1. Design Volume Calculations for Structural, Volume-Reduction BMPs

Pennsylvania Stormwater
Best Management Practices
Manual
Appendix D – Stormwater Calculations and
Methodology: Case Study
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Pennsylvania Stormwater Best Management Practices Manual
Appendix D
EXAMPLE 1: Control Guideline 1 for Residential 10-Lot Subdivision
This example describes a 10-lot residential subdivision in Blair County, Pennsylvania
with the following conditions:
1. In this 10-lot subdivision, on-lot structural BMPs provide volume reduction and
infiltration for the net increase in volume for the 2-year, 24-hour storm event.
Peak rate calculations are developed by two different techniques. Because of
the relatively slow-draining soils and a small total infiltration area, increased
storage in the BMPs or downstream detention is required to mitigate the peak
rate of runoff for the larger storm events.
2. The same design is then revised to incorporate Non-structural BMPs to reduce
the requirements of the structural BMPs. Adjusted volume calculations are
provided.
3. In addition, the 10-lot subdivision is modeled with a dry detention basin for
conventional peak rate control for comparison. Finally, the site is routed with an
extended detention (ED) basin for ED of the 1-year storm and peak rate control
for the larger storms.
Follow Flow Chart A
Step 1
: Provide General Site Information (Worksheet 1)
In this example, the pre-development condition is a 10-acre site with 7 acres of meadow
and 3 acres of woods. The underlying soils are classified as hydrologic group “C”, and
the overall site slope is approximately 8%.
Step 2
: Identify sensitive natural resources (if applicable) and what areas will be
protected or maintained. (Worksheet 2).
Note: In this example, there are 3 acres of woodlands that are not protected.
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Step 3
: Estimate the benefits of Non-structural BMPs in the stormwater design
(Worksheet 3).
Note: In this example, Non-structural BMPs are not initially applied.
Step 4
: Based on the proposed design, estimate the increased volume of runoff
for the 2-year storm event, using the Cover Complex Curve Number method.
Using a weighted curve number is NOT acceptable
. Runoff volume should be
calculated based on major land use types and soil types (Worksheet 4).
The proposed development includes 10 residential lots, each covering 0.91 acres.
Step 5
: Design and incorporate Structural and Non-Structural BMPs that provide
volume control for the 2-Year volume increase (Worksheet 5).
Note: In this example, Rain Gardens and Infiltration Trenches are placed on
each lot
.
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Calculations are provided to demonstrate that the required volume is provided. The
storage volume is calculated for each rain garden and infiltration trench. The total
volume is indicated on Worksheet 5 and compared to the volume requirement for
CG1 of the net increase in runoff volume for the 2-year storm (Worksheet 4).
For this example, the net increase in runoff volume for the 2-year storm is
approximately 25,913 ft
3
, and the combined storage provided by the rain garden and
infiltration trench BMPs is approximately 26,020 ft
3
, so the volume requirement of
CG1 has been met.
Step 6
: Demonstrate Peak Rate Control for the 2-year through 100-year events.
o If Conditions for Peak Rate mitigation can be met, detailed Peak Rate
Analysis and Flood Routing can be waived (Worksheet 6). This example
does not meet those conditions because it has 2 acres of impervious
cover. The maximum impervious area for a waiver is 1 acre.
o If Conditions for Peak Rate mitigation cannot be met, detailed Peak Rate
Analysis and Flood routing is required.
One of the challenges designers often face in using many BMPs throughout the site
is that traditional engineering models and methods of peak rate calculation do not
lend themselves to this type of design. As a result, designers often include BMPs for
volume control, infiltration, or water quality, and then add detention measures.
These detention measures may be greatly oversized because the volume-reduction
and detention benefits of the BMPs and the effects of slowing the movement of
runoff from the site are not accounted for. Chapter 8 provides a discussion titled
“Guidelines: Volume Credits for Detention Routing” that proposes several options for
considering the volume and rate mitigation benefits of multiple volume-reducing
BMPs.
