HomeMy WebLinkAboutDRAINAGE REPORT - 16-00377 - 11 W 7th S - BYUI Blasting for New Field
Brigham Young University Idaho
Playfield Complex Renovations – Phase 2
111 W 7th South
Rexburg, ID 83440
Drainage Report
March 29, 2016
The information contained in this report was
prepared by and under the direct supervision
of the undersigned:
LPD Engineering, PLLC
911 Western Ave, Suite 420
Seattle, WA 98104
Contact: Laurie J. Pfarr, P.E.
(206) 725-1211
BRIGHAM YOUNG UNIVERSITY IDAHO
PLAYFIELD COMPLEX RENOVATIONS – PHASE 2
DRAINAGE REPORT
TABLE OF CONTENTS
Section 1 – Project Overview ............................................................................................................................ 1
Section 2 – Existing Conditions Summary ....................................................................................................... 1
Section 3 – Permanent Stormwater Control Plan ......................................................................................... 2
Pre-developed Site Hydrology........................................................................................................................................... 2
Developed Site Hydrology, Field ...................................................................................................................................... 3
Developed Site Hydrology, Parking Lot/Tennis Courts ............................................................................................. 3
Field Drainage/Flow Control Design ............................................................................................................................... 4
Parking Lot Drainage............................................................................................................................................................ 5
Conveyance System Analysis and Design ....................................................................................................................... 5
Section 4 – Construction Stormwater Pollution Prevention Plan (SWPPP) ........................................... 6
FIGURES
Figure 1: Vicinity Map
Figure 2: Existing Conditions
Figure 3: Proposed Conditions
Figure 4: Tennis Courts
APPENDICES
Appendix A – Design Drawings
Appendix B – Design Calculations and Supporting Information
Appendix C – Construction Stormwater Pollution Prevention Plan
BRIGHAM YOUNG UNIVERSITY IDAHO
PLAYFIELD COMPLEX RENOVATIONS – PHASE 2
DRAINAGE REPORT
MARCH 29, 2016
SECTION 1 – PROJECT OVERVIEW
This drainage report is for the phase 2 of the playfield complex renovations at Brigham Young
University Idaho. The site address is at 111 W 7th S in Rexburg, Idaho. The site is located in
Section 31, Township 6 North, Range 40 East, Boise Meridian. The site is located toward the
southwest corner of campus, on one parcel (#RPRSVSC0000040) with an area of approximately
39.43 acres. Refer to Figure 1 - Vicinity Map for site location.
The proposed project improvements include the installation of a synthetic turf field with an
underdrain system, to the southwest of the playfields constructed in Phase 1. The parking lot will
expand to the south, and a corresponding access path to the proposed field will be constructed.
There is a proposed volleyball area to the east of the field. Additionally, away from the field area,
the existing tennis courts adjacent to the student health center will be demolished and the emptied
area will be restored with grass. The total project disturbance area will be approximately 360,744
square feet (8.282 acres), while the total new plus replaced impervious area will be 47,892 square
feet (1.099 acres).
This drainage report will be in accordance with the City of Rexburg Standards and the Catalog for
Best Management Practices for Idaho Cities and Counties, by the Idaho Department of
Environmental Quality, dated September 2005.
SECTION 2 – EXISTING CONDITIONS SUMMARY
The existing project area consists of an open natural grass area to the southwest of the fields
constructed in Phase 1 of the project. The main project area is toward the southwest end of campus,
to the west of a student housing development. The playfield complex site is also bounded by East
7th S to the north, S Center Street to the east (with existing parking lot) and open lawn area to the
south. The existing tennis courts to be demolished are approximately a mile to the northeast from
the field site, adjacent to the student health center, located southwest of the intersection of E Viking
Street and South 2nd E. See Figure 2 – Existing Conditions for a layout of the existing project area.
The site of the proposed field is mostly natural grass covered with some open storage areas and
dirt/gravel paths for access. The existing topography generally slopes down from east to west, with
the existing slope in the proposed field footprint ranging from 1.0% (east side) to 5.8% (west side).
The existing parking lot has a falling slope from southeast to northwest at approximately 4.6%.
Previous soil investigations have found the existing site soils to be Ririe silt loam soils overlaying
basalt rock. The permeability of these soils is very low, and thus fill will be placed to raise the
LPD Engineering, PLLC Page 2 BYU Idaho – Playfield Complex Renovations Phase 2
Drainage Report, March 29, 2016
proposed field to a slope of around 0.6%. It is assumed that infiltration and subsurface drainage is
not possible within the native soil.
In the existing parking lot, there is are multiple catch basins and an 8-inch storm drain line that
conveys runoff north to a detention pond north of the fields installed in Phase 1. The stormwater
from the parking lot addition will be discharged into the existing 8-inch system. Stormwater from
the proposed field will be discharged into the existing drainage infrastructure to the west, serving
the adjacent student housing development. The proposed field’s conveyance system will be
connected to a manhole within the housing development parking lot and stormwater will be
conveyed into the existing 12-inch downstream conveyance system. Stormwater within this
conveyance system will discharge into detention ponds along the east side of University Boulevard.
SECTION 3 – PERMANENT STORMWATER CONTROL PLAN
According to correspondence with the City of Rexburg, the proposed synthetic turf field (105,940
square feet, 2.432 acres) will be modeled as 100% grass. All proposed hard surfaces outside of
field and landscaping were modeled as 100% impervious. See the following Table 1 and Figure 3 –
Proposed Conditions for a breakdown of the new plus replaced impervious surfaces.
Table 1 – New Plus Replaced Impervious Surfaces
Square Feet Acres
Walkway Path 19,605 0.450
Volleyball Area 14,019 0.322
Parking Lot 14,268 0.327
Total New Plus Replaced Impervious Surface 47,892 1.099
Pre-developed Site Hydrology
Existing runoff sheet flows down slope, in a westerly direction, toward the existing student housing
development, where it is collected in the existing piped stormwater system within the student
housing parking lot. The existing fields installed in Phase 1 and their underdrain systems will not
be affected by this project. As stated previously, the existing 8-inch storm line and catch basin
system, along the west side of the existing parking lot, conveys stormwater flows north to the
existing detention pond.
