HomeMy WebLinkAboutEngineer's OWTS Design PlanMay 1, 2025
502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704-0311 • Fax (970) 704-0313
S OPRIS E NGINEERING • LLC civil consultants
Broadhurst, Christopher J & Tamara Marie
PO Box 1132,
Carbondale, CO 81623
C/O Jeff Parrington
President, Tally Ho Construction
www.tallyhoconstruction.com
"jeff" <jeff@tallyhoconstruction.com>
RE: Design Report–Onsite Wastewater Treatment System (OWTS), Broadhurst Residence, 1420 County Road
107, Carbondale, CO, Garfield County, Colorado
SE Job No. 35041.01 Parcel No: 239322300114. Garfield County OWTS Permit No._____________
Dear Jeff and Chris:
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 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 permit/construction
plan “C1-OWTS-Plan”, dated 05-01-2025. that was prepared with GIS, LIDAR, Survey boundary information and
aerial mapping. The design criteria and system sizing information are summarized below.
Existing and Proposed Conditions
The proposed development to replace the existing garage to the southeast portion of the 10-acre parcel. The
proposed OWTS will serve the new garage/shop structure with a ½ bath that may have a future 2-bedroom small
apartment ADU addition in the future.
The subject 10-acre residential development area existing topography and site conditions are derived from county
GIS database, aerial mapping, and lidar topography. The site is situated in existing irrigated agricultural land
primarily consisting of meadow and hayfield grasses south of the existing developed facilities. The proposed
development area is in the transition between a strong slope (30% to 45%) and a broad gently sloped (6% to 10%.)
meadow that generally trends downgradient to the south. The proposed OWTS field will be constructed parallel to
the contours on the south faced sloped terrain below the garage shop. The property’s water supply is from an
existing domestic well (DWR permit number 82425-F, located over 100 feet west of the proposed OWTS field.
The proposed development will consist of the construction of a 7,500 S.F garage/accessory structure with a ½ bath
and a future a 2- bedroom residential apartment dwelling on the 1500 S.F. on the mezzanine floor. The proposed
improvements will include associated 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
OWTS plan.
Summary 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
Broadhurst Residence
1420 County Rd 107, Carbondale, CO
SE Job No. 35041.01
May 1, 2025
Page2
utilize a 1,000 gallon two-compartment concrete or alternative reinforced plastic septic tank to be installed off the
east side of the proposed structure. Sealed ribbed PVC risers with lids will be installed on the new septic tank along
with appropriate pipe connection to the new tank. 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
adjacent to the accessory structure.
The soil treatment area (STA) 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 a
minimum 1.5% slope, to the STA 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.
Design 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 gal/person/day, No PF
Max. Design flow (Qd)gallons/day = (# of people) x (avg. flow) gal/person/day.
Design flow Qd = 4*75=300 gpd
Septic Tank Design
The septic tank capacity required for a minimum 48 hour retention time is calculated as follows:
V = 300 gal/day * 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 Testing
A subsurface soil investigation and site assessment was performed by Sopris Engineering on April 17, 2025. The
soils in the proposed field envelope area were sampled and characterized by application of the USDA visual/tactile
soil texture method analysis from samples in two profile pits. The soils below approximately 6 inches of topsoil
consist of medium dense silt loam texture soil with slightly blocky to granular structure to the profile pit depth of 8
feet. The soils contain less than 5% 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 water or redox
conditions was observed in the profile pits. Seasonal high Groundwater levels are expected to be below 20 feet from
the existing surface grades on the moderatley sloped 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 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
absorption field consisting of gravelless infiltration chamber units.
Broadhurst Residence
1420 County Rd 107, Carbondale, CO
SE Job No. 35041.01
May 1, 2025
Page3
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.
Long Term Acceptance Rate (LTAR):
Considering the application of 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) = 450 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) = 300 gpd x 1.0 x 0.7 = 350 S.F.
LTAR 0.6 Gal/S.F./day
Number of Quick-4 Infiltrator chambers 4'x3'x12"chambers:
350 S.F. = 29 chambers; Use 30 chambers
12 S.F./Chamber
Number of Equivalent Arc chambers 5'x3'x12"chambers:
350 S.F. = 23 chambers; Use 24chambers
15 S.F./Chamber
Use 30 Quick-4 Infiltrator chambers in 2 trenches 62' long by 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-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 installation will be inspected, field measured,