HomeMy WebLinkAboutSDP200900036 Study Stormwater Management Plan 2009-03-18 Page 1 of 1
.fie- , wi-AAN
Philip CusterDAWN/ "$
From: Wagner, Kenneth [kwagner@Dewberry.com]
Sent: Wednesday, March 18, 2009 9:27 AM eGar�� •"'�
To: Philip Custer N opA Ce 1( �t 1 I i,5�
Cc: Fitzgerald Barnes 1
Subject: Monticello High School grass to turf field conversion
Attachments: The Park Example SWM.pdf
Philip,
This email is a follow-up from our Monday afternoon meeting with you at Albemarle County. Attached is the last
Turf field project stormwater management narrative that Dewberry had prepared for UVA that was approved by the
Virginia (Central Office) Department of Conservation and Recreation. Included is the narrative, and supporting
information. Not included is the detailed routing,which could be provided upon your request.
Let me give you a little background on that project. The turf field project at UVA was addressed as a stand alone
project in providing stormwater management due to extensive complications associated with modifing the existing
stormwater facility. This is the same approach that Dewberry would be using to address stormwater on the proposed
Monticello High School football field from grass to turf conversion. Any modification to the existing stormwater
management facility at Monticello High School is not anticipated with this project.
In the attached narrative, water quantity storage was address by allowing the stone beneath the field to store excess
runoff and the perforated discharge pipes acting as a control orifice, water quality was addressed by defining the
reduction in fertilizer not needed for a turf field.
It was discussed with DCR that Dewberry could have provided a smaller orifice to dupicate a BMP application to store
the associated Chesapeake Bay Water Quality Volume and provide a 30-hour drawdown, but it was concluded and
approved by DCR that this was not necessary, given the reducition in fertilizer or direct reduction in phosphurous
runoff from the site.
If you should have any questions please do not hesitate in calling me to discuss the attached computations.
Thank you and best regards,
Ken Wagner
Project Manager
Dewberry
4180 Innslake Drive
Glen Allen, Virginia 23060
p: 804-290-7957
f: 804-290-7928
www.dewben.com
Visit Dewberry's website at www.dewberry.com
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UNIVERSITY OF \-IR(; 1\
THE PARK TURF FIELD Pilal LC [
FIELD CONVERSION
FROM GRASS TO TURF
STORIVEN'VATER MANAGEMENT AN
piepated For:
University of Virginia
Charlottesville,Virginia
Prepared By:
1)ENVBERRN & DkVIS, INC
4180 Irinslake 1)ri‘
(;leri Mien, Virginia 23060
Apri' 20(18
!?rif. Detovberry
•
PROJECT NARRATIVE
University of Virginia
North Grounds Recreation at the Park
Field Conversion from Grass to Turf
Stormwater Management Analysis
Background
The University of Virginia(Client)contracted with Dewberry&Davis,Inc.(Dewberry)to complete
a stormwater management analysis from a site/civil perspective of converting grass fields 2,3,4,and
5 into synthetic turf fields. The existing fields are located at The Park on the grounds of the
University of Virginia in Charlottesville, Virginia, within a chain link fence adjacent to the public
shelter. The fields are currently owned and maintained by the Department of Intramural Recreational
Sports(MRS). Figure 1 in Tab 4 under Supporting Illustrations is a map that shows the location of
the project site.
The DIRS anticipates utilizing Carolina Green Corporation(CGC)as the contractor to construct and
complete this project.
Existing Conditions Analysis
Dewberry obtained GIS and archive drawings from the Client to gain an understanding of the
existing drainage and storm water management at the project site. Upon review of the provided
information, Dewberry identified that the existing storm water management detention facility that
currently serves the site has limiting site constraints that eliminates the ability for any existing
facility expansion or modifications. Dewberry has documented the existing conditions and explains
the reasoning behind not modifying the existing facility. This information can be found in Tab 6 of
this report. Therefore, given the existing facilities constraints and the requirement to provide
stormwater runoff controls, Dewberry worked with the installer to incorporate an innovative
modification to the turf system that will allow for non-impeded natural infiltration of the stormwater
into the soil at the fields.
Design Requirements
Based on discussions with the Virginia Department of Conservation and Recreation(DCR),it is our
understanding that Dewberry will he required to show how the new synthetic turf field provides both
water quantity and water quality control.