In this example, some of those techniques are applied, including: Composite BMP
and Travel Time Adjustment with Volume Diversion.
For the Composite BMP example, the volume and discharge of the multiple BMPs
(ten rain gardens and ten infiltration trenches) are combined to create a “synthetic”
storage reservoir with a composite stage-storage-discharge curve. The post-
development runoff hydrograph for the entire site is routed into the composite
storage reservoir represented by the combined stage-storage-discharge
characteristics of the many BMPs. The routed discharge from this “synthetic
reservoir” is then used to size the required detention facility for the site to meet the
peak rate attenuation requirements of the 1- to 100- year storm events. This method
allows the designer to “take credit” for the storage/detention volume and infiltration
occurring in the many BMPs, and to reduce the size of the downstream detention
facility that will be built. The method is limited because it does not
provide adequate
consideration of the effect that many BMPs have on how fast
water travels from and
across the site. Since the peak of the runoff hydrograph is strongly influenced by
how fast water travels across the site (or the Time of Concentration, T
c
), this method
is somewhat conservative.
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Appendix D
For the Travel Time Adjustment example, the post-development Time of
Concentration (T
c
) is increased to take into consideration the amount of time it takes
for runoff to move through
the various BMPs. Both structural and non-structural
BMPs can significantly slow the movement of water and reduce the peak flow rate.
In this approach, the total storage of the volume-reduction BMPs (in cubic feet) is
divided by the peak flow rate (calculated without the BMPs in place, in cubic feet per
second) for the 100-year storm event to estimate how long it will take for water to
move “through” the BMPs. This estimated time where runoff is essentially slowed by
the BMP is added to the original post-development T
c
in determining the post-
development runoff hydrograph. Because the T
c
increases, the calculated peak rate
of flow for the site will be lower and the required downstream detention facility will be
smaller. To account for the actual storage and infiltration of the volume-reducing
BMPs (trenches and rain gardens), a diversion is incorporated into the modeling
framework.
Residential 10 Lot Subdivision – Part 2
In this example, the same 10-lot residential subdivision is evaluated, but the design
has been revised to incorporate Non-structural BMPs. These non-Structural BMPs
include:
Maintaining the existing 3 acres of woods (BMP 5.4.1, Protect Sensitive/Special
Value Features and BMP 5.6.1, Minimize Total Disturbed Area). This has the
effect of reducing the volume and rate of runoff that must be managed.
Because
this area remains undisturbed, there is no requirement to manage the
volume of runoff. The total area considered in Worksheet 4 is reduced
from 10 acres to 7 acres.
Reducing the amount of cleared and disturbed area in the construction of the
homes (BMP 5.6.2, Minimize Soil compaction). Rather than clearing and grading
the entire site, approximately one-half of the proposed lawn area on the lots will
not be graded and stripped of topsoil. This area will be protected from heavy
equipment movement during construction, but much of this area will be converted
into lawn as part of the development. A portion of the site (approximately ½ an
acre) will be planted in meadow mix (BMP 5.6.3, Re-vegetate Using Native
Species). Protecting these areas from grading and compaction during
construction maintains their ability to both absorb rainfall and slow the rate of flow
across the site.
To encourage this practice, a “volume credit” is given under
BMP 5.6.2. This reduces the volume of runoff to be managed in structural
BMPs.
Shortening the house setbacks and driveway lengths reduces the amount of
impervious cover (BMP 5.5.1, Cluster) as does reducing the street width (BMP
5.7.1 Reduce Street Imperviousness).
The benefit of BMPs 5.5.1 and 5.7.2 is
significant – the amount of impervious area is reduced from 2 acres to 1.6
acres, and the total site imperviousness is reduced from 20% to 16%.
