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Discussion of the Boeing Creek Basin Plan

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Discussion of the Boeing Creek Basin Plan Council Meeting Date: January 7, 2013 Agenda Item: 8(c) CITY COUNCIL AGENDA ITEM CITY OF SHORELINE, WASHINGTON AGENDA TITLE: Boeing Creek Basin Plan Update DEPARTMENT: Public Works PRESENTED BY: Mark Relph, Public Works Director Jesus Sanchez, Operations Manager Brian Landau, Surface Water and Environmental Service Manager ACTION: ____ Ordinance ____ Resolution ____ Motion _X_ Discussion ____ Public Hearing PROBLEM/ISSUE STATEMENT: The adopted 2011 Surface Water Master Plan emphasized the role of basin planning to improve the management of the City’s surface water and infrastructure. The City completed its first basin plan for the Thornton Creek basin in 2009. The City is currently conducting basin plans in the Storm Creek and Boeing Creek basins to assess the basin conditions including drainage, erosion, infrastructure condition, water quality, and aquatic habitat. The assessment includes identification of problems and programmatic management actions to address the problems. The programmatic management actions may include capital projects, repair and replacement of infrastructure, improved maintenance, outreach programs or other corrective actions. The final draft Boeing Creek Basin plan report was completed in December 2012. Tonight’s presentation includes the findings of the Boeing Creek Basin Plan and the recommended management actions to address the drainage, infrastructure, water quality, and aquatic habitat issues in the basin. RESOURCE/FINANCIAL IMPACT: There is no resource or financial impact associated with this discussion. The programmatic management actions identified in the Boeing Creek Basin Plan (draft completed in October 2012) will be prioritized with other management actions identified within the other surface water basin plans in the City and may be proposed in the City’s future Capital Improvement Plan (CIP) budget process. All operational and capital expenditures are funded through the Surface Water Utility Fund. RECOMMENDATION No action is required at this time. This item is for Council discussion. The basin plan recommended programmatic management actions and associated costs will be prioritized as part of the 2014 CIP process. Approved By: City Manager -__DT__ City Attorney - __IS/FC__ 000077 INTRODUCTION The City is completing a Boeing Creek Basin Plan to assess conditions in the basin including drainage, erosion, water quality, and habitat. The assessment includes identification of problems and programmatic management actions to address the problems. These programmatic management actions may include capital projects, repair and replacement of infrastructure, maintenance, monitoring, outreach programs, and other potential solutions. BACKGROUND The Storm Creek and Boeing Creek Basin Plan studies began in mid-September 2011. The scope of the basin plans is to assess surface water conditions in streams and within buried infrastructure (i.e. pipes and catch basins) so that comprehensive strategies that include maintenance, repair and replacement, capital, and outreach programs can be used to address problems. These problems and management strategies will be prioritized so they can be implemented over a period of time, according to need and resource availability. The basin plans includes specific studies on hydrology and drainage, erosion, water quality, infrastructure condition assessment, and aquatic habitat (i.e. streams and wetlands). The primary issues in the Boeing Creek basin are related to past development practices that didn’t consider the detrimental effects of increased stormwater flows and flow durations on small streams such as Boeing Creek. Stormwater runoff from intensely developed upland areas on the Aurora Avenue corridor have contributed to instability and erosion in Boeing Creek, as well as water quality concerns. Efforts to mitigate and minimize development-related impacts have occurred over the last several decades with construction of dams, detention facilities and addition of material to stabilize the bed and banks of the creek. The City has addressed most of the major flooding issues in the Boeing Creek basin with implementation of several drainage improvements projects over the last five years, including 3rd Avenue drainage improvements, Pan Terra pond retrofit, Midvale Avenue North drainage improvements, and Darnell Park improvements. DISCUSSION The following discussion provides an update on some of the initial findings of the basin plan, including hydrology, infrastructure condition, and water quality, Basin Hydrologic Analysis A hydrologic model was developed for Boeing Creek as part of the basin planning effort. Basin geology, topography, land cover (impervious surfaces and vegetation), and historic precipitation records were used as inputs to the model to simulate rainfall-runoff relationships under different sizes of rainfall events. The model was used to identify a preliminary 100-year floodplain boundary for planning purposes only, and identification of potential basin-wide stormwater management issues and solutions. 