HomeMy WebLinkAboutLARIMER COUNTY DETENTION COMPLEX, SHERIFF'S ADMINISTRATION BUILDING - SPECIAL SITE PLAN ADVISORY REVIEW - 37-99 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORT7. REFERENCES
1. City of Fort Collins, "Storm Drainage Design Criteria and Construction Standards",
(SDDCCS), dated May 1984.
2. "Final Drainage Report for Larimer County Detention Center Expansion", TEC,
February 1998.
3. "Final Drainage Report and Erosion Control Plan -Advanced Energy Buildings 7 &
8", Park Engineering Consultants, April 7, 1999
4. Soil Survey of Larimer County Area, Colorado. United States Department of
Agriculture Soil Conservation Service and Forest Service, 1980.
5. "Soils and Foundation Investigation, Larimer County Sheriffs Adminsitration
Building, Lot 24 Prospect Business Park, Fort Collins, Colorado", CT JThompson,
Inc., October 19, 1999.
6. "Spring Creek Basin Master Drainageway Plan", Engineering Professionals, Inc.,
March 1988.
7. Urban Drainage and Flood Control District, "Urban Storm Drainage Criteria
Manual", Volumes 1 and 2, dated March 1969, and Volume 3, dated September
1992.
Final Drainage and Erosion Control Report Page 11
Sheriff's Administration Building November 24, 1999
all
6.
strips. The detention pond has a relatively flat bottom that helps slow the velocity of runoff
water and aids sediment settling. The retention pond will serve as a settling pond for
sediment from runoff water from the parking areas.
EROSION CONTROL DURING CONSTRUCTION
6.1 Erosion and Sediment Control Measures
Erosion and sedimentation will be controlled during construction by inlet filters, silt fences,
gravel construction entrances, seeding and mulching and sediment traps. These measures
are designed to limit the overall increase in sediment yield due to construction activities.
During overlot and final grading, the soil will be roughened and furrowed perpendicular to
the prevailing winds. The placements of these erosion control measures are shown on the
Overlot Grading, Drainage and Erosion Control Plan included with this report.
6.2 Permanent Stabilization
All soils exposed during land disturbing activity (stripping, grading, utility installations,
stockpiling, filling, etc.) shall be kept in a roughened condition by ripping or disking along
land contours until mulch, vegetation or other permanent erosion control is installed. No
soils in areas outside project street rights of way shall remain exposed by land disturbing
activity for more than thirty (30) days before required temporary or permanent erosion
control (e.g. seed/ mulch, landscaping, etc.) is installed.
Rnal Drainage and Erosion Control Repoli Page 10
Sheriff's Administration Building November 24, 1999
1 5.
The size of Pond B was set equal to twice the total volume of runoff from the tributary area
generated during a 2-hour, 100-year storm (3.67" total rainfall). The Colorado Urban
Hydrograph Procedure (CUHP) was used to calculate the volume of excess runoff to be 1.4
ac-ft. Thus, the minimum retention volume was set to be 2 x 1.4 ac-ft (from CUHP) = 2.8
ac-ft. The proposed Pond B has a volume of 2.9 ac-ft at an elevation of 4899.3 ft-msl and
3.8 ac-ft at an elevation of 4900 ft-msl. Thus, ample retention volume is being provided in
the event that two 2-hour, 100-year storms occur back-to-back.
The volume of excess runoff during a 24-hr, 100-year storm was also analyzed. The rainfall
amount for this storm (5.49") was obtained from Matt Fater with the City of Fort Collins
Stormwater Utility. The volume of excess runoff for this storm was calculated to be 2.8 ac-
ft. Thus, ample retention volume is available for one 24-hour, 100-year storm event with
0.7-feet of freeboard. If the proposed retention pond overflows, water will exit on the
southern portion of the pond and flow south following historical flow patterns.