Rooftop leaders will also be disconnected, but because the disconnected roof
leaders will discharge into the Rain Gardens and Infiltration Trenches, the 75-foot
overland flow requirement will not be met, and so no additional volume reduction
credit is given. Existing trees will also be protected, but because this area is
addressed under BMP 5.6.1 (Minimize Total Disturbed Area) additional credit for
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Appendix D
protecting trees is not given. In other words, credit for a measure (structural or non-
structural) can only be taken once.
Following the same Design and Calculation Process for the design with Non-
Structural BMPs is as follows:
Step 1
: Provide General Site Information (Worksheet 1). The Existing Site
conditions are the same.
Step 2
: Identify sensitive natural resources (if applicable) and what areas will be
protected or maintained. (Worksheet 2).
Note: In this example, there are 3 acres of woodlands that ARE protected.
Therefore, the overall site area contributing to runoff volume requirements is
reduced from 10 acres to 7 acres.
Step 3
: Estimate the benefits of Non-structural BMPs in the stormwater design
(Worksheet 3).
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Appendix D
In this example, Woods are maintained, lot setbacks and driveway lengths are
reduced, the street width is reduced, and areas of lawn are protected from topsoil
removal and compaction. Portions of lawn are replaced with meadow. Rain
Gardens and Infiltration Trenches are placed on each lot
,
however, these BMPs
are reduced in size. The proposed development still includes 10 residential lots.
Note: Direct volume credit can be calculated for certain Non-Structural BMPs.
In this example, a volume credit of approximately 2,900 ft
3
is provided by
creating lawns and meadows in areas that have NOT been cleared of topsoil
and have been protected from compaction during construction.
Step 4
: Based on the proposed design, estimate the increased volume of runoff
for the 2-Year storm event, using the Cover Complex Curve Number method.
Using a weighted curve number is NOT acceptable
. Runoff volume should be
calculated based on major land use types and soil types (Worksheet 4).
Note: Because a number of Non-structural BMPs are applied (as discussed
above), the stormwater management volume requirement is reduced from
25,913 ft
3
to 18,088 ft
3
. This is a 30% reduction in the volume requirement.
Step 5
: Design and incorporate Structural and Non-Structural BMPs that provide
volume control for the 2-Year volume increase (Worksheet 5).
Calculations are provided to demonstrate that the required volume is provided. The
storage volume is calculated for each rain garden and infiltration trench. The total
volume is indicated on Worksheet 5 and compared to the volume requirement for
CG1 of the net increase in runoff volume for the 2-year storm (Worksheet 4).
For this example that includes Non-Structural BMPs, the volume requirement has
been reduced and so the Structural BMPs are reduced in size. The volume
requirement for the original design (without Non-structural BMPs) was 25,913 ft
3
. By
incorporating the Non-structural BMPs, this volume requirement has been reduced to
15,199 ft
3
(including the non-structural volume credits). Correspondingly, the
structural BMPs have been reduced in size: the rain gardens are reduced from
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Appendix D
1,820 ft
2
to 1,070 ft
2
each, and the infiltration trenches are reduced from 1,500 ft
2
to
875 ft
2
.
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Pennsylvania Stormwater Best Management Practices Manual
Appendix D
Part 1 – Structural BMP Design
Date:
Project Name:
10 Lot Residential Subdivision
Municipality:
Smith Township
County:
Blair County
Total Area (acres):
10
Major River Basin:
http://www.dep.state.pa.us/dep/deputate/watermgt/wc/default.htm#newtopics
Watershed:
Purdy Creek
Sub-Basin:
Nearest Surface Water(s) to Receive Runoff:
Tributary to Purdy Creek
Chapter 93 - Designated Water Use:
HQ
http://www.pacode.com/secure/data/025/chapter93/chap93toc.html
Impaired according to Chapter 303(d) List?