LPD Engineering, PLLC Page 3 BYU Idaho – Playfield Complex Renovations Phase 2
Drainage Report, March 29, 2016
Developed Site Hydrology, Field
Rain falling upon the proposed synthetic turf field will be collected in an underdrain system. No
surface runoff is anticipated from the field surface due to the permeable nature of the materials.
The synthetic turf surface includes a tufted fiber and woven synthetic backing that is perforated to
allow a minimum vertical infiltration rate of 20 inches per hour. The synthetic turf is infilled with a
mixture of rounded sand particles and granulated rubber that do not interlock and are also highly
permeable and has a minimum vertical infiltration rate of 20 inches per hour. Below the synthetic
turf surface is a top course and base course permeable crushed aggregate constructed of a specially
graded sand and gravel mixture that results in a permeable base below the field. The vertical
infiltration rates through the synthetic turf are tested by an independent third party testing agency
after manufacture of the turf. Testing of the permeable aggregate occurs on site after placement and
compaction.
The flow restrictor is proposed and designed to temporarily back stormwater up into the field
permeable base aggregate, and a portion of the perforated lateral drain lines, and the collector drain
line. The permeable base aggregates are assumed to have a void ratio of 30% by volume. Due to
the surface slope of the field, there is limited storage volume available within the base course
section beneath the field. Therefore, an additional volume of permeable aggregate has been
provided underneath the base course material toward the west side of the field to provide the
required additional detention storage volume. Within this report, this additional volume will be
noted as the “gravel detention basin.” An 18-inch perforated storm pipe will be placed at the
bottom of the gravel detention basin to hydraulically connect it with the flow control structure.
Stormwater will percolate vertically and gradually horizontally through the base crushed rock layer
to the flat drain pipe system. Refer to the field cross section on Sheet F-2.4 in Appendix A. Once
stormwater reaches the perforated flat drain pipe, it will either discharge directly into the gravel
detention basin or be conveyed to collector pipes along the north and south perimeters of the field,
which are tributary to the gravel detention basin. The flow control structure will be located in the
northwest corner of the field, beneath the synthetic turf surfacing. The outlet from the flow control
structure will discharge to the proposed rock swale, which conveys flow to the existing piped
system at the student housing development at the bottom of the hill.
A proposed french drain will be located east of the field to collect upstream stormwater flows and
convey them around the field to the proposed rock swale. Additionally, along the west side of the
volleyball area, a berm will be created to provide additional storage upstream of the field by
temporarily storing stormwater prior to spilling over to the French drain.
Developed Site Hydrology, Parking Lot/Tennis Courts
There will be 40 new parking stalls added to the south end of the existing lot (151 stalls) along S
Center Street. The project proposes to install two more catch basins and 8-inch pipe that will
connect to the existing parking lot stormwater conveyance system. The existing detention pond, at
the north end of the phase 1 field, was analyzed for capacity. This detention pond was installed in
Phase 1 and is in good functional condition. See the “Parking Lot Drainage” section of this report
for additional information on the stormwater modeling used to determine the detention volume
required.
LPD Engineering, PLLC Page 4 BYU Idaho – Playfield Complex Renovations Phase 2
Drainage Report, March 29, 2016
The existing tennis courts will be demolished and the area will be an open lawn area. Based on
proposed grading, drainage will sheet flow from southeast to northwest, at anywhere from 3-11%
slope. A StormShed analysis was conducted to compare 50-year peak flows from the existing and
proposed conditions, see Appendix B for output report. The results yielded an existing 50-year
peak flow of 2.948 cubic feet per second (cfs) and a developed 50-year peak flow of 1.155 cfs.
Because the unmitigated flows are being reduced as a result of impervious area becoming grass,
there are no detention requirements for the tennis courts. See Figure 4 – Tennis Courts, for a visual
representation of the tennis court component of the project.
Field Drainage/Flow Control Design
The proposed discharge control mitigates for stormwater flow through the field subgrade can vary
depending on the characteristics of the material. The intent is to mitigate the stormwater discharge
and regulate it in a manner such that it closely resembles the existing conditions.
As approved by the City of Rexburg, the StormShed 2G SCS Model was used to design the flow
restrictor for the synthetic turf field. Due to the proposed drainage infrastructure to intercept
upstream flows, the field surface itself will be the only area tributary to flow control (2.432 acres,
grass). The other impervious areas sans the proposed parking lot expansion (0.770 acres,
impervious) were entered as a bypass basin in the StormShed modeling. The additional parking
area will not be included in the StormShed model because it will be routed to the existing detention
pond north of the Phase 1 fields (see next section). The output reports from the StormShed model
are included in Appendix B of this report.
At the farthest reach from the point of compliance, the amount of time stormwater takes to travel
the drainage path is described as the Time of Concentration (Tc). In the existing conditions, the Tc
for the natural grass site area is approximately 29.65 minutes, as shown on Figure 2 – Existing
Conditions.
For the proposed conditions, the predicted time of concentration for similar fields calculated using
Darcy’s Law has shown that the proposed condition would provide a significantly greater Tc than
the existing, due to the permeable nature of the field sub-base. However, based upon variability in
the delivered granular sub-base material composition, an anticipated worst case, theoretical Tc for
the proposed conditions was input into the stormwater model as 10 minutes. All impervious areas
in the proposed conditions have a Tc of 5 minutes. Refer to Figure 3 – Proposed Conditions.
Idaho is within a Type-2 rainfall distribution. The StormShed program uses 24-hour precipitation
values for the recurrence interval of the 2-year, 10-year, and 50-year storm events based upon the
project location. These precipitation values entered into the StormShed model were based on
isopluvial 24-hour precipitation maps for the state of Idaho, prepared by the United States
Department of Commerce. The 24-hour precipitation values for the 2-year, 10-year, and 50-year
are 1.2 inches, 1.7 inches, and 2.6 inches, respectively.