Proposed Synthetic Turf Field and Under-drain System
The proposed turf system will consist ofsynthetic turf with a rubber-in-fill over 8-inches of sand and
stone, which from top to bottom includes the synthetic turf rubber mat, I-inch of sand, 2-inches of
#78 stone,and 5-inches of#57 stone over a pervious liner on an existing excavated soil surface that
allows for infiltration or ground water recharge over the 347 feet by 515 feet field area. A typical
synthetic turf profile from the installer is part of this report as Figure 3 in Tab 4. The
manufacturer's data states that the turf with rubber backing has an infiltration rate of 10
inches of rain per hour and the sand gravel base has an infiltration rate of 18 inches of rain per
hour. The void area in the sand and gravel per Luck Stone calculation is approximately 50% for
sand and 42%for#78 and 457 stone. This void area between the stones will provide the storage area
for the storm events to infiltrate into the soils and provide an under-drain system that can carry off
any excess flow not recharged back into the water table.
The COC recommends that the collector lines on a field of this size and slope be oriented at the low
side(building side)and low end(double softball field end)of the field area. The design would also
run three lines across the field cutting the area in quarters just to collect some volume periodically
and not have the excess runoff move through the stone for the entire diagonal length of the field.
The lateral lines would be 6"perforated HDPE and the long side collector would be 12". This would
tie into the storm system at the low corner in what appears to he a 12" RCP, based on the GIS.
Figure 3 in Tab 4 is a typical layout of the collection lines.
Typical excavation for the collection lines would be a 12" wide by 12" deep trench for the 6"
collector pipe and 18"wide by 18"deep for the 12"collector pipe. The slope of the field allows for
plenty offal.]in the ditch without having to create fall in the trench,so depth o f trench would remain
constant.
The slope of the existing field is approximately 2% at a cross angle from corner to corner. This
provides for an approximate 1.25% slope down each X and Y axis. This will be the approximate
slope of the collector lines.
Addressing On-site Stormwater Management Requirement Summary(both Quality and Quantity)
Water Quality is addressed below, in that the proposed turf system does not prevent or impede
natural infiltration into the soils through use of a pervious (porous) liner. Water Quantity is also
detailed below,in that when thc soils can not infiltrate fast enough,an under-drain system has been
installed to carry the excess runoff to the existing storm outfall and stonnwater management facility.
In Tab 3 of this report the proposed turf system routing documents that the post-development flows
are controlled within the stone and under-drain system to release below the pre-development flow
conditions, in the event that the soils cannot infiltrate the storm runoff fast enough.
Detailed Proposed Water Quality Control
The water quality for the project is addressed within the workings of the various components of the
new synthetic turf system. The turf itself allows for the storm runoff to be absorbed at a rate of 11-
inches per hour,storing water in the sand and stone layers and infiltrating back into the soils beneath
the stone through a pervious liner. The new turf system will operate as a natural grass surface does
today allowing for the ground water to be recharged. The new turf system will allow for infiltration
back into the surrounding soils. The existing soils information and geotechnical information for the
project site is included in this report in Tab 5. This information shows that sandy silt is 3.5 to 4.0-
feet below the surface,according to the boring log documents. The textural soil guide is included to
show the ability of infiltration for the sandy silt under soils.
The following information provided from[)IRS,shows the amount of fertilizer used per year at The
Park and documents the anticipated reduction by the grass to turf conversion of fields 2,3,4,and 5.
These are the University's "Record of Actual Nutrient Application" that arc maintained for
University verification and submission to DCR.The Fertilizer Analysis symbol numbers in order are
(N) Nitrogen(P)Phosphorus (K) Potash.
Date Fertilizer Analysis ft Bags Bag Size Totals
9-15-06 31-0-0 20 50Ibs 10001bs (all Park fields)
10-20-06 31-0-0 20 501bs 10001bs (all Park fields)
11-1-06 6ozi I 000 sq ft 82 gals(turf area only)
3-10-07 46-0-() 13 SOlbs 650 lbs (all Park fields)
4-15-07 46-0-0 13 501bs 650 lbs(all Park fields)
5-10-07 3oz/1000 sq ft/.03 lbs N per 1000 41 gals(turf area only)
9-10-07 25-5-0 12 50Ibs (Application beyond one-year)
The above quantities are the total application rates for the two softball fields, an additional smaller
playing field,and the major field that we are proposing to convert.The field conversion from grass to
turf will reduce the above granulated fertilizer applications by 2/3 of the totals. The above
application rate totals 3300 lbs for the year of granulated fertilizer for all the fields. The 2/3 adjusted
total would result in a 2200 lbs per year reduction of the granulated fertilizer application by the
proposed grass to turf field conversion. The above liquid application fertilizer was calculated for the
field conversion area only and will total 123 gallon of reduced application a year by the proposed
grass to turf field conversion.
