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HomeMy WebLinkAboutBLD2020-00281 - 05 STORMWATER PLAN 311 W. 10th Street, Port Angeles, WA 98362 (360) 912-5191 www.gounitedengineering.com February 8, 2021 STORMWATER, EROSION, AND SEDIMENT CONTROL PLAN FOR PARCEL #990400106, GAMBLE LN., PORT LUDLOW, WASHINGTON 98382 1.0 BACKGROUND AND SCOPE OF WORK United Engineering, Inc. was retained by Jim Williams to prepare a Stormwater, Erosion, and Sediment Control Plan for a single-family residential project located on Jefferson County Tax Parcel #990400106. A three-bedroom single family residence is proposed to be constructed. This plan will conform to the 2019 Stormwater Management Manual for Western Washington. 2.0 SITE CONDITIONS Existing Conditions The site has an overall slope of roughly 20% from the west downward to the east. The parcel is currently undeveloped. The surrounding parcels are developed with single family homes on similar parcel sizes. Existing site vegetation includes grass and brush. Soil Conditions Per the United States Department of Agriculture Natural Resou rces Conservation Services (NRCS) Web Soil Survey, site soils for are as follows: Upper half of the site: • Alderwood gravelly sandy loam • Hydrologic soil group Type B • Depth to the water table of 18” – 36” • Infiltration rate of 0.00 to 0.06 inches per hour Lower half of the site: • Swantown gravelly loam • Hydrologic soil group Type D • Depth to the water table of 6” – 12” • Infiltration rate of 0.00 to 0.06 inches per hour Field soil logs on file from December 1989 show mainly medium to fine s ands on the lower half of the site. Mottling was present at depths of about 12” and compact soils were found at depths of about 43”. On the uphill, westernmost portions of the site field investigation found the soils to be medium to fine sands to depths of 72” or more. Based on the soil logs and NRCS soils information for the site, infiltration could be possible in westernmost portions of the site, the location of the septic system drainfield along with the proposed layout of the home and driveway, prohibit any infiltration BMPs from being placed east of the house location. 3.0 DRAINAGE DESIGN New Development The proposed single-family residence will result in 3,934 square feet of new impervious surfaces, of which about 3,064 square feet will be non-pollution generating impervious surfaces (new roof), and about 870 square feet will be pollution generating impervious surfaces (new concrete driveway and walkways). The project results in less than 5,000 square feet of new impervious surfaces, so Minimum Requirements 1-5, as outlined in the Department of Ecology’s 2019 Stormwater Management Manual for Western Washington (SWMMWW), apply. Minimum Requirement #1 – Preparation of Stormwater Site Plans See Appendix I. Minimum Requirement #2 – Construction Stormwater Pollution Prevention For description of Best Management Practices (BMPs) listed below for Elements #1 through #9, and except where specifically noted otherwise, refer to the 2019 SWMMWW. Element #1 – Preserve Vegetation/Mark Clearing Limits Before land disturbing activity begins, the clearing limits must be marked with silt fence (BMP C233). The native top soil and existing vegetation must be retained in an undisturbed state to the maximum degree practical. Element #2 – Establish Construction Access Unless a written variance is granted in advance, a stabilized construction entrance (BMP C105) for this project shall be provided via Gamble Ln. See the Erosion & Sediment Control Plan and referenced detail sheets, Appendix I, for specific location and details for the stabilized construction entrance. Any sediment tracked off the site must be cleaned thoroughly at the end of each day, or more frequently if determined necessary, by shoveling, sweeping, transporting to a controlled sediment disposal area, street washing, and/or other necessary and appropriate measures. Element #3 – Control Flow Rates Dispersion systems must be constructed in advance to serve roofs, pavements, sidewalks, and other impervious areas. Activities such as roof gutter and downspout installation, paving, and similar construction activities, must be promptly completed in the proper sequence to control flows and minimize the potential for erosion and sedimentation. Silt fence (BMP C233), wattles (BMP C235), and/or other appropriate BMPs must be used as necessary for control of flow rates. Element #4 – Install Sediment Controls Silt fence must be installed and maintained throughout the duration of the project. The location of the silt fence is shown on the Erosion and Sediment Control Plan and must be in place in all areas where sediment could potentially leave the site. Silt fence (BMP C233), wattles (BMP C235), oil-absorbent-rated fabric catch basin insert bags, and/or other appropriate BMPs must be used as necessary for control of sediment. Element #5 – Stabilize Soils Soils must not remain exposed and unworked for more tha n the time periods set forth below: • May 1 to September 30: 7 days. • October 1 - April 30: 2 days. Bare soils, particularly bare slopes, and soil stockpiles must be protected from erosion with sediment trapping measures. To the maximum extent possible, stockpiles must be located away from storm drain inlets, waterways, and drainage channels. Temporary and permanent seeding (BMP C120), mulching (BMP C121), nets and blanketing (BMP C122), plastic covering (BMP C123), top-soiling and/or composting (BMP C125), and/or other appropriate BMPs must be used as necessary for stabilizing of soils. Additionally, post-construction soil quality and depth enhancement (BMP T5.13; refer to Building Soil; Guidelines and Resources for Implementing Soil Quality and Depth BMP T5.13, 2016 Edition) must be implemented to improve infiltration capacity of the newly created and restored lawn areas. Element #6 – Protect Slopes Blanketing or other effective temporary measures shall be taken to prevent erosion. Slope construction and slope stabilization should be scheduled during dry weather (usually May 1 to September 30) to minimize potential for erosion. Temporary and permanent seeding (BMP C120), mulching (BMP C121), nets and blanketing (BMP C122), plastic covering (BMP C123), and/or other appropriate BMPs must be used as necessary for protection of slopes. Element #7 – Protect Drain Inlets Inlet protection