VOLUME II – Construction Stormwater Pollution Prevention


1.1 Purpose of this Volume

Volume II focuses on managing stormwater impacts associated with construction activities. Best management practices (BMPs) that are properly planned, installed, and maintained can minimize stormwater impacts, such as heavy stormwater flows, soil erosion, water-borne sediment from exposed soils, and degradation of water quality, from on-site pollutant sources. Ecology’s Construction Stormwater General Permit, Ecology’s Municipal Stormwater Permits, and the City of Olympia (City) require the implementation of the BMPs listed in this volume (See Chapter 2.)

Volume II addresses the planning, design, and implementation of BMPs before and during construction projects. A collaborative planning process with all project proponents (owners, designers, contractors, engineers), and compliance reviewers is critical. Such a process can result in a high-quality, cost-effective project with excellent environmental protection. It can also minimize unnecessary risk associated with some traditional construction practices. By planning your project phasing, you will better manage your contractor’s schedule and materials.

The construction phase of a project is usually a temporary condition, ultimately giving way to permanent improvements and facilities. However, construction work may take place over an extended period of time. Ensure that all of your management practices and control facilities are of sufficient size, strength, and durability to outlast the longest possible construction schedule and the worst anticipated rainfall conditions.

Linear projects, such as roadway construction and utility installations, may present a unique set of stormwater protection challenges. You can adapt or modify many of the BMPs discussed in this volume to provide the controls needed to address these projects. It may be advantageous to phase portions of long, linear projects and apply all necessary controls to individual phases.

The Construction Stormwater Pollution Prevention Plan (SWPPP) serves as a tool for the site operator to manage the site and to avoid immediate and long-term environmental loss. Implementing a Construction SWPPP, designed in accordance with Chapter 3 and Chapter 4 of this volume, can provide a number of benefits. These include limiting adverse effects on the environment, improving the relationship between the contractor and the permitting authority staying on schedule, and saving money otherwise spent on repairing erosion.

Many of the BMPs contained in this volume can be adapted and modified to provide the erosion and sediment controls needed for other activities such as mining.

1.2 Content, Organization, and Use of this Volume

Volume II consists of four chapters that address the key considerations of preparing and implementing the Construction SWPPP. Volume II should be used in developing SWPPPs, which are a required component of Drainage Control Plans (see Volume I, Chapter 3).

Chapter 1 highlights the importance of construction stormwater management in preventing pollution of surface waters. The chapter briefly lists the 13 elements (12 elements listed in the Construction General Permit and 1 additional element covering Low Impact Development) of pollution prevention, and discusses erosion and sedimentation processes and impacts. Users should refer to Chapter 1 for an overview of construction stormwater issues.

Chapter 2 contains the regulatory requirements that apply to construction sites and their stormwater discharges. The Department of Ecology’s (Ecology) National Pollutant Discharge Elimination System (NPDES) discharge permits are discussed. Chapter 2 lists Washington’s Water Quality Standards pertaining to construction stormwater and explains how they apply to field situations. Users should consult Chapter 2 to determine how regulatory requirements apply to a construction sites, including permit requirements. Volume I, Section 1.6 contains more information about the relationships of this manual to the various levels of regulatory requirements.

Chapter 3 presents a step-by-step method for developing a Construction SWPPP and details the 13 elements. It includes lists of suggested BMPs to meet each element. Chapter 3 encourages the examination of all conditions that could affect a project’s stormwater control systems during the project construction phase. Users should read Chapter 3 to determine the organization, content, and development of a Construction SWPPP.

Chapter 4 contains BMPs for construction stormwater control and site management. The first section of Chapter 4 contains BMPs for source control. The second section addresses runoff, conveyance, and treatment BMPs. Use various combinations of these BMPs in the Construction SWPPP to satisfy each of the 13 elements applying to the project (WAC 173-201A-510). Users should also refer to Chapter 4 to design and document application of these BMPs to the project construction site.

1.3 Thirteen Elements of Construction Stormwater Pollution Prevention

The 13 Elements listed below must be considered in the development of the Construction SWPPP unless site conditions render the element unnecessary. If an element is considered unnecessary, the Construction SWPPP must provide the justification.

These elements cover the general water quality protection strategies of limiting site impacts, preventing erosion and sedimentation, and managing activities and sources.

The 13 Elements are:

1.    Preserve Vegetation/Mark Clearing Limits

2.    Establish Construction Access

3.    Control Flow Rates

4.    Install Sediment Controls

5.    Stabilize Soils

6.    Protect Slopes

7.    Protect Drain Inlets

8.    Stabilize Channels and Outlets

9.    Control Pollutants

10.    Control Dewatering

11.    Maintain BMPs

12.    Manage the Project

13.    Protect Low Impact Development BMPs

A complete description of each element and associated BMPs is given in Chapter 3.

