4.2.2 Minimum Requirement #2: Construction Stormwater Pollution Prevention Plan (SWPPP)

All new development and redevelopment projects are responsible for preventing erosion and discharge of sediment and other pollutants into receiving waters.

Projects that result in 2,000 square feet or more of new plus replaced hard surface area, or that disturb 7,000 square feet or more of land must prepare a Construction Stormwater Pollution Prevention Plan (SWPPP) as part of the Stormwater Site Plan (see Vol I–4.2.1 Minimum Requirement #1: Preparation of Stormwater Site Plans).

Projects below those thresholds (listed above) are not required to prepare a Construction SWPPP, but must consider all of the Construction SWPPP elements (listed below) and develop controls for all Construction SWPPP elements that pertain to the project site.

4.2.2.1 General Requirements

The Construction SWPPP shall include a narrative and drawings that describe and reference all BMPs to be implemented. The Construction SWPPP narrative shall include documentation to explain and justify the pollution prevention decisions made for the project. Each of the 13 Construction SWPPP elements must be considered and included in the Construction SWPPP unless site conditions render the element unnecessary and the exemption from that element is clearly justified in the narrative of the SWPPP.

Clearing and grading activities for developments shall be permitted only if conducted pursuant to an approved SDAP in accordance with KCC 12.10.030. These permitted clearing and grading areas and any other areas required to preserve critical or sensitive areas, buffers, native growth protection easements, or tree retention areas, shall be delineated on the site plans and the development site.

The Construction SWPPP shall be implemented beginning with initial land disturbance and until final stabilization. Sediment and erosion control BMPs shall be consistent with the BMPs contained in Volume II, Chapter 2.

Seasonal Work Limitations: From October 1 through April 30 (typically the wet season), clearing, grading, and other soil disturbing activities shall only be permitted if shown to the satisfaction of the director that silt-laden runoff will be prevented from leaving the site through a combination of the following:

1. Site conditions including existing vegetative coverage, slope, soil type and proximity to receiving waters, and

2. Limitations on activities and the extent of disturbed areas, and

3. Proposed erosion and sediment control measures.

The following activities are exempt from the seasonal clearing and grading limitations:

• Routine maintenance and necessary repair of erosion and sediment control BMPs,

• Routine maintenance of public facilities or existing utility structures that do not expose the soil or result in the removal of the vegetative cover to soil,

• Activities where there is 100% infiltration of surface water runoff within the site in approved and installed erosion and sediment control facilities.

4.2.2.2 Construction SWPPP Elements

Element 1: Preserve Vegetation/Mark Clearing Limits

• Before beginning land disturbing activities, including clearing and grading, clearly mark all clearing limits, sensitive areas and their buffers, and trees that are to be preserved within the construction area.

• Retain the duff layer, native topsoil, and natural vegetation in an undisturbed state to the maximum degree practicable.

Element 2: Establish Construction Access

• Limit construction vehicle access and exit to one route, if possible.

• Stabilize access points with a pad of quarry spalls, crushed rock, or other equivalent BMPs, to minimize tracking of sediment onto public roads.

• Locate wheel wash or tire baths on site, if the stabilized construction entrance is not effective in preventing tracking sediment onto roads.

• If sediment is tracked off site, clean the affected roadway(s) thoroughly at the end of each day, or more frequently as necessary (for example, during wet weather). Remove sediment from roads by shoveling, vacuum sweeping, or picking up and transporting the sediment to a controlled sediment disposal area.

• Conduct street washing only after sediment is removed in accordance with the above bullet.

• Control street wash wastewater by pumping back on site, or otherwise prevent it from discharging into systems tributary to waters of the State.

Element 3: Control Flow Rates

• Protect properties and waterways downstream of development sites from erosion and the associated discharge of turbid waters due to increases in the velocity and peak volumetric flow rate of stormwater runoff from the project site.

• Where necessary to comply with the bullet above, construct stormwater infiltration or detention BMPs as one of the first steps in grading. Ensure that detention facilities function properly before constructing site improvements (e.g., impervious surfaces).

• If permanent infiltration BMPs are used for temporary flow control during construction, protect these facilities from siltation during the construction phase.

Element 4: Install Sediment Controls

Design, install, and maintain effective erosion controls and sediment controls to minimize the discharge of pollutants.

• Construct sediment control BMPs (sediment ponds, traps, filters, etc.) as one of the first steps in grading. These BMPs must be functional before other land disturbing activities take place.

• Minimize sediment discharges from the site. The design, installation and maintenance of erosion and sediment controls must address factors such as the amount, frequency, intensity and duration of precipitation, the nature of resulting stormwater runoff, and soil characteristics, including the range of soil particle sizes expected to be present on the site.