In conclusion, the ability of the turf system gravel layer and pervious liner to act as a slow release
infiltration system and the reduction of the use of fertilizer on the field prove that no additional
measures would need to be provided to address water quality. Dewberry believes that the sand and
stone system associated with the synthetic field's media sub-base, the fact that the field will be used
for recreational sports that do not permit vehicular traffic with the exception of an extreme
circumstance, and that the new field no longer requires 2200 pounds of additional granulated
fertilizer and 123 gallons of liquid fertilizer a year,will provide sufficient evidence that DCR should
accept that the new synthetic field provides the required water quality improvements over the
existing grass field without providing additional measures.
DCR's Chesapeake Bay water quality performance worksheet calculations are provided in Tab 2 of
this report. It documents that this project does not change the impervious cover percentage with
development.
Detailed Proposed Water Quantity Control
Within the typical field section and header pipe layout described above, Dewberry calculated how
this system provides water quantity control through the sand and stone layers and under drain system.
In the event that the soils cannot infiltrate the storm runoff fast enough. These layers will provide
approximately 51,227 cf or 1.176 acre-feet of storage through voids around the sand and stone
media, and the header pipes will discharge at or below the current runoff rate of the existing grass
fields. This is proven in the stormwater management routing in this report. The voids in the sand
and stone base surface acts as a storage area long enough to allow the soils to infiltrate and then
excess runoff is released in the header pipe system that acts as the control structure. This will
ultimately control the post developed turf conditions to below the pre-developed grass condition
runoff rates. These facts should satisfy DCR that the new synthetic turf field meets or exceeds the
storm water quantity controls associated with the existing grass fields. A detailed storm water
management routing that simulates these conditions is provided in Tab 3 of this report.
This study estimated the time of concentration for both pre-and post-development to be 10 minutes
across the proposed site development area. The pre-development"C"value for the existing grass
surface is estimated to be 0.30. In order to develop a routed computer model the proposed post
development had to consider that the soil was not infiltrating fast enough and the excess runoff has
developed a 0.90 "C" value condition. The only way to computer model the site is to change the
post-development "C" value. The 12-inch header collector pipe serves as the controlled release
structure and the pond storage is the void in the stone. This routing produced the following results.
The 2-year storm produces a post-development runoff"Q"of 8.31 cfs,the turf system releases a
of 3.85 cfs,which is below the pre-developed 2-year storm runoff"Q"of 6.35 cfs.The 10-year storm
produces a post-development runoff"Q" of 14.40 cfs. the turf system releases a "Q"of 4.06 cfs,
which is below the pre-developed 2-year storm runoff"Q" of 10.64 cfs. Therefore, Stormwater
Management Water Quantity requirements arc met.
PROPOSED DESIGN INFORMATION &
CALCULTIONS
- STONE/SAND LAYER STORAGE 'VOLUME
LUCKSTONE STONE VOID ( HRT
- PROPOSED DESIGN DRAINAGE AREA MAP
- IDE TABLE
- CHESAPEAKE BAY PERFORMANCE BASED WATER
QUALITY CALCULATION WORKSHEETS
1
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Last Revision Date 01/05/2005
Properties:
Coarse A ore aces
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Absorption 0.60 0.61 039 0.87 0.81
Unit weight Apvii 102 _ 102 102 102 98
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4 DATE
orairei TITLE SHEET NO.
