VOLUME I – Core Technical Requirements and Site Planning


1.1 Objective

The objective of this manual is to provide guidance on the measures necessary to control the quantity and quality of stormwater produced by new development and redevelopment, such that they comply with water quality standards and contribute to the protection of beneficial uses of the receiving waters. Application of appropriate core requirements and Best Management Practices (BMPs) identified in this manual are necessary but sometimes insufficient measures to achieve these objectives. (See Section 1.7, Effects of Urbanization).

Water quality standards include:

•    Chapter 173-200 of the Washington Administrative Code (WAC) , Water Quality Standards for Ground Waters of the State of Washington

•    Chapter 173-201A WAC, Water Quality Standards for Surface Waters of the State of Washington

•    Chapter 173-204 WAC, Sediment Management Standards

This manual identifies core requirements for development and redevelopment projects of all sizes and provides guidance concerning how to prepare and implement Drainage Control Plans. These requirements are, in turn, satisfied by the application of BMPs from Volumes II through V. Projects that follow this approach will apply reasonable, technology-based BMPs and water quality-based BMPs to reduce the adverse impacts of stormwater. This manual is applicable to all types of land development – including residential, commercial, industrial, and roads. Manuals with a more-specific focus, such as a Highway Runoff Manual, may provide more appropriate guidance to the intended audience

Where requirements in this document are also covered in any other law, ordinance, resolution, rule, regulation, or similar, the more restrictive shall govern.

If any provisions of this manual, or their application to any person or property, are amended or held to be invalid, the remainder of the provisions of this manual, in their application to other persons or circumstances, shall not be affected.

Federal, state, and local permitting authorities with jurisdiction can require more stringent measures that are deemed necessary to meet locally established goals, state water quality standards, or other established natural resource or drainage objectives.

This manual has been adopted by City of Olympia (City) ordinance and has force of law. Failure to comply may trigger administrative or enforcement action, and result in project delays, fines or criminal penalties.

The Administrator of this Manual is the City of Olympia Public Works Director or their designee. General technical questions or interpretation of this Manual should be directed to Public Works – Water Resources Program staff.

This manual contains both uniform requirements, which are normally preceded by “shall,” “will” or “must,” and situation-specific requirements, which are normally preceded by “may.” For any project, the City Stormwater Manual Administrator is authorized to determine if situation-specific requirements are applicable.

This manual can also help to identifying options for retrofitting BMPs in existing developments. Retrofitting stormwater BMPs into existing developed areas will be necessary in many cases to meet federal Clean Water Act and state Water Pollution Control Act (Chapter 90.48 RCW) requirements.

The Washington State Department of Ecology (Ecology) does not have guidance specifically for retrofit situations (not including redevelopment situations). Application of BMPs from this manual is encouraged. However, there can be site constraints that make the strict application of these BMPs difficult.

1.2 Applicability to City of Olympia

This stormwater manual applies to all of the City of Olympia (City). This manual is designed to be equivalent to Ecology’s 2012 Stormwater Management Manual for Western Washington (Ecology stormwater manual) with 2014 revisions. The Ecology stormwater manual was originally developed in response to a directive of the Puget Sound Water Quality Management Plan (PSWQA 1987 et seq.). The Puget Sound Water Quality Authority (since replaced by the Puget Sound Partnership, PSP) recognized the need for overall guidance for stormwater quality improvement. It incorporated requirements in its plan to implement a cohesive, integrated stormwater management approach through the development and implementation of programs by local jurisdictions, and the development of rules, permits and guidance by Ecology.

The Puget Sound Water Quality Management Plan included a stormwater element (SW-2.1) requiring Ecology to develop a stormwater technical manual for use by local jurisdictions. This manual was developed to meet this requirement. Ecology has found that the concepts developed for the Puget Sound Basin are applicable throughout western Washington.

Information describing how this manual relates to the Puget Sound Water Quality Management Plan (now the Puget Sound Action Agenda) is included in Volume I, Section 1.6.4.

1.3 Overview of Manual Content

To accomplish the objective described in Volume I, Section 1.1, the manual includes the following:

•    Core Requirements that cover a range of issues, such as preparation of Drainage Control Plans, pollution prevention during the construction phase of a project, control of potential pollutant sources, treatment of runoff, control of stormwater flow volumes, protection of wetlands, and long-term operation and maintenance. The Core Requirements applicable to a project vary depending on the type and size of the proposed project.

•    Best Management Practices (BMPs) that can be used to meet the core requirements. BMPs are schedules of activities, prohibitions of practices, maintenance procedures, managerial practices, or structural features that prevent or reduce pollutants or other adverse impacts to waters of Washington State. BMPs are divided into those for short-term control of stormwater from construction sites, and those addressing long-term management of stormwater at developed sites. Long-term BMPs are further subdivided into those covering management of the volume and timing of stormwater flows, prevention of pollution from potential sources, and treatment of runoff to remove sediment and other pollutants.