Yes
http://www.dep.state.pa.us/dep/deputate/watermgt/wqp/wqstandards/303d-Report.htm
No
X
List Causes of Impairment:
Is project subject to, or part of:
Municipal Separate Storm Sewer System (MS4) Requirements?
Yes
No
X
Existing or planned drinking water supply?
Yes
No
X
If yes, distance from proposed discharge (miles):
Approved Act 167 Plan?
Yes
No
X
Existing River Conservation Plan?
Yes
http://www.dcnr.state.pa.us/brc/rivers/riversconservation/planningprojects/
No
X
Worksheet 1. General Site Information
INSTRUCTIONS: Fill out Worksheet 1 for each watershed
http://www.dep.state.pa.us/dep/deputate/watermgt/wc/Subjects/StormwaterManagement
/Approved_1.html
http://www.dep.state.pa.us/dep/deputate/watermgt/wc/Subjects/StormwaterManagement
/GeneralPermits/default.htm
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Appendix D
INSTRUCTIONS:
YES
3
0
30
PROTECTED
AREA (Ac.)
EXISTING NATURAL
SENSITIVE RESOURCE
Worksheet 2. Sensitive Natural Resources
Waterbodies
1. Provide Sensitive Resources Map according to non-structural BMP 1.1 in
Section 5.0 Non-Structural BMPs. This map should identify waterbodies,
floodplains, riparian areas, wetlands, woodlands, natural drainage ways, steep
slopes, and other sensitive natural features.
2. Summarize the existing extent of each sensitive resource in the Existing
Sensitive Resources Table (below, using Acres).
3. Summarize Total Protected Area as defined under BMPs in Section 5.0.
4. Do not count any area twice. For example, an area that is both a floodplain
and a wetland may only be considered once.
Natural Drainage Ways
Steep Slopes, 15% - 25%
TOTAL AREA
(Ac.)
MAPPED?
yes/no/n/a
Floodplains
Riparian Areas
Wetlands
Woodlands
Steep Slopes, over 25%
Other:
Other:
TOTAL EXISTING:
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5.1
0Ac.
5.2
0Ac.
5.6
0Ac.
TOTAL
0
Ac.
Site Area
minus
Protected
Area
=
10
-
0
=
5.3 Protect/Utilize Natural Flow Paths
Flow Path/Depression
ft
2
x 1/4" x 1/12
=
ft
3
5.7 Minimum Soil Compaction
Lawn
ft
2
x 1/4" x 1/12
=
ft
3
Meadow
ft
2
x 1/3" x 1/12
=
ft
3
3.3 Protect Existing Trees
For Trees within 100 feet of impervious area:
Tree Canopy
ft
2
x 1/2" x 1/12
=
ft
3
For Trees within 20 feet of impervious area:
Tree Canopy
ft
2
x 1"
x 1/12
=
ft
3
5.1 Disconnect Roof Leaders to Vegetated Areas
For Runoff directed to areas protected under 3.1 and 3.2
Roof Area
ft
2
x 1/3" x 1/12
=
ft
3
For all other disconnected roof areas
Roof Area
ft
2
x 1/4" x 1/12
=
ft
3
5.2 Disconnect Non-Roof impervious to Vegetated Areas
For Runoff directed to areas protected under 3.1 and 3.2
Impervious Area
ft
2
x 1/3" x 1/12
=
ft
3
For all other disconnected roof areas
Impervious Area
ft
2
x 1/4" x 1/12
=
ft
3
0
ft
3
* For use on Worksheet 5
Stormwater Management Area
This is the area that requires
stormwater management
TOTAL NON-STRUCTURAL VOLUME CREDIT*
VOLUME CREDITS
10
Worksheet 3. Nonstructural BMP Credits
Area of Protected Sensitive/Special Value Features (see WS 2)
Area of Riparian Forest Buffer Protection
Area of Minimum Disturbance/Reduced Grading
PROTECTED AREA

