HomeMy WebLinkAboutPOUDRE SCHOOL DISTRICT 1999 ELEMENTARY - SITE PLAN ADVISORY REVIEW - 19-97 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORT (3)fThe storage -discharge for the Wildwood Townhomes Detention Pond (302) was compiled
from information in the drainage report and from as -built documentation for the site. Discharge
from the pond is controlled by a staged concrete weir which was intended to meet the allowable 10-
and 100-year allowable discharges for the site; runoff from the pond is directed to Corbett Drive and
is the routed to the PJHS pond:as gutter flow. The stage -discharge rating curve for the pond was
calculated using the weir overtopping component of the HY-8 model for the as -built condition.
Since the as -built survey information compared favorably with the design grading plan, the stage -
storage curve given in the drainage report was assumed to be valid. The stage -discharge and stage -
storage rating curves were combined to form a storage -discharge rating curve for the pond. The
results of the SWMM analysis indicate that the 100-year discharge is 25.2 cfs which is considerably
greater than the design discharge of 8.3 cfs cited in the drainage report. This discrepancy is
primarily due to two factors: (a) the mass -diagram method which was used in the original design
tends to over -estimate pond releases, thus underestimating required storage volumes; and (b) the
original design did not consider inflows from the Charter Hospital Detention Pond.
The resulting discharge from the off -site detention ponds and undetained runoff from Corbett
' Drive was routed into the PJHS Detention Pond to determine the operation of the pond with the
incorporation of the off -site flows. The results of the analysis indicate that the spillway section
would be used for the 100-year event and that the discharge from the PJHS Pond would be 47.2 cfs
which corresponds to a unit runoff rate of 0.7 cfs/acre. The active storage volume would be 6.1 'acre-
feet with a 100-year water surface elevation of 4923.9 ft, msl which provides 0.6 feet of freeboard.
The depth of overtopping in the spillway section would be approximately 0.3 feet which corresponds
to a discharge of 23.4 cfs through the spillway.
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where Q is the discharge (cfs), C is the weir coefficient (2.6), L is the width of the spillway section
(feet), and H is the flow depth (set at 0.25 feet). A weir coefficient of 2.6 was chosen to represent
the characteristics of a grass -lined spillway with a relatively low slope of approximately 2.5 percent.
The results indicate that a 55-foot wide spillway section would be required. The runoff through the
spillway section will be directed east toward the west flowline of Corbett Drive. Calculations for
the design of the spillway section are given in Appendix A.3. The SWMM input and output for the
design of the spillway section is given in Appendix B.4.
1 2.3 Developed Condition Analysis for both On- and Off -Site Flows
The hydrologic evaluation of the with -project PJHS Detention Pond, as described in the
previous section, was subsequently performed to determine the operation of the pond with additional
off -site runoff from the Charter Hospital and Wildwood Townhomes Detention Ponds, and from the
east side of Corbett Drive. The off -site areamas subdivided into three subbasins which include: (a)
Charter Hospital (Subbasin 103); (b) Wildwood Townhomes (Subbasin 104); and (c) the area
tributary to the east side of Corbett Drive which includes fringe areas from Wild Wood Farms,
t Filings 2 and 3 (Subbasin 105). On -site detention for the Charter Hospital and the Wildwood
Townhomes is represented by Detention Ponds 301 and 302, respectively. Two conveyance
elements (Elements 201 and 202) were included to represent routing along Corbett Drive to the
PJHS Pond. A description of the calculation of the storage -discharge rating curves for the two
detention ponds is given in the following paragraphs. Calculations for the development of subbasin,
' conveyance element and detention pond parameters for the off -site tributary area is given in
Appendix A.4.
