Reducing Stormwater Runoff Impacts

Powell Butte is a 578-acre nature park of vast meadowlands and forests, located in southeast Portland, OR, within easy reach of city dwellers. The park is bounded roughly by SE Powell Blvd. to the north, SE 141st Ave. on the west, SE 162nd Ave. on the east, and the Springwater Trail Corridor on the south (Figure 1). The butte, an extinct cinder cone volcano, rises near the headwaters of Johnson Creek – an urban creek with remnant populations of native salmon and steelhead. It is Portland’s second-largest park.

In the late 1800s, the large meadow area was cleared and an orchard planted. In 1925, the city of Portland purchased the land for future water reservoirs, but continued to lease the northeast portion of the property to Henry Anderegg, a farmer and owner of Meadowland Crest Dairy, until 1948 when the farming was discontinued. However, dairy cattle were permitted to graze on the acreage to preserve the pastures.

In the mid-1970s the Portland Water Bureau (PWB) prepared a development plan for Powell Butte that called for the construction of four, 50-million gallon underground reservoirs to be located at the north end of the butte. In 1981, the first, and currently only, 50-million gallon reservoir was built and still serves as one of the hubs of PWB’s distribution system. Also, the Powell Valley Water District had three reservoirs on the butte prior to its acquisition by PWB in 2005. In 1987, the city officially established Powell Butte as a nature park and it was opened to the public in 1990.

Since the early 1990’s, Powell Butte has been co-managed by both PWB and Portland Parks and Recreation (PPR). PWB owns the Butte and manages water facilities, and PPR manages the security, nature, and recreation functions.

Existing conditions
Despite all it has to offer as a nature park, a large portion of the northeast quadrant of the butte slopes toward Meadowland Mobile Home Park (MMHP), a community of approximately 200 residents. Since it was developed in 1988, MMHP has historically experienced flooding and saturated ground conditions each winter during severe storm events (Figure 2). The relatively impermeable soils and high local groundwater conditions exacerbate the problem.

Figure 2

The primary stormwater conveyance system in this portion of the butte consists of sheet (overland) flow to a series of collection channels and culverts conveying stormwater runoff north towards the 158th Avenue Access Road. A collection channel along the south side of this access road conveys runoff in a westerly direction. A portion of this runoff is diverted through several culverts (Figure 3) and is discharged through open outfalls into MMHP. Runoff is conveyed from these outfalls via overland flow towards MMHP (Figures 4 and 5), where the flooding has occurred in the past. The remaining runoff is conveyed along the access road collection channel in a westerly direction towards the 158th Avenue entrance to the park. The area contributing runoff includes approximately 1.2 acres of impervious surface, which consists of existing reservoirs and associated crushed-rock driving surfaces.

Figure 3

In a “good faith” effort to help mitigate the impacts to downstream properties, PWB retained Kennedy/Jenks Consultants to provide design services for reducing the impact to these downstream properties. The design of the facility would focus on the following critical issues:

  • Reduce the impact to downstream properties by re-directing existing runoff in a westerly direction toward a new surface infiltration facility west of the SE 158th Avenue access road; and
  • Size and design the proposed infiltration facility in accordance with the requirements of the City of Portland Bureau of Environmental Services’ Stormwater Management Manual (Manual).

Regulatory requirements

As the city of Portland is developed, impervious surfaces create increased amounts of stormwater runoff during rainfall events, disrupting the natural hydrologic cycle. Without stormwater management, these conditions erode stream channels, prevent groundwater recharge and often result in combined sewer overflows (CSOs) and basement sewer back-ups. Parking lots, roadways, rooftops and other impervious surfaces increase the pollution levels and temperature of stormwater that is transported to streams, rivers and groundwater resources. Implementing measures to mitigate the impacts of increased runoff will help protect Portland’s water resources, which in turn will provide great benefit to human health, fish and wildlife habitat, recreational resources and drinking water.

Figure 4

All development and redevelopment projects are subject to the requirements of the Manual. The purpose of this manual is to provide stormwater management principles and techniques that help preserve or mimic the natural hydrologic cycle, minimize sewer system problems and improve water quality. The Manual provides developers and design professionals with specific requirements for reducing the impacts of stormwater from new development and redevelopment.

Strategies for meeting the requirements in the Manual depend on a number of site factors, including infiltration capacity, available infrastructure, proposed development plans, and the drainage basin the proposed development is in. The applicant’s ability to effectively use the design standards in the Manual depends on a demonstrated understanding of the development site’s ecology and of the upstream and downstream impacts resulting from stormwater management improvements. The standards addressed in the Manual are intended to make site-specific improvements to properties across the city and to comprehensively manage stormwater by watershed. Stormwater management is critical to maintaining and enhancing the city’s livability and improving watershed health. The Manual allows the city of Portland to protect both watershed resources and infrastructure investments with every land improvement. As each development and redevelopment project meets the requirements of the Manual, it will contribute to achieving these important citywide goals.

[inline:Figure 5.jpg=Figure 5]

The Manual presents three methodologies for sizing stormwater management facilities: Simplified Approach, Presumptive Approach and Performance Approach. The Simplified Approach may be used for projects with less than 10,000 square feet (.23 acre) of impervious area. This approach is most appropriate for private, small-scale residential development. It is not allowed for use on large, complex projects or on projects that have multiple catchments that, when combined, exceed 10,000 square feet of new impervious area.

