HomeMy WebLinkAboutSDP200200133 Calculations 2003-01-24 7
BMP Calculations
for Biofilter
at
Greenbrier Service Center
Prepared by
DAVID C. WYANT, P. E.
4686 GARTH ROAD
CROZET, VA 22932
January 21, 2003
RECEIVED IN
PLANNING JAN.24,2003
BMP Calculations
for Biofilter
at
Greenbrier Service Center
January 21, 2003
I. Narrative
The purpose of this plan is to describe the stormwater/BMP measure proposed
for parcel TMP 61-148. A biofilter is proposed in the parking area of the
parcel.
The owner of Greenbrier Service Center, LLC proposes to rebuild a garage
on Gasoline Alley. The previous garage (1,868 sq. ft.) was destroyed in a fire
during the month of September 2002. The office (676 sq. ft.) was saved
from the fire. The new structure is 1512 sq. ft. larger than the structure
destroyed by the fire making the total square footage equal 4,056 sq. ft..
Minimal grading at the back of the building to allow access to the 3 back
bays of the garage and over the parking areas and roadways to get water
flow in the proposed biofilter and retention pond will be required. Runoff
from the entire site should flow to the biofilter and retention pond with this
minimal grading. The treated runoff will discharge into a manhole that
enters a 18-inch CMP that exits the property to a manhole across the
Southern Railway property to the east. Additional runoff from parcels
above this property and treated on those parcels will pass through the
biofilter.
The owner of the property presently has a Special Use.Permit to rent and
sell rental vehicles. He proposes to expand the area used to store and sell
these vehicles. In order to expand the owner will request an amendment to
his Special Use Permit at the end of January 2003. Included in that
amendment he will request the parking of two trailers for hauling used tires
to be recycled. This use has been conducted on this property for a number
of years.
II. Existing Site Conditions
The site is located west of the Southern Railway tracks, east of Rio Road,
and on Gasoline Alley (Rt. 650). The entire site is behind a car wash and
service station on Rio Road. The operations on the site are screened from
the travelers on Rio Road because of these businesses as well it is in a low
location. There are several existing drop inlets on the site as well as a
flared stormwater inlet.
The topography is rolling and at the bottom of a drainage area. Drainage is
piped off the site into a manhole on the east side of the Southern Railway
tracks.
The major portion of the parcel is open (63%) and the remainder covered with
impervious surfaces (pavement and rooftops). All slopes and open areas are
vegetated and established.
III. Adjacent Areas
The property is surrounded by all C1 or HC zoned. parcels. Beyond the
businesses on the west side is-Rio Road. On the east is the Southern Railway
Company. None of the adjacent parcels of land will be affected by the
proposed work.
IV. So Is
The soils at this site are Culpeper fine sandy loam. The Culpeper sandy loam
soils are deep, strongly sloping, and well drained soils with the surface layer
about 8 inches thick and being brown and yellowish brown fine sandy loam. The
subsoil is yellowish red clay loam and red day and sandy clay loam for 29
inches. Below the subsoil to a depth of 50 inches is reddish yellow fine sandy
loam. Hard rock exists at a depth of about 50 inches. These soils have a
moderate permeability as well as available water capacity.. The hazard of
erosion at the surface is severe, i.e. the erosion factor, K, is 0.37. The subsoil
has a moderate shrink-swell potential.
V. Calculations
Biofilter Design
Note: The biofilter is designed to handle the conditions (area and flow) when
the Special Use Permit amendment is approved. The entire site will drain
into the biofilter, but 41,810 sq. ft. (0.96 ac.) of the site is impervious.
Treating the first flush (0.5 in.) from a storm event, where the removal rate
is less than 50%, requires that 2.5% of the impervious area above the
biofilter be achieved at the biofilter floor. The required removal amount
for this job is 36% (see the attached worksheet). Therefore, 1,045 sq. ft.
is required and 1045 sq. ft. of floor area of the biofilter is provided at
elevation 439.0 ft. The top of the DI is 440.95 ft., so the floor is 1.95 ft.
below the overflow into the DI. The floor area was lowered to accommodate
the storage necessary to handle the 2-yr. pre-construction storm event
from the parcels above TMP 61-148 and TMP 61-148 rather than keep the
storage above the floor at 1.0 ft. The berm height around the biofilter is
442.0 ft., which is 1.05 ft. above the DI top. The calculations follow:
Quantity
Pre-construction
A = 1.64 ac.
c = 0.2 (grass)
c = 0.85 (roof, drives, and walks)
Impervious area = 26,567 sq. ft. (37.3%)
Open area = 44,712 sq. ft. (62.7%)
c (composite) _ (0.2) (0.627) + (0.85) (0.373) = 0.44
L= 330 ft. (top of lot by frame shed to biofilter by'northwest corner of
parcel.)