IThe storage -discharge rating curve for the Charter Hospital Detention Pond (301) was
compiled based on information given in as -built documentation for the site. The pond has a staged
' outlet structure intended to limit the discharge for both the 10- and 100-year events to the discharges
specified in the Master Plan. The discharge from the pond is directed to the.detention pond for the
' Wildwood Townhomes. The discharge and storage volume values given in the as -built
documentation for the 10- and 100-year events were used in the SWMM model rating curve. Initial
SWMM calculations showed a 100-year water surface greater than the design elevation.
tConsequently, an additional ordinate was added to the rating curve, using the orifice equation
methodology given in the Charter Hospital drainage report, to represent the pond release just prior
to the overtopping of the pond embankment. The results of the analysis show that the 100-year
discharge from the pond would be 5.9 cfs (compared to the allowable 5.5 cfs) and that the pond
embankment would not overtop.
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The design procedure for the modification to the outlet structure utilized the orifice equation, the
weir equation and the UDSEWER pipe hydraulic analysis program and is presented in detail in
Appendix A.3.
The result of this design procedure is the placement of orifice restrictor plates on both the
front opening and the outlet pipe to control the 10- and 100-year pond discharges, respectively. An
orifice plate will be placed on the front opening to cover the upper 1.23 feet of the existing 2.13 foot
diameter opening. An additional orifice plate will be placed over the 24"H x 38"W HERCP outlet
to cover the upper 0.93 feet of the. pipe. The pond embankment is assumed to be graded up
approximately 1.5 feet to elevation 4924.5 ft, msl at a slope of 3H:1V along the southern frontage
of the pond and at a slope of 5H:1 V along the eastern and northern edges of the pond. A diagram
of the proposed outlet structure modifications and the proposed berm grading plan are presented on
Sheet 1.
The results of the SWMM modeling for the developed condition pond outlet configuration
with on -site runoff shows that the pond will meet the allowable discharge criteria for both the 10-
and 100-year events. The 10-year discharge from the pond would be 8.7 cfs which corresponds to
a unit runoff of 0.2 cfs/acre. The active storage volume would be 2.7 .acre-feet at the 10-year water
surface elevation of 4922.1 ft, msl. The 100-year discharge from the pond would be 22.0 cfs which
corresponds to a unit runoff of 0.5 cfs/acre. The active storage volume would be 5.0 acre-feet at the
100-year water surface elevation of 4923.4 ft, msl. The SWMM input and output for the developed
condition on -site detention pond analyses are given in Appendices B.2 and B.3 for the 10- and 100-
year events, respectively.
The City of Fort Collins Criteria mandates the design of an overflow spillway to evacuate
runoff from the detention pond assuming that the outlet structure is completely blocked. The crest
of the spillway was set at elevation 4923.65 ft, msl, which is 0.25 feet above the 100-year water
surface. The overtopping water surface elevation was set at elevation 4923.9 ft, msl, which is 0.5
feet above the 100-year water surface and would provide 0.6 feet of freeboard. An additional
SWMM analysis was performed to determine the 100-year discharge out of the pond given those
operational criteria. A conceptual three point stage -storage -discharge rating curve was developed
to allow no outflow up to the crest elevation of 4923.65 ft, msl (at a volume of 5.6 acre-feet). The
volume at the design elevation of 4923.9 ft, msl was set at 6.1 acre-feet; the discharge was iterated
using the SWMM model to determine the spillway discharge at the design water surface.
The results of the of the SWMM analysis indicate a discharge of 18 cfs would flow through
the spillway section at elevation 4923.9 ft, msl assuming that the outlet structure is completely
blocked. The width of the overflow section was sized using the weir equation:
Q= C•L•H"
(1)
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' discharge rating curve for the pond. Calculations for the development of the existing condition
storage -discharge rating curve are presented in Appendix A.2.
' The results of the analysis show the PJHS Detention Pond to be releasing at a peak rate of
38.4 cfs with a water surface elevation of 4922.9 ft-msl for the 100-year event. This corresponds
' to an active storage volume of 4.0 acre-ft (above the normal irrigation storage level). The results of
the analysis show that the pond does not overtop during the 100-year storm event with the current
I outlet structure, but that the release rate is not commensurate with the McClellands Master Drainage
Plan and that there is essentially no freeboard provided in the pond. A release rate of 0.5 cfs/acre
' is required for the 100-year design storm; the 100-year discharge of 38.4 cfs associated with the as -
built outlet configuration corresponds to a unit release of 0.87 cfs/acre. The SWMM model input
' and output for the analysis of the existing condition pond with on -site developed condition inflows
is given in Appendix B.1.
