Grants Pass Stormwater
Management Manual

WHAT IS LOW IMPACT DEVELOPMENT?

Also known as Green Infrastructure, Green Development Practices, or Alternative Storm Water Management Systems, LID is a pattern of land development that preserves natural resources and promotes managing stormwater where it falls. LID relies on carefully selected techniques for reducing, receiving and cleaning stormwater runoff to protect and improve water availability and quality.

The Puget Sound Partnership defines LID as “a stormwater and land use management strategy that strives to mimic pre-disturbance hydrologic processes of infiltration, filtration, storage, evaporation and transpiration by emphasizing conservation, use of on-site natural features, site planning and distributed stormwater management practices that are integrated into a project design.”

WHAT IS A BMP?

The EPA defines a best management practice (BMP) as "a device, practice, or method for removing, reducing, retarding, or preventing targeted stormwater runoff constituents, pollutants and contaminants from reaching receiving waters.”

Despite widespread use of “best,” these practices may not be effective unless properly applied:

• Use several BMPs together, not just one.
• Select BMPs appropriate for the site’s opportunities and constraints and for the project’s objectives.
• Apply BMPs during the appropriate phase(s) of the project. Some BMPs are important during more than one project phase.

You will learn more about specific BMPs later in this site.

WHY USE LOW IMPACT DEVELOPMENT?

• It can cost less to build and maintain.
• It reduces water pollution, which supports community health and welfare with better drinking-water quality, reduced treatment costs and cleaner waterways for recreation and commerce.
• It reduces local flooding during storms, except at sites with a history of flooding.
• It’s the most accepted path to meeting a community’s water quality goals as defined by an MS4 (National Pollutant Discharge Elimination System [NPDES] Municipal Separate Storm Sewer System) permit, or the Total Maximum Daily Load (TMDL) program.

In 2009 the EPA wrote, “Until recently, stormwater programs established to address water quality objectives have been designed to control traditional pollutants that are commonly associated with municipal and industrial discharges, e.g., nutrients, sediment and metals. Increases in runoff volume and peak discharge rates have been regulated through state and local flood control programs. Although these programs have merit, knowledge accumulated during the past 20 years has led stormwater experts to the conclusion that conventional approaches to control runoff are not fully adequate to protect the nation’s water resources.”

Traditional development versus low impact development patterns on a residential subdivision, where each site has 110 lots. Parcels are clustered to preserve valuable existing trees and reduce impervious areas, cut and fill are significantly reduced, and relatively small-scale, BMPs are distributed throughout the site to manage rainfall and runoff where it is generated.

BMP SIZING METHODS

To design stormwater facilities, select either the Simplified Sizing Approach or the Engineered Design Approach. Each has a unique process to review and approve plans. Final selection of an approach is subject to City approval. The City may require a different approach upon review.

Some stormwater facilities require a specific approach. A combination of approaches may be used for a single project, but review and submittal requirements will be for the more intensive approach.

Simplified Sizing Approach

This approach is for projects with less than 10,000 square feet (0.23 acre) of total new or redeveloped impervious area on private property, including but not limited to roofs, concrete, asphalt, pavers, compacted gravel, compacted clay, plastic liners and clogged landscape fabric. The landowner or any other qualified, licensed professional or contractor may perform the sizing.

This approach is most appropriate for small-scale residential development, typically with limited professional design services. It is not allowed for:

• Large, complex projects
• Projects with multiple catchments totaling more than 10,000 square feet of new or redeveloped impervious area
• Projects that include a public right of way or are a Public Improvement (Municipal Code).

BMPs using this approach may require the Engineered Design Approach upon review by the City.

Engineered Design Approach

When any of the conditions for the Simplified Sizing Approach are not met, use the Engineered Design Approach. This approach may also be required as determined by the City Engineer. It requires a licensed engineer to design BMPs based on the criteria in Sections “Stormwater Management: Water Quality” and “Stormwater Management: Flow Control.” Submission of the Chapter 5 LID forms is not required with this approach.

Selecting an Approach

On this page is a "Sizing Methods" logic tree for selecting a sizing approach.

First determine if the manual applies based on the footprint of new or replaced impervious surface area. If the manual applies, select an approach.

If all the conditions in the “Simplified Sizing Approach” section and Chapter 2 requirements are met, the Simplified Sizing Approach may be used at the discretion of the City Engineer. Otherwise, use the Engineered Design Approach.

