14.48.140 - Small parcel minimum requirements.

The following new development shall be required to control erosion and sediment during construction, to permanently stabilize soil exposed during construction, to comply with small parcel requirements 1 through 4, to prepare a small parcel erosion and sediment control plan, and may need to prepare a small parcel drainage plan/sketch (Mason County's public works department can provide additional instructions, guidance, and examples if a small parcel drainage plan is deemed necessary): (a) creation or addition of less than two thousand square feet of impervious surface area; (b) land disturbing activities of less than one acre.

(1)

Small Parcel Requirement #1 Construction Access Route. Construction vehicle access shall be, whenever possible, limited to one route. Access points shall be stabilized to minimize the tracking of sediment onto public roads.

Wherever construction vehicle access routes intersect paved roads, provisions must be made to minimize the transport of sediment (mud) onto the paved road. If sediment is transported onto a road surface creating a hazard, the roads shall be cleaned immediately, and thoroughly cleaned at the end of each day. Sediment shall be removed from roads by shoveling or sweeping and be transported to a controlled sediment disposal area. Street washing shall be allowed only after sediment is removed in this manner.

(2)

Small Parcel Requirement #2 Stabilization of Potentially Erodible Denuded Areas. Soil stabilization. All exposed and unworked soils with erosion potential shall be stabilized by suitable application of BMPs, including but not limited to sod or other vegetation, plastic covering, mulching, or application of ground base on areas to be paved. All BMPs shall be selected, designed and maintained in accordance with an approved manual. From October 1 through April 30, no soils shall remain exposed for more than two days. From May 1 through September 30, no soils shall remain exposed for more than seven days.

(3)

Small Parcel Requirement #3 Protection of Adjacent Properties. Adjacent properties shall be protected from sediment deposition by appropriate use of vegetative buffer strips, sediment barriers or filters, dikes or mulching, or by a combination of these measures and other appropriate BMPs.

(4)

Small Parcel Requirement #4 Maintenance. All erosion and sediment control BMPs shall be regularly inspected and maintained by the owner or authorized agent to ensure continued performance of their intended function.

(5)

Small Parcel Requirement #5 Other BMPs. As required by the director, other appropriate BMPs to mitigate the effects of increased runoff shall be applied.

(Ord. 81-08 (part), 2008).

14.48.150 - New development and redevelopment—Minimum requirements.

Development of individual, detached, single-family residences and duplexes exceeding two thousand square feet of impermeable surfacing must have a small parcel erosion and sediment control plan, and a residential site improvement plan (which includes a written description of the proposed work and development) prepared and submitted for county review and approval.

Nonresidential development exceeding five thousand square feet of impermeable surfacing and/or land disturbing activities of one acre or more must have the large parcel ESC plan (if required) and the permanent stormwater quality control (PSQC) plan (if required) prepared and stamped by a licensed civil engineer.

(1)

New Development.

(A)

All new nonresidential development that includes the creation or addition of five thousand square feet, or greater, of new impervious surface area, and/or land disturbing activity of one acre or greater, shall comply with minimum requirements #1 through #11 in subsections (3) through (13) of this section.

Compliance shall be demonstrated through the implementation of an approved stormwater site plan consisting of a large parcel ESC plan and a PSQC plan, as appropriate.

(B)

All new nonresidential development that includes the creation or addition of five thousand square feet, or greater, of new impervious surface area, and land disturbing activity of less than one acre, shall comply with minimum requirements #2 through #11 in subsections (4) through (13) of this section and the small parcel minimum requirements found in Section 14.48.140 above. This category of development shall also prepare a stormwater site plan that includes a small parcel erosion and sediment control plan.

Compliance shall be demonstrated through the implementation of an approved stormwater site plan that includes a small parcel erosion and sediment control plan and a PSQC plan.

(C)

All new residential development and redevelopment (individual, detached, single family residences and duplexes) that includes the creation or addition of two thousand square feet, or greater of new impervious surface area, shall prepare a small parcel erosion and sediment control plan which complies with minimum requirements #1 through #5 in Sections 14.48.140(1) through 14.48.140(5), and shall prepare a residential site improvement plan which includes a written description of the proposed work and development. The small parcel erosion and sediment control plan along with the residential site improvement plan should show and describe how development impacts are proposed to be mitigated.

Compliance shall be demonstrated through the implementation of approved erosion and sediment control plan, and a residential site improvement plan.

(2)

Redevelopment.