2.2 With -Project Configuration Evaluation/Design for On -Site Flows
' The design of modifications to the PJHS detention pond system was performed to bring the
I pond into conformance with the allowable discharge criteria set forth in the McClellands Master
Plan. Three alternatives were investigated at a conceptual level to provide additional detention
' storage volume and limit the 100-year discharge to the allowable levels:
(a) provide on -site detention on the elementary school site, modify the outlet using
' orifice restrictor plates, and construct a berm approximately 1 foot above the existing
spill elevation for freeboard;
' (b) lower the normal irrigation water surface in the pond, modify the outlet using orifice
restrictor plates, and construct a berm approximately 1 foot above the existing spill
elevation for freeboard; and
' (c) modify the outlet using orifice restrictor plates and construct a berm approximately
' 1.5 feet above the existing spill elevation to provide additional storage volume and
freeboard.
' Alternative C was chosen by the School District as the. most cost-effective and desirable solution.
Only the design and analysis for the selected alternative is discussed in the following paragraphs.
The detention pond outlet structure will be modified to restrict the 10- and 100-year
discharges to the allowable rate specified in the Master Plan; an earthen berm will be constructed
to provide additional storage volume and one foot of freeboard above the 100-year water surface
�! elevation in the PJHS Detention Pond in order to accommodate on -site flows and meet City criteria.
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II. HYDRAULIC EVALUATION/DESIGN OF THE PJHS DETENTION POND
2.1 Existing Condition Analysis for On -Site Flows
The evaluation of the current operation of the PJHS Detention Pond was performed using the
SWMM hydrologic model. At the direction of the City of Fort Collins Stormwater Utility, the PJHS
Detention Pond is to be analyzed and designed to meet the release rate criteria considering only on -
site flows. Off -site runoff is intended to pass through the pond undetained.
For this analysis, the on -site area was subdivided into three subbasins which include: (a) the
1999 Elementary School Site (Subbasin 100); (b) the future City Park and the existing PJHS athletic
field (Subbasin 101); and (c) the existing Preston Junior High School Site (Subbasin 102). A single
conveyance element (Element 200), was included to route runoff from the 1999 Elementary site to
the PJHS Detention Pond (Pond 300). Both the City Park/PJHS athletic field and the PJHS sites
were assumed to drain directly into the detention pond. Subbasin and conveyance element
parameters were determined from the development site plans and were reaffirmed with field visits.
Calculations for the development of subbasin and conveyance element parameters is given in
Appendix A. 1. The overall site plan, subbasin delineation and schematic diagram for the SWMM
model are shown on Sheet 1.
The PJHS Detention Pond serves a dual purpose as both an irrigation water storage pond and
a storm drainage detention pond. The normal water surface in the pond is controlled by a�concrete
weir at the entrance to the concrete outlet box structure. The existing PJHS Detention Pond outflow
structure is depicted on Sheet 1. Discharge from the PJHS Detention Pond outlet structure is. initially
controlled by a 2.13 foot diameter opening in the front face of the concrete box structure and a 57-
inch by 41-inch grated inlet on the top of the box structure. The outlet from the concrete box
structure is a 24"H x 38"W HERCP which conveys runoff east under Corbett Drive to the outfall
channel for the Wild Wood Farms development. The design plans for the PJHS Detention Pond
show an emergency spillway at elevation 4923.7 ft, msl with a berm to provide one foot of freeboard
at elevation 4924.7 ft, msl. With the revised datum, the design spillway and berm elevations would
be approximately 4922.9 ft, msl and 4923.9 ft, msl, respectively. The as -built survey and field
reconnaissance indicate that overtopping spills from the pond would begin at elevation 4922.9 ft-msl
and that the berm to provide additional freeboard was not constructed.
