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Civil Engineering - Stormwater

With incidents of flooding increasing in recent years, BHC Consultants has provided Washington State with a diverse range of services to help in responding to and preparing for Stormwater issues. These services include:

 

Stormwater Project Briefs

City of Snohomish CSO Reduction Project Phase 2A, City of Snohomish, Snohomish, WASnohomish CSO 2A
BHC was responsible for planning level analysis and design for separation of the sewer and stormwater collection systems within the gravity CSO basin, and design of the new water quality treatment facility with an outfall to the Snohomish River for the City of Snohomish. The project included preparing an Engineering Pre-Design Report that complied with the terms of the Municipal Stormwater Capacity Grant for pre-construction planning and design obtained by the City under a Washington State Department of Ecology (Ecology) program. The options analysis performed showed that installing a stormwater treatment wetland in the abandoned wastewater lagoon was the most appropriate best management practice (BMP) stormwater treatment facility for this area. The Western Washington Hydrology Model (WWHM2012) was used to size the stormwater treatment wetland. Plans, specifications and an opinion of probable cost were prepared to the 90% design level for this project.

City of Snohomish Combined Sewer Overflow Management Plan, City of Snohomish, Snohomish, WA
BHC updated the CSO Management Plan for the City of Snohomish.The project included hydraulic modeling of the combined sewer system in its current condition as well as modeling future stormwater separation projects. An EPA SWMM model was built and calibrated with existing rain and flow data to analyze the existing system and find points of constriction during rain events. BHC identified and modeled potential solutions to bring the outfalls into compliance. Additional tasks included an evaluation of the existing CSO pump station for hydraulic capacity. Recommendations and conceptual costs for proposed operational modifications and system improvements were prepared along with a standard operating procedure for managing lagoon levels at the WWTP and pumping rates at the CSO pump station.

South Park Green Stormwater Infrastructure Project, South Park Area Redevelopment Community, WAS Park GSI
BHC was responsible for planning level analysis and design of three bioswales in an area of the South Park Community that is part of an unincorporated portion of King County. Some of the tasks included the site evaluation and survey review, involvement in community relations, hydrologic modeling, design, and development of the King County Special Use Permit, construction drawings and Temporary Sediment and Erosion Control Plan drawings. The modeling was performed in WWHM 2012. Interactions with both the residents and King County Roads were an integral part of this project.

2015 Sewer, Water, and Stormwater System Plan Updates, City of Monroe, WA
In compliance with the state mandated schedule to update City-wide Comprehensive Plans it was necessary to update the Utility System Planning documents as supplement to this planning obligation.

BHC was retained to update the Sewer General Plan, the Water System Plan and the Stormwater Comprehensive Plan in a timely fashion so that all three documents would be completed to coincide with the City-wide Comprehensive Plan. Updating all three documents at the same time allow an opportunity to combine common elements which resulted in cost and time savings. The utility system plans were then incorporated and appended to the Comprehensive Plan.

The Stormwater Comprehensive Plan included the identification of the probable cause of localized flooding and system backups in the areas that the City indicated to be areas of concern. Field investigations, system analysis and modeling utilizing WWHM2012 were used to develop potential local solutions for these areas. Planning level designs utilizing infiltration galleries and bioswales were prepared for the four main areas of concern.

Phased Engineering Consulting Services for CSO Planning, Design, and Construction in CSO Areas, Seattle Public Utilities (SPU), Seattle, WA
BHC is the basin lead for the Henderson and Magnolia basins as part of the Phased Engineering Services for CSO Planning, Design and Construction project for SPU. This project involves options analysis for addressing CSO overflows in the Henderson, Magnolia, Montlake, Madison Park, and Duwamish basins. Tasks include coordinating with hydraulic modelers, alternatives analysis, and design development to determine the most feasible solution to reduce CSO events to less than one per year. The Magnolia 60 basin project includes rehabilitating Pump Station No. 22 to convey more flow and replacing the existing force main. Henderson 49 includes modifications to hydraulic structures to optimize and increase storage within the CSO basin. The current phase of the project includes 10 percent design drawings, alternatives analysis reporting, cost estimating, and coordination with SPU.