Since a clayey, gravelly sand layer lies 1 to 6-feet below the bottom of the proposed retention
pond, it is expected that water in the pond will drain relatively quickly. An analysis of the
infiltration. time for the 2.8 ac-ft of storage was completed using Horton's infiltration
equation. This infiltration equation is used in the program UDSWM2-PC and is
recommended by the city of Fort Collins and the Urban Drainage and Flood Control District.
The time of complete infiltration varies depending on the area over which infiltration occurs.
The analysis used surface areas of 1.2, 0.75 and 0.40 acres, which approximately represent
the top surface area of the pond when full, half full and almost, empty. These infiltration
areas gave a retention time in the pond of 55, 88, and 166 hours (2.3, 3.7, and 6.9 days),
respectively. Thus, the expected amount of time that it will take Pond B to infiltrate after
two consecutive 2-hour, 100-year storm events is less than 7 days. In the event that the pond
bottom seals up and no longer infiltrates, pumping of the pond may be required if it becomes
a nuisance. Maintenance of the proposed retention pond will be the responsibility of Larimer
County. Calculations for the retention pond sizing and infiltration rates are included in
Appendix B.
WATER QUALITY
The quality of stormwater runoff will be enhanced on the Sheriff's Administration site
through the use of Best Management Practices (BMP's). Some of the BMP'S include
minimizing the amount of directly connected impervious area and using irrigated grass buffer
Final Drainage and Erosion Control Report Page 9
Sheriff's Administration Building November 24, 1999
from Pond A was set so that the developed peak flow from the site south of the high point
on Midpoint Drive is equal to or less than historic peak flows. The historic peak flow is from
historic basins HI and H2. Runoff from subbasin Hl will continue to the south through the
undeveloped lot to the south and then to Prospect Business Park Lot 26. This lot diverts a
portion of the flows to the west and a portion of the flows to the east into Midpoint Drive.
It is estimated that approximately 80% of the flow is carried to the east to Midpoint Drive.
Thus, the total historic runoff from the site to Midpoint Drive south is approximately 0.2 cfs
(subbasin H2) + 1.7 cfs x 80% (subbasin Hl) = 1.6 cfs. The allowable release from the pond
is then 1.6 cfs (historic) —1.1 cfs (subbasin 106) = 0.5 cfs.
Pond A was sized using the Rational Volumetric (FAA) Method. The tributary area to Pond
A is 3.4 acres with a composite CI00 of 0.44. Given a release rate of 0.5 cfs, the required
detention volume is 0.21 ac-ft. The available volume is 0.24 ac-ft at a water surface
elevation (WSEL) of 4902.0. This requires ponding at the low point of the entrance drive
to a maximum depth of 6". This depth of ponding is within the allowable street flow depth
set by Larimer County and City of Fort Collins criteria. Release from the pond will be
controlled through a 4" PVC outlet pipe. The emergency spillway for Pond A will be to the
east onto Midpoint Drive. Detention volume and outlet sizing calculations are included in
Appendix B.
4.2 Retention Pond B
A retention pond is being proposed west of the site on the adjoining property owned by
Larimer County. As stated in Section 3.1, the retention pond is a temporary pond until a
master drainage plan for the Larimer County Detention Center Midpoint Campus can be
completed and permanent detention facilities with acceptable drainage outlets can be
designed.
The geotechnical investigation (CTUThompson, Inc., 1999) indicates that the groundwater
surface lies at a depth of 8 to 9 feet at an elevation of approximately 4894.0. This
investigation was conducted at a time that is expected to experience the highest groundwater
elevation. Thus, the proposed pond was graded so that the lowest point is a minimum of 1-
foot above the groundwater. The geotechnical investigation also indicates that the site is
underlain a 19-foot layer of medium dense to dense, clayey gravelly sands approximately 8-
feet below the existing ground (approx. elevation 4893-4874 ft).
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Sheriff's Administration Building November 24, 1999
r
subbasins and Design Points (DP's) associated with this site. Qwr in the table includes
applicable carry over flows.
