HomeMy WebLinkAboutMOUNTAINRIDGE FARM PUD, FIRST FILING FINAL - 18 92C - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTTHIS FINAL STORM DRAINAGE STUDY AND REPORT FOR MOUNTAIN RIDGE
FARM P.U.D. FIRST FILING WAS PREPARED BY LAND DEVELOPMENT
SERVICES USING THE CITY OF FORT COLLINS STORM DRAINAGE DESIGN
CRITERIA, dated May, 1984.
The results of this drainage study are reported
text and supporting documents, and are shown in
the Construction Drawings submitted concurrently
report.
Signed this 1$ih—d$Y—Q£—Ju1y, 1994.
Dennis L. Donovan, P.E. 4
Land Development Services •.
309 W. Harmony Road ••....••'
Fort Collins, Colorado 80526
Phone (303) 225-9244
in the enclosed
further detail on
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MOUNTAIN RIDGE FARM P.U.D. FIRST FILING
FINAL STORM DRAINAGE REPORT
I. GENERAL INFORMATION
Mountain Ridge P.U.D. First Filing is a proposed residential
subdivision in the City of Fort Collins, Colorado. The site is
generally located on the west side of South Shields Street
between Harmony Road and Horsetooth Road in southwest Fort
Collins. A general Vicinity Map is provided at the end of this
report.
A preliminary storm drainage report for the entire Mountain
Ridge Farm property was previously prepared by this office. This
filing is the first of three to five total filings anticipated
for the project. The entire property consists of the majority of
the South 1/2 of the Northeast 1/4 of Section 34, T7N, R68W,
Sixth Principal Meridian, Larimer County, Colorado.
A 14 acre park site has previously been purchased by the
city in the southwest corner of the overall 80 acre site, and a 6
acre townhome project (Cobblestone Corners) is under development
in the northeast corner of the site.
The proposed development for the first filing consists of 58
single family residential lots on approximately 22 acres. The
residential use planned for the property is consistent with the
approved Master Plan. The first filing, or phase, of development
is.the portion of the site on the west side of Shields Street to
Wabash Street. Wabash will be constructed with the first filing,
as well as three cul—de—sac streets and the widening of Shields
adjacent to Mountain Ridge Farm.
The proposed development is bounded on the north by the
Cobblestone Corners residential project, on the west by vacant
land that will be future phases of Mountain Ridge Farm, and on
the south by vacant land. Across Shields Street to the east is
an existing church and additional vacant land that is part of the
Four Seasons residential development.
Mountain Ridge Farm P.U.D. is a part of the properties
identified in the city designated McClelland — Mail Creek and
Foothills Storm Drainage Basins. Almost all of the first filing
is located in the McClelland — Mail Creek "Master Drainage
Basin", but a small portion on the north side of the site (i.e.
the front yard areas of a few lots fronting on Wabash) falls
within the Foothills Drainage Basin.
City funded reports on both basins are quite lengthy, and
are not attached as a part of this report. Both reports are
available at the Stormwater Utility Department of the City of
Fort Collins.
The biggest impact of these studies and reports are the
identified need to construct Regional Detention facilities within
the Mountain Ridge Farm P.U.D. The most recent description of
those improvements are contained in a report titled "FINAL DESIGN
,5e /
MOUNTAIN RIDGE FARM P.U.D. FIRST FILING
FINAL STORM DRAINAGE REPORT
OF REGIONAL DETENTION POND 247 AND OPEN CHANNEL (SHIELDS TO
BENTHAVEN STREETS) FOR THE McCLELLANDS AND MAIL CREEK BASIN -
FORT COLLINS* COLORADO" by RBD Inc., Engineering consultants.
Previously, RBD prepared "FEASIBILITY ANALYSIS FOR REGIONAL
DETENTION FACILITIES ADJACENT TO MOUNTAINRIDGE P.U.D. FOR THE
McCLELLANDS AND MAIL CREEK BASIN FORT COLLINS, COLORADO". Both
reports are considered background information to this drainage
report. Sheets A-1 thru A-4 of the construction drawings for the
first filing show the regional improvements required.
An existing irrigation ditch (i.e. the Pleasant Valley and
Lake Canal irrigation ditch) crosses the property from north to
south approximately 1,200 feet west of Shields Street. The
irrigation ditch is discussed in detail in the above mentioned
reports. The ditch is presumed full of water during the 100
year, or "Major Storm", so it is not a significant factor in the
storm drainage plan.