•    Guidance on how to prepare and implement Drainage Control Plans. The Drainage Control Plan is a comprehensive package of plans, reports, and site drawings that describes existing site conditions, explains development plans, examines potential off-site effects, identifies applicable Core Requirements, and proposes stormwater controls for both the construction phase and long-term stormwater management. The project proponent submits the Drainage Control Plan to state and city authorities with jurisdiction, who use the plan to evaluate a proposed project for compliance with stormwater requirements.

1.3.1 Volume Content

Volume I of this manual serves as an introduction and covers several key elements of developing the Drainage Control Plan. The remaining volumes of this manual cover BMPs for specific aspects of stormwater management. Volumes II through V are organized as follows:

•    Volume II covers BMPs for short-term stormwater management at construction sites.

•    Volume III covers hydrologic analysis and BMPs to control flow volumes from developed sites.

•    Volume IV addresses BMPs to minimize pollution generated by potential pollution sources at developed sites and post construction inspection and maintenance of stormwater facilities.

•    Volume V presents BMPs to treat runoff that contains sediment or other pollutants from developed sites.

1.3.2 Organization of Volume I

Following this introduction, Volume I contains three additional chapters. Volume I, Chapter 2 identifies the Core Requirements for stormwater management at all new development and redevelopment projects. Volume I, Chapter 3 describes the Drainage Control Plan, and provides step-by-step guidance on how to develop these plans. Volume I, Chapter 4 describes the process for selecting BMPs for long-term management of stormwater flows and quality. Appendices are included to support these topics. Volume I also includes the Glossary for all five volumes of the stormwater manual.

1.4 How to Use this Manual

This manual has applications for a variety of users. Project proponents should start by reading Volume I, Chapter 3. It explains how to complete drainage control plans.

Note that this stormwater management manual is based on Ecology’s Stormwater Management Manual for Western Washington (2012).

Other Federal, State, and local permits may refer to the Ecology Manual or the BMPs contained in that manual. For example, the Industrial Stormwater General Permit and the Construction Stormwater General Permit refer to this manual. In those cases, affected permit-holders or applicants should use Ecology’s manual for specific guidance on how to comply with those permit conditions.

1.5 Development of Best Management Practices for Stormwater Management

1.5.1 Best Management Practices (BMPs)

The method by which the manual controls the adverse impacts of development and redevelopment is through the application of Best Management Practices.

Best Management Practices are defined as schedules of activities, prohibitions of practices, maintenance procedures, and structural and/or managerial practices, that when used singly or in combination, prevent or reduce the release of pollutants and other adverse impacts to waters of Washington State. The types of BMPs are source control, treatment, and flow control. BMPs that involve construction of engineered structures are often referred to as facilities in this manual. For instance, the BMPs referenced in the menus of Volume V, Chapter 3 are called treatment facilities.

The primary purpose of using BMPs is to protect beneficial uses of water resources through the reduction of pollutant loads and concentrations, through reduction of discharges (volumetric flow rates) causing stream channel erosion, and through reductions in deviations from natural hydrology. If it is found that, after the implementation of BMPs advocated in this manual, beneficial uses are still threatened or impaired, then additional controls may be required.

1.5.2 Source Control BMPs

Source control BMPs typically prevent pollution, or other adverse effects of stormwater, from occurring. Ecology further classifies source control BMPs as operational or structural. Examples of source control BMPs include methods as various as using mulches and covers on disturbed soil, putting roofs over outside storage areas, and berming areas to prevent stormwater run-on and pollutant runoff.

It is generally more cost effective to use source controls to prevent pollutants from entering runoff, than to treat runoff to remove pollutants. However, since source controls cannot prevent all impacts, some combination of measures will always be needed.

1.5.3 Treatment BMPs

Treatment BMPs include facilities that remove pollutants by simple gravity settling of particulate pollutants, centrifugal separation, filtration, biological uptake, and media or soil adsorption. Treatment BMPs can accomplish significant levels of pollutant load reductions if properly designed and maintained.

1.5.4 Flow Control BMPs

Flow control BMPs typically control the volume rate, frequency, and flow duration of stormwater surface runoff. The need to provide flow control BMPs depends on whether a development site discharges to a stream system or wetland, either directly or indirectly. Stream channel erosion control can be accomplished by BMPs that detain runoff flows and also by those which physically stabilize eroding streambanks. Both types of measures may be necessary in urban watersheds. Only the former is covered in this manual.

Construction of a detention pond is the most common means of meeting flow control requirements. Construction of an infiltration facility is the preferred option but is feasible only where more porous soils are available.

The concept of detention is to collect runoff from a developed area and release it at a slower rate than it enters the collection system. The reduced release rate requires temporary storage of the excess amounts in a pond with release occurring over a few hours or days. The volume of storage needed is dependent on:

1.    The size of the drainage area.

2.    The extent of disturbance of the natural vegetation, topography, and soils and creation of effective impervious surfaces (surfaces that drain to a stormwater collection system).

