HomeMy WebLinkAboutEngineer's OWTS Design PlanApril 24, 2025
502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704-0311 • Fax (970) 704-0313
S OPRIS E NGINEERING • LLC civil consultants
Richard and Vicky Nash
965 Westbank Road,
Glenwood Springs, CO 81601
richard@nashconstructioncompany.com;
RE: Design Report–Onsite Wastewater Treatment System (OWTS), Nash Residence, 965 Westbank Rd,
Glenwood Springs CO, AKA Westbank Ranch Subdivision #1 Lot 15, Garfield County, Colorado
SE Job No. 30177.01 Parcel No: 218535301015. Garfield County OWTS Permit No._____________
Dear Richard and Vicky:
This letter presents the applicable findings regarding 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 prior 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 permit/construction plan “C1-OWTS-
Plan”, dated 04-24-2025. that was prepared with GIS, LIDAR, Survey Plat information and aerial mapping. The
design criteria and system sizing information are summarized below.
Summary Existing Conditions
The subject property is a developed site consisting of a single-family residence, detached garage, utilities, driveway
and established landscaping. The detached garage will be expanded with an ADU. The OWTS, as described here,
will serve the proposed ADU. The current OWTS system is recorded by permit # SEPT-05-216880 issued 08/02/21.
Summary Design Criteria
The new OWTS is designed for a treatment capacity of 150 gallons per day based on the design wastewater flow to
serve a minimum 1-bedroom ADU in accordance with Table 6-2 values, Section 43.6, A.4. The new OWTS will
utilize a minimum 1,000 gallon two-compartment reinforced plastic or alternative concrete septic tank to be installed
off the west side of the proposed structure. 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 STA will consist of leaching chamber trenches that provide 180 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 a minimum 1.5%
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-1 OWTS plan.
Nash Residence
965 Westbank Road, Glenwood Springs, CO
SE Job No. 30177.01
April 24, 2025
Page2
Design Flow
The design flow is calculated as follows for the minimum design capacity of an equivalent 1-bedroom additional
dwelling unit. The OWTS is designed for a treatment capacity of 150 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 1 bedrooms at 2 persons per bedroom equating to 2 persons.
The Design flow is calculated based on residential usage of 75 gal/person/day, No PF
Max. Design flow (Qd)gallons/day = (# of people) x (avg. flow) gal/person/day.
Design flow Qd = 2*75=150 gpd
Septic Tank Design
The septic tank capacity required for a minimum 48 hour retention time is calculated as follows:
V = 150 gal/day * 2 = 300 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 Testing
A subsurface soil investigation and site assessments were performed by Sopris Engineering on May 1, 2021 and
June 7 2021. The shallow soils in the previously excavated chamber trench field, excavation for the replacement
tank and soils observed in the deep crawl space area were sampled and characterized by application of the USDA
visual/tactile soil texture method analysis. Soils were observed in the 2-3 feet deep trenches excavated for the
original replacement field. A 2.5 shallow hand dug profile hole was dug in the vicinity of the field and the
approximately 6 feet deep vertical foundation excavation walls were observed and sampled at 3 and 6 foot depths.
The soils below 6 inches of topsoil consist of medium dense slightly sandy silt loam texture soil with slightly blocky
to granular structure to a depth of 6 feet. The soil profile contained less than 35% scattered gravel or rock content to
the 6-feet depth observed below the surface grades. No evidence of free water was encountered in the excavations.
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 and foundation, are
characterized as a soil type 2 consisting of silt loam texture with slightly blocky granular structure. This soil has an
effective loading rate for conventional soil treatment of 0.6 Gal/S.F./day for a level 1 conventional treatment system.
The equivalent percolation rate is assumed to be 20-30 minutes per inch. The soils are suitable for a shallow
conventional shallow absorption field consisting of gravelless infiltration chamber units. Prior to field excavation
and placement of materials, during the time of construction, additional observations of soils in the proposed tank and
trenches will be performed to confirm initial soil characterization and direct placement of specified components.
The findings from the observations and construction inspections will be included in the As-constructed
documentation.
Treatment-Soil Treatment Unit/ Absorption System Design
The treatment/absorption 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.
Nash Residence
965 Westbank Road, Glenwood Springs, CO
SE Job No. 30177.01
April 24, 2025
Page3
Long Term Acceptance Rate (LTAR):
Considering the application of the state regulation 43:
Receiving Wastewater Treatment Level 1;
LTAR; (Table 10-1 Soil type 2 loading rate for a silt loam is (0.6 gal./S.F./day);
Design Flow; Qd = flow (gal/day) =150 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) = Qd x L.F.1 x L.F.2 .: A(sf) = 150 gpd x 1.0 x 0.7 = 175 S.F.
LTAR 0.6 Gal/S.F./day
Number of Quick-4 Infiltrator chambers 4'x3'x12"chambers:
175 S.F. = 14.583 chambers; Use 15 chambers
12 S.F./Chamber
Number of Equivalent Arc chambers 5'x3'x12"chambers:
350 S.F. = 11.667 chambers; Use 12 chambers
15 S.F./Chamber
Use 15 Quick-4 Infiltrator chambers in 3 trenches 22' long by 3' wide with 5 chambers units per trench plus two end
caps as delineated on the plan. Alternative use 12 Arc chambers in 3 trenches 20' long by 3' wide with 4 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-OWTS construction
meeting is essential and required prior to installation of the OWTS. All septic system components shall be staked in
the field for approval by the Engineer prior to excavation.
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 installations will be inspected, field measured,
reported and delineated in the certification letter and record drawing. The contractor needs to photo document all
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