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ECO-3       Protecting Our Soil, Water & Air

Erosion & Sediment Control Training & Site Supervision

Download Snohomish County Erosion Control Guidelines

Snohomish County Procedure 3044

STANDARDS FOR CONSTRUCTION STORMWATER POLLUTION PREVENTION PLAN

 This procedure outlines the selection process used to prepare the plans for Construction Stormwater Pollution Prevention (SWPPP).

1.)    Erosion Control Plans.

2.)    Terms Used to Select the Appropriate Level of SWPPP

3.)    The Decision Matrix Shall Be Used to Determine the Level of SWPPP

4.)    Winter Grading Requests Shall Consider the Following BMPs as Minimum

Appendix A    Erosion Sensitivity of Soils

Appendix B    Construction SWPPP

Appendix C    Chemical Treatment & Experimental Processes

 1.         Erosion Control Plans.    TOP

Erosion control during the construction phase of land development has been known as Temporary Erosion and Sediment Control (TESC). Best Management Practices (BMPs), ongoing inspection, re-evaluation, and revision to the plans during construction may be necessary to prevent dirty water from leaving the construction site.

2.         Terms Used to Select the Appropriate Level of SWPPP    TOP

·        Dirty water is defined as stormwater run-off that does not meet the standards in 7.53 SCC or WAC 210A.  In most cases, this means a turbidity level of 5 ntu over the background water quality level.  The terms “dirty water”, “turbid water”, and “silty water” mean the same for this policy and procedure.

·        Downstream location for critical areas to be identified and addressed in the risk assessment is 1/4 mile for non-ESA streams or bodies of water, wetlands and other critical areas.  For projects that are tributary to ESA streams, the risk assessment downstream shall not be limited to a ¼ mile, but shall extend as far as needed so that risk of turbid water from the site is negligible, de minimis or meets water quality standards.

·        Erosion hazard of the soils and parent material is shown in tables in the Appendix A.  Erosion hazard is based on soil type, permeability, underlying geology and slope.  The soils are as classified by the Soil Conservation Service Soil Survey of Snohomish County Area.  The actual soil on site shall be used in determining the erosion hazard.  When there is more than one type of soil on the site, the soil that has the greater erosion hazard shall be used for the site.

·        Critical areas and buffers are defined in the Critical Areas Regulations (CAR) 32.10 SCC.  Risk factors are based on the proximity to the site and the risk of temporary or permanent damage.  The worst case shall be assumed when assigning risk.

·        Site area is the area of disturbed soil (i.e. graded soil) by each drainage basin on the site.  Undisturbed site is not counted as site area.  To determine risk factor when developing by phases, previously exposed soil on the site is required to have effective cover or established vegetation to be counted as undisturbed again.  Bare soil outside of the limits of the project (on adjacent properties) is not counted.  For road and utility construction, the site area is the disturbed area.

·        Slope is based on the existing or finished grade slope.  Average slope is determined for the site by using the time of concentration line as the horizontal length and the vertical difference along said line.  Slope is the vertical difference divided by the horizontal length expressed as a percent (%). The overall site risk is based on the highest risk slope being disturbed with an area of 5,000 sqft or more.

·        Standard Best Management Practices (BMPs) are those that are in the DOE or currently adopted Snohomish County Stormwater Manual.  They do not include chemical treatment or other measures that may require a separate permit from DOE.

·        Stormwater Pollution Prevention Plan (SWPPP) is the term used by the Washington State Dept. of Ecology (DOE) for the documented plan to identify, and implement measures to prevent and control contamination of point source discharges of stormwater.  In the past, for construction projects, a similar, but limited, plan was called an Erosion and Sediment Control Plan and Temporary Erosion and Sediment Control Plan (TESC). For construction plans, one element in the plans and drainage report will be the construction SWPPP.

·        Vegetation or effective cover is when the ground has natural permanent growth sufficient to resist erosion during normal winter rainstorm events.

·        Winter grading means grading between October 1 and March 31.  This term is the same as Wet season grading.

 3.         The Decision Matrix Shall Be Used to Determine the Level of SWPPP:   TOP

The risk category, and whether winter grading is proposed on a site, are key elements in the decision matrix. The predicted level of SWPPP that will be required for normal projects is shown.

 If a project was planned and approved for grading between April 1^st and September 30^th and the developer requests that grading be allowed to continue, revised plans must be approved.

