HomeMy WebLinkAbout0.8.18 Drainage, 4.08.05 (7) (e)(iii) DRAINAGE
(iii) 'The proposed method in which storm drainage will be handled, demonstrating that adjoining
property owners would not be damaged by the development; and (A97 -I09)
DRAINAGE REPORT — Sopris Engineering
54
May 19, 2006
Ron Liston
Land Design Partnership
918 Cooper Ave
Glenwood Springs, CO 81601
RE: Reserve at Elk Meadows, Garfield, Colorado
SE Job # 25019.01
Dear Ron:
I have evaluated the preliminary drainage for the proposed Reserve at Elk Meadows. The analysis is based
upon the site plan dated 5/01/06.
Reserve at Elk Meadows is located in Garfield County, about 1'/ miles South of Glenwood Springs,
Colorado. The proposed development lies along County Road 117 (Four Mile Road). The project is divided
by Four Mile Road.
The planned development consists of 189 single-family lots and a community facility (preservation of some
existing structures). The lots range approximately one quarter acre in size. The lots lie in the "flatter" areas
of the site. Additionally, the development includes significant open areas. The total area of the subject
property is approximately 506 acres. The majority of the lot development lies in a valley setting with upland
slopes radiating to the west, north and south. Slopes in the valley area generally vary from 2 to 10 percent,
whereas the slopes on the upland areas may exceed 30 percent.
Based on the location of this development and the proposed activity, the purpose of the drainage study is
three -fold:
• First, to estimate the stormwater peak runoff rates that may be expected due to development.
Pre -development is defined as the existing condition prior to any development construction.
Post -development is referred to as The Reserve at Elk Meadows, with all lots developed with
infrastructure roadways.
• Second, to provide detention for any increase in developed peak runoff rates, above the historic
peak runoff rates, such that stormwater leaving the site does not increase.
• Third, to meet compliance of approved drainage standards per Land Use Regulations for
Garfield County.
Existing On-site Hydrologic Conditions -
The subject site consists of 506 acres. The parcel generally consists of undeveloped property with native
vegetation and irrigated pasture. Existing vegetation consists predominately of sage, oak and other brush
with scattered juniper trees. Four Mile Creek flows along the southern side of the project and has eroded a
narrow, steep canyon below high level outwash terrance. Other than Four Mile Road, there are not any
improved roads across the property.
502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704-0311 • Fax
(970) 704-0313
SOPRIS ENGINEERING • LLC
civil consultants
Reserve at Elk Meadows
SE Project No. 25019
May 19, 2006
Page 2 of 3
Several irrigation ditches are located on the site. The Four Mile ditch is proposed to be piped through most
of the development.
The overall land slopes generally from west to east and slightly to the south. In the portion of the site, west
of Four Mile Road, the slopes vary from more than 30 percent on the side slopes to about 10 percent in the
flatter valley areas. The portion of the site east of Four Mile Road generally slopes between 2 percent to 5
percent within the irrigated pasture area.
100 -year Floodplain Affecting Onsite Development -
The site is not affected by Four Mile Creek. The proposed development as proposed is a large distance from
Four Mile Creek and well above any 100 year floodplain. We reviewed a drainage study dated December
1982 and The Federal Emergency Management Agency (FEMA) Panel Number 080205 1445 dated January
3, 1986 and both studies indicate that the proposed development is well outside the 100 year floodplain. No
additional evaluation was done along Four Mile Creek. The water treatment and sanitary sewer treatment
facility, if needed, will need to be placed out of the 100 year floodplain. Per the studies and a site visit, it is
evident that there is adequate area for both structures to avoid the 100 year floodplain. The future evaluation
will need to be done for the State permits for the water treatment and sanitary sewer treatment. Several
small ephemeral stream channels occur within the project site that merit attention as to potential flooding
along the drainage.
Existing Runoff -
Five drainage basins are identified across the site, which accounts for the property that is proposed to be
developed and the off-site tributary area that flows onto the project site. See Exhibit 1- Existing Drainage
Map, Sheet 1 of 5, which depicts the existing topography and drainage channels. Ten -foot contours are
presented for the off-site and on-site tributary areas. In general, surface water flows in an easterly direction,
with the majority of it collected and discharged into Four Mile Creek.
Off-site Hydrologic Conditions -
The site is affected by off-site stormwater from the west and slightly from the north of the site. All
stormwater from the south is intercepted by Four Mile Creek. The off-site drainage basins consist of natural
conditions, are undeveloped, and have similar hydrologic characteristics as the basins within the proposed
site. The drainage basin areas and peak runoff rates are listed above in the "On-site Hydrologic Conditions".