The Presumptive Approach is available for medium-to-large-scale residential and commercial projects on either private or public property of any size. Its application is required for projects with a total impervious area that exceeds 10,000 square feet (.23 acre), where the Simplified Approach is no longer applicable. It can also be applied to size facilities on smaller projects where the more detailed hydrologic calculations will allow the applicant to size a facility more accurately by taking measured infiltration rates and other more specific design factors into account.

The Performance Approach is available for projects with unique circumstances that require analysis that go beyond the capabilities of the Simplified and Presumptive Approaches. It may be used to address specific site conditions or project needs, or to apply a new or emerging design technology. Detailed engineering calculations must be provided as evidence of the proposed design’s performance with respect to achievement of the SWMM’s stormwater requirements.

Infiltration facility design

After reviewing the three methodologies, it appeared that a vegetated swale constructed over an infiltration trench, using the Presumptive Approach, would be the most appropriate for use on this project. Field infiltration tests conducted prior to design indicated that the soils near the 158th Avenue entrance are conducive to infiltration (Figure 6). A falling head infiltration test was conducted, and the documented infiltration rate was 99 feet per hour in a layer of well-graded gravelly sand at a depth of 9 to 10-feet below the existing ground surface at the location of the proposed infiltration facility. The Presumptive Approach Calculator (PAC) limits the infiltration rate to a maximum of 20 inches per hour. As such, this is the infiltration rate used in the design of the infiltration facility. Elsewhere, in the outfall and swale areas, a conservative infiltration rate of two inches per hour was used. This results in a composite maximum infiltration rate of 13.1 inches per hour.

Figure 6

The Manual includes the PAC, a spreadsheet that can be used to perform sizing calculations (based on impervious area) for vegetated surface facilities with impervious catchment areas up to 1 acre in size. If the impervious catchment area exceeds one acre, appropriate alternative software that follows the same principles as the PAC must be used.

The Santa Barbara Urban Hydrograph (SBUH) methodology is a common software used to estimate runoff generated from storm events. The computer program “HYD,” developed by King County, Washington Department of Public Works, was used for the SBUH calculations for this project.

In accordance with the surface infiltration design requirements of the Manual, a 10-year storm (3.4 inches of rainfall over 24 hours, NRCS Type 1A rainfall distribution) was used to generate the runoff flow rates presented. A conservative estimate of 5 minutes was used for the time of concentration (Tc). A pervious area Curve Number of 86 was used, based on local soil conditions, and an impervious area Curve Number of 98 was used, which is generally the industry standard for impervious surfaces.

Figure 7

Figure 7 presents the inflow and outflow hydrographs for the 10-year, 24-hour storm event for the drainage basin. The inflow hydrograph, generated from the HYD program, represents the inflow into the stormwater facility from the new storm drainage piping. The outflow hydrograph represents the infiltration (outflow) into the ground.

Figure 8

At the early stages of the inflow hydrograph, the incoming runoff infiltrates into the ground until the incoming runoff exceeds the composite maximum infiltration rate of 13.1 inches per hour. After the incoming stormwater exceeds this amount, runoff accumulates in the infiltration facility until shortly after the peak inflow, 1.07 cubic feet per second, is reached. After this point, the inflow is less than the maximum infiltration rate and the runoff stored gradually infiltrates into the ground.

Figure 9

The area between the inflow and outflow hydrographs represents the volume that must be contained within the infiltration facility. Using the HYD program, it was determined that 1,240 cubic feet of storage would be needed. Using a trench section eight-feet wide by six-feet deep (Figure 8) and a void ratio of 0.30 for the infiltration fill, 86 feet of infiltration trench would be required. As an additional measure of safety, the design length was 100 feet, resulting in an actual storage capacity of 1,440 cubic feet. An eight-foot wide by 13-foot long rip-rap channel was designed at the outfall for energy dissipation, and the remainder of the infiltration trench is covered by a swale with plantings.

This innovative engineering project reduced the impact to downstream properties by re-directing existing stormwater runoff from a residential community that has historically experienced flooding and saturated ground conditions each winter during severe storm events, to a new surface infiltration facility (Figure 9). The facility was designed in accordance with underground injection control (UIC) regulations. In exchange for the improvements, PWB received property and a permanent disposal point for draining the 3-million and 7-million gallon reservoirs (recently obtained from the acquisition of the Powell Valley Road Water District). Construction began in the spring of 2009 and was completed in the fall of 2009, before the wet weather season began.

About the author:
Aaron Eder, P.E., is a project manager in the Portland office of Kennedy/Jenks Consultants, one of the west coast’s leading water and wastewater treatment and design firms. He is a licensed professional civil engineer in Oregon and Washington. Eder has been instrumental in Kennedy/Jenks recently being awarded several high-profile projects with the city of The Dalles and the city of Portland Water Bureau. He is currently serving as the project manager for the city of The Dalles’ Terminal Reservoir and Pump Station project. (503) 423-4016

Related Articles