4.         Winter Grading Requests Shall Consider the Following BMPs as Minimum:   TOP

When the design engineer requests a preliminary staff review of the preliminary plans for the SWPPP, they may schedule a meeting to discuss their evaluation of risk factors, level of SWPPP and other issues or requirements for winter grading approval.  Minimum requirements established for construction SWPPPs for the plans to be considered for winter grading approval are:

·        Extra provisions must be used to prevent tracking of mud onto roads from wheels of equipment.  Provisions shall include at a minimum: wheel washing, sweeping equipment on site, rock entrances (or ATB) to connect to employee and equipment parking areas.  For large sites or sites that have more than 10 workers, additional measures should be considered;

·        Geo-technical fabric needs to be under all construction entrances with rock surfaces;

·        Straw or other material to cover any portion of the site that is exposed to work must be on site and ready to use. Weather forecasts shall be used to govern whether work is started and stopped. In general, the exposed soil on site shall be covered when it rains and at the end of the day. The site shall be covered if the next day is a non-working day;

·        Except for installation and maintenance of erosion control BMPs, all site work generating exposed soil will cease during weather conditions when rain generates runoff and causes erosion;

·        Phasing of earthwork is to be used to minimize areas of exposed soil;

·        Plans shall make provisions to prevent concentrated drainage flows over slopes;

·        The plan will contain a maintenance and inspection schedule with identified responsible person(s) ;

·        A monitoring plan, with independent testing, shall be part of the quality assurance plan for compliance.  Monitoring reports shall be available on site and provided to the County when requested and at least monthly;

·        Preserve vegetated buffers and forest areas wherever possible;

·        Limited grading on roads and flatter areas of the site for phased work is encouraged;

·        Provisions need to be in place to address dirty water in detention and sediment ponds.  For example, a provision to pump dirty water to undisturbed areas or forest on site (or on adjacent property, with an easement) when needed to prevent dirty water discharge;

·        Provide treatment system for storm water with necessary regulatory agency permits.

 Appendix A   TOP

Erosion Sensitivity of Soils

Using the SCS Survey for Snohomish County Table 14 Physical and Chemical Properties of Soils, the K factors are an index of erosion potential.  The K factor fits into the Universal Soil Loss Equation^1. as a value for the erodibility rate.  The higher the number the more easily eroded the soil is.  Only the more common soil types found in the urban growth area is listed in the table. Use the SCS values for the B-horizon (second soil layer) to estimate the erosion hazard rating for unlisted soils.

 Parent materials that lie below the soil level need to be assessed for erosion hazard based on the geologic formations that are affected by the grading.  When the grading of the project is deeper than the soil, the characteristics of the parent material are used to determine the erosion hazard value.

 ^1.  Universal Soil Loss Equation              A = R x K x LS x C x P

A =          The computed soil loss in tons (dry weight) per acre from a given storm period

R =          The rainfall erosion index for the given storm period in units of ft-ton per acre-hr.

                K =          The erodibility value, defined as the erosion rate in tons per acre per unit of R for a  specific soil in continuous fallow condition on a 9% slope having a length of 72.6 ft.

L =          The slope length factor, defined as the ratio of soil loss from a specific field to that from a unit field having the same soil type and slope but with a length of 72.6 ft.

S =           The slope factor, defined as the ratio of soil loss from a specific field to that from a similar field having the same soil type and a slope gradient of 9%.

C =          The cropping management or cover factor defined as the ratio of soil loss from an area with specified cropping and management or vegetative cover to that from the same area but under fallow condition.

P =          The erosion control practice factor defined as the ratio of soil loss with a given practice to that with a straight row, up-and-down slope farming.

Appendix B   TOP

Construction SWPPP

 A Construction Stormwater Pollution Prevention Plan (SWPPP) needs to have construction plans and drainage report as part of the full or targeted drainage plan.  The drainage report will have a section that discusses the 12 elements as they apply to the project.

 At the time that this policy was prepared, the new (since 1992) DOE stormwater requirements for a SWPPP were in public comment.  The final version of SWPP requirements will be adopted as part of the new Snohomish County Stormwater Manual.  Most of these requirements are part of subsection 24.30.020 SCC.

 The 12 elements that are part of a Construction SWPPP are as follows:

1.      Mark Clearing Limits: Prior to clearing or disturbing the limits must be marked. This element is part of most normal construction plans as one of the first steps.

2.      Establish Construction Access: All erosion control plans shall install a stabilized construction entrance (or other method of preventing sediment transport onto the roads).  If a standard gravel construction entrance is proposed, use geo-textile fabric under the rock.  Note: a wheel wash is required for plans that propose winter grading.

3.      Detain Flows: Based on a downstream analysis it may be necessary to detain runoff from a site under construction.  It may be necessary to construct and use a detention pond to control flows during construction.

4.      Install Sediment Controls: If there is runoff from the construction site, sediment shall be removed from the water.  Note that the water quality standards must be met.

5.      Stabilize Soils: All exposed and non-worked soil shall be stabilized by use of BMP’s.  Note there are time periods of allowed exposure that depend on the season.  Groundcover, both temporary and permanent, needs to be part of the construction plans.

6.      Protect Slopes: Cut and fill slopes need to be protected from erosive flows and concentrated flows until permanent cover and drainage conveyance systems are in place.

7.      Protect Drain Inlets: All storm drain inlets require protection from sediment and silt laden water.

8.      Stabilize Channels and Outlets: Temporary and permanent conveyance systems shall be stabilized to prevent erosion during and after construction.  Culvert outlets require protection.

9.      Control Pollutants: The plan shall show how all pollutants, including waste materials and demolition debris, will be handled.  This includes maintenance of construction equipment, fertilizers, application of chemicals, and water treatment systems.

