Stormwater

Below are some of our representative projects, sorted alphabetically.

City of Anacortes,
Wastewater Treatment Plant Outfall Replacement

Skillings Connolly worked closely with the City to develop a plan to repair and replace approximately 175 LF of existing 24-inch reinforced concrete pipe with 24-inch HDPE pipe. The existing wastewater treatment plant’s sole outfall, located at the end of R Avenue on the Port of Anacortes property, had been experiencing effluent leakage along a section of the old outfall pipe, prior to the effluent reaching the multi-port diffuser discharge point. A combination of joint failure, settlement, and a prior unsuccessful attempt at slip-lining with an 18-inch HDPE pipe, caused the outfall to be in violation of the City’s NPDES agreement.

The repair required designing a temporary bypass system to maintain operation of the treatment plant during the repair (the plant only had a 4-hour shutdown window for the implementation of a bypass), coordination with the Port’s logistics throughout the project, design of a sheet-pile cofferdam and implementation of the fish-exclusion measures to comply with the environmental requirements set forth in the USACE permits and HPA, and providing construction management and on-site support to the City and the Contractor to mitigate for unknown conditions and prevent over-run of the fish-window mandated schedule.

Our experience in marine outfall construction, pumps, and hydraulic design allowed us to design an implementable temporary bypass solution which was critical to executing the repairs. We provided active construction support to help mitigate for unknown conditions discovered during construction. Our responsiveness and ability to communicate effectively allows us to reduce conflicts and prevent schedule impacts.

City of Chehalis,
Chehalis-Centralia Airport Pump Station Rebuild

Skillings Connolly, Inc. was retained by the City of Chehalis to provide professional services to assist in the project management, planning, design, and construction management for the Airport Pump Station Rebuild project located at the Chehalis-Centralia Airport.  The project was funded through a grant from the Chehalis Flood Authority and managed by the Recreation and Conservation Office (RCO).

During high runoff periods, the pump station conveys storm and flood water from the basin within the airport levee perimeter, as well as additional runoff from adjacent properties, to an existing conveyance channel outside the levee.  Constructed in the 1940s, the existing pump station was a simplex station, and during a 100-year flood event the pump motors and other electrical components were underwater.

The Airport has replaced the existing flood control pump station with a duplex pump station which has been set-up for simplex operation.  The pumps are vertical axial flow propeller flow type designed to pump at approximately 11,900 gallons/minute against a total dynamic head of 8.6 feet.  Motor size for the pumps is 50 horsepower.

The pump station and backup generator floor elevation are raised above the record flood elevation and future top of levee elevations to ensure that the pump motors and other electrical components remain above the 100-year flood elevation.  The goal of the pump station rebuild project is to have a reliable pump station that helps restore airport operations as soon as possible after a flood event.

We also provided construction management services, which included ensuring that the project is in compliance with the contract plans and specifications, responding to RFIs, performing submittal reviews, reviewing payment applications, performing periodic site inspections, and aiding the City in project closeout.

    • City of Olympia, State Avenue Road Improvements

Skillings Connolly designed roadway and utility improvements to facilitate this redevelopment, which widened the existing roadways and provided safe routes for bicycles and pedestrians. As part of the alternatives analysis, we incorporated stop signs to facilitate the movement of oversized freight from the City’s busy Port area.

City of Tumwater, Outfalls and Various LID Treatment Tasks

Skillings Connolly provided professional services to the City, for various stormwater-related tasks for three sites as follows.

Somerset Hill Drainage Treatment:  Skillings Connolly’s design mitigated the flow of untreated stormwater runoff water into Percival Creek; thereby, reducing the negative impacts to downstream waters.  The retrofitted stormwater treatment systems were sized to fit within the existing right-of-way.  We provided PS&E so that runoff from the western portion of Somerset Hill Drive could be treated via two filter units. The Filterra Bioretention System offers stormwater treatment technologies that optimize bioretention technology for pollutant removal.  Our engineers designed a rain garden to treat runoff from the eastern portion of the roadway.  The rain garden, an off-line facility, is located in a planter strip and tiered due to the slope.  A flow splitter placed in the drainage structure upstream diverts the 91st percentile flow and smaller events into the rain garden. Larger flows bypass to the existing drainage system. The rain garden has an infiltration rate of 2″ per hour.