(A)

Where nonresidential redevelopment of greater than or equal to five thousand square feet occurs:

The new development minimum requirements #1 through #11, subsections (3) through (13), shall apply to that portion of the site that is being redeveloped, and source control BMPs shall be applied to the entire site, including adjoining parcels if they are part of the project. A stormwater site plan shall be prepared.

(B)

In addition to the above requirements, where one or more of the following conditions apply, a stormwater site plan shall also be prepared that includes a schedule for implementing the minimum requirements to the maximum extent practicable, for the entire site, including adjoining parcels if they are part of the project. An adopted and implemented basin plan (minimum requirement #9) may be used to develop redevelopment requirements that are tailored to a specific basin.

(i)

Existing sites greater than one acre in size with fifty percent or more impervious surface.

(ii)

Sites that discharge to a receiving water that has a documented water quality problem. Subject to local priorities, a documented water quality problem includes, but is not limited to water bodies:

a.

Listed in reports required under Section 305(b) of the Clean Water Act, and designated as not supporting beneficial uses;

b.

Listed under Section 303(d) of the Clean Water Act as not expected to meet water quality standards or water quality goals;

c.

Listed in Washington State's Non-point Source Assessment required under Section 319(a) of the Clean Water Act that, without additional action to control non-point sources of pollution cannot reasonably be expected to attain or maintain water quality standards.

(iii)

Sites where the need for additional stormwater control measures has been identified through a basin plan, the watershed ranking process under Ch. 400-12 WAC, or through Growth Management Act planning.

(3)

Minimum Requirement #1: Erosion and Sediment Control. All new development and redevelopment that includes land disturbing activities of one acre or greater shall comply with erosion and sediment control requirements 1 through 14, below. Compliance with the erosion and sediment control requirements shall be demonstrated through implementation of a large parcel erosion and sediment control plan.

All new development and redevelopment that includes land disturbing activities of less than one acre shall comply with the small parcel minimum requirements found in Section 14.48.130, above. Compliance with the small parcel requirements shall be demonstrated through implementation of a small parcel erosion and sediment control plan.

The following erosion and sediment control requirements shall be met:

(A)

Erosion and Sediment Control Requirement #1: Stabilization and Sediment Trapping. All exposed and unworked soils shall be stabilized by suitable application of BMPs. From October 1 to April 30, no exposed and unworked soils shall remain unstabilized for more than two days. From May 1 to September 30, no exposed and unworked soils shall remain unstabilized for more than seven days. Prior to leaving the site, stormwater runoff shall pass through a sediment pond or sediment trap, or other appropriate BMPs.

(B)

Erosion and Sediment Control Requirement #2: Delineate Clearing and Easement Limits. In the field, mark clearing limits and/or any easements, setbacks, sensitive/critical areas and their buffers, trees, and drainage courses.

(C)

Erosion and Sediment Control Requirement #3: Protection of Adjacent Properties. Properties adjacent to the project site shall be protected from damage by sediment deposition.

(D)

Erosion and Sediment Control Requirement #4: Timing and Stabilization of Sediment Trapping Measures. Sediment ponds and traps, perimeter dikes, sediment barriers, and other BMPs intended to trap sediment on-site shall be constructed as a first step in grading. These BMPs shall be functional before land disturbing activities take place. Earthen structures such as dams, dikes, and diversions shall be seeded and mulched according to the timing indicated in erosion and sediment control requirement #1.

(E)

Erosion and Sediment Control Requirement #5: Cut and Fill Slopes. Cut and fill slopes shall be designed and constructed in a manner that will minimize erosion. In addition, slopes shall be stabilized in accordance with erosion and sediment control requirement #1.

Supplemental Guidelines: Consideration should be given to the length and steepness of the slope, the soil type, upslope drainage area, ground water conditions, and other applicable factors. Slopes which are found to be eroding excessively within two years of construction must be provided with additional slope stabilizing measures until the problem is corrected.

(i)

Roughened soil surfaces are preferred to smooth surfaces on slopes (see BMP E2.35 in Chapter II-5).

(ii)

Interceptors (see BMP E2.55 in Chapter II-5) should be constructed at the top of long steep slopes, which have significant drainage areas above the slope. Diversions or terraces may also be used to reduce slope length.

(iii)

Concentrated stormwater should not be allowed to flow down cut or fill slopes unless contained within an adequate temporary or permanent channel, or pipe slope drain (see BMP E2.25 in Chapter II-5).

(iv)

Wherever a slope face crosses a water seepage plane which endangers the stability of the slope, adequate drainage or other protection should be provided (BMPs E2.30 and E2.75 in Chapter II-5).