Drop = 456.4' - 441.0' = .15.4ft.
s = 4.7%
v = 3.5 fps (Plate 5-2, p. V-12)
t = 330 ft. / (3.5'fps) (60) = 1.6 min. , use 5 min.
i = 5.5 in/hr (2-yr. Storm)
Q = CIA
Q = (0.44)(5.5)(1.64) = 3.97 cfs for 2-year storm
Vol. _ (3.97)(60)(5) = 1191 cu.ft.
Post-construction
A = 1.64 ac.
c = 0.2 (grass)
c = 0.85 (roof, drives, and walks)
Impervious area = 41,810 sq. ft. (58.7%)
Open area = 29,469 sq. ft. (41.3%)
c (composite) _ (0.2) (0.413) + (0.85) (0.587) = 0.58
L= 330 ft. (top of lot by frame shed to biofilter by northwest corner of
parcel)
Drop = 456.4' - 441.0' = 15.4 ft.
s = 4.7%
v = 3.5 fps (Plate 5-2, p. V-12)
t = 330 ft. / (3.5 fps) (60) = 1.6 min. , use 5 min.
i = 7.1 in/hr (10-yr. Storm)
Q = ciA
Q = (0.58)(7.1)(1.64) = 6.75 cfs for 10-year storm
Vol. _ (6.75)(60)(5) = 2,026 cu.ft.
Difference between Pre and Post
Difference = 2026 - 1191 = 835 cu. ft.
Discharge from Upper Parcels
Note: Since the design engineer for the upper parcels had to retain and not
exceed a 2-yr pre-construction storm event, then the maximum 2-yr.
discharge from the off-site detention pond is taken into consideration and
designed for storage in the biofilter.
Q = 1.11 cfs (Laser Car Wash)
T = 13min.
Vol. req. _ (1.11) (60) (13) = 866 cu. ft.
Total for the biofilter is: 835 + 866 = 1701 cu. ft. above the biofilter
material floor. Area of biofilter floor = 1045 sq. ft.
Min. depth of biofilter (min.) below top of DI = 1701 / 1045 = 1.6 ft., so set
top of biofilter equal 439.0 ft.
Top of Berm = 442.0 ft.
Top of DI = 440.95 ft. (at least 1 ft. of freeboard above DI)
Top of Filter Media = 439.0 ft: (at least 1 ft. of storage above media)
Top of aggregate/ perforated pipe = 436.5 ft.
Inv. of Pipe into DI = 435.8 ft.
Bottom of aggregate layer = 435.5 ft.
Attached is the short version BMP Worksheet with calculations for pollutant
removal. Using a biofilter is the best form of stormwater quality treatment.
The attached BMP worksheet indicates that the required removal is 36%.
The biofilter with the underdrain pipe system and trees and shrubs will meet
the required removal.
Biofilter Requirements
1. The biofilter shall have a surface area of least 1045 sq. ft. and have 2:1
sides.
2. The biofilter soil mix shall be 2.5 ft. thick.
3. The biofilter soil mix must be Luck Bio-filter Mix or tested and
approved equivalent (50% sand, 30% top soil, and 20% leaf mulch). For
more information please call Water Resources personnel at
(434) 296-5861.
4. Three red maple (acer rubrum) and three pin oak (Quercus palustris) 15
ft. on center and having a minimum caliper at planting of 1 inch shall be
placed evenly throughout the biofilter. Twelve shrubs placed 8 ft. on
center and spread evenly throughout the biofilter are required as well
as the trees.
Pipe Exiting DI
18" CMP
L = 322.38'
s = 4.8%
a = 1.77 5q. Ft.
P = 4.71 Ft.
r = 0.38Ft.
n =. 0.015
v = (1.49/n) [(r * 0.67) (s * 0.5)] _ (1.49/0.015) (0.52) (0.22) = 11.32 fps
Q = a v = (1.77) (11.32) = 20.03 cfs > 5.08 cfs for 2-yr. storm event
Pipe can also handle 10-yr. post storm event discharge from parcels above'
(4.94 cfs) and this parcel (6.75 cfs).