The City of Rexburg requires that the peak runoff control for the proposed 50-year/24-hour storm
be limited to the existing 50-year/24-hour storm. In the SCS model, both the existing open grass
area and proposed synthetic turf field were modeled as pervious (grass).
LPD Engineering, PLLC Page 5 BYU Idaho – Playfield Complex Renovations Phase 2
Drainage Report, March 29, 2016
The results of the modeling in SCS with the StormShed 2G software program indicates that a
combination flow control riser with a circular orifice will provide the discharge control that release
stormwater at peak rates that closely resemble the existing peak flow rates from the existing site
area. The following table shows a comparison of the existing and proposed flows:
Table 2 – Comparison of Existing and Proposed Flows
Event Q Ex
(cfs)
Q Dev.
Bypass (cfs)
Q Dev.
Detention. (cfs)
Q total
(cfs)
50 year 4.47 2.20 2.27 4.47
As seen from Table 2, the flow control structure and gravel detention basin were sized so that the
combination of the 50-year peak flows from the bypass basin and the detention basin was matching
that of the existing conditions. The resulting flow control structure would be a 12-inch riser with a
7-inch diameter orifice. The required detention volume was approximately 2,932 cubic feet of
storage. Due to the 30% void ratio in the gravel detention basin, this would equate to about 9,773
cubic feet of total volume. The actual gravel detention basin will have dimensions of 310’ x 8.67’ x
3.67’. See the Playfield Drainage Plan (Sheet F-1.7), gravel detention basin detail (Sheet F-2.5),
and flow control structure detail (Sheet F-2.5) in Appendix A.
Parking Lot Drainage
Based on the previous drainage report for Phase 1 of the playfield complex, dated March 6, 2006,
4,452 cubic feet of detention was required for what is now the existing parking lot. This
requirement was addressed with a stormwater detention pond, located north of proposed Phase 1
fields (SW of intersection of E 7th S and S Center Street). Based on the previous design, the live
storage has a depth of two feet. Based on the shape of the pond, the available live storage volume
was conservatively estimated to be 5,760 cubic feet. This is an extra 1,308 cubic feet of storage
available for Phase 2 of the project.
To determine the amount of additional detention volume required for the parking lot addition, an
analysis was conducted using StormShed 2G. The parking lot area (0.328 acres, impervious) was
assigned a conservative time of concentration of 5 minutes, while the predeveloped condition was
considered grass. The model was designed to limit the proposed 50-year flow to the existing 50-
year flow. The required detention volume was found to be approximately 485 cubic feet. Thus, the
existing detention pond would have sufficient capacity to detain the extra impervious parking area
created by this project. See the associated StormShed report in Appendix B.
Conveyance System Analysis and Design
An analysis of the onsite conveyance system was performed for the 8-inch outlet pipe for the field,
10-inch storm line in the parking lot (outlet to existing detention pond), and the 12-inch connection
to the existing storm system at the downstream end of the proposed rock swale. Refer to the
Conveyance Analysis Spreadsheet in Appendix B.
Just as the detention system sizing, each pipe will be sized to convey the 50-year peak storm event.
StormShed 2G was used to determine the 50-year peak runoff rate. The complete output from the
program is included within Appendix B.
LPD Engineering, PLLC Page 6 BYU Idaho – Playfield Complex Renovations Phase 2
Drainage Report, March 29, 2016
The field outlet pipe will only be conveying the field area of 2.432 acres. For the 8-inch pipe at an
18.6% slope connecting to the proposed rock swale, the full-flow capacity calculated using
Manning’s Equation was found to be 6.18 cubic feet per second (cfs). This was compared to the
50-year peak runoff rate based on the appropriate tributary basin, as calculated using StormShed.
The post-developed 50-year peak flow was found to be 5.20 cfs. Thus, the collector pipe would
have adequate conveyance capacity for the 50-year storm event.
The 10-inch storm line in the parking lot was analyzed similarly, with a tributary basin consisting
of the additional parking lot, existing parking lot, as well as the portions of existing sidewalk
sloping toward the east (total 2.347 acres). The 50-year peak flow found using StormShed was 6.71
cfs. The full flow capacity for the outlet to the existing detention pond, a 10-inch pipe at 7.5%
slope, would be 7.11 cfs. Therefore, the existing conveyance system would have adequate capacity
for the parking area added in this project.
Finally, the 12-inch connection to the existing storm system at the adjacent student housing
development will be ran at minimum 6% slope, giving a full-flow capacity of 10.34 cfs. This pipe
will be conveying flow from the field, bypass impervious areas (excluding proposed parking lot),
and the remaining tributary grass areas from the hillside. The 50-year peak flow from StormShed
was found to be 9.96 cfs, and thus the pipe will have adequate conveyance capacity.
SECTION 4 – CONSTRUCTION STORMWATER POLLUTION PREVENTION
PLAN (SWPPP)
The SWPPP narrative for this project can be found in Appendix C. The SWPPP is based on the
2005 Idaho Stormwater BMP Catalog and the BYU-Idaho Stormwater Pollution Prevention Plan
for Construction Activities Generic Plan. The project includes Temporary Erosion and
Sedimentation Control (TESC) plans, included on Sheets F-1.2 (Field), F-5.1 (Parking Lot), and F-
6.2 (Tennis Courts) of Appendix A.
The TESC plan includes a temporary sediment settling tank. A minimum volume was calculated
using the methodology from the 2005 Idaho Stormwater BMP Catalog. To determine the storage
volume of the sediment tank, the pump discharge (gallons per minute [gpm]) is to be multiplied by
16. As mentioned throughout the manual in other BMP design criteria, the 2-year developed peak
flow was used to determine the appropriate pump capacity. Using StormShed 2G, the 2-year peak
flow for the main Phase 2 site (field, parking) was found to be 4.94 cfs, or approximately 2,218
gpm. Thus, the necessary sediment tank volume would need to be 35,488 gallons. The sediment
tank for the tennis courts were sized similarly, yielding a required volume of 3,216 gallons. Refer
to the Sediment Sizing Worksheet and StormShed output attached in Appendix B. Temporary
sump and pumps will be installed as necessary, and then stormwater will be pumped to the
temporary sediment settling tank.