Generally, the Mountain Ridge Farm First Filing site slopes
from west to east at about 1.0 percent. Off -site storm drainage
will enter the site from the west and the southwest. The
contributing drainage basins and flows are identified on the
Master Drainage Basin Reports. The on -site storm drainage system
is designed to accommodate the storm water flows from off -site,
as well as provide the facilities for the storm water flows
originating within Mountain Ridge P.U.D.
The site has previously been processed for residential
development through the City of Fort Collins. Major obstacles to
the development occurring per previous plans seem to stem from a
shallower water table than desirable; the depth and cost of a 15
inch "trunk/outfall" sanitary sewer system that will relieve the
currently, and/or in the future, inadequate sewer system for
projects that exist to the northwest of Mountain Ridge; and the
size and appropriate cost sharing of the Regional Detention
Facilities in the McClelland Basin.
Basin wide improvements are'required for all of the City
Designated Storm Drainage Basins. Certain basin storm drainage
improvements are identified in each of the Drainage Basin Utility
Construction Plans. Drainage Basin Fees are collected in each
City designated basin to offset the anticipated cost of basin
wide improvements. Information regarding the City designated
McClelland - Mail Creek and Foothills Basins can be obtained in
current form at the City Stormwater Utility Department.
Meetings have been held between the City, owner, developer,
and this office to discuss these issues. It appears that the
detrimental aspects of the site have been resolved and that the
development is ready to proceed, with positive impacts on the
city's storm drainage system resulting.
l
MOUNTAIN'RIDGE FARM P.U.D. FIRST FILING
FINAL STORM DRAINAGE REPORT
The project will allow an existing storm drainage problem to
be resolved. Currently, the basins contributing storm runoff to
the Shields Street crossing of the McClellands - Mail Creek Basin
would overtop Shields during a Major Storm and create flooding
problems downstream. After the Mountain Ridge Farm Regional
Improvements have been completed, the extent (even possibility)
of downstream flooding will be mitigated.
The storm drainage exhibits supporting this report are as
follows:
1. Sheets 13 and 14 of the Construction Drawings, which show the
Neighborhood Grading Plan for the First Filing. These sheets are
identified as Exhibits A and B for this report.
2: Sheet 15 of the Construction Drawings, which shows the Storm
Drainage and Erosion Control Plan for the first filing and
upstream areas contributing storm water runoff directly to the
streets and detention pond in the first filing. This plan shows
the drainage subbasin boundaries, flow arrows, slopes, culverts,
inlets, and the basic storm system in general. Erosion control
measures for the first filing are also noted on the plan. This
sheet is identified as Exhibit C for this report.
Although not included in this report, the information
contained in Sheets A-1 thru A-4 are germain to the storm
drainage plan, and will be included in the revised final report.
Generally speaking, erosion control measures for the first
filing are as follows:
A. Re -seed all disturbed areas (Permanent grasses in the
detention area and temporary crop vegetation in the overlot
grading area.
B. Straw hay bales installed during construction and prior
to installation of the 4 foot wide concrete trickle channel in
the low point of the detention pond. After the concrete pan is
placed, sand bags should be placed in the same locations to
mitigate siltation problems. The pond will also function as a
settling basin during the construction phase.
C. Gravel filters shall be placed in front of all inlets
and sidewalk culverts during the construction and maintained
during the post -construction phase for up to two years.
II. Proposed Storm Drainage Plan
The proposed drainage plan for the Mountain Ridge Farm
P.U.D. First Filing is in compliance with the City's Master
Drainage Plans. The on -site subbasin boundaries are close to
MOUNTAIN RIDGE FARM P.U.D. FIRST FILING
FINAL STORM DRAINAGE REPORT
those boundaries depicted on the Master Plans. The storm
drainage plan for Mountain Ridge Farm P.U.D. First Filing is in
compliance with the City of Fort Collins criteria.
All of the subbasins within the Mountain Ridge Farm first
filing are identified at Exhibit C, which is the Storm Drainage
and Erosion Control Plan. Additional grading and drainage
information is provided at Exhibit A and B, which are the two
sheets describing the Neighborhood Grading Plan. These sheets
are included with this report and were also submitted separately
as a part of the final plan submittal.
Storm runoff from subbasins on the east side of the Pleasant
Valley and Lake Canal irrigation ditch are designated as Subbasin
#Is 1 through 14. These are the subbasins that originate within,
or will directly affect, the first filing improvements.
