Introduction

The City of Lake Stevens frequently experiences flooding in the low‐lying areas north and east of Lake Stevens with some roads being overtopped by flood waters several times a year. Flood management has been an ongoing issue for the City, but other related issues are also important including lake level management, stream health and protection of aquatic species. In 2020, the City commissioned a study with Davido Consulting Group to identify and assess existing hydrologic issues then develop and evaluate alternatives to address the identified issues. Links to the executive summary and study in its entirety are at the bottom of this page.

Lake Level and Outlet Issues

The study found and, in many cases, confirmed the following:

• The existing weir does not provide hydraulic control of the lake level during the wet season months, October through May. Instead, the stream channel configuration (width/height/slope etc.) is the control during the wet season. During the dry season (May through September), lake levels often drop below the bottom of the weir not allowing control or maintenance of low flows in the lake outlet and Catherine Creek.

Figure 1: Graph of Tabulated Lake Elevations Relative to Target Elevations (2008-2019)

• During intense rain events the outlet channel along Hartford Drive lacks capacity to accommodate high flows. This is further exacerbated by the lack of outlet control at the weir during the wet season and potentially a backwater issue from the Catherine Creek floodplain/wetland north of the fire station.
• Low-lying areas where transportation and conveyance infrastructure intersect, specifically at 18th Ave NE and 20th Ave NE, the roadways tend to flood during intense rainfall events. Causes included inadequate culvert sizes, sags or broken conveyances, and low roadway elevations.

Figure 2: Lake Stevens Outlet Channel Existing Conditions

Project Prioritization

Several project alternatives to address these issues were developed conceptually and ranked according to the following criteria:

1. Project provides better management of the lake level and lake outlet flows.
2. Project reduces the severity and occurrence of flooding.
3. Project improves stream channel habitat and overall health.

This triage-style approach is necessary to ensure the scarce resources of the City provide the highest benefit to the community and environment.

Priority 1: Lake Outlet Control and Outlet Channel Restoration 

Replace lake outlet weir and upper channel modifications. Weir will provide lake level and stream channel control for both high and low lake levels and the channel will be modified to improve habitat and channel capacity. Planning level cost estimate for design and construction is $1.65 million.

Priority 2: Catherine Creek Confluence 

Additional modeling and analysis is necessary to understand the interaction of the lake outlet stream at its confluence with Catherine Creek, the backwater condition from the wetland north of the Fire Station and the impact on Hartford Drive residences and roadway. Cost of additional study is $100K.

Priority 3: Hartford Drive Flood Mitigation and Stream Channel Restoration (Alternative 2C & 5D): 

Improve channel capacity, elevate Hartford Drive above the floodplain, and improve stormwater infrastructure near the fire station. This solution may potentially address backwater issue from Catherine Creek wetland. This will alleviate flooding of Hartford Drive. Planning level cost estimate for design and construction is $939K.

Priority 4: Replace Culvert and Inadequate Stormwater Infrastructure at 20^th St. NE and Roadway Grading (Alternative 5C):

Replace the culvert under 20^th St. NE with box culvert, modify stormwater infrastructure to improve conveyance, and roadway grading. This will alleviate flooding experienced on 20th St. NE and adjacent private property. Planning level cost estimate for design and construction is $1.28 million.

Priority 5: Replace Culvert and Inadequate Stormwater Infrastructure at 18th St. NE and Roadway Grading (Alternative 5B): 

Replace the culvert under 18^th St. NE with box culvert, modify stormwater infrastructure to improve conveyance, and roadway grading. Planning level cost estimate for design and construction is $1.78 million.

Conclusion

Despite the City’s best efforts, the lake level and outlet flows cannot be managed during the dry and wet seasons with the existing weir. Further, the lake outlet stream confluence with Catherine Creek likely creates a backwater condition along Hartford Drive causing flooding in the downtown area. This flooding is exacerbated by inadequately sized culverts and stream channels. Several alternatives have been prioritized and cost estimates have been prepared as part of this study. A pragmatic approach to address these issues has been formulated:

1. Improve management of lake level and lake outlet flows by replacing the weir and improving the outlet stream channel capacity.
2. Reduce the severity and occurrence of flooding by replacing undersized culverts and improving stream channel capacity.
3. Improve stream channel habitat and health as an element of each of the projects.

The planning level cost estimate to design and construct the project alternatives is $5.74 million. These projects can be designed and constructed over several years using local stormwater funds and grants.

Additional operational recommendations to alleviate on-going flooding concerns include the following:

1. Continue robust catch basin inspection and maintenance program to ensure that catch basins and enclosed stormwater conveyance infrastructure is free and clear of debris, especially in advance of predicted rainfall.
2. Continue beaver management program to alleviate back-ups and conveyance blockages due to beaver dams.
3. Remove vegetation cut from outlet channel banks during mowing operations, rather than allowing material and debris to fall into channel. This will keep channel open for conveyance.

Lake Stevens Lake Outlet Study Executive Summary

Lake Stevens Outlet Study

Lake Stevens Outlet Study Attachments

Lake Stevens Outlet Study Project Summary Sheets

Lake Outlet Study Presentation to City Council - 11/10/2020