LPD Engineering, PLLC Page 7 BYU Idaho – Playfield Complex Renovations Phase 2
Drainage Report, March 29, 2016
As further described in the SWPPP narrative, erosion control BMP elements in the project include
the following:
• General Construction BMPs, BMPs 1-6
• Housekeeping BMPs, BMPs 7-11,13
• Inlet Protection, BMP 31
• Check Dams, BMP 32
• Fiber Rolls, BMP 35
• Silt Fence, BMP 36
• Portable Sediment Tank, BMP 39
• Temporary Swale, BMP 40
The TESC elements shown are intended to be the minimum allowable. The NPDES permit will
require periodic inspection of the TESC elements to confirm they are holding up and continuing to
function as intended. During construction, the contractor is responsible for upgrading these
facilities as necessary. The implementation of the TESC plan and construction maintenance,
replacement and upgrading of the TESC facilities are the responsibility of the contractor, per the
contract documents. The TESC facilities will be constructed prior to and in conjunction with all
clearing and grading activity and in a manner in which sediment or sediment laden water does not
leave the project site, enter the drainage system, or violate applicable water quality standards.
FIGURES
Figure 1: Vicinity Map
Figure 2: Existing Conditions
Figure 3: Proposed Conditions
Figure 4: Tennis Courts
APPENDIX A
Design Drawings
APPENDIX B
Design Calculations and Supporting Information
StormShed Output – Tennis Courts
Event Match Q (cfs) Peak Q (cfs) Peak Stg (ft) Vol (cf) Vol (acft) Time to Empty
2 year 1.2743 0.3908 0.0929 93.68 0.0022 25.33
10 year 1.8744 0.6782 0.2625 264.57 0.0061 25.33
50 year 2.9476 1.1549 0.7552 761.20 0.0175 25.33
Record Id: Predeveloped
Design Method SCS Rainfall type TYPE2
Hyd Intv 10.00 min Peaking Factor 484.00
Abstraction Coeff 0.20
Pervious Area 0.10 ac DCIA 0.95 ac
Pervious CN 90.00 DC CN 98.00
Pervious TC 1.52 min DC TC 5.00 min
Pervious CN Calc
Description SubArea Sub cn
Open spaces, lawns,parks (>75% grass) 0.10 ac 90.00
Pervious Composited CN (AMC 2) 90.00
Pervious TC Calc
Type Description Length Slope Coeff Misc TT
Sheet Short prairie grass and lawns.: 0.15 19.00 ft 26.00% 0.1500 1.20 in 1.52 min
Pervious TC 1.52 min
Directly Connected CN Calc
Description SubArea Sub cn
Impervious surfaces (pavements, roofs, etc) 0.95 ac 98.00
DC Composited CN (AMC 2) 98.00
Directly Connected TC Calc
Type Description Length Slope Coeff Misc TT
Fixed Tennis Courts 5.00 min
Directly Connected TC 5.00min
Record Id: Postdeveloped
Design Method SCS Rainfall type TYPE2
Hyd Intv 10.00 min Peaking Factor 484.00
Abstraction Coeff 0.20
Pervious Area 1.05 ac DCIA 0.00 ac
Pervious CN 90.00 DC CN 0.00
Pervious TC 23.59 min DC TC 0.00 min
Pervious CN Calc
Description SubArea Sub cn
Open spaces, lawns,parks (>75% grass) 1.05 ac 90.00
Pervious Composited CN (AMC 2) 90.00
Pervious TC Calc
Type Description Length Slope Coeff Misc TT
Sheet Short prairie grass and lawns.: 0.15 304.00 ft 7.00% 0.1500 1.20 in 23.59 min
Pervious TC 23.59 min
50-year peak flow in Developed Conditions is less than 50-year peak flow in Existing
Conditions, and therefore no detention is required for the tennis court area.
