Stormwater report highlights impact of MLT growth on water quality

(Photo courtesy of the City of Mountlake Terrace.)

The effects of future population growth on water quality – including stormwater runoff and related pollution – were among the concerns identified by Mountlake Terrace Stormwater Program Manager Laura Reed during a report to the city council earlier this month.

The city’s stormwater plan must be updated every six years to identify and meet new state environmental requirements, address stormwater infrastructure needs, alleviate flooding and improve water quality. Reed reviewed the city’s 2025-2031 Stormwater Comprehensive Plan during the Jan. 9 city council meeting.

Stormwater staff have identified possible impacts of  expected population growth in the city over the next six years and into the following decades.

Stormwater Program Manager Laura Reed discusses the role of trees in mitigating flooding and heat during the Jan. 9, 2025, city council meeting.

Long-range concerns:

– Pollution risk increases with increased population

– More inspection resources will be needed for new public and private stormwater systems

– Small developments fall below thresholds for flow control and water quality treatment

– Climate change: Increasing storm intensity likely to cause more flooding; heat island effects

– Planning for future fish passage barrier removals (including budget-level cost)

– Consideration of regional stormwater facilities

Reed said that increased pollution is inevitable, as “just having more cars on the street” increases chemical runoff from the roads. However, it can be contained with the proper infrastructure.

Another concern expressed in the Comprehensive Plan was single parcel developments. Reed explained that they are not large enough to trigger the flood control system, and that multiple are a “death of a thousand cuts scenario.”

Reed said that climate change can exacerbate flooding issues with more severe weather fluctuations and intense storms. That, in turn, causes more flooding, hotter summers and the development of heat islands in urban areas. The city’s new permitting standards have been designed to help compensate for the anticipated changes.

A heat map of Mountlake Terrace shows the hottest areas, known as heat islands. (Image courtesy of the City of Mountlake Terrace.)

City staff has proposed the following ecology updates to permit requirements to mitigate pollution and climate change effects in the coming years:

– Tree canopy goals, policies and documentation

– Reduction of polychlorinated biphenyls (PCBs) and per- and polyfluoroalkyl substances (PFAS), also known as forever chemicals.

– Water quality treatment retrofits for existing developments

– Smaller road projects to trigger water quality treatment

– An updated stormwater technical manual for new development and redevelopment

– Additional documentation for the street sweeping program and outfall mapping

Reed said that trees will have multiple uses as climate change progresses. Not only will they help cool heat islands, where temperatures have consistently reached 94 degrees in the summer, they will help soak water into the ground and stabilize the earth with their roots.

However, increased severe weather and population will increase the load on the city’s current stormwater infrastructure, which Reed said was “pretty small.” Smaller systems tend to go into bypass mode more frequently when overloaded.

Reed explained that a regional facility with surrounding cities is one option to meet future stormwater needs. She mentioned that there are caveats, such as developers’ expenses, which are passed on to consumers.

A map showing the location of proposed capital improvement projects. (Image courtesy of the City of Mountlake Terrace.)

Proposed stormwater capital improvement projects to address the new requirements:

– 220th Street Southwest/70th Avenue West water quality upgrades

– Complex/deep storm pipe repair

– Sheet pile dam removal

– Wildermere pond retrofit

– Main Street Phase II stormwater improvements

Reed said the Hall Creek Sheet Pile was built in the 1980s to prevent sediment from entering Lake Ballinger. Removing it will eliminate a fish barrier and mitigate flooding on 216th Street Southwest. Reed said the sheet pile was so effective that it created a backwater effect that “floods us out on the regular.”

Wildermere Pond is a detention pond built in 1980 but has filled with sediment and no longer serves its function. Currently, the pond exceeds the state’s water quality standard concerning bacteria.

Stormwater utility rates will go up by 1.5% once a year from 2027 – 2030. (Image courtesy of the City of Mountlake Terrace.)

While all these improvements do not mean an instant price increase for customers, rates will increase in two years, she said.

“The stormwater fund is healthy enough to where we don’t need to raise rates until 2027,” Reed said.

The utilities will see a 6% increase spread over four years. Starting in 2027, the rates will increase by 1.5% annually until 2030. New stormwater rates are based on the anticipated future revenue requirements from the stormwater comprehensive plan.

Reed said that the cost of the stormwater plan was budgeted at $188,000 and was delivered for $148,000.

“So, we’re $40,000 under budget, which I’m happy about,” Reed said.

A public hearing is scheduled for Feb. 6.

Click here to view the presentation materials.

To view the Stormwater Comprehensive Plan, click here.

— By Rick Sinnett

  1. There is no real estate in this city for small developments and if there were as a builder I can tell you most contractors avoid this city even for remodel work because of your inspectors.

  2. I just finished a major remodel of my house and MLT inspectors were wonderful, especially Greg and Maggie!

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