Development Drainage Plan -
Access roads will link the residential development with County Road 117. Roadways are planned to be
paved with ditches constructed on each side of the roadway to convey stormwater runoff from the lots and
roads. The conveyed stormwater will then drain to the proposed grassy swalls and detention basins, where
detention and infiltration areas will be provided to reduce peak runoff rates and accommodate additional
stormwater volumes generated from development. See Exhibit 2 — Proposed Drainage Map, Sheets 2 — 5,
which depicts land use culverts, detention, step pools, infiltration trenches and general stormwater
management elements. These areas are designed to provide for detention and water quality treatment.
The development consists of 189 lots. Lot sites range approximately one quarter in size. Remaining areas
of the proposed development include undisturbed open areas and roadways.
Reserve at Elk Meadows
SE Project No. 25019
May 19, 2006
Page 3 of 3
Due to the size of the lots and amount of open space, a majority of the site will remain in the native state.
Detention
A minimum of nine detention/infiltration ponds are proposed. The detention or infiltration facilities vary in
size. Each basin will have a low-level outlet pipe to release flows at a controlled rate. Spillways should be
constructed when storms in excess of the 25 -year occur. This will minimize any erosion of the detention
basin slopes.
Interior step pond detention is proposed in some areas to interconnect the released drainage from the
detention basins. The developed peak runoff rate will then be equal or less than the historic peak runoff rate
leaving the site.
Summary
It is our opinion the above preliminary drainage concept and drainage facilities will meet Garfield County's
drainage standards.
The results from this preliminary study suggest that no long-term, adverse impacts to drainage are
anticipated with the development of the Reserve at Elk Meadows.
On-site peak discharge will increase slightly with development. The additional increase in stormwater
volumes will be provided in the proposed detention/infiltration ponds within select basins.
Since surface disturbance is proposed only within the lots and the roadways, the historical drainage pattern
will be maintained.
If you have any questions or need any additional information, please give me a call.
Sincerely,
Sopris Engineering, LLC
Yancy Nichol, P.E.
Principal
PROPERTY BOUNDARY (TYP.)
FOUR MILE ROAD
(COUNTY RD 117)
FOUR MILACREEK
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EXISTING DRAINAGE AREA
-- EXISTING 10' CONMUR
EXISTPJG PAYED R.:1A0
PROPOSED CULVERT
pas-nNo RIPARIAN WOODLAND
TO PLMAIN
EXISTING WOODLAND TO REMAIN
PROPOSED AGRICULTURAL/PASTURE
PROPOSED RESIDENTIAL 1/4 ACRE
PROPOSED ROW/EASEMENT
PROPOSED AGRICULTURAL/PASTURE
N1, Aura
RITVISiON
CAR, IELD COUNTY
RESERVE AT ELK MEADOWS
=BIT 2 - PROPOSED DRAINAGE MAP
MD ITN
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DATE 05.41.A4
PILE NO.
16018.01
SUM8
OP 6
RESERVE AT ELK MEADOWS
EXHIBIT 2 PROPOSED DRAINAGE MAP -UPPER MEADOW
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SOPRIS ENGINEERING - LLC
CIVIL CONSULTANTS
502 MAIN STREET, SUITE A3
CARBONDALE, COLORADO 81623
(970) 704-0311
25019-DRG.DWG 05/01/06
RESERVE AT ELK MEADOWS
EXHIBIT 2 PROPOSED DRAINAGE MAP -SOUTH MEADOW
SHEET 4 OF 5
SCALE: 1"=400'
SOPRIS ENGINEERING - LLC
CIVIL CONSULTANTS
502 MAIN STREET, SUITE A3 •
CARBONDALE, COLORADO 81623
(970) 704-0311
25019-ORG.OWG 05/01/06
\\1
RESERVE AT ELK MEADOWS:
EXHIBIT 2 PROPOSED DRAINAGE MAP -EAST MEADOW
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SHEET 5 OF 5
FOUR MILE ROAD
(COUNTY ROAD 117)
DETENTION/INFILTRATION TRENCH
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SCALE: 1"=400'
FOUR MILE CREEK
SOPRIS ENGINEERING - LLC
CIVIL CONSULTANTS
502 MAIN STREET, SUITE A3
CARBONDALE, COLORADO 81623
(970) 704-0311
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♦ 9
25019—DRGDWG 05/01/06