10.  Control De-Watering: The water from de-watering systems for trenches, vaults and foundations shall be discharged into a controlled system.

11.  Maintain BMPs: The plan shall provide for inspection and maintenance of the planned and installed construction BMPs as well as their removal at the end of the project.

12.  Manage the Project: The plan shall outline how the site shall be managed for erosion control.  It needs to cover phasing, training, pre-construction conference, coordination with utilities and contractors, monitoring and reporting.  It shall provide for notice of problems, revisions during construction and contingency planning.  One of the most important elements in the management of the project is planning for contingencies, based on the risk of exposure during phases of the development.  It is essential that planning is ongoing throughout the life of the project.

 When consideration for winter grading is requested, the construction SWPPP shall contain a plan for stormwater sampling locations, background measurements, and a periodic reporting schedule.  The reporting schedule shall at a minimum require samples during every storm event that generates runoff and site inspection condition reports on the installed BMPs.  The monitoring and sampling are to be done in a professional manner consistent with current sampling protocols and reporting requirements.  The sampling points are to be shown on a map and marked on the ground.

 The checklist below identifies items that need to be added to the drainage plans and report when the Construction SWPPP is incorporated in the full drainage plan.

 Level 1 SWPPP

·        Identification of potential pollutants and the plan to control them;

·        Identification of items that are applicable to the site and required when winter grading is requested as a special section in the report;

·        Section on inspection, maintenance, and management of the project.

 Level 2 SWPPP

·        All of the items shown for Level 1 SWPPP;

·        Detailed analysis of the elements required by the code and Construction SWPPP and discussion on how the proposed BMPs and construction management team will be formed to manage the project.  Note: this level of detail is not appropriate at the preliminary development or project review stage.  When the developer has selected the project team and contractor to construct the project they need to outline how the project will be managed for this section.  In the case of County construction projects this step would occur after bid award and before notice to proceed;

·        Section on monitoring and sampling proposed and other items related to maintenance, inspection and management of the project.

 Level 3 SWPPP

·        All of the elements shown for Level 1 & 2 SWPPPs;

·        Detailed description of how the construction management team will address all of the elements of the SWPPP;

·        Identification of special design considerations and items that are applicable to the site and required when winter grading is requested.  This will be a special section in the report;

·        A project design meeting will be scheduled to review the plan with a consensus required in order to proceed with the plan documentation and required permits and agreements to implement the plan;

·        If needed, when no consensus is reached, the County will convene a review panel of technical experts who are not connected with the development or review process to arbitrate the issues for which consensus is not reached.

Appendix C   TOP

Chemical Treatment & Experimental Processes

 This section is reserved for future criteria to be used for approval of chemical assisted clarification of stormwater.  At this time, the approval of stormwater treatment with polymers and cat-ion agents is done on a case-by-case basis.  The County will work with the technical proposals and DOE to review and approve the use of chemical water treatment.  Their use, to start the formation of flocculation and precipitation of the fine clays and other particles in stormwater, requires an individual permit from the DOE.

 Because most of the construction disturbance is on till soils, such as Alderwood, which covers the majority of the urban growth area of Snohomish County, new tools are needed to reach the target of 5 ntu’s in runoff water.

 To date, chemical systems used fall into three general categories:

1.      Ground application of Poly-acrilamides (PAM),

2.      Cat-ion exchange batch processes with settling in clarifying ponds, and

3.      Chemical treatment with filtering.

 The SWPPP proposing to use chemical treatment to clarify the stormwater followed by release into a stormwater system, stream or other surface water body must be specified early in the development approval process.  This is needed because the use needs to be included as one of the options addressed by SEPA.  The information contained in the supporting documentation at the preliminary stage needs to be thorough, complete and specific.  It needs to outline the process, testing and other parameters proposed for quality assurance, contingency plans, and experience of the proposed operators of the process.

 Minimum elements in the SWPPP for chemical treatment are:

·        Proposed polymer or chemicals to treat the stormwater;

·        Bench test plan to identify the dosage rates and range of variables anticipated for the site;

·        Bioassay proposal by independent lab;

·        Solids or sludge handling and disposal plan;

·        Engineering design aspects of the process with a description of:

ü      Chemical feed system, mixing, sampling, testing and lab equipment

ü      Stormwater conveyance and process system, pipes, pumps and ponds, power supply, backup systems;

·        Logs, records and sampling program with predetermined sample points;

·        Emergency and contingency plans which identify probable failures;

·        Quality assurance program and audits by independent testing lab;

·        Normal operations reporting program and emergency plans;

·        Cleanup plans for potential chemical spills or equipment failures;

·        Identification of the key personnel responsible to ensure that the treatment program is implemented correctly and adjustments made when needed to respond to changes;

·        Treatment operators shall demonstrate professional experience or certification.

 When the process is approved, if the general contractor and owner have not been involved in the detailed information in the SWPPP, an education program for all parties involved with the construction will be one element of the SWPPP.  The education program will be ongoing and periodic during construction so that as key personnel and subcontractors change, they will be kept informed about the system and their role in keeping the plan working.

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