Table 3.1 - Developed Conditions
DRAINAGE SUMMARY TABLE
Design
Point
Tributary
Sub -basin
Area
(ac)
C (10)
C (100)
tc (10)
(min)
tc (100)
(min)
Q(10)tot
(cfs)
0(100)tot
(cfs)
101
1.38
0.10
0.13
7.4
7.3
0.59
1.5
2
102
2.01
0.52
0.65
7.8
6.4
4.37
11.9
1
101 +102
3.39
0.35
0.44
11.8
8.7
4.21
12.2
3
103
0.32
0.82
1.00
5.0
5.0
1.29
3.2
104
0.98
0.55
0.69
11.3
8.8
1.95
5.5
4
103+104
1.30
0.62
0.77
5.2
5.0
3.80 .
10.0
5
105
0.84
0.45
0.56
9.5
8.3
1.45
3.9
6
106
0.14
0.59
0.74
5.0
5.0
0.41
1.1
4.
3.6 Storm Drainage Structure Design
Storm drainage structures for this site includes a sidewalk culvert and a storm outlet pipe for
Pond A. The capacity of the sidewalk culvert and the storm pipe was determined using the
computer program FlowMaster, which uses Manning's Equation. The proposed 4' concrete
sidewalk culvert has capacity to convey peak flows from the 100-year storm. The outlet
pipe for Pond A was sized using the orifice equation to determine the diameter needed to
appropriately control flows out of the pond.
All hydraulic calculations are included in Appendix B, and the location of all storm drainage
structures is shown on the Drainage and Erosion Control Plan included in the back pocket
of this report.
POND DESIGN
4.1 Detention Pond A
Detention Pond A is located on the east side of the site and will outlet onto Midpoint Drive.
Pond A intercepts and detains stormwater runoff from subbasins 103 and 104. The release
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Sheriff's Administration Building November 24, 1999
3.2 Off -site Flow Routing
There are no off -site flows being routed through the Sheriff's Administration Building Site.
3.3 Onsite Flows
The site was graded so that stormwater runoff is directed away from the proposed building.
The entire site except for Basins 105 and 106 will be directed into the proposed ponds (Pond
A & B). Details of the pond design are included in Section 4. A qualitative summarization
of the drainage patterns within each sub -basin is provided in the following paragraphs.
Subbasin 101 includes the area of Retention Pond B.
Runoff from Subbasin 102 is conveyed via sheet flow and gutter flow through the
parking lot to the west and to the proposed Retention Pond B.
Runoff from Subbasin 103 is conveyed via sheet flow and gutter flow through the
proposed parking lot to the east to a mid -block cross pan and sidwalk culvert located
at design point 3.
Subbasin 104 includes the area of Detention Pond A and the proposed building
which is drained via roof drains to Pond A.
Runoff from Subbasin 105 is conveyed via overland flow, sheet flow and gutter flow
to Midpoint Drive and the existing concrete cross pan at design point 5. The cross
pan flows to an existing grass -lined swale and then to the existing Vipont Ponds.
Subbasin 106 includes the east portion of the entrance drive. Runoff is conveyed via
sheet flow and gutter flow to Midpoint Drive and then south to an existing inlet and
then to the City of Fort Collins ponds and finally to the Cache La Poudre River.
3.5 Hydrologic Analysis of the Proposed Drainage Conditions
The Rational Method was used to determine the 10-year and 100-year peak runoff rates for
each subbasin. Runoff coefficients were assigned utilizing Table 3-2 of the SDDCCS
Manual. Rainfall intensity was based on the recently adopted design storm information for
Fort Collins. Details of the hydrologic calculations associated with each subbasin are
included in Appendix A. Table 3.1 provides a summary of the resulting peak flows for all
Final Drainage and Erosion Control Report Page 6
Sheriff's Administration Building November 24, 1999
3.
�I
PROPOSED DRAINAGE PLAN
3.1 Drainage Design Options
Many options for grading and stormwater management were considered for the Sheriff's
Administration Building Site. The site is constrained by the lack of underground stormwater
drainage facilities adjacent to the property. Installing underground stormwater pipe was
considered but is not cost effective for the small amount of allowable discharge from the site.