Logic tree for selecting sizing methods.

Site Planning Tools

This manual has three tools for site assessment and design.

Site Assessment Checklist. This is a comprehensive list of environmental, social and financial considerations for making decisions about best practices. Six steps, each with its own checklist items, are critical for planning:

1. On-site natural resources
2. On-site infrastructure/built environment
3. Off-site natural resources
4. Off-site infrastructure/built environment
5. Municipal, state and federal guidelines/laws
6. Programmatic requirements

BMP Suitability Matrix. This table guides gathering preliminary information to help choose appropriate BMPs based on natural conditions, land uses and water-quality considerations.

LID Implementation Form. This form steps through a preferred stormwater hierarchy, with the most preferred BMPs appearing first. The LID Worksheets help determine which information is most critical to implementing a BMP, and assist with its sizing.

The LID site design process could be consolidated into nine basic steps.

STORMWATER MANAGEMENT HIERARCHY

Choose BMPs based on their ability to mimic pre-development hydrology, following the preferred hierarchy:

1. Lay out the site to minimize impacts to natural resources and to minimize impervious areas.
2. Prevent runoff by intercepting, evaporating and/or infiltrating rainfall.
3. Reduce runoff using BMPs with surface storage (such as ponding) that infiltrate and to a lesser extent, intercept and evaporate runoff and the rainfall they receive.
4. Reduce runoff using BMPs with underground storage that infiltrate runoff.
5. Reduce runoff using flow-based BMPs without storage (i.e. conveyance) that infiltrate lower volumes and to a lesser extent, intercept and evaporate runoff and rainfall.
6. If the first five choices are not feasible, then the only remaining LID choice is to improve the water quality of runoff without significantly reducing the volume, using a lined BMP or proprietary device. This is not preferred. (See Chapter 1.)
7. To manage stormwater water quality and flow control on larger sites when LID facilities are not sufficient, use a Wet Pond, Extended Wet Pond, or Dry Detention Pond. (See Chapter 4.) Consider steps 1-6 before using ponds, especially at sites with higher infiltration rates.

LID IMPLEMENTATION FORM REQUIREMENTS

Complete the LID Implementation Form at the beginning of a project, during site planning. Revisit it when detailing a set of plans for construction.

If using the Simplified Sizing Approach to design BMPs, submit all LID Forms for the permit application package:

• Project Summary Form
• LID Forms A through C once.
• LID Forms D through J for each basin
• A relevant Worksheet for each BMP

Refer to Chapter 3 for further requirements.

If using the Engineered Design Approach to design BMPs, LID Forms and worksheets are not required. The permit application package should include the following along with a drainage report:

• Information on project summary form
• Any sizing factors used to design BMPs on-site

Refer to Chapter 3 for further requirements.

Example runoff prevention BMPs applied to a single-family home:

• Minimize Building Footprint (with a two-story house, impervious area reduced)
• Minimize Front Setbacks (impervious area reduced)
• Vegetated Roof BMP (50% prevented, 50% runoff)
• Restored Soils to lawn BMP (100% managed)
• Porous Pavement (Rainfall) BMP (100% prevented)

Some sites could generate pollutants that stormwater quality measures would not address. They have characteristics and uses involving oil and grease, toxic hydrocarbons, heavy metals, toxic compounds, solvents, abnormal pH levels, nutrients, organics, bacteria, chemicals or suspended solids. This chapter presents source controls for managing such pollutants at their source.

Industrial facilities may be subject to additional requirements through State of Oregon NPDES permits or as outlined in Oregon Administrative Rules (OAR) 340 Division 041. The Municipal Code has lists of prohibited discharges to the City’s storm sewer system.

The requirements and implementation of this chapter is in additionto applicable water-quality and flow-control requirements.

Applicants may propose alternatives to the source controls identified in this chapter, with an additional review process.

Site Uses and Characteristics That Trigger Source Controls

Projects with the following site uses and characteristics are subject to the design methodologies of this chapter:

• Fuel dispensing facilities and surrounding traffic areas
• Above-ground storage of liquid materials
• Solid-waste storage areas, containers and trash compactors
• Outdoor storage of bulk materials
• Equipment and/or vehicle washing facilities
• Stormwater and groundwater management for development on land with suspected or known contamination

Applicants are required to address all the site characteristics and uses listed in this chapter. For example, if a development includes both a fuel dispensing area and a vehicle washing facility, the source controls in both sections will apply.