As of November 1st, Mercer Island's new Residential Code is in effect. While there has been a lot of focus on what these codes mean to residents, another code that homeowners and developers may not be aware of (but is something that should be emphasized even more) is the Updated Stormwater Management Standards.

These updated standards come directly from the Federal Clean Water Act and Washington's Department of Ecology, requiring cities to comply with the National Pollutant Discharge Elimination System (NPDES) for stormwater control standards. Mercer Island is behind on adopting these standards, but as of this past July, the new standards have been approved and are now in effect.

What this means for Mercer Island residents is that the city will be holding them accountable for having better storm management systems in place at their home, and these standard will affect building new structures as well. Homes must now use a system of Low Impact Development (LID) to ensure that stormwater is being drained in a way that does not cause polluted water runoff. This essentially comes down to needing a certain amount of land for water to slowly flow into the ground (otherwise known as bioretention, like rain gardens), compared to the square feet of impervious, hard surfaces on a property.

If you're currently looking to remodel or build your home and you live in a city with specific Stormwater Management Standards -- like Mercer Island, Seattle, or other places in the Puget Sound -- it is vital to research these standards before creating your budget. Each city has a different ratio of impervious surfaces to bioretentive areas; not considering these standards or the cost of bioretention / LID can impact your costs if you don't plan for it.

Take one of our clients in Seattle, for example -- they had to decide whether or not to scale back their budget when they found out that adding or replacing impervious surfaces will trigger a fee. In those cases, we have to get creative to come up with solutions that comply with the standards while also coming in under budget.

The best way to prepare yourself for these costs is to educate yourself on the new codes, understand what LID means, and ensure that your current home or new construction will comply with the codes.

What is Low Impact Development (LID)?

The City of Mercer Island gives a clear overview of what LID means in the context of sustainable development:

"One approach to sustainable development is called Low Impact Development (LID). Without development, rainfall naturally evaporates, absorbs into the ground, or is taken up by the roots of plants and trees. Now, with an influx of buildings, infrastructure, and lawns, rainfall is becoming stormwater runoff. It often carries pollutants to streams, rivers, and lakes. LID is a set of techniques that mimic natural watershed hydrology by slowing, evaporating/transpiring, and filtering water before it reaches a stream or Lake Washington.

LID contrasts with traditional drainage techniques that collect and transport water to streams quickly; these traditional techniques can damage stream channels and degrade water quality. LID uses various land planning and design practices to conserve and protect natural resources and reduce infrastructure costs."

Rain gardens are a popular option for LID in the Puget Sound area. Below is an example of a typical Rain Garden:

Credit: 12,000 Rain Gardens in Puget Sound

LID is something that residents can't ignore any longer, especially if they want to ensure that their homes meet the new city codes.

Here are Mercer Island's updated minimum requirements for Stormwater Management:

Preparation of Stormwater Site Plans
- Additional requirements for soil/infiltration testing and stormwater site plan submittals
Construction Stormwater Pollution Prevention (SWPP/TESC)
- Protect LID BMPs
Source Control of Pollution
Preservation of Natural Drainage Systems and Outfalls
On-Site Stormwater Management
- LID is required where feasible (starting at a threshold of 2,000 sf new plus replaced hard surfaces or a net increase of 500 sf or more of impervious surface.)
- Provide stormwater detention following the 1992 DOE Manual methodology where LID is not feasible and the downstream drainage system includes a watercourse or there is a capacity problem identified in the conveyance system. Refer to the detention sizing table. A fee-in-lieu of detention may be allowed by the City Engineer in some cases.

Projects that create > 5,000 sf of new plus replaced impervious surface, must meet requirements 1 through 5 above and the following:

Runoff Treatment (>5,000 sf of Pollution Generating Hard Surfaces, such as a driveway)
Flow Control (using 2014 DOE Manual)
Wetland Protection
Operation and Maintenance

The city acknowledges that LID isn't always feasible due to steep slopes and other geographic issues, but in those cases, there are alternative management practices to manage runoff.

Using LID Techniques

With the new Stormwater Management Standards, there are several ways to implement LID at your own home or when developing new property. Using even one or two of these techniques can improve the quality of the water, reduce flooding, and slow down the amount of stormwater runoff coming from polluted areas, such as the streets. These options range from being quite expensive to reasonably priced -- you can even do research into the best types of plants for bioretention. Here are the LID Techniques recommended by City of Mercer Island:

Minimize/eliminate impervious surfaces
Retain site vegetation
Amend soils with compost to improve water retention
Construct bioretention swales or cells (natural areas that have specifically-chosen plans and engineered soils that slow, filter, and absorb water)
Use of permeable pavement or pavers for roadways, driveways, and walkways
Installation of green roofs and/or rooftop gardens
Installation of cisterns or rain barrels to hold and reuse rain water

A rain garden that the City of Mercer Island recommends as an easy way to create bioretentive surfaces.

If you live in Seattle and want to learn more about how to adapt to the Stormwater Management Standards, visit Seattle's Green Stormwater Infrastructure website for more information.

With our upcoming Case Study House on Mercer Island, we'll be addressing how to adapt a 1960s rambler to be compliant with the new standards. Learn more about how to save money on site development, while also creating compliant structures, in our upcoming Case Study posts!