(F)

Erosion and Sediment Control Requirement #6: Controlling Off-site Erosion. Properties and waterways downstream from development sites shall be protected from damage by erosion due to increases in the volume, velocity, or peak flow rate of stormwater runoff from the project site.

(G)

Erosion and Sediment Control Requirement #7: Stabilization of Temporary Conveyance Channels and Outlets. All temporary on-site conveyance channels shall be designed, constructed and stabilized to prevent erosion from the expected velocity of the peak flow from a two-year, twenty-four hour frequency storm for the developed condition. Stabilization adequate to prevent erosion of outlets, adjacent streambanks, slopes and downstream reaches shall be provided at the outlets of all conveyance systems.

(H)

Erosion and Sediment Control Requirement #8: Storm Drain Inlet Protection. All storm drain inlets made operable during construction shall be protected so that stormwater runoff shall not be discharged offsite or be introduced into any fill area without first being filtered or otherwise treated to remove sediment.

(I)

Erosion and Sediment Control Requirement #9: Underground Utility Construction. The construction of underground utility lines shall be subject to the following criteria:

(i)

No more than five hundred feet of trench shall be opened at one time, unless provisions are made to protect against adverse stormwater impacts.

(ii)

Where consistent with safety and space considerations, excavated material shall be placed on the uphill side of trenches.

(iii)

Trench dewatering devices shall discharge into a sediment trap or sediment pond.

(J)

Erosion and Sediment Control Requirement #10: Construction Access Routes. Construction vehicle access shall be, whenever possible, limited to one route. Access points shall be stabilized to minimize the tracking of sediment onto public roads.

Wherever construction vehicle access routes intersect paved roads, provisions must be made to minimize the transport of sediment (mud) onto the paved road. If sediment is transported onto a road surface creating a hazard, the roads shall be cleaned immediately, and thoroughly cleaned at the end of each day. Sediment shall be removed from roads by shoveling or sweeping and be transported to a controlled sediment disposal area. Street washing shall be allowed only after sediment is removed in this manner.

(K)

Erosion and Sediment Control Requirement #11: Removal of Temporary BMPs. All temporary erosion and sediment control BMPs shall be removed within thirty days after final site stabilization is achieved or after the temporary BMPs are no longer needed. Trapped sediment shall be removed or stabilized on site. Disturbed soil areas resulting from removal shall be permanently stabilized.

(L)

Erosion and Sediment Control Requirement #12: Dewatering Construction Sites. Trench dewatering devices shall be discharged in a manner that will not adversely affect flowing streams, drainage systems, or offsite property. Water discharged from dewatering pumps shall be routed through a sediment pond or trap unless it is clear.

(M)

Erosion and Sediment Control Requirement #13: Control of Pollutants Other Than Sediment on Construction Sites. All pollutants other than sediment that occur on-site during construction shall be handled and disposed of in a manner that does not cause contamination of stormwater.

(N)

Erosion and Sediment Control Requirement #14: Maintenance. All temporary and permanent erosion and sediment control BMPs shall be maintained and repaired as needed and in a timely manner to assure continued performance of there intended function. All maintenance and repair shall be conducted in accordance with an approved manual.

(O)

Erosion and Sediment Control Requirement #15: Financial Liability. Performance bonding, or other appropriate instruments, may be required to ensure compliance with the approved erosion and sediment control plan.

(4)

Minimum Requirement #2: Preservation of Natural Drainage Systems. Natural drainage patterns shall be maintained, and discharges from the site shall occur at the natural location to the maximum extent practicable.

(5)

Minimum Requirement #3: Source Control of Pollution. Source control BMPs shall be applied to all projects to the maximum extent practicable. Source control BMPs shall be selected, designed, and maintained according to an approved manual. An adopted and implemented basin plan (minimum requirement #9) may be used to develop source control requirements that are tailored to a specific basin, however, in all circumstances, source control BMPs shall be required for all sites.

(6)

Minimum Requirement #4: Runoff Treatment BMPS. All projects shall provide treatment of stormwater. Treatment BMPs shall be sized to capture and treat the water quality storm, defined as the six month, twenty-four hour storm. The first priority for treatment of stormwater shall be to infiltrate as much as possible of the water quality design storm into the ground. (Infiltration BMPs practices shall be in accordance with Chapter III-3 of the PSWQA Manual) Pretreatment of stormwater prior to infiltration into the ground may be required in cases where:

(A)

The stormwater contains high concentrations of undesirable dissolved chemicals that can move through soil.