Thus, options that enabled surface discharge onto Midpoint Drive were considered. Option
A was to grade the property so that the entire site drained to the east towards Midpoint Drive.
This option required a large amount of fill to raise the west portion of the site and required
a large detention area adjacent to Midpoint Drive. Taking area out of the lot to the south for
detention was not desirable nor was moving the proposed building away from Midpoint
Drive. Thus, Option A was not selected.
Option B was to grade the site with a high point in the middle of the site and allow a portion
of the site to drain to the east and a portion to the west. The landscape area between
Midpoint Drive and the proposed building was adequate to accommodate a small detention
pond with an outlet onto Midpoint Drive. Both detention and retention were considered to
attenuate runoff from the portion of the site draining to the west. A detention pond is
preferred. However, designing an outfall for the pond was a problem. Due to grade
constraints, it was impossible to construct a detention pond outlet that connected with the
existing concrete trickle pan that runs behind Prospect Business Park Lots 27 to 33.
Constructing an outlet that discharges into the existing Burlington Northern Railroad ditch
is a possibility. However, obtaining a permanent drainage easement and evaluating the
hydraulics of the existing ditch is to be completed in a future master drainage plan for the
1mimer County Detention Center. Thus, constructing a retention pond as a temporary
drainage solution until a permanent outfall can be found was determined to be the best
option.
To insure that the current proposal will not preclude future drainage options without
retention, the proposed grade of the parking lot was evaluated with respect to the existing
concrete trickle pan behind Prospect Business Park Lots 27 to 33. The grade from the
proposed low point in the west parking area to the existing concrete pan in Lot 27 is 0.46%
indicating that positive drainage off of the site is possible.
Final Drainage and Erosion Control Report Page 5
Sheriffs Administration Building November 24, 1999
2.2 Historic Drainage
The existing site is covered with native grasses. The site consists of approximately 3.69
acres and can be divided into three drainage subbasins. A small ridge runs along the northern
property line so that the majority of the site drains in a southern direction at slopes ranging
from 1 to 3 percent. This area is included in historic basin Hl. The southern boundary of
subbasin HI follows the limits of the proposed grading. Another ridge runs north to
southeast on the eastern portion of the site. The area to the north of the high point in
Midpoint Drive drains to an existing mid -block cross pan located at historic design point H3.
This cross pan drains to a grass -lined swale that outlets into the existing Vipont Pond which
outlets into the City of Fort Collins ponds and then to the Poudre River. The area that drains
south of the high point in Midpoint Drive (subbasin H2) is carried approximately 2100-ft
south to an existing inlet on Midpoint Drive that outlets into the City of Fort Collins ponds
and then to the Poudre River.
' The historic peak flow rates for this site were calculated using the rational method for the 10-
year and 100-year storm. Calculations can be found in Appendix A and are summarized in
Table 2.1. An exhibit showing the delineation of the historic subbasins is included in the
back pocket of this report.
I
t
Table 2.1 Historic Conditions
DRAINAGE SUMMARY TABLE
Design
Point
Tributary
Sub -basin
Area
(ac)
C (10)
C (100)
tc (10)
(min)
tc (100)
(min)
O(10)tot
(cfs)
O(100)tot
(cfs)
HISTORIC BASINS
H1
H1
3.07
0.10
0.13
30.5
29.7
0.7
1.7
H2
H2
O.21
0.10
0.13
11.5
1 11.2
1 0.1
1 0.2
H3
H3
1.00
0.34
0.42
9.9
1 9.2
1 1.3
1 3.4
Final Drainage and Erosion Control Report Page 4
Sheriff's Administration Building November 24, 1999
No Text
and, therefore, this site is located within the Poudre River Drainage Basin. The Poudre River
�j Drainage Basin does not have a City of Fort Collins master plan and detention is not required
for this basin. However, per Latimer County requirements, releases from the site must not
�1 cause negative impact to downstream drainage facilities and adequate conveyance must be
■i shown from this site to a major drainage outfall. Thus, detention requirements for this.site
were based on capacity of the downstream drainage facilities and releasing runoff at historic
r; rates.