Licensed to: LPD PLC
StormShed Output: Detention – Field
Event Match Q (cfs) Peak Q (cfs) Peak Stg (ft) Vol (cf) Vol (acft) Time to Empty
2 year 1.2320 0.8935 0.4529 456.50 0.0105 24.67
10 year 2.3263 1.4350 1.1652 1174.49 0.0270 24.67
50 year 4.4743 2.2677 2.9085 2931.78 0.0673 24.67
Record Id: Predeveloped
Design Method SCS Rainfall type TYPE2
Hyd Intv 10.00 min Peaking Factor 484.00
Abstraction Coeff 0.20
Pervious Area 3.18 ac DCIA 0.00 ac
Pervious CN 90.00 DC CN 0.00
Pervious TC 29.65 min DC TC 0.00 min
Pervious CN Calc
Description SubArea Sub cn
Open spaces, lawns,parks (>75% grass) 2.41 ac 90.00
Open spaces, lawns,parks (>75% grass) 0.77 ac 90.00
Pervious Composited CN (AMC 2) 90.00
Pervious TC Calc
Type Description Length Slope Coeff Misc TT
Sheet Short prairie grass and lawns.: 0.15 286.00 ft 3.50% 0.1500 1.20 in 29.65 min
Pervious TC 29.65 min
Record Id: Postdeveloped
Design Method SCS Rainfall type TYPE2
Hyd Intv 10.00 min Peaking Factor 484.00
Abstraction Coeff 0.20
Pervious Area 2.43 ac DCIA 0.00 ac
Pervious CN 90.00 DC CN 0.00
Pervious TC 10.00 min DC TC 0.00 min
Pervious CN Calc
Description SubArea Sub cn
Open spaces, lawns,parks (>75% grass) 2.43 ac 90.00
Pervious Composited CN (AMC 2) 90.00
Pervious TC Calc
Type Description Length Slope Coeff Misc TT
Fixed Developed Turf Field 10.00 min
Pervious TC 10.00 min
Bypass Event Summary
Event Peak Q (cfs) Peak T (hrs) Hyd Vol (acft) Area (ac) Method Raintype
2 year 0.9761 12.01 0.0610 0.7720 SCS TYPE2
10 year 1.4185 12.01 0.0930 0.7720 SCS TYPE2
50 year 2.2059 12.01 0.1515 0.7720 SCS TYPE2
Record Id: Bypass
Design Method SCS Rainfall type TYPE2
Hyd Intv 10.00 min Peaking Factor 484.00
Abstraction Coeff 0.20
Pervious Area 0.00 ac DCIA 0.77 ac
Pervious CN 0.00 DC CN 98.00
Pervious TC 0.00 min DC TC 5.00 min
Directly Connected CN Calc
Description SubArea Sub cn
Impervious surfaces (pavements, roofs, etc) 0.77 ac 98.00
DC Composited CN (AMC 2) 98.00
Directly Connected TC Calc
Type Description Length Slope Coeff Misc TT
Fixed Impervious Surface 5.00 min
Directly Connected TC 5.00min
Record Id: Gravel Detention Basin
Descrip: Detention Basin @ west end of field Increment 0.10 ft
Start El. 100.0000 ft Max El. 103.6667 ft
Length 96.0000 ft Width 35.0000 ft
Catch 30.0000 (void ratio)
Record Id: D-002
Descrip: Orifice Increment 0.10 ft
Start El. 100.0000 ft Max El. 103.6670 ft
Orif Coeff 0.62 Lowest Orif El. 100.00
Lowest Diam 7.0000 in Dist to next 0.0000 ft
Match 50-year Flow (predeveloped) = 4.47 cfs
Detained 50-year flow (developed) = 2.27 cfs
Bypass 50-year flow = 2.20 cfs
Detained + Bypass (total) = 4.47 cfs
Required Detention Volume = 2,932 cubic feet
Volume of Gravel (30% void ratio) Required = 9,773 cubic feet
Gravel Detention Basin Provided = 310’ x 8.667’ x 3.667’ = 9,851 cubic feet
Licensed to: LPD PLC
StormShed Output: Detention – Parking Lot
Event Summary
Event Match Q (cfs) Peak Q (cfs) Peak Stg (ft) Vol (cf) Vol (acft) Time to Empty
2 year 0.1563 0.2982 0.0709 192.92 0.0044 24.50
10 year 0.2937 0.4298 0.1053 286.47 0.0066 24.67
50 year 0.5610 0.5567 0.1782 484.72 0.0111 24.67
Record Id: Predeveloped
Design Method SCS Rainfall type TYPE2
Hyd Intv 10.00 min Peaking Factor 484.00
Abstraction Coeff 0.20
Pervious Area 0.33 ac DCIA 0.00 ac
Pervious CN 90.00 DC CN 0.00
Pervious TC 20.46 min DC TC 0.00 min
Pervious CN Calc
Description SubArea Sub cn
Open spaces, lawns,parks (>75% grass) 0.33 ac 90.00
Pervious Composited CN (AMC 2) 90.00
Pervious TC Calc
Type Description Length Slope Coeff Misc TT
Sheet Short prairie grass and lawns.: 0.15 215.00 ft 5.00% 0.1500 1.20 in 20.46 min
Pervious TC 20.46 min
Record Id: Postdeveloped
Design Method SCS Rainfall type TYPE2
Hyd Intv 10.00 min Peaking Factor 484.00
Abstraction Coeff 0.20
Pervious Area 0.00 ac DCIA 0.33 ac
Pervious CN 0.00 DC CN 98.00
Pervious TC 0.00 min DC TC 5.00 min
Directly Connected CN Calc
Description SubArea Sub cn
Impervious surfaces (pavements, roofs, etc) 0.33 ac 98.00
DC Composited CN (AMC 2) 98.00
Directly Connected TC Calc
Type Description Length Slope Coeff Misc TT
Fixed Additional Parking Lot 5.00 min
Directly Connected TC 5.00min
Record Id: Detention
Descrip: Prototype Record Increment 0.10 ft
Start El. 100.0000 ft Max El. 105.0000 ft
Length 80.0000 ft Width 34.0000 ft
Catch 100.0000
Record Id: Orifice
Descrip: Prototype Structure Increment 0.10 ft
Start El. 100.0000 ft Max El. 105.0000 ft
Orif Coeff 0.62 Lowest Orif El. 100.00
Lowest Diam 7.0000 in Dist to next 0.0000 ft
Required Detention= 485 cubic feet
Available Detention in Existing Detention Pond = 1,308 cubic feet
Licensed to: LPD PLC
BYU Idaho - Phase 2 3/29/2016
Conveyance Analysis Spreadsheet
Pipe Run Size Mannings
N
Plan
Slope Qfull Tributary Basins Tributary Area Impervious Grass Qtrib, 50-year
(StormShed)% Full
(inches)(ft/ft)(cfs)(acres)(acres)(acres)(cfs)
Outfall from Field 8 0.011 0.186 6.18 Field Area 2.432 0.000 2.432 5.20 84%
