Hybrid Wetpond/Wetland at Yauger Park LID Stormwater Facility

The stormwater wetland/wetpond functioned as one of the primary treatment BMP’s for the incoming pollutants and management of flow control at the Yauger Park LID facility. The pallet of appropriate stormwater plants that were integrated into the design of this BMP helped increase biofiltration creating a larger storage than traditional designs. This technology was mainly chosen over the traditional stormwater pond, because it was previously shown to increase pollutant removal through biofiltration by plant life, the sizing considerations created a larger amount of runoff that could be handled, and the recreation and environmental benefits of creating a habitat for birds, insects, etc, at a community facility. In addition the city was able to meet the requirements of Low Impact Development that have been set forth by NPDES permit.

The codes stated in the chapter 9 and 10 of Volume V of the city stormwater manual were the local interpretations of the Department of Ecology SMMWW, and were the way by which both the bioswale and wetpond were successfully permitted. Further detail of the bioswale permitting is provided in the rain garden/bioswale case study. Design of the wetpond/wetland facilities conformed to Volume V chapter 10. The bioswale used standard geotextile fabric to perform. The wetpond on request by the city of non-PVC fabric integrated HDPE geomembrane fabric to hold water year round.

The SMMWW was used as the overarching design of stormwater facility and was the primary construction and design guidelines on the wetland portion of the BMP. Prescriptive thresholds were set and triggered such as a renovation requirements, as stated in the Minimum Technical Requirements. Plant selections suggestion and educational requirements were met through a direct following of the codes. Soil depths and post construction soil quality were prescriptively met for both the city and state minimum requirements. Measures for temporary erosion and sediment control for the construction of the facility were implemented under the Construction Stormwater General Permit. Examples included fence barriers and removal of wheel wash from vehicles.

Lastly, the limitations on the loading of downstream Percival Creek as it functioned as a 303d waterway, from the State Environmental Protection Act (SEPA) required that volumes of runoff and pollutants would not increase. A SEPA Determination of Non-Significance (DNS) was issued by the City of Olympia acting as Lead Agency.

By employing Best Management Practices (BMP) to demonstrate the effectiveness of innovative biofiltration techniques and incorporating these renovations of a regional stormwater facility in SW Olympia the city was able to manage the stormwater runoff from a 500 acre basin.  Since the project was a retrofit project not all of the technical requirements outlined in the Stormwater Management Manual for Western Washington Minimum needed to met. However as a large facility that also functions as flow control and treatment for runoff tributary to Percival Creek , the city voluntarily took on the requirements of on-site Stormwater Management.

When using this specific technology of wetpond, updated BMP’s from the 2005 SMMWW functioned as proven technology by the Department of Ecology, and led the specific path by which wetland/wet pond facility was created and functions. As long as the standards for each BMP were met the different technologies, could be incorporated to maximize flow control and treatment. Bioretention swales needed to show that  flow velocity would be greatly reduced so weirs were installed. Flow velocities calculations by David Consulting Firm showed  that the average velocity in the main cells of bioretention swale had reduced the initial velocity  of at it’s highest of 1.76 ft/s  to  0.84 ft/s using the ‘Clear Creek Solutions Model’. From this the size and shape of the wetland facility could be extrapolated and demonstrates how each technology is both independent and dependent on technologies surrounding it.

Another permitting technicality was the stormwater wet pond/wetland facility was a hybrid facility and did meet city code requirements of wetland facilities (BMP T10.30 -Stormwater Treatment Wetland) of being no deeper than 2.5 feet. This was allowed by the city along with other deviations  in the hybrid facility except for the Distribution of Water Depths as stated in table 10.2 of BMP 10.30. From David Consulting firm’s report:

“The following goals and guidelines were used: Maintain a deep pool near the outlet and match existing outlet pipe elevations. It is acknowledged that this depth exceeds the guidelines for maximum depth of 2.5 feet in a stormwater wetland, as set forth in the City’s BMP T10.30 – Stormwater Treatment Wetlands. This criterion was deemed appropriate for the design of a hybrid wetpond/wetland. During design, the various criteria outlined in BMP T10.30 were evaluated for this site, and it was agreed that the Yauger Park facility could deviate from the requirements due to the existing constraints and hybrid design. The exception is that the water depths ratios in Table 10.2 were to be followed.”

When it came to the shape and depth of the pond the engineers designed the wetland “vigorous, varied, and economical plantings…Design a complex shape with undulation and irregular edges. Design for sediment storage and sediment removal. ” Also included was the need to install a geomembrane to allow the wetpond to function year round.

Since this facility would handle large volume, a maintenance plan was created. Every several years the wetland soil would need to be excavated out and soil treated. A warning fabric was also designed within the system such that maintenance workers would not puncture the geotechnical fabric. Plant life as well as bark layers would have to replaced based on storm outputs and needs.

The wetpond facility soil mix was one of the key bioretention facilities to use a bioretention soil mix designed specifically for maximum biofiltration. The mix was required to increase filtration, microbial breaking down of pollutants, and plant absorption of pollutants. A prescriptive approach was taken in  making sure the soil mix had correct depth, acidity, organic matter, and dry weight. Soil testing showed that the mixture fell within the bioretention mix  requirements.As a requirement to wetlands in the City of Olympia Manual, landscapers and designers created a model wetland habitat under a “deep immersion zone”. The common plant material that has dual function as habitat and filtration used at this site was bull rush.

Yauger Park LID Project effectively demonstrates  interconnectivity of appropriate plants essential for filtering effluent material from the built environment by incorporate an underzone area for wetland plants.  The common wet pond plant, bull rush functioned as a dual habitat and filtration. Snags help meet a demonstrative purpose for habitat at this project too.

Yauger Park LID project main goals was to increase capacity of the stormwater treatment facility, flow control, and water quality treatment while still maintaining a recreational facility.  With the use of biofiltration stormwater facilities, the amount of stormwater treatment was greatly increased in a very dense area. The facility increased it’s capacity by 20% , and greatly  increased the removal of pollutants via natural processes. What largely stands out about this LID project is the dense integration of BMP’s within each other. Backups within the BMP’s mean that even when facing a large storm event, it has a high likelihood of handle that event. Stormwater wetlands and bioretention swales can greatly increase the storage capacity and  pollutant removal while also controlling large water velocities that would otherwise have reached Percival Creek.

By 2016, Low Impact Development will become a required practice by municipal and commercial entities. In order to avoid being fined large amounts by the Department of Ecology, stormwater BMP’s like the ones used at Yauger Park Project can be used by other jurisdiction.

While noted that the function may have a high maintenance program this wetland/wetpond BMP’s and one like it, the natural benefits to the environment, in the way on natural habitat created, outweigh the downturns of upkeep. The landscaping and habitat that is created for park users is highly valued by local users.

1. Probable Construction Cost Update

An Engineer’s Opinion of Probable Cost was developed for the project using the City of Olympia construction cost database, and estimated at $2,814,000 without State Sales Tax. The estimated construction cost for Additive Bid Schedule #1 for the bicycle shelter construction is $49,000 without State Sales Tax.