E Street:  Skillings Connolly provided PS&E so that most of the runoff could be treated by a stormwater treatment wetland.  The wetland and its pre-settling cell were sized according to the procedure given the Drainage Design and Erosion Control Manual (DDECM) Volume V Section 6.1.1.  The wetland cell has an average depth of 1.5 feet. The plantings were a blend of Red-Osier Dogwood, Small Fruited Bulrush, and Hardstem Bulrush.  Located at the northeast quadrant of the Capital Boulevard and E Street intersection, this Skillings Connolly design mitigated the flows of untreated runoff water into the Deschutes River.  By treating the stormwater and changing the point of discharge to a dispersion type outfall, our team reduced the negative impacts to downstream waters, which ultimately includes Budd Inlet.

Cleveland Avenue Drainage Treatment:  Skillings Connolly designed a single, large biofiltration swale /equalization basin for Cleveland Avenue.  This included sizing the facilities, grading the sites, and developing PS&E.  The team also completed the engineer’s cost estimate.

City of Tumwater, Tumwater Boulevard (Airdustrial Way) Improvement Project

The City of Tumwater asked Skillings Connolly to improve 1 . 1 miles of arterial running east and west between Interstate 5 and Capitol Boulevard.  This route is a critical transportation link to the Interstate for a rapidly developing area of the City of Tumwater and the Port of Olympia.  The existing roadway was widened as needed to provide additional lanes for vehicles and for bicycles.  Curb and gutter, sidewalks, illumination, and landscaping were added throughout the project length.  The existing storm drainage system was upgraded to meet current standards for detention and treatment of roadway runoff.

  • Island County, Columbia Beach Drive Marine Outfall Extension

In late 2011, Island County improved the storm drainage system located along Columbia Beach Road from just southwest of the intersection of Berg Road and Columbia Beach Road to an existing MH located on the west side of Columbia Beach Road near the ferry landing.  The northerly outfall handles a portion of the stormwater runoff that would normally drain through the existing southerly outfall that was subject to being buried by migrating beach sand. This did not prevent all of the flooding.

A neighborhood located on Columbia Beach Drive had been experiencing intermittent flooding, particularly during the winter months. This was largely due to the inability of the existing marine outfall to drain freely, as the outfall terminus was buried under approximately five feet of sand and gravel. The existing southerly outfall consisted of 250 linear feet of 18-inch diameter concrete pipe.  The outfall ended approximately at the elevation 0.0 NAVD88.

To alleviate the flooding, Skillings Connolly provided the PS&E and the permit exhibits necessary for extending the existing concrete stormwater outfall.  As an interim solution, the County asked Skillings Connolly to model the capacity of the northerly outfall to see what its capacity is and how it could reduce flooding.  The northerly outfall was modeled using Autodesk® Storm and Sanitary Analysis.

The drainage basin and the flows for various storm events were observed and verified prior to starting the work. The County’s drainage basin map showed a basin of 79.3 acres draining to the existing outfall.  Based on site visit observations and review of the drainage basin maps, it appeared that the drainage basin was actually larger by almost 26 acres than what was shown on the basin map.

The model was run for 6-month, 2-year, and 10-year storm events; with tides ranging from elevation 8.2 NAV88 Datum which equates to an elevation of 10 MLLW Datum to an elevation 11.3 NAV88 Datum which equates to an elevation of 13.1 MLLW Datum.  Based on the model, the flow for a 10-year storm event is 4.61 cubic feet/second (cfs). Assuming that the ditch and the inlets to the original twin 12-inch diameter culverts were to be kept clean, the model showed that the old system could only handle approximately 4.64 cfs. At this time, the system would begin to overflow.  Any minimal clogging of the inlet further diminished the capacity of the twin pipes and the system.