Combined Sewer Overflow (CSO) Pump Station, City of Snohomish, WACSO PS
BHC provided preliminary engineering, final design, and services during construction for facilities to intercept and control existing combined sewer overflows (CSO) in the City of Snohomish. The new facilities include a new 6,500 gpm combined CSO and stormwater pump station, nearly 6,200 feet of 18-inch and 24-inch force main and gravity sewer pipe, 3,000 feet of 18- to 30-inch stormwater pipe, and 1,200 feet of 12-inch water main. Special design considerations include stabilization of the pump station site using a stone column system, architectural pump station features to conform to the historic neighboring community, and location of separate sanitary sewer, stormwater, and potable water pipelines within a relatively narrow existing City street. The pump station consists of two suction lift pump sets, one at 6,500 gpm (sewer), and one at 5,400 gpm (storm).

A key factor in the project was the crossing of an SR-9 interchange under a WSDOT bridge. We secured WSDOT approval of a Channelization Plan and Franchise Agreements. We also convinced WSDOT to agree to closing off-ramps and on-ramps to SR-9 for weeks at a time to allow for phased, efficient pipeline installation. A six-stage traffic control plan was developed to maintain and detour traffic while pipe installation work proceeded.

BHC was the lead design firm for the project and retained several subconsultants for specialized design and permitting services. Subconsultant disciplines included surveying, geotechnical engineering, electrical engineering, architectural services, wetlands specialists, traffic control and archeologists. BHC also provided services during construction including field observation, office engineering, and the preparation of an operations/maintenance manual.

Permits obtained included a Corps Nationwide Permit (wetlands), Shoreline Permit, Biological Evaluation (endangered species), JARPA, SEPA Checklist, and WSDOT agreements. Mitigation for wetland disturbance included 3.2 acres of wetland planting, at a cost of $219,000.

Riverfront Development, Stormwater Collection Facility Design, City of Everett, WA
BHC was responsible for the design of the stormwater infrastructure required to support the 70 -acre Everett Riverfront Development, a proposed retail, commercial, and residential development on the former Everett Landfill and adjacent sites along the Snohomish River. BHC designed the stormwater quality treatment and high flow bypass facilities including several flow splitting and pretreatment vortex screens and 6 filter cartridge vaults. Filter vaults are 8 feet deep and range in size from 12 to 35 feet wide.

Local Flood Mitigation Drainage Improvements Program, City of Bothell, WA
BHC completed a Conceptual Analysis and Preliminary Engineering/Design of stormwater drainage system improvements to address local flooding associated with four separate projects located within the City of Bothell. The local flooding problems were caused by one or more of the following: 1) existing conveyance system capacity limitations; 2) undersized roadway culverts; 3) root intrusion – pipe maintenance/integrity issues; 4) incomplete/inadequate drainage collection facilities; 5) discharge location issues; and/or 6) others. The purpose of BHC’s initial work was to identify and analyze potential solution(s) to the problems including a conceptual-level design of recommended improvements to a sufficient level of detail to determine required facilities, establish a preliminary layout, assess potential environmental documentation and permitting requirements, as well as estimate a planning-level construction cost for the proposed project. BHC’s efforts also included field reconnaissance, researching existing facilities and City records, as well as performing drainage area hydrologic and hydraulic modeling and analyses utilizing WWHM and XPSWMM software to develop comprehensive and cost effective solutions. Potential solutions included: a) modifying, rehabilitating or replacing existing or installing new collection/conveyance system piping, culverts, swales, and/or appurtenances; b) modifying existing catch basins for improved local drainage; c) enlarging/modifying/replacing an existing sedimentation basin, it’s outlet flow control, and/or overflow system; and/or d) installing energy dissipation and/or erosion control features at the discharge locations (if warranted).

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