How rapidly the water is allowed to leave the detention pond, i.e., the target release rates.

The 1992 Ecology manual focused primarily on controlling the peak flow release rates for recurrence intervals of concern – the 2, 10, and 100-year rates. This level of control did not adequately address the increased duration at which those high flows occur because of the increased volume of water from the developed condition as compared to the pre-developed conditions.

To protect stream channels from increased erosion, it is necessary to control the durations over which a stream channel experiences geomorphically significant flows such that the energy imparted to the stream channel does not increase significantly. Geomorphically significant flows are those that are capable of moving sediments. This target will translate into lower release rates and significantly larger detention ponds than the previous Ecology standard. The size of such a facility can be reduced by changing the extent to which a site is disturbed.

In regard to wetlands, the goal is to not alter the natural hydroperiod. This requires the control of input flows such that the wetland is within certain elevations at different times of the year and short-term elevation changes are within the desired limits. If the amount of surface water runoff draining to a wetland is increased because of land conversion from forested to impervious areas, it may be necessary to bypass some water around the wetland in the wet season. (Bypassed stormwater must still meet flow control and treatment requirements applicable to the receiving water.) If however, the wetland was fed by local ground water elevations during the dry season, the impervious surface additions and the bypassing practice may cause variations from the dry season elevations.

Because Ecology found it difficult to model water surface elevation changes, especially for riverine and slope wetlands, the new regulatory strategy is to simply try to match the pre-project surface and ground water inputs that drive the water surface elevations in wetlands. Estimates of what should be done to match inputs requires the use of a continuous runoff model. It remains to be seen whether the available continuous runoff models are sufficiently accurate to determine successful flow management strategies. Even if the modeling approaches are sufficient, it will be a challenge to simulate pre-project hydrology after significant development has occurred.

1.5.5 Construction Stormwater BMPs and On-site Stormwater Management BMPs

Construction stormwater BMPs can be source control, treatment, or flow control BMPs. Examples include stabilized construction entrances, silt fences, check dams, and sediment traps. Volume II contains temporary construction stormwater BMPs.

On-site stormwater management BMPs can be either treatment or flow control BMPs. BMPs in this category serve to infiltrate, disperse, and retain stormwater runoff on-site. Examples include bioretention, rain gardens, and permeable pavements in Volume V, Chapter 5. Other examples include downspout infiltration, downspout dispersion, and perforated sub-out connections in Volume III, Chapter 3.

1.6 Relationship of this Manual to Federal, State, and Local Regulatory Requirements

1.6.1 The Manual’s Role as Technical Guidance

This manual has been adopted by City ordinance and has force of law. Failure to comply may trigger administrative or enforcement action, and result in project delays, fines or criminal penalties.

Current law and regulations require the design, construction, operation and maintenance of stormwater systems that prevent pollution of State waters. The Manual is a guidance document which provides the City, State and Federal agencies, developers and project proponents with a stormwater management strategy to apply at the project level. If this strategy is implemented correctly, in most cases it should result in compliance with existing regulatory requirements for stormwater – including compliance with the Federal Clean Water Act, Federal Safe Drinking Water Act and State Water Pollution Control Act.

The Manual provides generic, technical guidance on measures to control the quantity and quality of stormwater runoff from new development and redevelopment projects. These measures are considered to be necessary to achieve compliance with State water quality standards and to contribute to the protection of the beneficial uses of the receiving waters (both surface and ground waters). Stormwater management techniques applied in accordance with this Manual are presumed to meet the technology-based treatment requirement of State law to provide all known available and reasonable methods of treatment, prevention and control (AKART; RCW 90.52.040 and RCW 90.48.010).

This technology-based treatment requirement does not excuse any discharge from the obligation to apply additional stormwater management practices as necessary to comply with State water quality standards. The State water quality standards include: Chapter 173-200 WAC, Water Quality Standards for Ground Waters of the State of Washington; Chapter 173-201A WAC, Water Quality Standards for Surface Waters of the State of Washington; and Chapter 173-204 WAC, Sediment Management Standards.

Following this Manual is not the only way to properly manage stormwater runoff. A municipality may adopt, or a project proponent may choose to implement other methods to protect water quality; but in those cases, they assume the responsibility of providing technical justification that the chosen methods will protect water quality (see Volume I, Section 1.6.3, Presumptive versus Demonstrative Approaches to Protecting Water Quality below).

1.6.2 More Stringent Measures and Retrofitting

Federal, State, and local government agencies with jurisdiction can require more stringent measures that are deemed necessary to meet locally established goals, State water quality standards, or other established natural resource or drainage objectives. Water cleanup plans or Total Maximum Daily Loads (TMDLs) may identify more stringent measures needed to restore water quality in an impaired water body.

