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Home My WebLink AboutOWTS DesignAugust 5,2021 Harvey, Christopher R & Rebecca J 993 CR227, Rifle, CO 81650 C/O Jeff Parrington President, Tally Ho Construction www.tallyhoconstruction.com 'leff " <jeff@tallyhoconstruction.com> RE: Design Report-Onsite Wastewater Treatment System (OWTS), Harvey Residence, 993 County Frd. 227, Silt, CO, Lot 2 Hoffmeister Exemption, Garfield County, Colorado SEJobNo.31154.01ParcelNo:217906100631'Garfie1dCountyowTSPermitNo.- Dear Jeff and Chris: This letter report presents the applicable findings in regard to the design and construction of a new Onsite Wastewater Treatment System (OWTS) at the above referenced Site, in support of your OWTS permit application to Garfield County. The OWTS design is based on recent site assessment observations by Sopris Engineering of existing physical site features, conditions, geotechnical soils evaluation with respect to the proposed residential usage and applicable regulatory design criteria in accordance with State and County OWTS Regulations. The design recommendations, specifications and construction details are delineated on the attached OWTS permilconstruction plan that was prepared with GIS, LIDAR, Survey Plat information and aerial mapping. The design criteria and system sizing information is summarized below. Existins and Pronosed Conditions The proposed development area is situated on undeveloped meadow adjacent to existing residential development to the south and equipment storage and agricultural land to the north in the central portion of the 3-acre Parcel. The proposed OWTS will serve a new garagelshop structure with a small residential apartment. The subject 3-acre residential development area existing topography and site conditions are derived from county GIS data base, aerial mapping, Lidar topography and survey information by Bookcliff survey. The site is situated in existing inigated agricultural land primarily consisting of meadow and hayfield grasses north of the existing developed facilities. The proposed development area in a broad gently sloped (6Vo to I07o. ) concave meadow that generally trends downgradient to the west. The proposed OWTS field will be constructed parallel to the contours on the southwesterly faced sloped terrain. The property's water supply is from an existing domestic well (DWR permit number 49861-F that is augmented under a West Divide Water Conservancy District Contract No. 980225SSH (a), 1998, located over a 100 feet southwest of the proposed O'WTS field. The proposed development will consist of the construction of a2,400 S.F two-story garage /accessory structure with a L,360 S.F. 1- bedroom residential apartment dwelling on the upper floor. The proposed site improvements will include associated driveway, auto court, utilities and a new onsite wastewater treatment system. The development will be based on applicable site design criteria and include appropriate grading, drainage, and site construction management. The proposed development improvements and site layout plan is shown on the attached OV/TS plan. 502 Main Street a Suite A3 a Carbondale, C0 B1 623 a 970) 704-031 1 a Fax (970) 704-031 3 SoPRrs tttGmttnllG o 11G civil consultants Harvey Residence oV/Ts, 993 County Rd227, Silt, CO SE Job No. 31154.01 August 5,2021 PageZ Summarv Design Criteria The new OWTS is designed for a treatment capacity of 300 gallons per day based on the design wastewater flow to serve a minimum 2-bedroom residence in accordance with Table 6-2 values, Section 43.6, A.4. The new OWTS will utilize a minimum 1,000 gallon two-compartment concrete or alternative reinforced plastic septic tank to be installed off the northwesterly corner of the proposed structure. Sealed concrete or ribbed PVC risers with lids will be installed on the new septic tank along with appropriate pipe connection to the new tank utilizing the appropriate fitting adaptor rings for the tank and riser. The new 1,000 gallon 2-compartment septic tank will be installed with appropriate sanitary tees, outlet effluent filter with risers and lids at the surface. A gravity schedule-40 or SDR-35 sewer pipe section with two-way cleanout will be installed between the tank inlet on the gravity sewer line and the accessory structure. The STU will consist of leaching chamber trenches that provide 350 S.F. of total absorption area. The septic tank effluent will be gravity discharged through a new 4-inch schedule-40 or SDR-35, installed at minimum I.5Vo slope, to the STU via a distribution box. The effluent will be equally distributed from the distribution box via 4" distribution pipes connected to the head end cap of each chamber trench. Observation ports with caps will be installed on the proximal and distal ends of each chamber trench. The design is in compliance with the current County regulations with design calculations based on the soil type, texture and structure with an appropriate long term acceptance rate (LTAR). The designed system meets all required setbacks and will be installed within the general location indicated on the plan. Our design is outlined below and delineated on the attached C-l OWTS plan. Desisn Flow The design flow is calculated as follows for the minimum design capacity of an equivalent 2-bedroom residential dwelling. The OWTS is designed for a treatment capacity of 300 gallons per day based on the design wastewater flow in accordance with Table 10-1 values, Section 43.6, A.4. Regulation 43: Minimum population based on 2 bedrooms at 2 persons per bedroom equating to 4 persons The Design flow is calculated based on residential usage of 75 gaVpersonlday, No PF Max. Design flow (Qd)gallons/day = (# of people) x (avg. flow)gaVperson/day. Design flow Qd = 4*75=300 gpd Septic Tank Desisn The septic tank capacity required for a minimum 48 hour retention time is calculated as follows: V = 300 gallday * 2 = 600 gallons. Use a minimum 1,000 gallon 2-compartment septic tank. The tank to be installed with risers and access lids set a minimum 3 inches above finish grade surface. Sub Surface Conditions and Testins A subsurface soil investigation and site assessment was performed by Sopris Engineering in on July 30,2021.The soils in the proposed field envelope area were sampled and