Parking Lot (Outfall to Existing
Detention Pond)10 0.011 0.075 7.11 Existing parking lot, portions of existing
sidewalk, and proposed parking 2.347 2.347 0.000 6.71 94%
Connection to Existing Storm
System 12 0.011 0.06 10.34 Field, bypass impervious areas (except
proposed parking), existing hillside 7.076 0.701 6.375 9.96 96%
StormShed Output: Conveyance – Field Outlet
Postdeveloped Event Summary
Event Peak Q (cfs) Peak T (hrs) Hyd Vol (acft) Area (ac) Method Raintype
2 year 1.5153 12.04 0.0923 2.4320 SCS TYPE2
10 year 2.7745 12.04 0.1700 2.4320 SCS TYPE2
50 year 5.1986 12.03 0.3275 2.4320 SCS TYPE2
Record Id: Postdeveloped
Design Method SCS Rainfall type TYPE2
Hyd Intv 10.00 min Peaking Factor 484.00
Abstraction Coeff 0.20
Pervious Area 2.43 ac DCIA 0.00 ac
Pervious CN 90.00 DC CN 0.00
Pervious TC 10.00 min DC TC 0.00 min
Pervious CN Calc
Description SubArea Sub cn
Open spaces, lawns,parks (>75% grass) 2.43 ac 90.00
Pervious Composited CN (AMC 2) 90.00
Pervious TC Calc
Type Description Length Slope Coeff Misc TT
Fixed Developed Turf Field 10.00 min
Pervious TC 10.00 min
Licensed to: LPD PLC
StormShed Output: Conveyance – Parking (Outlet to Ex Detention Pond)
Postdeveloped Event Summary
Event Peak Q (cfs) Peak T (hrs) Hyd Vol (acft) Area (ac) Method Raintype
2 year 2.9675 12.01 0.1856 2.3470 SCS TYPE2
10 year 4.3125 12.01 0.2827 2.3470 SCS TYPE2
50 year 6.7064 12.01 0.4606 2.3470 SCS TYPE2
Record Id: Postdeveloped
Design Method SCS Rainfall type TYPE2
Hyd Intv 10.00 min Peaking Factor 484.00
Abstraction Coeff 0.20
Pervious Area 0.00 ac DCIA 2.35 ac
Pervious CN 0.00 DC CN 98.00
Pervious TC 0.00 min DC TC 5.00 min
Directly Connected CN Calc
Description SubArea Sub cn
Impervious surfaces (pavements, roofs, etc) 2.35 ac 98.00
DC Composited CN (AMC 2) 98.00
Directly Connected TC Calc
Type Description Length Slope Coeff Misc TT
Fixed Parking area, portion of ex sidewalk 5.00 min
Directly Connected TC 5.00min
Licensed to: LPD PLC
StormShed Output: Conveyance – Connection to Existing System
Upstream - Postdeveloped Event Summary
Event Peak Q (cfs) Peak T (hrs) Hyd Vol (acft) Area (ac) Method Raintype
2 year 1.1515 12.00 0.1780 3.8700 SCS TYPE2
10 year 1.9214 12.33 0.3098 3.8700 SCS TYPE2
50 year 3.6839 12.33 0.5698 3.8700 SCS TYPE2
Record Id: Upstream - Postdeveloped
Design Method SCS Rainfall type TYPE2
Hyd Intv 10.00 min Peaking Factor 484.00
Abstraction Coeff 0.20
Pervious Area 3.17 ac DCIA 0.70 ac
Pervious CN 90.00 DC CN 98.00
Pervious TC 44.13 min DC TC 5.00 min
Pervious CN Calc
Description SubArea Sub cn
Open spaces, lawns,parks (>75% grass) 3.17 ac 90.00
Pervious Composited CN (AMC 2) 90.00
Pervious TC Calc
Type Description Length Slope Coeff Misc TT
Sheet Short prairie grass and lawns.: 0.15 665.00 ft 7.00% 0.1500 1.20 in 44.13 min
Pervious TC 44.13 min
Directly Connected CN Calc
Description SubArea Sub cn
Impervious surfaces (pavements, roofs, etc) 0.70 ac 98.00
DC Composited CN (AMC 2) 98.00
Directly Connected TC Calc
Type Description Length Slope Coeff Misc TT
Fixed Upstream Proposed Impervious 5.00 min
Directly Connected TC 5.00min
Downstream - To Swale Event Summary
Event Peak Q (cfs) Peak T (hrs) Hyd Vol (acft) Area (ac) Method Raintype
2 year 0.2951 12.25 0.0295 0.7740 SCS TYPE2
10 year 0.5594 12.22 0.0545 0.7740 SCS TYPE2
50 year 1.0778 12.22 0.1047 0.7740 SCS TYPE2
Record Id: Downstream - To Swale
Design Method SCS Rainfall type TYPE2
Hyd Intv 10.00 min Peaking Factor 484.00
Abstraction Coeff 0.20
Pervious Area 0.77 ac DCIA 0.00 ac
Pervious CN 90.00 DC CN 0.00
Pervious TC 30.31 min DC TC 0.00 min
Pervious CN Calc
Description SubArea Sub cn
Open spaces, lawns,parks (>75% grass) 0.77 ac 90.00
Pervious Composited CN (AMC 2) 90.00
Pervious TC Calc
Type Description Length Slope Coeff Misc TT
Sheet Short prairie grass and lawns.: 0.15 385.00 ft 6.00% 0.1500 1.20 in 30.31 min
Pervious TC 30.31 min
Total Tributary 50-year Flow
3.684 cfs (Upstream) + 1.078 cfs (Downstream) + 5.199 (Field, see previous output) = 9.96 cfs
Licensed to: LPD PLC
Sediment Tank Sizing Calculations
Per the 2005 Idaho Stormwater BMP Catalog (BMP 39, page 129)
Project Name:BYU Phase 2
Field/Parking Area
Required Sediment Tank Volume (Vol.):
Vol. = Q * 16
Where:Q =2-year developed flow rate from StormShed
(pump discharge in gpm)
Calculation:multiplier =16
Q =2218 gpm
Required Volume =35488.0 gallons
Volume Provided (See Sheet F-1.2, Appendix A)
37,800 gallons
StormShed Output: TESC Sizing – Field/Parking Area
Postdeveloped Event Summary
Event Peak Q (cfs) Peak T (hrs) Hyd Vol (acft) Area (ac) Method Raintype
2 year 4.9427 12.00 0.3188 7.2330 SCS TYPE2
10 year 8.6770 12.00 0.5590 7.2330 SCS TYPE2
50 year 15.8252 12.00 1.0368 7.2330 SCS TYPE2
Record Id: Postdeveloped
Design Method SCS Rainfall type TYPE2
Hyd Intv 10.00 min Peaking Factor 484.00
Abstraction Coeff 0.20
Pervious Area 6.13 ac DCIA 1.10 ac
Pervious CN 90.00 DC CN 98.00
Pervious TC 10.00 min DC TC 5.00 min
Pervious CN Calc