The site is also included in the Larimer County Midpoint Drive Campus Site Master Plan,
�I completed by EDAW (1999). According to the EDAW report, the site is above the 100-year
floodplain elevation of Spring Creek and the Poudre River. This report also indicated that
future stormwater detention facilities could outlet to the west into the Burlington Northern
Railroad right of way. Because a conceptual master drainage study for the entire campus has
�I yet to be completed, the viability of this option has not been fully investigated. Therefore,
the proposed storm drainage plan for the west side of the site is an "interim" condition and
is further discussed in Section 11 below.
2. EXISTING SITE CHARACTERISTICS
2.1 Soils
The soils for this site are comprised of Caruso clay loam (22) and Table Mountain loam
(105) (USDA, 1980). The soils and foundation investigation for the site (CTUThompson,
1999) found stiff to very stiff sandy clay from a depth of 0 to 9 feet. Caruso clay loam
�- consists of nearly level soils on low terraces and bottom lands. Some areas are, however,
somewhat sloping. Pertinent characteristics of this soil include slow runoff, a slight hazard
of wind erosion, and a moderate hazard of water erosion. Caruso clay loam is categorized
in Hydrologic Group C. Table Mountain loam consists of nearly level soils on low terraces
and bottom lands. Runoff is slow and the hazard of wind and water erosion is slight. Table
Mountain loam is categorized in Hydrologic Group B. Figure 2 shows the site overlaid on
the soils map from the "Soil Survey for Larimer County Area, Colorado", prepared by the
United States Department of Agriculture Soil Conservation Service.
Final Drainage and Erosion Control Report Page 2
Sheriff's Administration Building November 24, 1999
�I
INTRODUCTION
1.1 Project Location & Description
The Larimer County Detention Center Sheriff's Administration Building site is located on
Lot 24 of the Prospect East Business Park in the Northwest One -Quarter of Section 20,
Township 7 North, Range 68 West of the Sixth Principal Meridian, in the City of Fort
Collins, Unimer County, Colorado. The site is bounded by Midpoint Drive to the east and
the Larimer County Detention Center to the north. To the south lies Prospect East Business
Park Lot 25 and to the west lies Spring Creek Farm Lot 6, which are both undeveloped and
currently owned by Larimer County. A vicinity map is included after the Table of Contents
of this report.
This project includes the construction of the proposed Larimer County. Sheriff's
Administration building and associated parking lots. Future expansions of the proposed
building and parking areas are also tentatively planned at this time. The project is part of the
Larmier County Detention Facility Campus Master Plan. This report describes the proposed
drainage and erosion control plan for the proposed construction. The drainage plan includes
consideration of all on -site and tributary off -site runoff.
1.2 Design Criteria & Methods
This report was prepared to meet or exceed the submittal requirements established in the
" Larimer County Storm -Water Management Manual" (LCS WM Manual), dated April 1979.
Where applicable, the criteria established in the "Urban Storm Drainage Criteria Manual"
(UDFCD), 1984, developed by the Denver Regional Council of Governments, was used.
The Rational Method was used to calculate peak 10-year minor and 100-year major runoff
values. Detention volume requirements were determined using the Rational Volumetric
(FAA) method. Local drainage facilities such. as curb inlets and storm pipes were designed
to convey peak flows.. The new rainfall criteria for the City of Fort Collins as amended by
Ordinance 42.199 was used in all hydrologic calculations.
1.3 Master Drainage Basin
The Spring Creek Master Drainageway Plan, completed by Engineering Professionals, Inc.
(1988) indicates the Sheriff's Administration building lies outside of the Spring Creek Basin,
Final Drainage and Erosion Control Report Page 1
Sheriffs Administration Building November 24, 1999
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SHERIFF'S ADMINISTRATION SITE
NOT TO SCALE
TABLE OF CONTENTS
PACE
1.