1.4 Erosion and Sedimentation Impacts

Soil erosion and the resulting sedimentation produced by land development impacts the environment, damaging aquatic and recreational resources, as well as aesthetic qualities. Erosion and sedimentation ultimately affect everyone.

Common examples of the impacts of erosion and sedimentation are:

•    Natural, nutrient-rich topsoils erode. Re-establishing vegetation is difficult without applying soil amendments and fertilizers.

•    Silt fills culverts and storm drains, decreasing capacities and increasing flooding and maintenance frequency.

•    Detention facilities fill rapidly with sediment, decreasing storage capacity and increasing flooding.

•    Sediment clogs infiltration devices, causing failure.

•    Sediment causes obstructions in streams and harbors, requiring dredging to restore navigability.

•    Shallow areas in lakes form rapidly, resulting in growth of aquatic plants and reduced usability.

•    Nutrient loading from phosphorus and nitrogen attached to soil particles and transported to lakes and streams cause a change in the water pH, algal blooms, and oxygen depletion, leading to eutrophication and fish kills.

•    Water treatment for domestic uses becomes more difficult and costly.

•    Turbid water replaces aesthetically pleasing, clear, clean water in streams and lakes.

•    Eroded soil particles decrease the viability of macro-invertebrates and food-chain organisms, impair the feeding ability of aquatic animals, clog gill passages of fish, and reduce photosynthesis.

•    Sediment-clogged gravel diminishes fish spawning and can smother eggs or young fry.

Costs associated with these impacts may be obvious or subtle. Some are difficult to quantify, such as the loss of aesthetic values or recreational opportunities. Restoration and management of a single lake can cost millions of dollars. Reductions in spawning habitat, and subsequent reduction in salmon and trout production, cause economic losses to sport fisheries, traditional Native American fisheries, and the fishing industry. The maintenance costs of man-made structures and harbors are readily quantifiable. Citizens pay repeatedly for these avoidable costs in their tax dollars.

Effective erosion and sediment control practices on construction sites can greatly reduce undesirable environmental impacts and costs. Being aware of the erosion and sedimentation process is helpful in understanding the role of BMPs in controlling stormwater runoff.

1.5 Erosion and Sedimentation Processes

1.5.1 Soil Erosion

Soil erosion is defined as the removal of soil from its original location by the action of water, ice, gravity, or wind. In construction activities, soil erosion is largely caused by the force of falling and flowing water. Erosion by water includes the following processes (see Figure 1.5.1):

Raindrop Erosion: The direct impact of falling drops of rain on soil dislodges soil particles so that they can then be easily transported by runoff.

Sheet Erosion: The removal of a layer of exposed soil by the action of raindrop splash and runoff, as water moves in broad sheets over the land (not confined in small depressions).

Rill and Gully Erosion: As runoff concentrates in rivulets, it cuts grooves called rills into the soil surface. If the flow of water is sufficient, rills may develop into larger gullies.

Stream and Channel Erosion: Increased volume and velocity of runoff in an unprotected, confined channel may cause stream meander instability and scouring of significant portions of the stream or channel banks and bottom.

Soil erosion by wind creates a water quality problem when dust is blown into water. Dust control on paved streets using washdown waters, if not conducted properly, can also create water quality problems.

View Figure 1.5.1 Types of Erosion.

1.5.2 Sedimentation

Sedimentation is defined as the gravity-induced settling of soil particles transported by water. The process is accelerated in slower-moving, quiescent stretches of natural waterbodies or in treatment facilities such as sediment ponds and wetponds.

Sedimentation occurs when the velocity of water in which soil particles are suspended is slowed for a sufficient time to allow particles to settle. The settling rate depends on the soil particle size. Heavier particles, such as sand and gravel, settle more rapidly than fine particles such as clay and silt. Sedimentation of clay soil particles is reduced due to clay’s relative low density and electro-charged surfaces, which discourage aggregation. The presence of clay particles in stormwater runoff can result in highly turbid water, which is not amenable to treatment by settling.

Turbidity, an indirect measure of soil particles in water, is one of the primary water quality standards in Washington State law (WAC 173-201A-200). Turbidity is increased when erosion carries soil particles into receiving waters. Treating stormwater to reduce turbidity can be an expensive, difficult process with limited effectiveness. Any actions or prevention measures that reduce the volume of water needing treatment for turbidity are beneficial.