shall also be placed in all existing and new catch basins and inlets that could potentially receive stormwater flows directly from the site. Storm drain inlet protection (BMP C220), use of oil-absorbent-rated catch basin inlet fabric insert bags, and/or other appropriate BMPs must be used as necessary for inlet protection. Inlet protection must not obstruct vehicle traffic on the site. Element #8 – Stabilize Channels and Outlets This project does not have any channels or outlets where concentrated flows could leave the site and enter natural or man-made bodies of water or wetlands. Element #9 – Control Pollutants All chemicals, liquid products, petroleum products, and other materials that have the potential to pose a threat to human health or the environment must be provided with secondary containment and protection from weather, vandalism, and/or other adverse circumstances. Secondary containment means placing tanks and/or containers within an impervious structure capable of containing 110% of the volume contained in the largest tank or container within the containment structure. Fueling, maintenance, and repair of heavy equipment and vehicles must include use of spill prevention and control measures. Washout of concrete trucks, dumping of excess concrete, and similar activities shall not be performed on-site. Washout of concrete trucks, dumping of excess concrete, and similar activities shall only be performed in off-site locations specifically approved for such activities. Proper concrete handling (BMP C151), saw cutting and surfacing pollution prevention (BMP C152), proper material delivery, storage, and con tainment (BMP C153), and/or other appropriate BMPs must be used as necessary for control of pollutants. Any and all spills associated with this project, either on site or off site, must be reported to the proper authorities and thoroughly cleaned up to me et applicable City, County, State, and/or Federal standards. Element #10 – Control Dewatering In the unlikely event that dewatering is required, water from dewatering operations shall be discharged in such a manner that it does not result in erosion and sedimentation. Special care shall be taken to avoid sedimentation of existing and new stormwater control facilities. Element #11 – Maintain BMPs All temporary and permanent erosion and sediment control BMPs must be maintained and repaired as needed to assure continued performance of their intended function in accordance with BMP specifications. All temporary erosion and sediment control BMPs shall be removed within 30 days after achieving final site stabilization or after the temporary BMPs are no longer needed. An adequate supply of appropriate spare materials shall be kept on site for maintenance of BMPs (BMP C150). Element #12 – Manage the Project Accurate and timely BMP installation, monitoring, and maintenance, SWPPP preparation and up-dating as necessary, and record-keeping of construction SWPPP activities for the project, shall be under the supervision of a Certified Erosion and Sediment Control Lead (BMP C150 and BMP C160). To the extent possible, work shall be schedule to take advantage of normal seasonal dry weather conditions (BMP C162). Element #13 – Protect Low Impact Development BMPs Stormwater control facilities shall be protected from sedimentation and other harmful processes. Silt fence (BMP C233), wattles (BMP C235), oi l absorbent fabric catch basin inlet bags, and/or other appropriate BMPs must be used as necessary to protect stormwater facilities. Minimum Requirement #3 – Source Control of Pollution All known, available and reasonable source control BMPs must be applied to this project to prevent stormwater from coming into contact with pollutants. Source control BMPs must be selected, designed, and maintained according to the 2014 SWMMWW. Construction-related pollution control is addressed above under Minimum Requirement #2, Element #9 above. Dust control may be achieved by sprinkling dry soils with water, but not to the point that the dust control water creates turbid runoff. Pollutant Source Specific BMP S411 – BMPs for Landscaping and Lawn Management shall be implemented per the SWMMWW and appendices in this report. Minimum Requirement #4 – Preservation of Natural Drainage Systems and Outfalls Stormwater will be dispersed to prevent concentration of runoff. Little to no impact will occur to any existing natural drainage channels. Minimum Requirement #5 – Onsite Stormwater Management Projects that trigger only Minimum Requirements #1 -#5 shall either use LID BMPs from List #1 or achieve the LID Performance Standard. For this project, BMPs from List #1 will be employed to meet this requirement. For lawn and landscaped areas BMP T5.13, Post -Construction Soil Quality and Depth will be applied to disturbed areas which will not be converted into impervious surface. An 8” layer of topsoil meeting the specifications detailed in the BMP explanation in the SWMMWW will be placed throughout the lawn area and other disturbed areas. Roof Areas will runoff will be discharged through an above ground dispersion pipe. This BMP should meet the intent of the BMP T5.10A – Downspout Dispersion Systems. A 9’ x 9’ x 4’ deep drywell will be built in the front yard area and will collect all roof runoff. Driveway runoff will be collected into a trench drain at the bottom of the driveway and routed to the 9’ x 9’ x 4’ deep drywell built in the front yard area. The location of the existing septic system makes any dispersion BMPs infeasible. The drywell has been sized to take on the runoff from the driveway area and the roof area. This method of stormwater management for the driveway was discussed with agreed to with county officials via Zoom conference call on November 19, 2020. 4.0 THE LIMITATIONS OF OUR INVESTIGATION AND FINDINGS The observations and conclusions of this report apply only to th e subject property and they are not transferable to nearby or adjoining property. This report was prepared for Jim Williams, and may be used by others only with permission. United Engineering, Inc. warrants this report was conscientiously prepared in accordance with the practice of professional civil engineering. No other warranty, either express nor implied, is provided. Respectfully, Zachary N Slota, P.E. Principal Engineer Digitally signed by Zachary N. Slota, P.E. Date: 2021.02.08 08:25:58 -08'00'