INTRODUCTION.......................................................................................................................................1
-
1.1 PROJECT' LOCATION & DESCRIPTION.........................................................................................................
1
1.2 DESIGN CRITERIA & METHODS..................................................................................................................
1
1.3 MASTER DRAINAGE BASIN ........................................................................................................................
1
- 2.
EXISTING SITE CHARACTERISTICS..................................................................................................
2
2.1 Solis.........................................................................................................................................................
2
3.
PROPOSED DRAINAGE PLAN...............................................................................................................
5
3.1 DRAINAGE DESIGN OPTIONS......................................................................................................................
5
3.2 OFF -SITE FLow ROUTING...........................................................................................................................
6
_
3.3 ONsTTEFLows...........................................................................................................................................6
3.5 HYDROLOGIC ANALYSIS OF THE PROPOSED DRAINAGE CONDITIONS.........................................................
6
3.6 STORM DRAINAGE STRUCTURE DESIGN.....................................................................................................
7
4.
POND DESIGN............................................................................................................................................
7
4.1 DETENTION POND A..................................................................................................................................
7
4.2 RETENTION POND B...................................................................................................................................
8
- 5.
WATER QUALITY.....................................................................................................................................
9
6. EROSION CONTROL DURING CONSTRUCTION...........................................................................10
6.1 EROSION AND SEDIMENT CONTROL MEASURES....................................................................................... 10
6.2 PERMANENT STABR ZATION.................................................................................................................... 10
7. REFERENCES..........................................................................................................................................11
' APPENDIX A - HYDROLOGIC CALCULATIONS
APPENDIX B - DETENTION & RETENTION POND DESIGN
APPENDIX C - EXCERPTS FROM OTHER REPORTS & CHECK FOR FUTURE OUTFALL
Final Drainage and Erosion Control Report Page ii
Sheriff's Administration Building November 24, 1999
j][I-Engineering,
Ltd. - -
2620 East Prospect Rd. Suite 190
Fort Collins, Colorado 80525
(970) 491-9888 ! FAX (970) 491-9984 -
_ www:jreng.com
November,24, 1999
Mark Peterson,'P.E.
' Larimer County Engineering
P.O. Box'1190
Fort Collins, CO 80522-1190
�. RE: 'Final Drainage and Erosion Control Report for the Larimer-County_ Sheriff s
= Administration Building
Dear Mark,
' We "are pleased.to submit to you for your review and approval, this Final Drainage and Erosion
Control Report for the Larimer County Sheriff's Administration Building. All computations
within this report have been completed in compliance with Larimer County and City of Fort -
' Collins Storm Drainage Design Criteria:
' We appreciate your time and consideration in reviewing this submittal. Please call if you have
any questions.
' Sincerely,
JR ENGINEERING
Op RED/S
' Prepared by, . Reviewed by, KLO'� :TF9
JWT Dav1 W. Klocke •........4
r7 Senior Project Mana SS/ONAIEN�`
Attachment
I
4935 North 30th Street 6020 Greenwood Plaza Blvd. 704 Fortino Blvd. West
I Colorado Springs, Colorado 80919 Englewood, Colorado 80111 Pueblo, Colorado 81008
(719) 593-2593 • FAX (719) 528-6613 (303) 740-9393 • FAX (303) 721-9019 (719) 583-2575 • FAX (719) 583-8119'
FINAL DRAINAGE AND
EROSION CONTROL REPORT
LARIMER COUNTY
DETENTION CENTER
SHERIFF'S ADMINISTRATION
BUILDING
November 24, 1999
Prepared for
Larimer County, Colorado Facilities Management
212 West Mountain Avenue
Fort Collins, Colorado
Prepared by
JR Engineering
2620 E. Prospect Road, Suite 190
Fort Collins, Colorado 80525
(970)491-9888
Job Number 9067.11