2026 Stormwater Comprehensive Plan
This project was funded by a flood reduction project grant from the Office of Chehalis Basin.
The City of Cosmopolis (City) has developed this Stormwater Comprehensive Plan (SWCP) as a first step in building a long-term roadmap for managing the City’s existing stormwater system. Historically, the City has experienced severe flooding driven by several interconnected challenges including low-lying topography, proximity to the Chehalis River, high rainfall, and both aging and undersized stormwater infrastructure. The City is home to approximately 2,300 residents and contains communities which could be defined as overburdened within Washington State: “...minority, low-income, tribal, or indigenous populations or geographic locations in Washington State that potentially experience disproportionate environmental harms and risks [due to frequent flooding]”. Historically, the City has operated with a limited budget; the City’s 2026 operations and maintenance budget totals $44,391.
In the State of Washington, the Department of Ecology (Ecology) manages municipal stormwater systems under the National Pollutant Discharge Elimination System (NPDES) Municipal Separate Storm Sewer System (MS4) program. Due to the size of the City’s system, Cosmopolis is not permitted under an MS4 permit; instead, it is managed through a mix of local ordinances, best management practices (BMPs), and voluntary compliance with state stormwater standards.
This SWCP was developed collaboratively by City staff and HDR Engineering, Inc. (HDR) in 2025 and 2026. This is the City’s first comprehensive stormwater planning document and is intended to provide an evaluation of the current City stormwater system leveraging staff knowledge, supplemented by hydrologic and hydraulic modeling analyses. Based on the results of these analyses, HDR has developed a 10-year Capital Improvement Plan (CIP) highlighting six major projects, as shown in Table ES-1. Project costs range from approximately $769,000 to $9.0 million per project, reflecting planning-level (Class 5) estimates. Additionally, these six projects were prioritized based on how strongly they address flooding risks, regulatory needs, system limitations, reliability concerns, and environmental improvements.
Table ES-1. Summary of recommended CIP projects
| Priority | Project Name | Planned construction year | Project cost |
|---|---|---|---|
| 1 | Mill Creek Tide Gate Study and Design | 2028 | $769,300 |
| 2 | Mill Creek Tide Gate Improvements | 2029 | $4,172,800 |
| 3 | Mill Creek Altenau St., H St., and G St. Culvert Crossing Replacement | 2030 | $8,998,300 |
| 4 | Mill Creek F St., E St. and C St. Culvert Crossing Replacement | 2035 | $8,681,600 |
| 5 | F St. NE of 1st St. Flooding and E St. Outfall Improvements | 2036 | $1,986,400 |
| 6 | J St. at 4th St. Flooding and 2nd St. Outfall Improvements | 2038 | $3,408,700 |
Project summary sheets for each of these recommended improvement projects have been developed to provide the City with a high-level scope of work for the project along with initial geotechnical, utilities, permits, and real estate and easement considerations. They are provided in Appendix D.
To support the City’s ongoing operations and jumpstart capital improvements planning, this SWCP evaluates the utility’s financial capacity to determine whether current revenues are sufficient and to identify potential rate adjustments necessary to ensure long-term system sustainability. This financial analysis highlights a structural gap between existing revenues and future costs; the City’s current stormwater revenue (~$70,000 annually) is insufficient to support rising operating costs and anticipated capital needs.
The City’s current capital funding strategy relies primarily on securing grants to implement major projects to address ongoing flooding issues. While this approach helps limit the need for large rate increases, it introduces risk because grant funding is not guaranteed and often cannot fund capital projects resulting from routine wear and tears of an aging system.
Without adjustments to rates and external grant support, projected costs will exceed revenues, leading to increasing financial deficits over time and growing flooding risks due to system failure. As a result, the City is encouraged to gradually increase rate-funded capital contributions to ensure ongoing system reliability and reduce the risk of costly emergency repairs, while continue applying for grants to fund big-ticket infrastructure upgrades. Results of this financial analysis suggest two primary rate-related changes:
- Consider implementing phased rate increases (5% annually for five years, followed by ~2% annually)
- Consider transitioning to a more equitable rate structure (based on impervious surface area)
In addition to these recommendations, external funding remains a practical supplemental resource for helping the City meet projected revenue needs. This SWCP highlights a variety of grants and loan opportunities for Washington State stormwater utilities. Typically, utilities serving overburdened or disadvantaged communities gain preferential access to funding and more favorable financial terms such as grants, principal forgiveness, and below market interest loans. In collaboration with the nearby Cities of Aberdeen and Hoquiam, Cosmopolis participated in a community analysis to better understand citywide demographics and how the City can establish effective stormwater engagement processes. This analysis is described in Appendix A.
DOCUMENTS (This section to be updated regularly as documents become available)