Municipal Low Impact Development Design and Implementation

For municipalities tasked with eliminating stormwater drainage, options tend to be expensive or regulatory steeple-chases with several iterations of reports, plans, approval processes, and permitting. A recent addition to the municipal toolbox of reducing stormwater runoff is the incorporation of Low Impact Development (LID) techniques that can serve as an alternative to the typical stormwater channelization and drainage systems.

The focus of LID is to improve water quality concerns with stormwater runoff; that is by using runoff on the site where it falls, as opposed to conveying the runoff to other locations and transporting pollutants, including sediments, with the runoff. Some of the techniques used include biofiltration and retention basins and swales, rain gardens, green streets, pavement minimization, and permeable pavements. In addition to reducing discharge into adjacent waterbodies, LID implementation can increase groundwater recharge and possibly lower demands on landscaping irrigation. Part of the driving force behind LID developments includes regulatory pressures from state and federal agencies. The Clean Water Act and National Pollutant Discharge Elimination System (NPDES) program require qualifying municipalities to develop and implement stormwater management programs under municipal separate storm sewer system (MS4) permits.

Exploring LID techniques may help a municipality decide if these kinds of practices would be beneficial, and incorporating LID as a stormwater discharge in the local planning codes can be a runoff reduction technique that can lower costs associated with MS4 water treatment and meet permitting planning requirements. In Oregon, the Department of Environmental Quality (DEQ) administers the MS4 program permits, and as part of the permitting requirements, some municipalities have developed stormwater management plans requiring new developments and redevelopments to implement LID where feasible.

LID used as a compliance tool in MS4 permitting

Depending on the size of the municipality, the DEQ can require either Phase I or Phase II MS4 permit, with Phase I sources have populations greater than 100,000, and Phase II sources with populations less than 100,000 and located within a Census-Bureau defined “Urbanized Area.” While the municipalities within these classifications are already aware of the MS4 requirements, entities experiencing growth may find themselves seeking waivers or become very familiar with MS4 permitting through necessary compliance.

For those entities already working within the MS4 permitting regime, LID programs within municipal control could offer compliance with permit conditions requiring “Minimum Control Measures” including:

1) Public Education and Outreach on Stormwater Impacts by identifying LID infrastructure with signage explaining the use and purpose of the bioswale, green street, permeable pavements, etc.;

2) Public Involvement/Participation through implementation LID design concepts in the local planning commissions;

3) Illicit Discharge Detection and Elimination through diverting some possible illicit discharges away from the traditional stormwater sewer system and identifying sources of the discharge closer to the originating site; and

4) Post-Construction Stormwater Management in New Development and Redevelopment through implementation of the localized LID policies and requirements, as readily identified to developers by clear planning and design standards.

For those entities not yet required to perform within the MS4 permitting regime, having LID concepts employed could serve as a technique to reduce the overall Total Maximum Daily Loads (TMDL) for stormwater sewer discharges, thereby possibly qualifying the entity for an MS4 permitting waiver under federal regulations.

LID as an alternative or compliment to Injection Systems

Rule authorized injection systems are subject to a laundry-list of certifications and requirements in order to protect groundwater supplies from contamination. Prior to authorizing an injection system, the DEQ requires developers to consider LID concepts when municipal stormwater sewer connections are unavailable, and certify that no other stormwater disposal method is appropriate. LID options offer a municipal or private entity the ability to remove stormwater without the degree of regulatory oversight as with injection systems, and can serve as either a primary way of disposing of stormwater or can complement injection system design by removing stormwater that cannot meet the quality requirements of the injection system regulations.

Issues with implementing LID

As with any proposed method of implementing local zoning regulations and design standards, LID may not be appropriate for the particular application. Planning and zoning codes should not be intended as static one-size-fits-all requirements. Issues could include high water tables, previous on-site contamination, or economic issues involving larger rights-of-way and private landowner compensation. Within the proposed local land use planning codes, drafters should incorporate flexibility into the application of LID requirements, while not making the exemptions so broad as to make implementation useless, should that be the desire of the local leadership. LID implementation could be a way for new development or redevelopment to reduce overall strormwater loads, and could be incentivized with a reduction in system development charges. While LID is not a cure-all to a municipality’s issues with stormwater drainage and treatment, it offers an alternative and an additional tool to use when stormwater discharge regulation is already an issue, or is easily visible on the horizon.