000078 Based on a review of service requests for the past 10 years and implementation of projects over the last five years, flooding problems in the Boeing Creek basin have largely been alleviated with one exception. City staff and the City’s 2011 Surface Water Master Plan Update both report a flooding problem in the vicinity of North 175th Street between Dayton Avenue North and Linden Avenue North. Non-chronic flood-related service calls have been primarily due to localized problems, such as debris-clogged catch basins or culverts, rather than system-wide flooding caused by lack of capacity (streams or pipes) to contain large flows. The basin plan includes a map that shows the extent of the 100-year floodplain (Attachment A). This 100-year floodplain analysis will be used to support a Letter of Map Revision to the existing FEMA floodplain for Boeing Creek. Long-term solutions to reduce peak flows in a largely built-out watershed will require the implementation of retention or infiltrative stormwater management techniques in suitable parts of the watershed in city-owned rights-of-way or as part of redevelopment efforts on large privately owned properties, such as Aurora Square, Crista Ministries, and Shoreline Community College, Shoreline Town Center, and the Interurban Trail corridor (Attachment B). As part of the basin plan study, staff evaluated the effects of implementing stormwater infrastructure retrofits in these areas of the basin. If all of these areas were retrofitted to current stormwater design standards, the result would be an approximate 20 to 25% reduction in peak flows as measured at the mouth of Boeing Creek. Additional flow reductions could be achieved through public works projects within the City’s right-of-way. For example, flow reduction and water quality treatment projects in Thornton Creek watershed include the recently constructed Cromwell Park stormwater wetland detention facility and the grant funded North Fork Thornton Creek LID Stormwater Retrofit Project which will design and construct LID stormwater facilities within the City’s ROW. This preliminary analysis provides staff with a framework to assess the benefits of stormwater retrofit projects associated with redevelopment within the Boeing Creek basin. Infrastructure Condition Assessment The Condition Assessment included inspection of 829 stormwater pipes with a total length of 94,140 feet within the Boeing Creek basin. The condition assessment included an overall conditions rating of the each pipe based on maintenance condition and structural condition. The initial findings indicate 8% of the pipes are in very poor condition and will require replacement within the next five or ten years (Attachment C). Water Quality Since 2007, the City has been monitoring water quality at two locations in Boeing Creek. The data indicate the water quality is of low to moderate concern. The primary constituents of concern in Boeing Creek are fecal coliform, dissolved oxygen, and nutrients (nitrogen and phosphorus). 000079 Habitat and Fish Passage The Boeing Creek basin has the largest contiguous forested area in the City, and water quality conditions are better in Boeing Creek than in any other monitored stream in the City. Additionally, Boeing Creek provides fish habitat (anadromous and resident fish, such as cutthroat trout, in the lower 0.5 mile, and only resident fish in the upper reaches), excellent riparian conditions, and forest canopy. Despite the excellent riparian corridor, aquatic habitat has been affected by stream channel bed and bank erosion, and multiple fish passage barriers (Attachment D) that have been in place for decades. There are at least eight fish passage barriers limiting the movement of anadromous fish entering Boeing Creek from Puget Sound. Approximately 2,300 feet of stream channel from the mouth of Boeing Creek to the Seattle Golf and Country Club water supply dam is open to anadromous fish. Upstream of the dam, there are numerous barriers. It would be a monumental effort to remove or reconfigure these barriers to facilitate passage for anadromous fish. However, incorporation of barrier removals into a long-term plan for improving Boeing Creek through peak flow and duration reductions (stormwater retrofit) and local habitat improvements would be a worthy goal. The cost for retrofit would need to be evaluated as to the overall benefit on the utility and therefore its ability to be funded through surface water utility rates. Summary of Preliminary Conclusions: The
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