(B)

The stormwater contains large amounts of sediment that might clog the infiltrative surfaces in the basin.

(C)

The soils are extremely pervious and will not properly filter the stormwater as in the case for some gravelly (type one) soils.

Stormwater treatment BMPs shall not be built within a natural vegetated buffer, except for necessary conveyance systems as approved by the director.

An adopted and implemented basin plan (minimum requirement #9) may be used to develop runoff treatment requirements that are tailored to a specific basin.

The objectives of providing stormwater treatment BMPs are, for both surface and ground waters, to protect the quality and quantity, to allow attainment of the designated uses, and to meet state standards promulgated under Chapter 173 of the Washington Administrative Code.

(7)

Minimum Requirement #5: Streambank Erosion Control. The requirement below applies only to situations where stormwater runoff is discharged directly or indirectly to a stream, and must be met in addition to meeting the requirements in Minimum Requirement #4, Runoff Treatment BMPs:

Stormwater discharges to streams shall control streambank erosion by limiting the peak rate of runoff from individual development sites to fifty percent of the existing condition two-year, twenty-four hour design storm while maintaining the existing condition peak runoff rate for the ten-year, twenty-four hour and one hundred-year, twenty-four hour design storms. As the first priority, streambank erosion control BMPs shall utilize infiltration to the fullest extent practicable, only if site conditions are appropriate and ground water quality is protected. Streambank erosion control BMPs shall be selected, designed, and maintained according to an approved manual.

Stormwater treatment BMPs shall not be built within a natural vegetated buffer, except for necessary conveyance systems as approved by the director, and maintained according to the 1992 manual.

An adopted and implemented basin plan (minimum requirement #9) may be used to develop streambank erosion control requirements that are tailored to a specific basin.

(8)

Minimum Requirement #6: Wetlands. Stormwater discharges to wetlands shall maintain the wetland's natural hydroperiod and flows to the extent needed to preserve or enhance its existing functions and values. Prior to proposing discharge of higher volumes of stormwater to a wetland, alternative discharge, detention, and infiltration practices located in areas outside the wetland shall be evaluated and employed by the project engineer where feasible and practicable.

These requirements apply to existing natural wetlands and wetlands created as mitigation for loss of wetland acreage. Wetlands constructed and operated solely for use as stormwater treatment/storage areas are exempt from these and most other restrictions that apply to natural wetlands. Constructed wetlands may lose this exemption if not operated and maintained as stormwater areas for three or more years. Other local, state, or federal wetland protection requirements may also apply.

(9)

Minimum Requirement #7: Water Quality Sensitive Areas. Where the Mason County commissioners or their designee determine that the minimum requirements do not provide adequate protection of water quality sensitive areas, either on-site or within the basin, more stringent controls shall be required to protect water quality. An adopted and implemented basin plan (minimum requirement #9) may be used to develop requirements for water quality sensitive areas that are tailored to a specific basin.

(10)

Minimum Requirement #8: Off-site Analysis and Mitigation.

Downstream Analysis May Trigger Additional Requirements. The project engineer shall provide a detailed qualitative analysis of the flow path of the discharge from the project site to the receiving water. This requirement shall apply to all projects where a drainage and erosion control plan is prepared, including those proposing retention facilities. This analysis shall include flow routing, and provide existing pipe and channel sizes and estimated capacities. In addition, the project engineer shall discuss any known or expected downstream erosion, flooding, or water quality problems, including those that may be caused by interflow from the proposed retention facility. The director or designee shall have the discretion to specify the distance and level of detail to be provided by the project engineer. In making this determination, the director or designee shall consider such factors as the relative size of the new development, availability of other hydrologic work for the drainage area, and the extent to which stormwater generated on the project site is to be infiltrated.

Based upon this analysis, the project engineer may determine or the director or designee may require that a quantitative analysis of the conveyance system be performed both upstream and downstream of the project site. A quantitative analysis will not be required for most projects that propose to infiltrate most or all stormwater. This analysis shall determine conveyance system performance for the appropriate design event(s) both with and without the proposed development. The director or designee shall have the discretion to specify the distance and level of detail to be provided by the project engineer. The director or designee shall consider factors such as the relative size of the new development, availability of other hydrologic work for the drainage area, and the results of the qualitative analysis, in making this determination.

The quantitative downstream analysis will include modeling the hydraulics of the proposed project and all other sources of runoff tributary to the receiving water body for the appropriate design event. The project engineer shall include an analysis of the impact of the twenty-four hour, one hundred-year event (in addition to "Design Event" analysis) for each component of the system including pond spillway.