Description SubArea Sub cn
Open spaces, lawns,parks (>75% grass) 6.13 ac 90.00
Pervious Composited CN (AMC 2) 90.00
Pervious TC Calc
Type Description Length Slope Coeff Misc TT
Fixed Field and Pervious Areas 10.00 min
Pervious TC 10.00 min
Directly Connected CN Calc
Description SubArea Sub cn
Impervious surfaces (pavements, roofs, etc) 1.10 ac 98.00
DC Composited CN (AMC 2) 98.00
Directly Connected TC Calc
Type Description Length Slope Coeff Misc TT
Fixed Parking Lot & Walkway 5.00 min
Directly Connected TC 5.00min
Sediment Tank Sizing Calculations
Per the 2005 Idaho Stormwater BMP Catalog (BMP 39, page 129)
Project Name:BYU Phase 2
Tennis Court Area
Required Sediment Tank Volume (Vol.):
Vol. = Q * 16
Where:Q =2-year developed flow rate from StormShed = 4.94 cfs
(pump discharge in gpm)
Calculation:multiplier =16
Q =201 gpm
Required Volume =3216.0 gallons
Volume Provided (See Sheet F-6.1, Appendix A)
4,000 gallons
StormShed Output: TESC Sizing – Tennis Court
Postdeveloped Event Summary
Event Peak Q (cfs) Peak T (hrs) Hyd Vol (acft) Area (ac) Method Raintype
2 year 0.4470 12.18 0.0400 1.0490 SCS TYPE2
10 year 0.8415 12.18 0.0738 1.0490 SCS TYPE2
50 year 1.6123 12.16 0.1418 1.0490 SCS TYPE2
Record Id: Postdeveloped
Design Method SCS Rainfall type TYPE2
Hyd Intv 10.00 min Peaking Factor 484.00
Abstraction Coeff 0.20
Pervious Area 1.05 ac DCIA 0.00 ac
Pervious CN 90.00 DC CN 0.00
Pervious TC 25.33 min DC TC 0.00 min
Pervious CN Calc
Description SubArea Sub cn
Open spaces, lawns,parks (>75% grass) 1.05 ac 90.00
Pervious Composited CN (AMC 2) 90.00
Pervious TC Calc
Type Description Length Slope Coeff Misc TT
Sheet Short prairie grass and lawns.: 0.15 305.00 ft 5.90% 0.1500 1.20 in 25.33 min
Pervious TC 25.33 min
Licensed to: LPD PLC
APPENDIX C
Construction Stormwater Pollution Prevention Plan (SWPPP)
Page 1
Brigham Young University Idaho – Playfield Complex Renovations – Phase 2
Construction SWPPP Narrative
March 29, 2016
BRIGHAM YOUNG UNIVERSITY IDAHO
PLAYFIELD COMPLEX RENOVATIONS – PHASE 2
CONSTRUCTION SWPPP NARRATIVE
MARCH 29, 2016
The following Construction Storm Water Pollution Prevention Plan (SWPPP) Narrative is for the
Brigham Young University Idaho Phase 2 Playfield Complex Renovations at 111 W 7th S in
Rexburg, Idaho (Tax Parcel No. RPRSVSC0000040). The narrative supplements the
Erosion and Sediment Control (ESC) plan. This narrative and the drawings address the
requirements of section 2.3 of the 2005 Catalog of Stormwater Best Management Practices for
Idaho Cities and Counties. This SWPPP will also adhere to the guidelines provided in the BYU-
Idaho Stormwater Pollution Prevention Plan (SWP3) for Construction Activities – Generic Plan.
Refer to the TESC plans (Sheets F-1.2, F-5.1, and F-6.2) and TESC details & notes (Sheet F-1.3
and F-1.4) for more information regarding any erosion or sedimentation control measures
involved in this project.
PROJECT DESCRIPTION
The proposed project improvements include the installation of a new synthetic turf athletic field,
along with accompanying seating, lighting, walkways, and retaining walls. Additionally, the
existing parking lot to the east will be expanded in the south direction. Existing tennis courts will
be demolished and the vacated area will be restored as grass. The total project disturbance area is
360,744 square feet (SF) [8.282 acres].
The project proposes approximately 47,820 SF (1.098 acres) of total new and replaced
impervious surface. Flow control will be provided by a flow control structure located on the
outlet of the proposed underdrain system for the field. The flow control structure will temporarily
backup stormwater into the network of underdrain piping and within the void space of the gravel
base layers, while releasing the flows at a rate that will satisfactorily meet the flow control
requirements. Refer to the project’s drainage report for more information.
EXISTING SITE CONDITIONS
The existing project area is to the southwest of four other playfields, constructed in Phase 1 of the
project. The project area is currently open grass area, with some space utilized for equipment
storage and access paths. The area south of the existing parking lot, which is the location of the
future expansion, has grass land cover. The project area is toward the southwest end of the BYU
campus, to the east of a student housing development, south of E 7th S, and west of S Center
Street.
The existing topography generally slopes down from east to west, with the existing slope in the
proposed field footprint ranging from 1.0% (east side) to 5.8% (west side). The existing parking
lot slopes down from southeast to northwest at approximately 4.6%. Previous soil investigations
have found the existing site soils to be Ririe silt loam soils overlaying basalt rock. The
permeability of these soils is very low, and thus fill will be placed to raise the proposed field to a
slope of around 0.6%.
