HomeMy WebLinkAboutPARAGON POINT PUD PRELIMINARY - 48 91A - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTPRELIMINARY DRAINAGE PLAN
FOSSIL RIDGE P.U.D.
FORT COLLINS, COLORADO
UNNAMED
TRIBUTARY
LEGEND
SUBCATCHMEN'T
El
CONVEYANCE ELEMENT
ig NODE
17 KNOWN HYDROGRAPH
FOSSIL CREEK
DETENTION POND _U/S
OF TRILBY ROAD
Figure 3.1. Schematic of the Developed Condition SWMM Model.
7
1981) for the areas upstream of the project area. Although the upstream area has
seen substantial topographic changes since the 1/4-section maps were produced,
the potential storage in this area is primarily confined to the channel which has
not undergone significant changes. The resulting stage -storage curve and the
combined storage -discharge curve are both given in the appendix.
The developed condition SWMM analysis (including Fossil Creek and the
unnamed tributary influences, and Trilby Road storage routing) indicates that the
100-year proposed condition discharge upstream of Trilby Road would be 3,839 cfs.
This is almost 7 percent higher than the Master Plan value of 3,600 cfs; this may
be attributed to the greater detail with which the proposed development was
analyzed. However, the Master Plan analysis did not include storage routing
upstream of Trilby Road; consequently, the developed condition discharge
downstream of Trilby Road was also reported to be 3,600 cfs. The storage routing
SWMM analysis for the current study determined that the 100-year discharge
downstream of Trilby Road would be 1,798 cfs; this information is summarized on
Sheet 3. This discharge corresponds to a ponded water surface elevation of
4896.2 ft; or 0.2 feet of overtopping for the proposed embankment at Trilby Road.
The 100-year peak discharges from the local subbasins are also shown on
Sheet 3. Except for the relatively flat and undeveloped Subbasin 107 which
exhibited a unit runoff of 2.2 cfs/ac, the developed condition unit runoff values
range from 3.2 to 5.2 cfs/ac. The developed -condition SWMM output is provided
in the appendix.
3.2 Developed Condition Flood Plain Delineation
The Master Plan cross sections, from the upstream face of Trilby Road to
the upstream end of the study reach, were used in the revised HEC-2 analysis for
the current study. The starting water surface elevation at the upstream face of
Trilby Road was specified equal to the SWMM maximum elevation of 4896.2 ft. The
peak discharge upstream of Trilby Road of 3,839 cfs was assumed constant
throughout the study reach. Although the discharge actually varies along the
reach, this level of detail is reserved for the final drainage study.
The HEC-2 results (provided in the appendix) indicate that backwater
influence is felt for at least the first 1,900 ft upstream of Trilby Road. The
water surface elevation varies from 4896.2 ft at Trilby Road to 4896.9 ft at the
0
upstream end of the proposed development. The developed condition 100-year flood
plain boundary is delineated on Sheet 2. Although the boundaries indicate that
the developed condition flood plain is more extensive than the existing
condition, it is noted that the Master Plan developed condition water surface
elevation upstream of Trilby Road was 4896.5 ft (0.3 ft higher than the developed
condition water surface determined for the current study).
3.3 Proposed Drainage Plan
The proposed drainage plan consists of a combination of street flow, cross
pans, curb inlets, pipes, culvert, roadside and perimeter ditches, and drainage
swales through the park site; this plan is detailed on Sheet 3. Although the
proposed grading plan is not shown on the sheet, it is anticipated that, to the
extent possible, final grades will closely match existing ground. It is noted
that detention is not indicated on the plan. This is the result of an existing
agreement with the City of Fort Collins that detention will not be required for
this site. As indicated in the results presented in the previous section, Trilby
Road should provided ample detention for the area.
4
IV. REFERENCES
1. Simons, Li & Associates, Inc., August 1982. "Fossil Creek Drainage Basin,
Master Drainageway Planning Study". Prepared for the City of Fort
Collins, Larimer County, and the Colorado Water Conservation Board.
2. Simons, Li & Associates, Inc., August 1982. "Fossil Creek Drainage Basin,
Master Drainageway Planning Study, Technical Addendum". Prepared for the
City of Fort Collins, Larimer County, and the Colorado Water Conservation
Board.
3. U.S. Department of Transportation, Federal Highway Administration,
September 1985. "Hydraulic Design of Highway Culverts", Hydraulic Design
Series No. 5, Report No. FHWA-IP-85-15, McLean, Virginia.