• Direct stormwater runoff from disturbed areas through BMP C241: Sediment Pond (Temporary) or other appropriate sediment removal BMP, before the runoff leaves a construction site or before discharge to an infiltration facility. Runoff from fully stabilized areas may be discharged without a sediment removal BMP, but must control flow rates per Element #3, Bullet #1.

• Locate BMPs intended to trap sediment on site in a manner to avoid interference with the movement of juvenile salmonids attempting to enter off-channel areas or drainages.

• Provide and maintain natural buffers around surface waters, direct stormwater to vegetated areas to increase sediment removal, and maximize stormwater infiltration, unless infeasible.

• Where feasible, design outlet structures that withdraw impounded stormwater from the surface to avoid discharging sediment that is still suspended lower in the water column.

Element 5: Stabilize Soils

• Stabilize exposed and unworked soils by application of effective BMPs that prevent erosion. Applicable BMPs include, but are not limited to: temporary and permanent seeding, sodding, mulching, plastic covering, erosion control fabrics and matting, soil application of polyacrylamide (PAM), the early application of gravel base on areas to be paved, and dust control.

• Control stormwater volume and velocity within the site to minimize soil erosion.

• Control stormwater discharges, including both peak flow rates and total stormwater volume, to minimize erosion at outlets and to minimize downstream channel and stream bank erosion.

• Soils must not remain exposed and unworked for more than the time periods set forth below to prevent erosion:

  • During the dry season (May 1–September 30): 7 days
  • During the wet season (October 1–April 30): 2 days

• Stabilize soils at the end of the shift before a holiday or weekend if needed based on the weather forecast.

• Stabilize soil stockpiles from erosion, protect with sediment trapping measures, and where possible, locate away from storm drain inlets, waterways and drainage channels.

• Minimize the amount of soil exposed during construction activity.

• Minimize the disturbance of steep slopes.

• Minimize soil compaction and, unless infeasible, preserve topsoil.

Element 6: Protect Slopes

• Design and construct cut-and-fill slopes in a manner to minimize erosion. Applicable practices include, but are not limited to, reducing continuous length of slope with terracing and diversions, reducing slope steepness, and roughening slope surfaces (e.g., track walking).

• Divert offsite stormwater (run-on) or ground water away from slopes and disturbed areas with interceptor dikes, pipes and/or swales. Manage offsite stormwater separately from stormwater generated on site.

• At the top of slopes, collect drainage in pipe slope drains or protected channels to prevent erosion. Temporary pipe slope drains must be sized to convey the flow rate calculated by one of the following methods:

  • Single Event Hydrograph Method: The peak volumetric flow rate calculated using a 10‑minute time step from a Type 1A, 10‑year, 24‑hour frequency storm.

  OR

  • Continuous Simulation Method: The 10‑year and peak flow rate, as determined by an approved continuous runoff model with a 15‑minute time step.

The hydrologic analysis must use the existing land cover condition for predicting flow rates from tributary areas outside the project limits. For tributary areas on the project site, the analysis must use the temporary or permanent project land cover condition, whichever will produce the highest flow rates. If using the Western Washington Hydrology Model (WWHM) to predict flows, model bare soil areas as “landscaped” area.

• Place excavated material on the uphill side of trenches, consistent with safety and space considerations.

• Place check dams at regular intervals within constructed channels that are cut down a slope.

Photo 2. Example of a Shallow Gradient Slope.

Example of shallow gradient slope with berm installed at downgradient edge to minimize silt-laden runoff onto the sidewalk.

Element 7: Protect Drain Inlets

• Protect all storm drain inlets made operable during construction so that stormwater runoff does not enter the conveyance system without first being filtered or treated to remove sediment.

• Clean or remove and replace inlet protection devices when sediment has filled one-third of the available storage (unless a different standard is specified by the product manufacturer).

Element 8: Stabilize Channels and Outlets

• Design, construct, and stabilize all onsite conveyance channels to prevent erosion from the flow rate calculated by one of the following methods:

  • Single Event Hydrograph Method: The peak volumetric flow rate calculated using a 10‑minute time step from a Type 1A, 10‑year, 24‑hour frequency storm.
  • Continuous Simulation Method: The 10‑year peak flow rate, as determined by an approved continuous runoff model with a 15‑minute time step.

  The hydrologic analysis must use the existing land cover condition for predicting flow rates from tributary areas outside the project limits. For tributary areas on the project site, the analysis must use the temporary or permanent project land cover condition, whichever will produce the highest flow rates. If using WWHM to predict flows, model bare soil areas as “landscaped” area.