This Manual is not a retrofit manual, but it can be helpful in identifying options for retrofitting BMPs to existing development. Retrofitting stormwater BMPs into existing developed areas may be necessary to meet federal Clean Water Act and state Water Pollution Control Act (Chapter 90.48 RCW) requirements. The Puget Sound Action Agenda, described in Volume I, Section 1.6.4, also includes prioritizing and implementing stormwater retrofits as one objective. In retrofit situations there frequently are site constraints that make the strict application of these BMPs difficult. In these instances, the BMPs presented here can be modified using best professional judgment to provide reasonable improvements in stormwater management.

1.6.3 Presumptive versus Demonstrative Approaches to Protecting Water Quality

Wherever a discharge permit or other water-quality-based project approval is required, project proponents may be required to document the technical basis for the design criteria used to design their stormwater management BMPs. This includes: how stormwater BMPs were selected; the pollutant removal performance expected from the selected BMPs; the scientific basis, technical studies, and(or) modeling which supports the performance claims for the selected BMPs; and an assessment of how the selected BMP will comply with State water quality standards and satisfy State AKART requirements and Federal technology-based treatment requirements.

The Manual is intended to provide project proponents, regulatory agencies and others with technically sound stormwater management practices, which are presumed to protect water quality and instream habitat – and meet the stated environmental objectives of the regulations described in this chapter. Project proponents always have the option of not following the stormwater management practices in this Manual. However, if a project proponent chooses not to follow the practices in the Manual then the project proponent may be required to individually demonstrate that the project will not adversely impact water quality by collecting and providing appropriate supporting data to show that the alternative approach is protective of water quality and satisfies State and federal water quality laws.

Figure 1.6.1 graphically depicts the relation between the presumptive approach (the use of this Manual) and the demonstrative approach for achieving the environmental objectives of the standards. Both the presumptive and demonstrative approaches are based on best available science and result from existing Federal and State laws that require stormwater treatment systems to be properly designed, constructed, maintained and operated to:

1.    Prevent pollution of state waters and protect water quality, including compliance with state water quality standards.

2.    Satisfy state requirements for all known available and reasonable methods of prevention, control and treatment (AKART) of wastes prior to discharge to waters of the State.

3.    Satisfy the federal technology based treatment requirements under 40 CFR part 125.3.

Under the demonstration approach, the timeline and expectations for providing technical justification of stormwater management practices will depend on the complexity of the individual project and the nature of the receiving environment. In each case, the project proponent may be asked to document to the satisfaction of the permitting agency or other approval authority that the practices they have selected will result in compliance with the water quality protection requirements of the permit or other local, State, or Federal water-quality-based project approval condition. This approach may be more cost effective for large, complex or unusual types of projects.

Project proponents that choose to follow the stormwater management approaches contained in Ecology approved stormwater technical manuals are presumed to have satisfied this demonstration requirement and do not need to provide technical justification to support the selection of BMPs for the project. Following the stormwater management practices in this Manual means adhering to the guidance provided for proper selection, design, construction, implementation, operation and maintenance of BMPs. This approach will generally be more cost effective for typical development and redevelopment projects.

Ecology lists approved equivalent stormwater management manuals on the Department of Ecology website.

The following sub-sections will explain the relationship of the manual to various programs, permits, and planning efforts.

Both the presumptive and demonstrative approaches are based on using best available science to protect water quality. See the glossary for definitions.

View Figure 1.6.1 Relation between Environmental Science and Standards in Regulation.

1.6.4 The Puget Sound Action Agenda

The Puget Sound Partnership’s 2014/2015 Action Agenda lays out the work needed to protect and restore Puget Sound into the future. It is intended to drive investment and action. The Plan identifies three strategic initiatives to help prioritize near-term actions. “Prevention of pollution from urban stormwater runoff” is one of the strategic initiatives.

The Plan includes 29 strategies to achieve recovery targets, 106 sub-strategies to provide a narrower focus for the strategies and to develop near-term actions. The plan identifies about 150 regional and 150 local near-term actions. The strategy most aligned with this manual is to “Prevent, reduce, and control the sources of contaminants entering Puget Sound.” Within that strategy, the sub-strategies and the near-term actions under these sub-strategies in which Ecology is identified as the “owner” of the action follows:

Sub-Strategy: Prevent problems from new development at the site and subdivision scale.

•    NPDES Permits: Ecology will issue municipal stormwater permits for western Washington and provide financial assistance to permittees for implementation, particularly for code changes, stormwater system mapping, operations and maintenance, inspections and enforcement. This will require additional resources to Ecology for permit oversight, technical assistance, and enforcement. Ecology will provide incentives to NPDES permittees who, by interlocal agreement, lead or carry out regional or watershed scale NPDES implementation

•    Stormwater Treatment Standards: Ecology will evaluate under which circumstances (i.e., for which pollutants, from which land uses) discharges to Puget Sound should be required to provide treatment beyond sediment removal (i.e., TSS removal) to help meet 2020 recovery targets.

•    Stormwater management outside permitted areas: Ecology, in coordination with DOH, will identify two high priority shellfish growing areas degraded by urban stormwater discharges and work with local governments and other key parties to reduce these impacts to the areas.