Subbasins on the west side of the irrigation ditch are not
designated in this report, as the ditch and regional facilities
on the west side of the ditch will be constructed independently
of the subbasins improvements on the east side of the ditch.
Should future phases of Mountain Ridge not be developed, the
homes to be built in the first filing lots will be protected from
flooding — if they are constructed to the recommended minimum
foundation and lot grading elevations depicted on the
Neighborhood Grading Plan.
Storm drainage runoff from all
side of the project, except Subbasin
into the proposed Detention Area on
Runoff from Subbasins 10, 11 and 12
Foothills basin as anticipated. The
which all other runoff from the site
No. 247 on the Master Storm Drainage
of the subbasins on the east
#Is 10, it and 12 will flow
the west side of Shields St.
flow northeasterly into the
detention facility into
will flow is designated Pond
Plan.
There are proposed sidewalk culverts at the end of Ridgeway
Court, Crestway Court, and Lakecrest Court. The subbasins
contributing storm runoff at those locations are identified on
the storm drainage chart, which is on Exhibit C and is included
as Table One on the following page. The design points, or points
of concentration, at those locations are identified as D.P. #Is
I, II, and III.
Where storm drainage runoff from individual subbasins come
together, the combined flows are also identified as Design
Points. Design points are identified for the 10 foot inlet on
the west side of Wabash (Subbasins 6 & 7) as D.P. # V, for the 5
foot inlet on the east side of Wabash (Subbasins 4 & 5) as D.P.
IV, and for the 5 foot inlet on the west side of Shields Street
as D.P. VI.
MOUNTAIN RIDGE FARM P.U.D. FIRST FILING
FINAL STORM DRAINAGE REPORT
Per the above mentioned feasibility analysis and final
design by RBD, a 27 inch Reinforced Concrete Pipe (RCP) culvert
will be installed under Shields St. to provide a storm water
release for the Detention Area (i.e. Pond # 247).
A 42 inch Reinforced Concrete Pipe (RCP) culvert will be
installed under Wabash and under the irrigation ditch to allow
detained flows to occur from Pond 278 downstream to Pond 247.
Details as to discharges, detention volumes, riprap details and
other matters relative to the ponds are included in the RBD
studies.
The Hydrologic Characteristics for the subbasins in Mountain
Ridge, as well as for the combined subbasins at the respective
Design Point are included on the following page as Table I.
PA C" 'J--- , 5 �
TABLE ONE•
Drainage
Subbasin
Designation
1
2
3
4
5
6
7
a
9
10
11
12
13
14
4 & 5
6&7
S-I
Area
C
TC
(Min)
(Ac)
Value
Sr)(100 vr)
0.39
0.7
10.0
10.0
1.29
O.G
10.0
10.0
2.13
0.6
10.0
10.0
0.61
0.7
10.0
10.0
1.17
0.6
10.0
10.0
5.33
0.5
20.6
26.2
2.25
0.5
19.5
17.5
1.78
0.4
.14.4
12.6
3.11
0.3
2.2.1
20.4
1.16
0.4
10.0
10.0
1.49
0.5
12:7
11.3
0.59
0.4
10.0
10.0
11.23
0.3
21.4
19.3
2.01
0.4
10.0
10.0
1.70
0.63
10.0
10.0
7.50
0.5
20.6
26.2
2.98
0.8
10.0
10.0
i (in/hr)
2YR '1"OOYR
2.5
7.2
2.5
7.2
2.5
7.2
2.5
7.2
2.5
7'.2_
1.5
4.5
1.9
5.6
2.1
6.5
1.7
5.1
2..5
7.2
2.3
6.0
2.5
7.2
1.0
5.6
2.2
6.6
2.5
7.2
1.5
4.5
2 .5
7. 2
Q (cfs)
2YR 100YR
0.7
2.5
1.9
7.0
3.2
11.5
1.1
3.8
1.0
6.3
4.0
15.0
2..1
7.9
1.5
5.8
1.6
5.9
1.2
4.2
1.7
6.3
0.6
2.1
6.1
23.6
1.0
6.6
2.6
10.1
5.7
21.3
6.0
21.5
Design
P o :I. n L•
z
zz
zzz
VI
IV
V
VII
MOUNTAIN RIDGE FARM P.U.D. FIRST FILING
FINAL STORM DRAINAGE REPORT
The Time of Concentrations were determined using the
equation for overland flow in the City design manual, and then by
adding the time of concentrated gutter flow. The time of
concentrated flow was estimated using Figure 3-2 "Estimate of
Average Flow Velocity for Use with the Rational Formula" provided
in the "Urban Hydrology for Small Watersheds" Technical Release
No. 55, USDA, SCS January, 1975.