With this information, a storage -discharge rating curve was calculated for use in the SWMM
model. A stage -discharge rating curve was calculated for a series of discharges using a combination
of the orifice equation, the weir equation and the HY-8 culvert hydraulic analysis program. The
stage -discharge rating curve was then combined with the stage -storage curve to produce a storage-
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The "Final Drainage and Erosion Control Study for the Southeast Junior High School Site"
[RBD Inc., April 9, 1993] provided design information for the PJHS Detention Pond and outlet
structure. In addition, an as -built survey of the PJHS Detention Pond was performed [King
Surveyors, Inc., May 5, 1997] to document the existing pond and outlet structure configuration. It
is noted that the results of the as -built survey indicate a datum which is approximately 0.8 feet lower
than the design datum for Preston Junior High. All design calculations for the proposed elementary
school and the analysis of the PJHS Detention Pond have been performed using the current datum.
The "Final Drainage Report for Charter Hospital" [RBD Inc.], the "Addendum to Final
Drainage Report for the Charter Hospital of Fort Collins" [RBD, Inc., June 20, 1989] and subsequent
drainage certification documents provided the design information for the on -site detention pond for
Charter Hospital.
The "Final Drainage and Erosion Control Study for Wildwood Townhomes" [RBD, Inc. ,
October 26, 1995] and subsequent drainage certification documents provided information about the
on -site detention pond for the Wildwood Townhome development.
1.3 Purpose and Scope of Study
The purpose of this study is to evaluate the operation of the PJHS Detention Pond in
conjunction with the development of the Proposed 1999 Elementary School. Modifications to the
pond grading and outlet structure are assessed to bring the pond into conformance with current City
and Master Plan Criteria. All hydrologic calculations were performed for fully -developed basin
conditions. It is noted that for the purposes of the hydrologic analyses, the as -built outlet
configuration refers to the existing, as -built PJHS detention pond; the with project outlet
configuration refers to the PJHS detention pond with proposed modifications.
In the context of the project and study area defined above, the scope of the study is limited
to the following elements:
(a) analyze the performance of the PJHS Detention Pond for as -built outlet configuration
considering only on -site developed condition runoff;
(b) develop modifications to the pond grading and/or outlet structure to bring the pond
into conformance with City criteria for on -site developed condition flows;
(c) analyze the performance of the with -project configuration of the PJHS Detention
Pond with the inclusion of both on- and off -site developed condition flows.
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Figure 1.1. Vicinity Map for the lyyy tiemenrary ;!pcnooi.
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' I. INTRODUCTION
1.1 General
The proposed 1999 Southeast Elementary School site is located within the Wild Wood Farm
development area, which is located southwest of Harmony Road and County Road 9. More
specifically, the site is located within the northeast quarter of Section 5, Township 6 North, Range
68. West, Larimer County, Colorado. The site is bounded by Preston Junior High School to the east,
Wildwood Townhomes to the north, undeveloped agricultural land to the west, and a proposed City
park to the south. A vicinity map is included as Figure 1.1.
This area is part of the McClellands Basin and, consequently, is subject to the conditions
specified in the McClellands Basin Master Drainage Plan [Greenhome & O'Mara, 1986]. Based on
information contained in the Master Plan, on -site detention is required to attenuate the peak.
discharge from the school site to a historic level of 0.5 cfs/acre for the 100-year storm event and 0.2
cfs/acre for the 10-year storm event.
Drainage from the proposed elementary school will be directed to an existing detention pond
located south of Preston Junior High School (PJHS). The pond collects runoff from a contributing
area of nearly 69 acres which consists of both on- and off -site flows. This detention pond was
constructed in conjunction with PJHS for the purpose of detaining and releasing on -site runoff as
dictated by the McClellands Basin Master Drainage Plan. The study site, which makes up 44 acres
of the tributary area and includes PJHS, the proposed 1999 Elementary School site, and the future
City park site directs undetained runoff to the PJHS pond. The remaining 25 acres tributary area
consists of off -site detained runoff from the Charter Hospital and Wildwood Townhome sites, as
well as undetained runoff from Corbett Drive and small adjacent areas.
This study is being done in conjunction with the preparation of engineering plans and the
Final Drainage Plan for the 1999 Elementary School which is currently being prepared by TST, Inc.