•    New development under earlier stormwater programs: Ecology will initiate a process to assess projected implications and impacts of current state law concerning the level of stormwater control from new development approved under earlier stormwater programs.

Sub-Strategy: Control Sources of Pollutants

•    Compliance assurance program: Ecology and local governments will increase inspection, technical assistance, and enforcement programs for high-priority businesses and at construction sites.

Sub-Strategy: Provide focused stormwater-related education, training, and assistance.

•    Low Impact Development training and certification: Ecology will provide focused training for local government staff on Low Impact Development project review, and inspections and approvals, as well as to local government staff and private sector on maintenance. Develop new professional certification for stormwater maintenance specialists. Provide business staff and contractors with training on source control, spill recognition, spill response, and erosion control.

The Action Agenda includes many other stormwater-related sub-strategies and near-term actions. The Action Agenda is available at the Puget Sound Partnership website.

1.6.7 Industrial Stormwater General Permit

Facilities covered under Ecology’s Industrial Stormwater General Permit (i.e. NPDES and State Waste Discharge General Permit for Stormwater Discharges Associated With Industrial Activities) must manage stormwater in accordance with specific terms and conditions including: the development and implementation of an Industrial Stormwater Pollution Prevention Plan (Industrial SWPPP), monitoring, reporting, and ongoing adaptive management based on sampling and inspections.

The Industrial Stormwater General Permit (ISGP) requires Industrial SWPPPs to include certain mandatory Best Management Practices (BMPs), including those BMPs identified as “applicable” to specific industrial activities in Volume IV and V of the this manual. Facilities with new development or redevelopment must evaluate whether flow control BMPs are necessary. BMPs must be consistent with this manual, or other stormwater management guidance documents that are approved by Ecology and incorporated into the ISGP. Facilities may also use alternative BMPs if their Industrial SWPPP includes documentation that the BMPs selected are demonstrably equivalent to practices contained in stormwater technical manuals approved by Ecology, including the proper selection, implementation, and maintenance of all applicable and appropriate best management practices for on-site pollution control.

Ecology’s Industrial Stormwater Webpage has a fill-in-the-blank Industrial SWPPP template for use by industrial facilities.

ISGP facilities are required to update their Industrial SWPPPs and perform corrective actions if stormwater monitoring results exceed “benchmark” or indicator values. Facilities that trigger corrective actions under the ISGP, or otherwise need to update their SWPPP, should consider:

1.    “Recommended” operational and structural source control BMPs listed in Volume IV.

2.    Treatment BMPs listed in Volume V.

3.    Erosion and sediment control BMPs listed in Volume II (e.g., if turbidity, sediment, or associated pollutants need to be addressed).

4.    Treatment BMPs that have been evaluated through Ecology’s TAPE or C-TAPE program.

5.    BMPs that are “demonstrably equivalent”, as defined by the ISGP.

1.6.9 Construction Stormwater General Permit

Coverage under the CSWGP is generally required for any clearing, grading, or excavating if the project site discharges:

•    Stormwater from the site into surface water(s) State, or

•    Into storm drainage systems that discharge to a surface water(s) of the State.


•    Disturbs one or more acres of land area, or

•    Disturb less than one acre of land area, if the project or activity is part of a larger common plan of development or sale.

Any construction activity discharging stormwater that Ecology and/or the City determines to be a significant contributor of pollutants to waters of the State may also require permit coverage, regardless of project size, at the discretion of the agency.

The permit requires application of stabilization and structural practices to reduce the potential for erosion and the discharge of sediments from the site. The stabilization and structural practices cited in the permit are similar to the core requirements for sedimentation and erosion control in Volume I of the SWMM.

The permit also requires construction sites within Western Washington to implement stormwater BMPs contained in stormwater management manuals published or approved by Ecology, or BMPs that are demonstrably equivalent. Volume II of this manual further describes the requirements and BMPs appropriate for managing construction site stormwater.

1.6.10 Endangered Species Act

With the listing of multiple species of salmon as threatened or endangered across much of Washington State, and the probability of more listings in the future, implementation of the requirements of the Endangered Species Act impacts urban stormwater management. Provisions of the Endangered Species Act can apply to stormwater management include the Section 4(d) rules, Section 7 consultations, and Section 10 Habitat Conservation Plans (HCP).

Under Section 4(d) of the statute, the federal government issues regulations to provide for the conservation of the species. A 4(d) rule may require new development and redevelopment to comply with specific requirements.

Under Section 7 of the statute, all federal agencies must insure that any action they authorize, fund, or carry out is not likely to jeopardize the continued existence of any endangered or threatened species (or a species proposed for listing), nor result in the destruction or adverse modification of designated critical habitat. The responsibility for initially determining whether jeopardy is likely to occur rests with the "action" agency. If an action "may affect" a listed species, the "action" agency must consult with National Oceanic and Atmospheric Administration Fisheries Service (NOAA Fisheries), or the U.S. Fish and Wildlife Service (USFWS) depending on the species involved, to determine whether jeopardy is likely to occur.