10
PRELIMINARY DRAINAGE PLAN
FOSSIL RIDGE P.U.D.
FORT COLLINS, COLORADO
PREPARED BY:
Lidstone & Anderson, Inc.
736 Whalers Way, Suite F-200
Fort Collins, CO 80525
" PREPARED FOR:
TST, Inc.
748 Whalers Way, Building D
Fort Collins, CO 80525
October 28, 1991
TABLE OF CONTENTS
I. PROJECT DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . 1
II. EXISTING DRAINAGE CONDITIONS . . . . . . . . . . . . . . . . . . . 3
2.1 Existing Condition Hydrology . . . . . . . . . . . . . . 3
2.2 Existing Condition Flood Plain Delineation . . . . . . . . . . 4
III. POST -DEVELOPMENT DRAINAGE CONDITIONS . . . . . . . . . . . . . . . 6
3.1 Developed Condition Hydrology . . . . . . . . . . . . . . . . 6
3.2 Developed Condition Flood Plain Delineation . . . . . . . . . 8
3.3 Proposed Drainage Plan . . . . . . . . . . . . . . . . . . . 9
IV. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
APPENDIX
ii
1. PROJECT DESCRIPTION
The Fossil Ridge development is a proposed residential subdivision
encompassing the majority of the SW 1/4 of Section 7, Township 6 North, Range 68
West, in the City of Fort Collins, Colorado. The development is bounded on the),
north by the Union Pacific Railroad (UPRR), on the east by Trilby Road, on the
south by Lemay Avenue, and on the west by the city -owned Southridge Golf Course.
A vicinity map of the project is shown in Figure 1.1. Both Fossil Creek and a
relatively large unnamed tributary pass through the development site
The proposed development consists primarily of single-family homes, with
approximately 20 percent of the housing being devoted to multi -family dwellings.
In addition to residential uses, the development includes both a park and
recreation area consisting of tennis and basketball courts, baseball and soccer
fields, and two wetland areas.
1
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Figure 1.1. Vicinity Map for the Fossil Ridge Drainage Study.
2
II. EXISTING DRAINAGE CONDITIONS
The limits of the historic drainage area associated with the proposed
development are illustrated on Sheet 1. Also shown is the boundary of the
proposed development. The development would encompass approximately 145 acres
with a 35 acre corridor along Fossil Creek remaining in its natural state. The
existing drainage area consists of 167.1 acres and can be characterized by
relatively rugged terrain fringing the historic Fossil Creek flood plain. To the
west of the creek, the native ground is relatively steep (slopes up to.10
percent); the eastern area consists of steep bluffs (slopes of about 25 percent).
Fossil 'Creek is a meandering, alluvial stream with numerous oxbows and zones of
significant bank erosion. The development layout provides a substantial buffer
between all structures and Fossil Creek which should minimize the potential for
damage due to meandering of the stream.
2.1 Existing Condition Hydrology
The proposed development is located within the study area which is included
in the Fossil Creek Drainage Basin, Master Drainageway Planning Study [1,211
completed August 1982. The Master Plan was used extensively in preparing this
preliminary drainage study. The Master Plan includes a comprehensive Denver
Urban Drainage Storm Water Management Model (SWMM) for characterizing hydrologic
conditions in the Fossil Creek basin. The Master Plan indicates that the
existing condition 100-year return period discharge near the upstream development
-year existing
boundary is 2,520 cfs. The Master Plan also cites the 100
condition discharge near the downstream boundary (Trilby Road) as 2,720 cfs. The
unnamed tributary in the area has a 100-year upstream inflow of 509 cfs (as
computed by LIdstone & Anderson) Inc.), and a 100-year discharge at the Fossil
Creek confluence of 522 cfs (from the Master Plan). These flow values are
indicated on Sheet I.
Also shown on Sheet 1 are the local drainage boundaries delineated for the
current study. These subbasins were modeled for the current study using the SWMM
model. Geometric parameters for the subbasins were determined from the V =100'
topographic map (contour interval of 1 foot) developed for this study from
photography dated September 1991; this map provides the base for Sheet 1.
3
Infiltration parameters, depression detention depths, roughness coefficients, and
the percent impervious for each type of land use was adopted from the Master
Plan. The Master Plan SWMM model indicates a constant infiltration rate of 0.40
inches/hour and an impervious percentage of one (1) for the current study area.