1.6 Factors Influencing Erosion Potential

The erosion potential of soils can be readily determined using various models such as the Flaxman Method or the Revised Universal Soil Loss Equation (RUSLE).

The soil erosion potential of an area, including a construction site, is determined by four interrelated factors (see Figure 1.6.1):

•    Soil characteristics

•    Vegetative cover

•    Topography

•    Climate

Collect, analyze, and use detailed information specific to the construction site for each of these four factors to provide the basis for an effective construction stormwater management system.

View Figure 1.6.1 Factors Influencing Erosion Potential.

The first three factors (soil characteristics, vegetative cover, and topography) are constant with respect to time until altered by construction. The designer, developer, and construction contractor should have a working knowledge of, and control over, these factors to provide high quality stormwater results.

The fourth factor, climate, is predictable by season, historical record, and probability of occurrence. While predicting a specific rainfall event is not possible, plan appropriate seasonal construction activity and use properly designed BMPs to minimize or avoid many of the impacts of construction stormwater runoff.

1.6.1 Soil Characteristics

The vulnerability of soil to erode is determined by soil characteristics:

Particle Size: Soils that contain high proportions of silt and very fine sand are the most erodible and are easily detached and carried away. The erodibility of soil decreases as the percentage of clay or organic matter increases; clay acts as a binder and tends to limit erodibility. Most soils with high clay content are relatively resistant to detachment by rainfall and runoff. Once eroded, however, clays are easily suspended and settle out very slowly.

Organic Content: Organic matter creates a favorable soil structure, improving its stability and permeability. This increases infiltration capacity, delays the start of erosion, and reduces the amount of runoff.

The addition of organic matter increases infiltration rates (and, therefore, reduces surface flows and erodibility), water retention, pollution control, and pore space for oxygen.

Soil Structure: Organic matter, particle size, and gradation affect soil structure, which is the arrangement, orientation, and organization of particles. When the soil system is protected from compaction, the natural decomposition of plant debris on the surface maintains a healthy soil food web. The soil food web in turn maintains the porosity both on and below the surface.

1.6.2 Vegetative Cover

Soil Permeability: Soil permeability refers to the ease with which water passes through a given soil. Well-drained and well-graded gravel and gravel mixtures with little or no silt are the least erodible soils. Their high permeability and infiltration capacity helps prevent or delay runoff.

Vegetative cover plays an extremely important role in controlling erosion by:

•    Shielding the soil surface from the impact of falling rain.

•    Slowing the velocity of runoff, thereby permitting greater infiltration.

•    Maintaining the soil’s capacity to absorb water through root zone uptake and evapotranspiration.

•    Holding soil particles in place.

Limiting the removal of existing vegetation and decreasing duration of soil exposure to rainfall events can reduce erosion. Give special consideration to preserving existing vegetation on areas with a high potential for erosion such as erodible soils, steep slopes, drainage ways, and the banks of streams. When it is necessary to remove vegetation, such as removing noxious weeds, revegetate these areas immediately.

1.6.3 Topography

The size, shape, and slope of a construction site influence the amount and rate of stormwater runoff. Each site’s unique dimensions and characteristics provide both opportunities for and limitations on the use of specific control measures to protect vulnerable areas from high runoff amounts and rates. Slope length, steepness, and surface texture are key elements in determining the volume and velocity of runoff. As slope length and/or steepness increase, the rate of runoff and the potential for erosion increases. Slope orientation is also a factor in determining erosion potential. For example, a slope that faces south and contains dry soils may provide such poor growing conditions that vegetation will be difficult to re-establish.

1.6.4 Climate

Seasonal temperatures and the frequency, intensity, and duration of rainfall are fundamental in determining amounts of runoff. As the volume and the velocity of runoff increase, the likelihood of erosion increases. Where storms are frequent, intense, or long, erosion risks are high. Seasonal changes in temperature, as well as variations in rainfall, help to define the period of the year when there is a high erosion risk. When precipitation falls as snow, erosion may not occur until the spring, when melting snow adds to the runoff, and erosion potential will be higher. Partially frozen ground reduces infiltration capacity. Rain-on-snow events are common in western Washington between 1,500- and 3,000-foot elevations.

Western Washington is characterized in fall, winter, and spring by storms that are mild and long lasting. The fall and early winter events saturate the soil profile and fill stormwater detention ponds, increasing the amount of runoff leaving the construction site. Shorter-term, more intense storms occur in the summer. These storms can cause problems if adequate BMPs have not been installed on site.