Where NOAA Fisheries or USFWS believes that jeopardy would result, it must specify reasonable and prudent alternatives to the action that would avoid jeopardy if any such alternatives are available. If the "action" agency rejects these, the action cannot proceed.

Under Section 10 of the ESA, through voluntary agreements with the federal government that provide protections to an endangered species, a non-federal applicant may commit an "incidental take" of individuals of that species as long as it is incidental to an otherwise lawful activity (such as developing land or building a road). This provision of the ESA may help resolve conflicts between development pressures and endangered species protection. A "Habitat Conservation Plan" (HCP) is an example of this type of agreement. Under an HCP, the applicant’s plan must:

•    Outline the impact that will likely result from the taking;

•    List steps the applicant will take to minimize and mitigate such impacts, and funding available to implement such steps; and

•    Include alternative actions the applicant considered and reasons alternative acts are not being used.

The federal government may grant a permit if it finds that the taking will be incidental; the applicant will minimize and mitigate impacts of taking; and the applicant will ensure that adequate funding for the conservation plan will be provided. The USFWS and NOAA Fisheries may require additional measures as necessary or appropriate for purposes of the plan.

1.6.11 Section 401 Water Quality Certifications

For projects that require a fill or dredge permit under Section 404 of the Clean Water Act, Ecology must certify to the permitting agency, the U.S. Army Corps of Engineers, that the proposed project will not violate water quality standards. In order to make such a determination, Ecology may do a more specific review of the potential impacts of a stormwater discharge from the construction phase of the project and from the completed project. As a result of that review, Ecology may condition its certification to require:

•    Application of the core requirements and BMPs in this manual; or

•    Application of more stringent requirements.

1.6.12 Hydraulic Project Approvals (HPAs)

Under Chapter 77.55 RCW, the Hydraulics Act, the Washington State Department of Fish and Wildlife has the authority to require actions when stormwater discharges related to a project would change the natural flow or bed of state waters. The implementing mechanism is the issuance of a Hydraulics Project Approval (HPA) permit. In exercising this authority, Fish and Wildlife may require:

•    Compliance with the provisions of this manual; or

•    Application of more stringent requirements that they determine are necessary to meet their statutory obligations to protect fish and wildlife.

1.6.13 Aquatic Lands Use Authorizations

The Department of Natural Resources (DNR), as the steward of public aquatic lands, may require a stormwater outfall to have a valid use authorization, and to avoid or mitigate resource impacts. Through its use authorizations, which are issued under authority of Chapters 79.105 through 79.140 RCW, and in accordance with Chapter 332-30 WAC, DNR may require:

•    Compliance with the provisions of this manual; or

•    Application of more stringent requirements that they determine are necessary to meet their statutory obligations to protect the quality of the state’s aquatic lands.

1.6.14 Requirements Identified through Watershed/Basin Planning or Total Maximum Daily Loads

A number of the requirements of this manual can be superseded by the adoption of ordinances and rules to implement the recommendations of watershed plans or basin plans. Local governments may initiate their own watershed/ basin planning processes to identify more stringent or alternative requirements. They may also choose to develop a watershed plan in accordance with the Watershed Management Act (Chapter 90.82 RCW) that includes the optional elements of water quality and habitat. As long as the actions or requirements identified in those plans and implemented through local or state ordinances or rules comply with applicable state and federal statutes (e.g., the federal Clean Water Act and the Endangered Species Act), they can supersede the requirements in this manual. The decisions concerning whether such locally derived requirements comply with federal and state statutes rest with the regulatory agencies responsible for implementing those statutes.

A requirement of this manual can also be superseded or added to through the adoption of actions and requirements identified in a Total Maximum Daily Load (TMDL) that is approved by the EPA. However, it is likely that at least some TMDLs will require use of the BMPs in this manual.

1.6.15 Underground Injection Control Authorizations

To implement provisions of the federal Safe Drinking Water Act (see Federal UIC regulations, 40 CFR, Part 144), Ecology has adopted rules (Chapter 173-218 WAC) for an underground injection control (UIC) program. For more information visit Ecology’s home page for the UIC program and “Guidance for UIC Wells that Manage Stormwater.”

According to WAC 173-218-030 UIC well is defined as “a well that is used to discharge fluids into the subsurface. A UIC well is one of the following: (1) A bored, drilled or driven shaft, or dug hole whose depth is greater than the largest surface dimension; (2) An improved sinkhole; or (3) A subsurface fluid distribution system (contains perforated pipe or similar structure).”

Depending upon the manner in which it is accomplished, the discharge of stormwater into the ground can be classified as a Class V injection well. For more information and for a listing on potential stormwater facilities that may have Class V classification refer to the memorandum.