Finally, the 100-year rainfall event (total depth of 2.24 inches) was also taken
from the Master Plan.
With existing hydrologic conditions well-defined in the Master Plan, the
existing condition hydrologic analysis for the current study was limited to the
local runoff for the development site. The schematic of the existing condition
hydrologic model is shown in Figure 2.1. The 100-year peak runoff from each
subbasin is indicated on Sheet 1. These discharges correspond to unit runoff
values of about 1.1 cfs/ac for the flatter Subbasins 101 and 103, and
approximately 2.3 to 3.0 cfs/ac for the other four, steeper subbasins. The SWMM
model output for existing conditions is included in the appendix to this report.
2.2 Existing Condition Flood Plain Delineation
The existing culvert on Fossil Creek at Trilby Road is an 8-foot diameter
corrugated metal pipe (CMP). While this culvert and road embankment cause a
significant obstruction to Fossil Creek flows, the UPRR embankment and culvert
(approximately 1100 feet downstream of Trilby Road) are even more restrictive.
Consequently, the Master Plan HEC-2 analysis of Fossil Creek indicates the UPRR
backwater area extends a distance of about 2500 feet upstream of Trilby Road.
The 100-year ponded water elevation upstream of Trilby Road is 4895.1 ft for
existing conditions. The 100=year water surface elevation increases to 4895.7
ft at the upstream end of the development site. The existing condition 100-year
flood plain boundary is delineated on Sheet 2.
It is noted that the cross sections and water surface elevations used to
plot the existing condition boundaries shown on the sheet were taken directly
from the Master Plan.
4
UNNAMED
FOSSIL CREEK TRIBUTARY
(104 17 102
105) L (103
111 / (19) (18
12
106
13
101
14
10
LEGEND
101 r SUBCATCHMENT 15 — --
13 CONVEYANCE ELEMENT
18 NODE
17 KNOWN HYDROGRAPH
Figure 2.1. Schematic of the Existing Condition SWMM Model.
5
III. POST -DEVELOPMENT DRAINAGE CONDITIONS
3.1 Developed Condition Hydrology
The developed condition SWMM model was constructed using the existing
condition subbasins with percent impervious values of 45 and 60 for the single -
and multi -family areas, respectively. The only significant boundary change was
the subdividing of existing -condition Subbasin 102 into two developed -condition
subbasins, one each north and south of the unnamed tributary. In addition, the
upstream inflow hydrographs for both Fossil Creek and the unnamed tributary were
included. It is noted that the Master Plan, 100-year peak discharges at these
points are 3,206 and 998 cfs, respectively. The SWMM model schematic for
developed conditions is shown in Figure 3.1.
Included in the development plan is the replacement of the existing Trilby
Road 8-ft diameter CMP with a 24'W x 6'H three -side reinforced concrete box
(RCB), and the raising of Trilby Road from elevation 4892.5 ft to 4896.0 ft. As
the improved embankment would be only slightly lower than the ponded water
surface caused by the UPRR (cited in the Master Plan for developed conditions as
4896.5 ft), and noting that Trilby Road may result in substantial upstream
ponding, the developed condition SWMM model includes routing of inflows through
the detention area upstream of Trilby Road. This is represented in the schematic
as Conveyance Element 21. The detention area upstream of Trilby Road was modeled
by: (a) evaluating the hydraulic characteristics of the proposed Trilby Road
culvert, and (b) determining the available storage upstream of Trilby Road.
The proposed 24'W x 6'H RCB was evaluated using the HY-8 computer program
which utilizes the Federal Highway Administration's criteria--for—analyzing
culverts [3]. Tailwater conditions at the culvert were based on the Master Plan
HEC-2 water surface elevations computed at the downstream face of Trilby Road.
The minimum Trilby Road elevation was assumed to be 4896.0 ft. The results of
the HY-8 analysis demonstrated that overtopping of the road would occur at a
discharge of about 1600 cfs; the HY-8 output and the stage -discharge curve for
the culvert are contained the appendix.
The stage -storage curve for the detention area upstream of Trilby Road was
developed from the 1"=100' topographic map of the area (September 1991), and the
Fort Collins 1/4-section topographic maps (NW 1/4 of Section 7, Township 6 North,
Range 68 West and NE 1/4, Section 12, Township 6 North, Range 69 West, dated
0