As a permanent solution to the drainage problems, the existing outfall was extended using High-Density Polyethylene Pipe (HDPE) to approximately 30 feet beyond the Mean Lower Low Water (MLLW) mark. By extending the pipe past the MLLW mark, the migrating beach sand can no longer cover the end of the pipe and the outfall now drains freely. In addition to the extension of the pipe, a storm structure with a tide gate was installed.

  • Island County, Gerdes Road North Outfall

The Gerdes Road North Outfall, on Camano Island in Island County, is located on the mid to upper beach-face and had been buried and obstructed by beach sediment accretion, resulting in surface water backing up within the drainage system. This inundated the surrounding coastal properties with stormwater. The County desired to alleviate or improve these recurring flooding issues and was considering using the agricultural area near the outfall location for detention during storm events that occur during high tide.

Skillings Connolly conducted an alternatives analysis to assess the existing conditions and determined that a deep-water outfall extension could drain the basin in the agricultural area by gravity or would require a pump station to alleviate the flooding. Several design options were evaluated based on the results of the preliminary analysis.

Skillings Connolly completed the hydraulic analysis which determined that the extension required a pump station. We designed the stormwater pumps station, completed the stormwater design report, and provided environmental documentation and permitting. Skillings Connolly also provided topographic survey and the PS&E to extend the outfall itself.

  • Island County, Dave Mackie Park Stormwater Improvements

Due to the Park’s history of flooding, Skillings Connolly modeled the existing stormwater system and basin in order to design a new stormwater drainage system that included a temporary pump station and an 18-inch permanent outfall.  Other elements of the project included seawall reconstruction, outfall pipe anchor design and construction, and ditch reconstruction.  Skillings Connolly prepared PS&E for the three phases included in this project. The first phase was to design an emergency pump station to immediately alleviate flooding issues at the park.  The second phase included design and permitting to provide temporary excavation at the end of the outfall to allow the existing outfall to function.  Located within a highly active coastal drift cell, the outfall has been routinely covered due to a high level of beach material deposition.  As on-going excavation of the existing outfall was not a sustainable long-term solution, a new outfall was designed that extended the pipe beyond the area of active beach drift.  The project included completion of underwater eel grass surveys and eel grass mitigation.  We prepared a Biological Evaluation to support the USACE permit for in-water excavation.  The Biological Evaluation included analysis of potential impacts to forage fish species, including surf smelt.  The presence of surf smelt breeding on the beach adjacent to the outfall created a challenge for construction, and until the construction of the new outfall is completed, it is necessary to provide consistent monitoring of the beach substrate to determine if forage fish have laid eggs prior to and during construction.

  • Island County, Maxwelton South Stormwater Pump Station Installation and Outfall

The residences of Maxwelton Road were experiencing flooding. To help alleviate flooding, Skillings Connolly designed a basin-wide drainage system which included modeling of the 120-acre drainage basin to estimate peak flows, development of PS&E, and providing estimates of construction and maintenance cost.

There was a large public involvement element to this project, as much of the work will needed to be done on private property and maintenance became the responsibility of the community association. To accurately estimate flows in this complex basin, it was necessary to quantify contributing flows from numerous springs in the low-lying reaches. As part of the public involvement process, we prepared information appropriate for the community association to make informed choices about potential governance systems capable of providing funds for the maintenance and upkeep of flood control infrastructure.

In addition, Skillings Connolly provided topographic survey, environmental studies, JARPA, and construction of the pump station.  Survey was done to locate spot elevations that were used to establish two-foot interval contours to aid in determining flow direction.

Saint Martin’s University Campus Stormwater Master Plan

Skillings Connolly provided hydrologic modeling to help develop the master plan for the 94-acre University campus. The plan also included analysis of existing and future build-out conditions, an evaluation of existing stormwater infrastructure, and a backwater investigation.