1.7 Effects of Urbanization

1.7.1 Background Conditions

Prior to the Euro-American settlement, western Washington primarily was forested in alder, maple, fir, hemlock and cedar. The area’s bountiful rainfall supported the forest and the many creeks, springs, ponds, lakes and wetlands. The forest system provided protection by intercepting rainfall in the canopy, reducing the possibility of erosion and the deposition of sediment in waterways. The trees and other vegetative cover evapotranspirated at least 40% of the rainfall. The forest duff layer absorbed large amounts of runoff releasing it slowly to the streams through shallow ground water flow.

1.7.2 Hydrologic Changes

As settlement occurs and the population grows, trees are logged and land is cleared for the addition of impervious surfaces such as rooftops, roads, parking lots, and sidewalks. Maintained landscapes that have much higher runoff characteristics typically replace the natural vegetation. The natural soil structure is also lost due to grading and compaction during construction. Roads are cut through slopes and low spots are filled. Drainage patterns are irrevocably altered. All of this results in drastic changes in the natural hydrology, including:

•    Increased volumetric flow rates of runoff

•    Increased volume of runoff

•    Decreased time for runoff to reach a natural receiving water

•    Reduced ground water recharge

•    Increased frequency and duration of high stream flows and wetlands inundation during and after wet weather

•    Reduced stream flows and wetlands water levels during the dry season

•    Greater stream velocities

View Figure 1.7.1 Changes in Hydrology after Development.

Figure 1.7.1 illustrates some of these hydrologic changes. As a consequence of these hydrology changes, stream channels are eroded by high flows and can lose summertime base flows. Increased flooding occurs. Streams lose their hydraulic complexity. Habitat is degraded and receiving water species composition is altered as explained below.

View Figure 1.7.2 Channel Stability and Land Use: Hylebos, East Lake Sammamish, Issaquah Basins.

Figure 1.7.2 (Booth and Jackson, 1997) illustrates one observed relationship between the level of development in a basin (as measured by effective, not total, impervious area), the changes in the recurrence of modeled stream flows, and the resultant streambank instability and channel erosion. These data show that even a crude measure of stream degradation, “channel instability,” shows significant changes at relatively low levels of urban development. More sensitive measures, such as biological indicators (see Volume I, Section 1.7.4), document degradation at even lower levels of human activity.

1.7.3 Water Quality Changes

Urbanization also causes an increase in the types and quantities of pollutants in surface and ground waters. Runoff from urban areas has been shown to contain many different types of pollutants, depending on the nature of the activities in those areas. Table 1.7.1, from an analysis of Oregon urban runoff water quality monitoring data collected from 1990 to 1996, shows mean concentrations for a limited number of pollutants from different land uses. (Strecker et al, 1997)

Table 1.7.1
Mean Concentrations of Selected Pollutants in Runoff from Different Land Uses

Land Use



Total Cu


Total Zn


Dissolved Cu


Total P


In-pipe Industry






Instream Industry






























Note: In-pipe industry means the samples were taken in stormwater pipes. Instream industry means the samples were taken in streams flowing through industrial areas. Samples for all other categories were taken within stormwater pipes.

The runoff from roads and highways is contaminated with pollutants from vehicles. Oil and grease, polynuclear aromatic hydrocarbons (PAH’s), lead, zinc, copper, cadmium, as well as sediments (soil particles) and road salts are typical pollutants in road runoff. Runoff from industrial areas typically contains even more types of heavy metals, sediments, and a broad range of man-made organic pollutants, including phthalates, PAH’s, and other petroleum hydrocarbons. Residential areas contribute the same road-based pollutants to runoff, as well as herbicides, pesticides, nutrients (from fertilizers), bacteria and viruses (from animal waste). All of these contaminants can seriously impair beneficial uses of receiving waters.

Regardless of the eventual land use conversion, the sediment load produced by a construction site can turn the receiving waters turbid and be deposited over the natural sediments of the receiving water.

The pollutants added by urbanization can be dissolved in the water column or can be attached to particulates that settle in streambeds, lakes, wetlands, or marine estuaries. A number of urban bays in Puget Sound have contaminated sediments due to pollutants associated with particulates in stormwater runoff.

Urbanization also tends to cause changes in water temperature. Heated stormwater from impervious surfaces and exposed treatment and detention ponds discharges to streams with less riparian vegetation for shade. Urbanization also reduces ground water recharge, which reduces sources of cool ground water inputs to streams. In winter, stream temperatures may lower due to loss of riparian cover. There is also concern that the replacement of warmer ground water inputs with colder surface runoff during colder periods may have biological impacts.

1.7.4 Biological Changes

The hydrologic and water quality changes result in changes to the biological systems that were supported by the natural hydrologic system. In particular, aquatic life is greatly affected by urbanization. Habitats are drastically altered when a stream changes its physical configuration and substrate due to increased flows. Natural riffles, pools, gravel bars and other areas are altered or destroyed. These and other alterations produce a habitat structure that is very different from the one in which the resident aquatic life evolved. For example, spawning areas, particularly those of salmonids, are lost. Fine sediments imbed stream gravels and suffocate salmon redds. The complex food web is destroyed and is replaced by a biological system that can tolerate the changes. However, that biological community is typically not as complex, is less desirable, and is unstable due to the ongoing rapid changes in the new hydrologic regime.