characteized by application of the USDA visuaUtactile soil texture method analysis from samples in two profile pits. Generally the soils below approximately 12 inches of topsoil consist of medium dense silt loam texture soil with slightly blocþ to granular structure to the profile pit depth of 8 feet. The soils contain less than lÙVo scattered gravel or rock content to the 8-feet depth explored below the surface grades. No free water was encountered in the pit excavations. No evidence of seasonal high ground Harvey Residence OWTS, 993 County Rd227, Silt, CO SE Job No. 31154.01 August 5,2021 Page3 water or redox conditions was observed in the profile pits. Seasonal high Groundwater levels are expected to be below 10 feet from the existing surface grades on the relatively flat terrain. The native soils sampled from 2-3 feet below the surface, in the vicinity of the proposed field, are characterized as a soil type 2 consisting of silt loam texture with granular structure. This soil has an effective loading rate for conventional soil treatment of 0.6 Gal/S.F./day for a level I conventional treatment system. The equivalent percolation rate is assumed to be 20-30 minutes per inch. The soils are suitable for a shallow conventional absorption field consisting of gravelless infiltration chamber units. Treatment-Soil Treatment Unil Absorption Svstem Desien The treatmenlabsorption field is designed based on utilizing the effective Long Term Acceptance Rate (LTAR) for the native soils and the application of appropriate loading factors for a soil treatment unit system utilizing gravity distribution to gravelless chamber trenches. The new treatment system and absorption field will consist of gravity distribution of septic tank effluent via a distribution box to individual chamber trenches with treatment across the native soil absorptive surface. Lons Term Accentance Rate (LTAR) Considering the application of the state regulation 43: Receiving Wastewater Treatment Level 1; LTAR; (Table 10-1 Soil type 2loading rate for a silt loam is (0.6 gal./S.F./day); Design Flow; Qd = flow (gaVday) = 300 gpd Loading factors; (Table 10-2, Gravity application trenches, L.F.1 = 1'0 ) (Table 10-3, Gravelless chambers, L.F.2 = 0.7) STU absorption treatment area; A = Area S.F.; A(sf) = Od x L.F.l x L.F'2 .: LTAR A(sÐ = 300 epd x 1.0 x 0.7 = 350 S.F 0.6 GaUS.F./day Number of Quick-4 Infiltrator chambers 4'x3'xt2" chambers 350 S.F. = 29 chambers; Use 30 chambers 12 S.F./Chamber Number of Equivalent Arc chambers 5'x3'xl2"chambers: 350 S.F. = 23 chambers; Use 24 chambers 15 S.F./Chamber Use 30 Quick-4Infiltrator chambers in 2trenches 62'longby 3'wide with 15 chambers units per trench plus two end caps as delineated on the plan. Alternative use 24 Arc chambers in 2 trenches 60' long by 3' wide with 12 chambers units per trench with ends caps. The septic tank effluent will be gravity distributed to the head of each chamber row via individual distribution pipes from a distribution box. Construction and Inspections Prior to construction of the permitted system, the engineer should be contacted by the contractor and owner well in advance to provide adequate time to discuss the system components with the contractor, answer questions, resolve any conflict issues and schedule inspection site visits based on construction progress. A pre-OIVTS construction meeting is essential and required prior to installation of the OWTS in the field for approval by the Engineer prior to excavation. Harvey Residence OWTS,993 County Rd227, Silt, CO SE Job No. 31154.01 August 5,2021 Page4 All septic system components shall be staked During initial construction the engineer will evaluate the soils in the absorption treatment field excavations to confirm soil conditions and make adjustments as needed. All septic system components and trench installations are to be approved by the Engineer prior to backfilling. County Regulations require that the Engineers of record perform site inspections of the permitted system during construction and provide "As-Constructed" documentation of the installed system to the County after construction is complete. All installation will be inspected, field measured, reported and delineated in the certification letter and record drawing. The contractor needs to photo document att OWTS construction phases and supply photos to the Engineer. The County Environmental Health department should be contacted at least 48 hours prior to final backfill of components to provide advanced notice of construction schedule in order to perform a final inspection of the in-place components. The County Environmental Health department should be contacted at least 48 hours prior to final backfill of components to provide advanced notice of construction schedule in order to perform a final inspection of the in place components. The Garfield County contact is: Ted White, P.E., Environmental Health Specialist Itr.Garfield County Public Health; twhite@garfield-county.com,970-945-6614x 8106 (main office) or970-230-2442 (mobile) OWS Operation and Maintenance Ownership of the system and responsibility for maintenance and repair will remain with the property Owner. The Owner is encouraged to retain the services of qualificd pcrsonnel to inspect the OV/TS and to perform all maintenance and repairs necessary to ensure that the system components are maintained in good operating condition and suitable vegetative cover is established on the fields. The components of the OIVTS system should inspected within 30 days of being placed into operation and should been inspected and maintained bi-annually. The tank, absorption field and other system components should be visually inspected bi-annually for debris, damage, leaks, or other potential problems. In general, for a properly utilized system, septic tanla should be pumped every 3 - Syears. The effluent filter should be cleaned every six months and at the time of pumping. Absorption fields should be maintained with suitable vegetative cover and kept free of root invasive plants. Positive surface drainage away from the absorption field should be maintained. If you have any question or need any additional information, Sincerely, SOPRIS ENGINEERING, LLC Paul E. Rutledge Design Engineer Y ::¡t3?? rJ¿1p 05 2t?l T¡JÄL Principal