Ecology. In 2019, the city began working on a design for construction with Herrera Environmental Consultants, Inc. Site constraints were first evaluated and, as anticipated, geotechnical borings proved infiltration to be infeasible within the soils below the parking lot. However, the design team embraced each roadblock to LID, viewing them as opportunities to motivate out-of-the-box thinking to find workable solutions.
Design details
After many iterations, the final design selected for construction entailed replacing the traditional parking lot surface with new porous hot mix asphalt having a depth of 3 inches. This asphalt was accented with permeable concrete-paver walkways and supported by 2-foot-deep permeable ballast rock and underdrains. Permeable ballast is hard, angular blast-rock that is sieved and washed to meet a specific size and gradation. The hardness and angled surfaces of the rocks cause them to lock together, providing structural support for the parking lot while also maintaining 30% void spaces available for stormwater storage.
Perforated underdrain pipe was laid out on the finished subgrade and connected to an engineered flow control structure. Designed to limit the rate at which water can leave the reservoir, the flow control structure thereby detains water in the aggregate void spaces and reduces the peak flow leaving the site during large storm events. The ballast rock also has a large, combined surface area that requires a reasonable amount of rainfall to fully wet before droplets coalesce and flow out of the facility. This surface area provides a measure of stormwater retention during routine storm events that was unanticipated during design.
Runoff from the parking lot fills and exits the storage reservoir, passes through the flow control structure, and enters a nearby landscaped depression, called a bioretention cell, where it receives treatment. Petroleum, tire materials, heavy metals, and other common stormwater pollutants are filtered out as runoff percolates down through the bioretention media, collects in an underdrain, and is conveyed downstream.
Depressed landscaped areas began replacing raised landscaped areas a number of years ago in medians and rights-of-way as engineers realized the same footprint can be used for flood mitigation. However, on this project, the stormwater entry point into the bioretention cell was brought back to the surface via a weathered steel channel where stormwater then overflows down onto a splash pad running along the bottom of the bioretention cell. The goal of this feature is to help the public visualize the stormwater management process and understand its importance as the reservoir drains slowly from the channel and onto the treatment cell, long after the rain event has passed.
Supplementing the design, a series of high-density polyethylene cisterns were added to intercept runoff from a large section of roof drain coming from City Hall and redirect it into the bioretention cell. The stormwater storage tanks help to mitigate peak flow downstream by spreading out the runoff release over a longer period of time. Stormwater collected by the tanks is used to irrigate native plantings in the bioretention cell as necessary and serves as a showcase for the city’s developing cistern rebate program — a way for the public to recoup installation costs after installing a similar setup to control their own roof runoff and thereby decreasing peak flow in the city’s stormwater conveyance system.
Construction and operation and maintenance
After 9 years of careful planning and incremental progress, the project finally broke ground on April 22, 2024. The construction contractor — Bruch & Bruch Construction Inc., of Port Angeles, Washington — completed the work 5 months later for a total cost of $1.54 million, just as the October rains began to fall.
The project team established an operations and maintenance program for the site at the end of the project to ensure long-term performance. However, the city makes adjustments as it continues to figure out how best to maintain the new facility. Traditional methods for managing snow and ice using sand and salt are not recommended for permeable pavements and can impair the ability of the bioretention cell to attract and retain heavy metals. Currently, the city is testing the use of an alternative liquid deicer made with sugar beet brine and deploying a propane weed burner on walkways during frosty mornings.
For routine maintenance, the city deploys a street sweeper with vacuum capabilities to pass through the parking lot once a month to remove any debris from the surface. However, a permeable pavement cleaning vehicle that also incorporates angled pressure washing and wash-water collection is needed to ensure long-term permeability. The city is working on a grant application to help offset the cost of a future equipment purchase.
LID’s practical potential
LID techniques for managing stormwater onsite are superior to typical detention and treatment vault systems in that they are able to infiltrate water into the subsurface and reduce runoff volumes. With this parking lot retrofit project, the city of Port Angeles was able to show how LID techniques can still be used for the purposes of stormwater treatment and detention despite the presence of poor infiltrative soils.
Additionally, LID techniques are often shallower and can be paired with landscaping features to provide aesthetic appeal and have educational opportunities. In today’s climate, developers must recognize and accommodate stormwater management as part of parking facilities and other projects, while recognizing that LID techniques can be viable solutions even in challenging locations.
VINCE MCINTYRE is the capital projects engineer for the city of Port Angeles, Washington. He can be reached at vmcintyr@cityofpa.us.