Significant and detectable changes in the biological community of Puget Sound lowland streams begin early in the urbanization process. May et al (1997) reported changes in the 5-10% total impervious area range of a watershed. Figure 1.7.3 from May et al (1997) shows the relationship observed between the Benthic Index of Biotic Integrity (B-IBI) developed by Kleindl (1995) and Karr (1991), and the extent of watershed urbanization as estimated by the percentage of total impervious area (% TIA). Also shown in the figure is the correlation between the abundance ratio of juvenile coho salmon to cutthroat trout (Lucchetti and Fuerstenberg 1993) and the extent of urbanization.

The biological communities in wetlands are also severely impacted and altered by the hydrological changes. Relatively small changes in the natural water elevation fluctuations can cause dramatic shifts in vegetative and animal species composition.

In addition, the toxic pollutants in the water column such as pesticides, soaps, and metals can have immediate and long-term lethal impacts. Toxic pollutants in sediments can yield similar impacts with the lesions and cancers in bottom fish of urban bays serving as a prime example.

A rise in water temperature can have direct lethal effects. It reduces the maximum available dissolved oxygen and may cause algae blooms that further reduce the amount of dissolved oxygen in the water.

View Figure 1.7.3 Relationship between Basin Development and Biotic Integrity in Puget Sound Lowland Streams.

1.7.5 The Role of Land Use and Lifestyles

The manual’s scope is limited to managing the surface runoff generated by a new development or redevelopment project. The manual does not intend to delve deeply into site development standards or where development should be allowed. Those are land use decisions that should not be directed by this stormwater manual. The manual applies after the decision to develop a site has been made. The manual can provide site development strategies to reduce the pollutants generated and the hydrologic disruptions caused by development.

The engineered stormwater conveyance, treatment, and detention systems advocated by this and other stormwater manuals can reduce the impacts of development to water quality and hydrology. But they cannot replicate the natural hydrologic functions of the natural watershed that existed before development, nor can they remove sufficient pollutants to replicate the water quality of pre-development conditions. Ecology understands that despite the application of appropriate practices and technologies identified in this manual, some degradation of urban and suburban receiving waters will continue, and some beneficial uses will continue to be impaired or lost due to new development. This is because land development, as practiced today, is incompatible with the achievement of sustainable ecosystems. Unless development methods are adopted that cause significantly less disruption of the hydrologic cycle, the cycle of new development followed by beneficial use impairments will continue.

In recent years, researchers (May et al, 1997) and regulators (e.g., Issaquah Creek Basin and Nonpoint Action Plan, 1996) have speculated on the amount of natural land cover and soils that should be preserved in a watershed to retain sufficient hydrologic conditions to prevent stream channel degradation, maintain base flows, and contribute to achieving properly functioning conditions for salmonids. There is some agreement that preserving a high percentage (possibly 65 to 75%) of the land cover and soils in an undisturbed state is necessary. To achieve these high percentages in urban, urbanizing, and suburban watersheds, a dramatic reduction is necessary in the amount of impervious surfaces and artificially landscaped areas to accommodate our preferred housing, play, and work environments, and most significantly, our transportation choices.

Surfaces created to provide “car habitat” comprise the greatest portion of impervious areas in land development. Therefore, to make appreciable progress in reducing impervious surfaces in a watershed, we must reduce the density of our road systems, alter our road construction standards, reduce surface parking, and rely more on transportation systems that do not require such extensive impervious surfaces (rail, bicycles, walking).

Reducing the extent of impervious surfaces and increasing natural land cover in watersheds are also necessary to solve the water quality problems of sediment, temperature, toxicants, and bacteria. Changing public attitudes toward chemical use and preferred housing are also necessary to achieve healthy water ecosystems.

Until we are successful in applying land development techniques that result in matching the natural hydrologic functions and cycles of watersheds, management of the increased surface runoff is necessary to reduce the impact of the changes. Figure 1.7.3 illustrates that significant biological impacts in streams can occur at even low levels of development associated with rural areas where stormwater runoff has not been properly managed. Improving our stormwater detention, treatment, and source control management practices should help reduce the impacts of land development in urban and rural areas. We must also improve the operation and maintenance of our engineered systems so that they function as well as possible. This manual is Ecology’s latest effort to apply updated knowledge in these areas.

The question yet to be answered is whether better management – including improved treatment and detention techniques – of the increased surface runoff from developed areas can work in combination with preservation of high percentages of natural vegetation and soils on a watershed scale to yield a minimally altered hydrologic and water quality regime that protects the water-related natural resources.

In summary, implementing improved engineering techniques and drastic changes in where and how land is developed and how people live and move across the land are necessary to achieve the goals in the federal Clean Water Act – to preserve, maintain, and restore the beneficial uses of our nation’s waters.