Page 2
Brigham Young University Idaho – Playfield Complex Renovations – Phase 2
Construction SWPPP Narrative
March 29, 2016
Runoff from the existing hillside sheets flows toward the west, where there is an existing storm
drain conveyance system in the adjacent student housing development. The parking lot has a
piped conveyance system along the west side.
POTENTIAL POLLUTANTS
The site is not a high-use or industrial site. The only pollutants emanating from the site would be
construction stormwater. Construction stormwater will be treated with the BMPs listed in the
Best Management Practices section of this SWPPP narrative.
CONSTRUCTION SCHEDULE
Construction is expected to begin in April 2016 and be completed by the end of September 2016.
Earthwork activities are not expected to take place in the wet season, October 1st to April 30th.
Should any wet weather conditions occur during construction, the contractor shall implement the
de-watering procedures outlined in this SWPPP and applicable BMPs including, but not limited
to Cover for Materials and Equipment (BMP 8), Mulching (BMP 15), Geotextile (BMP 17),
Matting (BMP 18), and Slope Roughening (BMP 25).
BEST MANAGEMENT PRACTICES (BMP)
• General Construction BMPs (BMP 1-6): The project shall adhere to all 6 general
construction BMPs. Construction access will be provided with a quarry spall entrance
emanating from the south end of the existing parking lot. The existing crushed rock path
will be used as well, supplemented by quarry spalls to meet the minimum width
requirements. The Contractor shall provide wheel wash if necessary.
• Housekeeping BMPs (BMPs 7-11, 13): The following general Housekeeping BMPs,
section 4 of Volume 2 of the 2005 Idaho Stormwater BMP Catalog shall be implemented:
Dust Control (BMP 7), Cover for Materials and Equipment (BMP 8), Stockpile
Management (BMP 9), Spill Prevention and Control (BMP 10), Vehicle/Equipment
Washing and Maintenance (BMP 11), Concrete Waste Management (BMP 13). There
will be no hazardous waste or sanitary/septic waste, so BMPs 12 and 14 are not
applicable to this project.
• Inlet Protection (BMP 31): Drainage structures in areas where no work occurs will
remain and will be protected; discharge points to the public storm drain main line will
also be protected. To prevent discharge of turbid water downstream, all existing catch
basins located within the disturbance area and outside of the disturbance area within
approximately 300 feet downstream of the site will be protected with storm drain inlet
protection. Proposed inlets will also be protected with catch basin inserts, and where
feasible, their outlets will be temporarily plugged until the site is stabilized. The
Contractor shall remove inlet protection at the end of the project without releasing
captured sediment into the storm system.
• Check Dams (BMP 32): Rock check dams will be constructed in the temporary drainage
swale to provide protection for the soils and reduce the runoff velocity down the slope.
Page 3
Brigham Young University Idaho – Playfield Complex Renovations – Phase 2
Construction SWPPP Narrative
March 29, 2016
• Fiber Rolls (BMP 35): Compost Filter Sock will be used on the parking lot to prevent
sediment-laden water from flowing north into the existing drainage system.
• Silt Fence (BMP 36): Silt fence will be used along the western edge of the project limits
to prevent slope erosion and prevent sediment-laden stormwater from entering the
downstream storm system.
• Portable Sediment Tank (BMP 39): A sediment tank will be provided to filter
sediment-laden water for the construction site. See the project’s drainage report for more
on the calculations for the sizing of the tank. For the field/parking area, there must be a
minimum of 35,488 gallons of storage. However, to be conservative, it has been
recommended that the contractor provide two 18,900 gallon Baker steel tanks or
approved equal for sediment settling. For the tennis court area, there is 3,216 gallons of
storage required. A single 4,000 gallon Baker poly tank or approved equal has been
recommended.
• Temporary Swale (BMP 40): Temporary drainage swales will be constructed to direct
sediment laden stormwater to the sump and pump, where it will be pumped to the
sediment tank. The swales will include rock check dams.
PERMANENT STABILIZATION
The site shall be stabilized, including slopes, exposed soils, and other erosion control BMPs per
the 2005 Stormwater Best Management Practices Catalog for Idaho Cities and Counties. The
BMPs for stabilization include, but are not limited to: Dust Control (BMP 7), Cover for Materials
and Equipment (BMP 8), Mulching (BMP 15), Hyrdomulching (BMP 16), Geotextile (BMP 17),
Matting (BMP 18), Topsoiling (BMP 20), Seeding (BMP 21), Slope Roughening (BMP 25),
Check Dams (BMP 32).
It is possible that some of the earthwork and grading may occur in wet weather conditions. The
site must be stabilized and no soils will be allowed to remain unstabilized for more than two days
between October 1st and April 30th. From May 1 through September 30, install cover measures to
protect disturbed areas that will remain unworked for seven days or more. By October 8, seed all
areas that will remain unworked from October 1 through April 30. Mulch all seeded areas.
ENGINEERING CALCULATIONS
A copy of any calculations performed during design of the project and relevant storm drainage
modeling discussions is included in the project’s Drainage Report.
MAINTENANCE
Standard BMP maintenance will be required in accordance with the 2005 Catalog of Stormwater
Best Management Practices for Idaho Cities and Counties and the TESC Notes (see Sheets F-1.2
and F-1.3 of the Design Drawings).
Page 4
Brigham Young University Idaho – Playfield Complex Renovations – Phase 2
Construction SWPPP Narrative
March 29, 2016
FINANCIAL/OWNERSHIP RESPONSIBILITIES
This property is owned and operated by Brigham Young University. The accepted low bidder on
the project will be responsible for posting a performance and payment bond with Brigham Young
University and thus will be the responsible party for any liability associated with erosion and
sedimentation impact. Below is the contact information for the owner’s representative of Brigham
Young University, Idaho campus:
Owner’s Representative: Kehl Murray
Title: Construction Project Coordinator & Duly Authorized Representative
Address: 285 University Operation Building, Physical Plant Way, Rexburg, Idaho 83440
Phone Number: (208) 496-2665