t::140 ew erry PRE AND POST
Voior APR 2008 DRAINAGE AREAS
Ap. _
Dcwb,,qiy N Davls, Inc PROJ. NO. PROJECT
4180 nalai.. Drive Glen Men VA 080G0 NORTH GROUND
6))804 200 7957 (I)604 2907928
,A o...dowt*rry Com ' 50013429 RECREATION AT THE PARK
i
IOF Table for Charlottesville(city); County(or City)#104
Intensities (in/hr) fit Dewberry
To(hr) To(min) 2-Year 5-Year 10-Year 25-Year 50-Year 100-Year
0.083 5 5.18 5.91 6.67 7:48 8.04 8.76
0.167 10 4.12 4.79 5.37 6,01 6.45 7.01
0.250 15 3,44 4.05 4.55 5.10 5.49 5,98
0,333 20 2.97 3.53 3.97 4.47 4.83 5.29
0.500 30 2.36 2.83 3.21 3.65 3.98 4.40
0.667 40 1.97 2.38 2.72 3.13 3.43 3.83
0.833 50 1.70 2.06 2.38 2.76 3.05 3.43
1 60 1.50 1.83 2.12 2.48 2.76 3.13
2 120 0.90 1.12 1.35 1.63 1.86 2,18
3 180 0.66 0.83 1.02 1.27 1.47 1.76
4 240 0.53 0.67 0.84 1.05 1.24 1.51
5 300 0.44 0.56 0.71 0.91 1.09 1.34
6 360 0.38 0.49 0.63 0.81 0.97 1.21
8 , 480 0.30 0.39 0.31 0.67 0.82 1.04
10 600 0.25 0.33 0.44 0.58 0.72 0,92
12 720 0.22 0.28 0.38 0.52 0.64 0.83
18 1080 0.16 0.21 0.28 0.40 0.50 0.67
24 1440 0.13 0.16 0.23 0.33 0.42 0,57
j 10.0 T1I , (
9 0 s Charlottesville(city); County(or City)#104, VA
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1
PER! r. \I,INCE-BASED WATER QUALM I XI ILATIONS APPENDIX 51)
Worksheet 1
Page I of 3
STEP I Determine the applicable arca (A)and the post--developed impervious cover
(iposd•
Applicable area (A)* = Y. , 9.acres
Post-development impervious cover:
structures = 0, acres
parking lot 0 acres
roadway = 0 acres
other:
acres
acres
Total — 0, )(-). acres
=(total post-development impervious cover A) x
The area subject to the criteria may vary from locality to locality. Therefore,
consult the locality for proper determination of this value.
STEP 2 Determine the average laud cover condition (1.
walershe ) or the existing
impervious cover
Average land cover condition(lwaie„hc,4):
If the locality has determined land cover conditions for individual watersheds within its
jurisdiction. use the watershed specific value determined by the locality as
watershed =
Otherwise, use the Chesapeake Bay default value:
I 6 '
5D-5
PERFORNIANCL-11ASED NA ATER QIIALITY CALCULATIONS APPENDIX 51)
Worksheet I
Page 2 of 3
Existing impervious cover
Determine the existing impervious cover of the development site if present.
Existing impervious cover:
structures = 0, )a acres
parking lot — 0 acres
roadway = 0 acres
other:
acres
acres
Total - 0 ))
acres
(total existing impervious cover=A*) x 100=
The arca should be the same as used in STEP 1.
STEP 3 Determine the appropriate development situation.
The site information determined in STEP 1 and STEP 2 provide enough information to
determine the appropriate development situation under which the performance criteria will
apply. Check (• j the appropriate development situation as follows:
Situation 1: This consists of land development where the existing percent impervious
cover(c ) is less than or equal to the average land cover condition
(1„„,,„4„a)and the proposed improvements will create a total percent
impervious cover(11,„„)which is less than or equal to the average land
cover condition(I„,„,,,d).
Ip.m '75 % %
5D-6
411110101MIP
PERFORMANCE-BASED NA „ ALCULATIONS APPENDIX 51)
Worksheet I
Pagc 3 o13
Situation 2: This consists of land development where the existing percent impervious
cover(1,,,„„g)is less than or equal to the average land cover condition
(1„„,"")and the proposed improvements will create a total percent
impervious cover(l„,) which is greater than the average land cover
condition (1„.„,„ku,d).
elistine % waierchrd %; and
poM (0 -watershed
Situation 3: This consists of land development where the existing percent impervious
cover is greater than the average land cover condition
% 1
reater%hrd 01
0
Situation 4: This consists of land development where the existing percent impervious
cover(I ) is served by an existing stomtwater management BMP(s)
that addresses water quality.
If the proposed development meets the criteria for development Situation 1,than the low
density development is considered to be the BMP and no pollutant removal is required.
The calculation procedure for Situation I stops here. if the proposed development meets
the criteria for development Situations 2,3, or 4, then proceed to STEP 4 on the
appropriate worksheet.
5P-7