The director or designee may impose stricter discharge and/or detention standards if the discharge from the project, evaluated in the context of other existing conditions in the drainage area, is reasonably expected to result in any of the following:

(A)

Flooding;

(B)

Loss of aquatic habitat due either to high or low flows;

(C)

Property damage;

(D)

Water quality problems;

(E)

Erosion;

(F)

Or an unacceptable interruption of vital services.

If the project engineer (or director or designee) determines that greater treatment, infiltration and/or storage volumes, lower release rates, or downstream improvements are needed, he/she shall specify project design criteria or other means to relieve the downstream problems (providing that such solution will not violate minimum standards established in the 1992 manual). Other means might include increases in downstream flow capacity and/or offsite detention and infiltration facilities, plans and financing for which will be subject to the approval of the director or designee.

(11)

Minimum Requirement #9: Basin Planning. Basin Plan Supersedes 1992 Manual. If a proposed project is located in a basin or sub basin for which the county has an adopted basin plan, stormwater requirements specifically identified in the basin plan shall take precedence over those provided in this chapter. However, all other elements detailed in this chapter shall continue to apply to such projects. Basin plans are required to be developed according to an approved manual.

(12)

Minimum Requirement #10: Operation and Maintenance. An operation and maintenance schedule shall be provided for all proposed stormwater facilities and BMPs, and the party (or parties) responsible for maintenance and operation shall be identified. An operation and maintenance (O&M) Covenant will be required to cover all privately owned and maintained stormwater facilities approved by the director. O&M covenant forms are available at the Mason County public works office. A copy of the completed instrument, shall be recorded with the county auditors' office by the proponent, and a copy of the recorded instrument is required to be submitted to the public works department prior to final approval of the completed permanent stormwater facilities.

(13)

Minimum Requirement #11: Financial Liability. Performance bonding, or other appropriate instruments shall be required for all projects to ensure compliance with these standards.

(14)

Variances from These Standards. Variances from these standards may be allowed at the discretion of the director or designee provided that the proponent will substantially meet flow control and water quality goals established by or implicit in these standards. Compliance with ordinance requirements shall normally occur within the project area, but may be performed as offsite mitigation in certain situations. Examples of cases when variances may be approved includes, but are not limited to, the following:

(A)

That the special conditions and circumstances exist which are peculiar to the land, such as size, shape, topography, or location, and that literal interpretation of these standards would deprive the property owner of rights commonly enjoyed by other properties similarly situated; or

(B)

That the site is being remodeled and certain site investigations would be destructive to existing structures; or

(C)

That remodels of existing projects which are either too small or so configured that in the director's or designee's opinion some requirements of the manual cannot practically be met; or

(D)

That public works or private sector projects are in an existing road right-of-way, which is not of adequate size to install preferred BMPs and for which right-of-way cannot be expanded because of encroaching structures or setbacks for existing structures.

All requests for variances must be submitted in writing to the director or designee, and must clearly state the specific section(s) of the ordinance from which a variance is requested and why.

(15)

Experimental BMPs. Experimental best management practices are defined as BMPs, which have not been tested and evaluated by the Department of Ecology in collaboration with local governments and technical experts. Some so-called experimental BMPs will likely be minor variations on an existing theme. In that case, Ecology would review and approve or disapprove the BMP in as timely a manner as possible. Where new designs are developed (examples of experimental BMPs may be found in Section I-2.17.2 of the PSWQA Manual), the review will be extended through the use of a standing committee of technical experts. These persons will review and comment on the practice, and Ecology will then determine whether or not these BMPs should be approved and/or added to the manual.

(A)

Approval of Experimental BMPs. Approval to use an experimental BMP may be granted subject to initial approval by the Department of Ecology and the local government. If such experimental BMPs prove useful they may be incorporated into later editions of the manual following appraisal of the results and appropriate technical review conducted by Ecology in collaboration with local governments and other interested parties. Approval to use an experimental BMP will only be granted when a suitable contingency plan using approved BMPs has been provided by the applicant to be used in the event that the experimental BMP does not perform adequately.

In addition, several experimental BMPs have been included in the 1992 manual. People may wish to use these BMPs on a trial basis, subject to approval by the local government and provision of a contingency plan. In any event, use of experimental BMPs is encouraged whenever applied research is being undertaken so that more information is made available to facilitate judgement on their applicability and possible adoption as an approved BMP.

(Ord. 81-08 (part), 2008).