The formula for overland flow Time of Concentration is:
Tc = C1.87 (1.1 - CCf) D1/21 / S1/3
The subbasins within Mountain Ridge P.U.D. First Filing are
relatively small. The maximum time of concentration for any of
the sub basins in this filing is 28.6 minutes for sub basin # 6.
A 10 minute minimum time of concentration was used for several of
the subbasins, since the overland and concentrated times of
concentration for those basins did not exceed 10 minutes.
The Time of Concentration for combined subbasins at the
Design Points is determined using the longest time of
concentration for any of the sub basins contributing at that
point. The longest time of concentration was again 28.6 minutes
during a two year storm and 26.2 minutes during a 100 year storm
at Design Point V, which is the 10 foot inlet on the west side of
Wabash.
The storm discharges are estimated by using the Rational
Method, where 0 = CCfiA in cubic feet per second. C values for
the 2 Year storm are increased by 125 percent when estimating the
100 Major storm discharge. "C" values (Runoff Coefficients) vary
from 0.3 to 0.7 depending on the amount of impervious surfaces
that will exist in the subbasins after home construction. The C
value for an area such as the greenbelt is 0.3, and for subbasin
# 1 with the large amount of street, driveway and roof surfaces,
the C value is 0.7.
Rainfall intensity (i) varies in the subbasins from 1.5 to
2.5 inches per hour for a two year storm and from 4.5 to 7.2
inches per hour for the 100 year storm.
III. DESCRIPTION OF STREET FLOWS AND DRAINAGE IMPROVEMENTS
Due to the relatively small area of the subbasins that will
contribute storm runoff from within Mountain Ridge Farm P.U.D.
First Filing, the concentration of discharges in the street
gutters will be .s,omewhat minor.
The largest flow in a gutter will
Just north of the inlet in Wabash, a 2
discharge of 4.0 cfs is estimated to be
and a 100 year "major" design discharge
estimated. The slope of the street at
percent.
be from subbasin # 6.
year "minor" design storm
flowing in the street,
of 15.0 cfs is
that location is 0.6
MOUNTAIN RIDGE FARM P.U.D. FIRST FILING
FINAL STORM DRAINAGE REPORT
According to Figure 4-1 of the design manual and using the
following criteria: Cross Slope of 2 %, so a Reciprocal (Z) of
50; a Roughness Coefficient (n) of 0.016 from Table 4-3; an
allowable depth of flow in the gutter of 6 inches during a two
year storm: and an eighty percent reduction factor, the gutter
will carry about 20 cfs before exceeding City criteria for the
two year, or Minor, storm.
That capacity is about three times the 2 year storm
discharge in the curb and gutter north of the 10 foot inlet on
the west side of Wabash. The 100 year storm can encroach all the
way across the crown of the street, according to City criteria.
In this filing, the maximum street gutter flows during a Major
(100 year) storm will be below the 6 inch depth allowable during
a 2 year storm. 'Obviously, street flows in Mountain Ridge P.U.D.
First Filing are well within City of Fort Collins criteria.
Storm drainage facilities, or improvements, for the project
include cross pans at the intersections where flows are
concentrated, storm drain inlets and storm sewers, sidewalk
culverts and the regional drainage facilities previously
discussed.
When completed, landscaped, and subject to a warranty period
for the improvements, the City of Fort Collins has agreed to
accept ownership of the drainage tracts for future maintenance of
the integrity of the drainage capacity.
Figure 5-2 of the City Drainage Criteria manual shows a
theoretical capacity of 1.2 cfs per foot of length within the
allowable 6 inch depth and 8 inch heigth of opening for a 2 year
storm. A Reduction Factor of 80 percent shows 0.96 cfs per foot
actual capacity. The inlets all been sized to accomodate the
estimated 2 year flows divided by 0.96 cfs per linear foot.
In summary, the proposed storm drainage for the Mountain
Ridge as reflected on the enclosed exhibits is consistent with
the Master Plan (subject to some modification), and the resulting
storm drainage system should adequately protect life and property
on the site.