• Provide stabilization, including armoring material, adequate to prevent erosion of outlets, adjacent stream banks, slopes and downstream reaches at the outlets of all conveyance systems.

Element 9: Control Pollutants

Design, install, implement and maintain effective pollution prevention measures to minimize the discharge of pollutants. The project proponent must:

• Handle and dispose of all pollutants, including waste materials and demolition debris that occur on site in a manner that does not cause contamination of stormwater.

• Provide cover, containment, and protection from vandalism for all chemicals, liquid products, petroleum products, and other materials that have the potential to pose a threat to human health or the environment. Onsite fueling tanks must include secondary containment. Secondary containment means placing tanks or containers within an impervious structure capable of containing 110% of the volume contained in the largest take within the containment structure. Double-walled tanks do not require additional secondary containment.

• Conduct maintenance, fueling, and repair of heavy equipment and vehicles using spill prevention and control measures. Such activities shall be carried out no closer than 100 feet from an open channel or stream. Clean contaminated surfaces immediately following any spill incident.

• Discharge wheel wash or tire bath wastewater to a separate onsite treatment system that prevents discharge to surface water, or to the sanitary sewer, with local sewer district approval.

• Apply fertilizers and pesticides in a manner and at application rates that will not result in loss of chemical to stormwater runoff. Follow manufacturers’ label requirements for application rates and procedures.

• Use BMPs to prevent contamination of stormwater runoff by pH modifying sources. The sources for this contamination include, but are not limited to: recycled concrete stockpiles, bulk cement, cement kiln dust, fly ash, new concrete washing and curing waters, waste streams generated from concrete grinding and sawing, exposed aggregate processes, dewatering concrete vaults, concrete pumping and mixer washout waters.

• Adjust the pH of stormwater if necessary to prevent violations of water quality standards.

• Ensure that washout of concrete trucks is performed off site or in designated concrete washout areas only. Do not wash out concrete truck drums or concrete handling equipment onto the ground, or into storm drains, open ditches, streets, or streams. Washout from small concrete handling equipment may be disposed of in a formed area awaiting concrete where it will not contaminate surface or ground water. Do not dump excess concrete on site, except in designated concrete washout areas. Concrete spillage or concrete discharge directly to ground water or surface waters of the State is prohibited. Do not wash out to formed areas awaiting infiltration BMPs.

• Obtain written approval from Ecology before using chemical treatment other than carbon dioxide (CO₂), dry ice, or food grade vinegar to adjust pH.

• Uncontaminated water from water-only-based shaft drilling for construction of building, road, and bridge foundations may be infiltrated provided the wastewater is managed in a way that prohibits discharge to surface waters. Prior to infiltration, water from water-only-based shaft drilling that comes into contact with curing concrete must be neutralized until pH is in the range of 6.5 to 8.5 (su).

Photo 3. Temporary Sand Bags Divert Construction Site Stormwater.

Temporary sand bags divert construction site stormwater runoff to inlet protected with a catch basin filter sock.

Element 10: Control De-Watering

• Discharge foundation, vault, and trench dewatering water, which have similar characteristics to stormwater runoff at the site, into a controlled conveyance system before discharge to BMP C240: Sediment Trap or BMP C241: Sediment Pond (Temporary).

• Discharge clean, non-turbid dewatering water, such as well-point ground water, to systems tributary to, or directly into surface waters of the State, as specified in Element #8, provided the dewatering flow does not cause erosion or flooding of receiving waters. Do not route clean dewatering water through stormwater sediment BMPs. Note that “surface waters of the State” may exist on a construction site as well as off site; for example, a creek running through a site.

• Handle highly turbid or otherwise contaminated dewatering water separately from stormwater.

• Other dewatering treatment or disposal options may include:

  • Infiltration.
  • Transport off site in a vehicle, such as a vacuum flush truck, for legal disposal in a manner that does not pollute state waters.
  • Ecology-approved onsite chemical treatment or other suitable treatment technologies.
  • Sanitary or combined sewer discharge with local sewer district approval, if there is no other option.
  • Use of a sedimentation bag that discharges to a ditch or swale for small volumes of localized dewatering.

Element 11: Maintain BMPs

• Maintain and repair all temporary and permanent erosion and sediment control BMPs as needed to ensure continued performance of their intended function in accordance with BMP specifications.

• Remove all temporary erosion and sediment control BMPs within 30 days after achieving final site stabilization or after the temporary BMPs are no longer needed.

Element 12: Manage the Project

• Phase development projects to the maximum degree practicable and take into account seasonal work limitations.