20.03 cfs > 11.69 cfs
• Short Version SMP Computations For Worksheets 2 -6
Albemarle County Water Protection Ordinance: Modified Simple Method
Plan: Greenbrier Service Center Water Resources Area: Development Area
Preparers David Wyant Date: 21 Jan-03
Project Drainage Area Designation Site
L storm pollutant export in pounds, L=[P(Pj)Rv/12][C(A)2.72]
Rv mean runoff coefficient, Rv=0.05+0.009(1)
Pj small storm correction factor,0.9
percent imperviousness
P annual precipitation,43"in Albemarle
A project area in acres in subject drainage area, 1 A=�, ,°Y^,;1:647
C pollutant concentration, mg/I or ppm target phosphorus
f factor applied to RR
✓ required treatment volume in cy,0.5"over imperv. area= A(1)43560(0.5/12)/27
RR required removal, L(post)-f x L(pre)
%RR removal efficiency, RR100/L(post)
,
- Impervious Cover Computation(values in feet 8�square feet)
!Item m._....-_......_.__.__ ._._,.._.. !
;pre-development {Area jpost-development 'Area I
Roads LengthWidth subtotal ! ���� Length Width subtotal y# II [. 0' .� '0] _ 01 �'0,
"5 0i i3Oi,'_ :0 01
1
Driveways Length Width o. subtotal 1 Length width_ no. subtotal _j
{ .f, 0 0; :'0 :,-: "0:, Oj - 01 i
,and walks ,t :o'"
s r': - -,
_0 �0
1 1Y,r�:w
=tiG„
O1 0
i 1
_ ry
{Parking Lots Area 1 Area2 y' Area 3 Area 4 !Area 1 A ea 2 Area 3 Area 4 j 1
23693 i � 0 23693 p 37424w::, , ;: •Ol 374241
1Gravel areas Area 1 Area 2 ;subtotal j
1 Area 1 Area 2 !subtotal i 1
1 ( `Y. Oa. . :' -01 Olx 0.70= i 0' .Y ,,3...__,Y0':;:1'..,'::y:i,:.. ..0 Olx 0.70= 1 01
!Structures Area no. subtotal i i Area no. subtotal i
7':2544i,37 :Al 25441 _�4Q56�; �:�`�'''; 1 _ 40561
i'
! 40561_ ::l3; 0 2544 0
Y 9 pasture ;:::;;- _ 7 _..... Arerr a
.•:_?. }.:,..,..,,<. :.;,: i
1Activel - razed & -Area j Area
1 s
!yards and cultivated turf 447121x 0.08= 3576.96 177' 29469-x 0.08- � 2357.521
[Active crop land Area I Area 1 ;
_ s^' S0.x 0.25 i 01 _ [ _ v{ x 0.25= s 01
Other Impervious Areas Area-1 Area 2 Area 3 1 ! Area 1' Area 2 Area 3 ;_E 1
% ;? '''1' :,r .- -7.', r .":'Oi 3301
330 . ;0 330' ' Y i
Impervious Cover :4Z* ;152%
'I(pre) I(post)
Rv(post) V 1
0.61 r,ii 68�2;j
New Development(For Development Areas,existing impervious cover<=20%)
j MC f P I(pre)*!Rv(pre) I L(pre) I L(post) I RR %RR I Area Type
":`'`0 70`<°,;? ::,:;1is00-€ >I^''2091 0.43 1 4.33 ] 6.10 1 1.78 :' %1 Development Area
r�F<T ''035"I):;::<'.'s`�'I 00.7; :.0%a 0.432.16 ' 3.05 i 089 :29.%< Drinking Water Watersheds
[Tµ' O74p:"l7.,.:66. :":''-" ;:`:1°Ad� 0.43 I .2.47 3:49 { 1.02 (;:;;[::z- :29°%: Other Rural Land I
*min.values
Redevelopment(For Development Areas,existing impervious cover>20%)
j C I f I I(pre)* I Rv(pre) I L(pre) L(post) RR 1 %RR I Area Type_
i:'1 ;.0:70:�:$' :0:90.1: :;' 2O% 0.43 4.33 1 6.10 2.21 ig,:ilt;p§st Development Area
`: 0 35 °<; :d0 85" ::;0% 0.43. 2.16 1 3.05 1.21 ;;40% DrinkingWater Watersheds
.2;4`a.` [ °:`,.:;.>. "`';_:;. ., ;;-:;, 7 t 3.49 1.39i'e ,, .4.` _... ,mother Rural Land
9;40rrj >,:;�.0$5'i. < t4iA;l 0.43 2.4 iid:' ,;-;:.;;;,lut •
• rev.30 May 2002 JMK