1.2 Previous Studies
The "Overall Drainage Plan for the Wild Wood Farm" [RBD Inc.,,July 6, 1988] and the
"Amended Overall Drainage Study for Wild Wood Farm" [RBD Inc., June 26, 1992] defined general
drainage patterns and presented preliminary designs for certain major drainage facilities within the
Wild Wood Farm development including the PJHS detention facility. The studies also established
design discharges for the major drainage facilities commensurate with the McClellands Master
Drainage Plan criteria.
TABLE OF CONTENTS
I. INTRODUCTION.............................................1
1.1 General ....:.............. ..................... 1
1.2 Previous Studies ........................................ 1
1.3 Purpose and Scope of Study ................................. 3
II. HYDRAULIC EVALUATION/DESIGN OF THE PJHS DETENTION POND .... 4
2.1 Existing Condition Analysis for On -Site Flows .................... 4
2.2 With -Project Configuration Evaluation/Design for On -Site Flows ........ 5
2.3 Developed Condition Analysis for both On- and Off -Site Flows ......... 7
Figure 1.1.
FIGURES/APPENDICES/SHEETS
FIGURES
Vicinity Map for the 1999 Elementary School ...................... 2
APPENDICES
Appendix A: SWMM Model Input Parameters
A. 1: Subbasin Parameters
A.2: Existing Condition PJHS Detention Pond Storage Discharge Rating Curve
A.3: Developed Condition PJHS Detention Pond Storage Discharge Rating Curve
AA: Off -Site Detention Pond Storage -Discharge Rating Curves
Appendix B: SWMM Model Input and Output
B.1: Existing Condition 100-Year Event (On -Site Flows Only)
B.2: Developed Condition 10-Year Event (On -Site Flows Only)
B.3: Developed Condition 100-Year Event (On -Site Flows Only)
BA: Overflow Weir Design 100-Year Event (On -Site Flows Only)
B.5: Developed Condition 100-Year Event (On- and Off -Site Flows)
SHEETS
Sheet 1: Drainage Basin Delineation and Detention Pond Modification Details.
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LIDSTONE & ANDERSON, INC.
Water Resources and Environmental Consultants
760 Whalers Way, Suite B-200
Fort Collins, CO 80525
August 6, 1997
Mr. Basil Hamdan
City of Fort Collins
Stormwater Utility
235 Mathews Street
Fort Collins, CO 80524
Re:, Regional Drainage Evaluation for the 1999 Elementary School
(LA Project No. COPR108)
Dear Basil,
Lidstone & Anderson, Inc. (LA) is pleased to submit this Regional Drainage Evaluation for the
1999 Elementary School. This report documents the analysis and redesign of the Preston Junior
High Detention Pond which was initially designed in 1993 by RBD, Inc. The design of revisions
to the pond were performed to meet the allowable discharge given in the McClellands Basin
Master Drainage Plan. The hydraulic and hydrologic evaluation of the site, documented herein,
was performed according to the specifications set forth in the City of Fort Collins SDDC Manual.
If you have any questions regarding the procedures and results given in this report, please feel free
to call us.
Sincerely,
LIDSTONE & ANDERSON, INC.
Chris pher L. Doherty, P.E.
Project Engineer
' CLD/GJK/tlt
Attachment
Grefanager
, P.E.
P je
(970) 226-0120
FAX: (970) 226-0121 E-MAIL: LAWater@FortNet.org
Branch Office: Box 27,Savery, Wyoming 82332
REGIONAL DRAINAGE EVALUATION
FOR THE
1999 ELEMENTARY SCHOOL
FORT COLLINS, COLORADO
PREPARED FOR:
Poudre School District R-1
2407 Laporte Avenue
Fort Collins, Colorado 80521-2297
SUBMITTED TO:
City of Fort Collins
Stormwater Utility
235 Mathews Street
Fort Collins, CO 80524
PREPARED BY:
Lidstone & Anderson, Inc.
760 Whalers Way, B-200
Fort Collins, Colorado 80525
(LA Project No. COPR108)
August 6, 1997