• Inspect, maintain and repair all BMPs as needed to ensure continued performance of their intended function. Projects regulated under the Construction Stormwater General Permit (CSWGP) must conduct site inspections and monitoring in accordance with Special Condition S4 of the CSWGP.

• Maintain, update, and implement the Construction SWPPP.

• Projects that disturb 1 or more acres must have site inspections shall be conducted by a Certified Erosion and Sediment Control Lead (CESCL). Project sites disturbing less than 1 acre may have a CESCL or a person without CESCL certification conduct inspections. By the initiation of construction, the Construction SWPPP must identify the CESCL or inspector, who must be present on site or on-call at all times.

• The CESCL or inspector (project sites less than 1 acre) must have the skills to assess the:

  • Site conditions and construction activities that could impact the quality of stormwater.
  • Effectiveness of erosion and sediment control measures used to control the quality of stormwater discharges.

• The CESCL or inspector must examine stormwater visually for the presence of suspended sediment, turbidity, discoloration, and oil sheen. They must evaluate the effectiveness of BMPs and determine if it is necessary to install, maintain, or repair BMPs to improve the quality of stormwater discharges.

• Based on the results of the inspection, construction site operators must correct the problems identified by:

  • Reviewing the SWPPP for compliance with the 13 construction SWPPP elements and making appropriate revisions within 7 days of the inspection.
  • Immediately beginning the process of fully implementing and maintaining appropriate source control and/or treatment BMPs as soon as possible, addressing the problems not later than within 10 days of the inspection. If installation of necessary treatment BMPs is not feasible within 10 days, the construction site operator may request an extension within the initial 10‑day response period.
  • Documenting BMP implementation and maintenance in the site log book (sites larger than 1 acre).

• The CESCL or inspector must inspect all areas disturbed by construction activities, all BMPs, and all stormwater discharge locations at least once every calendar week and within 24 hours of any discharge from the site. (For purposes of this condition, individual discharge events that last more than 1 day do not require daily inspections. For example, if a stormwater pond discharges continuously over the course of a week, only one inspection is required that week.) Project BMPs shall also be inspected within 24 hours of a rain event that exceeds 0.5 inch in a 24‑hour period. The CESCL or inspector may reduce the inspection frequency for temporary stabilized, inactive sites to once every calendar month.

• If a Construction SWPPP is found to be inadequate (with respect to erosion and sediment control requirements), then other BMPs may be required to be implemented, as appropriate. Construction is a complex and ever-changing process. The SWPPP shall be treated as a “living document” and shall be modified as needed during the construction process. All parties involved shall agree on the changes and acknowledge this on the master SWPPP kept at the project site.

• For small residential projects, a narrative checklist and site plan template can be used in lieu of preparing a full Construction SWPPP. Consult with DCD to determine if a project qualifies for this and for a copy of the narrative checklist and site plan template.

• Based on the information provided and/or local weather conditions, seasonal limitations on site disturbance may be expanded or restricted.

• The applicant shall evaluate, with input from utilities and other contractors, the stormwater management requirements for the entire project, including the utilities, when preparing the Construction SWPPP.

Element 13: Protect Low Impact Development BMPs

The primary purpose of On-site Stormwater Management is to reduce the disruption of the natural site hydrology through infiltration. BMPs used to meet I-3.4.5 MR5: Onsite Stormwater (often called LID BMPs) are permanent facilities.

• Protect all LID BMPs (including, but not limited to Bioretention, Rain Gardens, and Permeable Pavements) from sedimentation through the installation and maintenance of erosion and sediment control BMPs on portions of the site that drain into the LID BMPs. Restore the BMPs to their fully functioning condition if they accumulate sediment during construction. Restoring the BMP must include removal of sediment and any sediment-laden Bioretention/Rain Garden soils, and replacing the removed soils with soils meeting the design specification.

• Maintain the infiltration capabilities of LID BMPs by protecting against compaction by construction equipment and foot traffic. Protect completed lawn and landscaped areas from compaction due to construction equipment.

• Control erosion and avoid introducing sediment from surrounding land uses onto Permeable Pavements. Do not allow muddy construction equipment on the base material or pavement. Do not allow sediment-laden runoff onto permeable pavements or base materials.

• Permeable pavement fouled with sediments or no longer passing an initial infiltration test must be cleaned using procedures in accordance with this manual or the manufacturer’s procedures.

• Keep all heavy equipment off existing soils under LID BMPs that have been excavated to final grade to retain the infiltration rate of the soils.

Photo 4. Sand Bags Prevent Silt-Laden Flow.

Sand bags prevent silt-laden flow from entering the bioretention BMP. Green construction fencing prevents compaction due to foot traffic.