Impervious Surfaces, Streets, and Parking Facilities

Hard surfaces, also called “impervious surfaces,” generate stormwater runoff. This stormwater has negative environmental impacts because it channels pollution into the Puget Sound. In addition, costly public infrastructure is required to treat this stormwater. Reducing the amount of impervious surface can reduce the amount of stormwater, and therefore, pollution going into the Puget Sound.

The following policy issues are currently being discussed by the Planning Commission. The City welcomes your comments and thoughts on these issues. You can post your comments on these topics by clicking on the “comments” hyperlink below.

Limits on Impervious Surface
The existing code does not place limits on the amount of impervious surface. This means that new developments can, in theory, pave over most of the project site.   If the City places limits on the amount of impervious surface in new developments, developers will be required to use pervious pavements, which absorb stormwater or could leave part of their property undeveloped called "native vegetation areas." Private property owners would be required to maintain pervious pavements and native vegetation areas.   

Policy question: Should the City place limits on the amount of impervious surface in new developments? Should these limits be voluntary, incentive-based, or mandatory?

LID Streets
Streets make up the largest portion of impervious surface in urban areas. Local streets comprise 60-80% of all road surfaces alone. As a result, streets create higher stormwater volumes and pollutants than any other source. The LID discussion on how to make streets more stormwater-friendly is focused on the following factors: road layout, road width, and road surface.

Maintenance is the central issue with LID streets. City staff anticipate that pervious pavements used in LID streets will require more maintenance than streets currently do. In addition, rain gardens (bioretnetion facilities), which collect and treat stormwater, will also require more maintenance. The rain gardens will need to be maintained either by the City or by property owners.

Policy Question: Should LID streets be voluntary, incentive-based, or mandatory in new developments? Positive environmental impacts increase when LID streets become mandatory. However maintenance costs for the City or property owners also increase.

LID Parking Facilities
In terms of the total amount of impervious surface in urban areas, commercial parking areas are second only to streets. The LID discussion focuses on reducing the amount of impervious surface in parking lots by addressing the following three issues: pervious surfaces, rain gardens, and setting parking maximums.

We can expect reduced environmental impacts when LID (pervious surfaces and rain gardens) are installed in new parking lots. Since commercial parking lots are maintained by tenants or owners, staff does not expect an increase in City costs by requiring LID parking facilities. There may be a small increase in maintenance costs to property owners, if pervious pavements and raingardens are required in commercial parking lots. Although, there would also be some cost savings to property owners since the on-site stormwater pond would be smaller.

Most cities use parking minimum requirements as a way of ensuring that there is enough parking for each use. Over time, studies have shown that a large portion of commercial parking facilities, like the kind you would see in shopping centers, go unused. That is to say, at least 20% of the spaces are vacant most of the time, except when stores are busy, such as during the holidays. Because there is so much unused parking, many cities now set a limit on the number of parking spaces non-residential uses can have. Minimum parking requirements may stay in place, so that there is a range of parking requirements for each use (a minimum AND a maximum). This reduces the amount of impervious surface in urban areas, and therefore stormwater runoff. It also makes more property available for other uses, such as building area and landscaping.

Given these facts, there are two policy questions that need to be answered: (1) “Should LID parking facilities be voluntary, incentive-based, or mandatory? and (2) Should the City consider setting parking maximums for non-residential uses, i.e. putting a limit on the number of parking spaces for commercial, industrial, and office uses?