Pennsylvania Stormwater Best Management Practices Manual
Appendix D
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Appendix D
PROJECT:
10 Lot Subdivision
SUB-BASIN:
1
25,913
-
0
25,913
Proposed BMP*
Area
Storage
Volume
(ft
2
)
(ft
3
)
6.4.1
Porous Pavement
6.4.2
Infiltration Basin
6.4.3
Infiltration Bed
6.4.4
Infiltration Trench
10
6,000
6.4.5
Rain Garden/Bioretention
10
20,020
6.4.6
Dry Well / Seepage Pit
6.4.7
Constructed Filter
6.4.8
Vegetated Swale
6.4.9
Vegetated Filter Strip
6.4.10
Berm
6.5.1
Vegetated Roof
6.5.2
Capture and Re-use
6.6.1
Constructed Wetlands
6.6.2
Wet Pond / Retention Basin
6.6.3
Dry Extended Detention Basin
6.6.4
Water Quality Filters
6.7.1
Riparian Buffer Restoration
6.7.2
Landscape Restoration / Reforestation
6.7.3
Soil Amendment
6.8.1
Level Spreader
6.8.2
Special Storage Areas
Other
26,020
Total Structural Volume (ft
3
):
26,020
Structural Volume Requirement (ft
3
):
25,913
DIFFERENCE
107
* Complete BMP Design Checklist for each measure proposed
Note: rovide supporting Volume Calculations for each Structural BMP
(Required Control Volume minus Non-structural Credit)
WORKSHEET 5 . STRUCTURAL BMP VOLUME CREDITS
Non-structural Volume Credit (ft
3
) -
from Worksheet 3
:
Required Control Volume (ft
3
) -
from Worksheet 4
:
Structural Volume Reqmt (ft
3
)
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Appendix D
Supporting Calculations for Worksheet 5: Part 1 Structural BMPs
Design Volume Calculations for Structural, Volume-Reduction BMPs
1. Infiltration Trenches
:
Storage Volume
= Area x Depth to overflow x Void Space in Stone
= 1,500 ft
2
x 1.0 ft x 40%
= 600 ft
3
Infiltration Volume for “Volume Abstraction” in Routing Process:
= Infiltration Rate x Infiltration Area x Infiltration Period (assume 6 hours)
= 1/2 in/hour x 1,500 ft
2
x 6 hr x (1/12) ft/in
= 375 ft
3
Total “Volume Abstraction”
= Storage Volume + Infiltration Volume
= 600 ft
3
+ 375 ft
3
=
975 ft
3
2. Rain Gardens
Storage Volume
= Surface Storage + Soil Storage*
= (Area x Depth) + (Area x Soil Depth x 10%)
= (1,820 ft
2
x 1.0 ft) + (1,820 ft
2
x 1 ft x 10%)
= 2,002 ft
3
Infiltration Volume for “Volume Abstraction” in Routing Process:
= Infiltration Rate x Infiltration Area x Infiltration Period (assume 6 hours)
= 1/2 in/hour x 1,820 ft
2
x 6 hr x (1/12) ft/in
= 455 ft
3
Total “Volume Abstraction”
= Storage Volume + Infiltration Volume
= 2,002 ft
3
+ 455 ft
3
=
2,457 ft
3
Structural Volume Storage per Lot = Infiltration Trench + Rain Garden = 2,602 ft
3
* Assume 1 ft depth modified soil with 10% void space for water retention.
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Appendix D
The 2-Year Runoff Volume increase must be met in BMPs designed in accordance
Yes
with Manual Standards
No
Total Site Impervious Area may not exceed
1 acre.
Yes
Maximum Development Area is
10 acres
.
Yes
Maximum site impervious cover cannot be greater than 50%.
Yes
No more than 25% Volume Control can be in Non-structural BMPs
Yes
Infiltration BMPs must have an infiltration rate of 0.5 in/hr.
Site Area
Percent
Impervious
Total
Impervious
10 acre
10%
1 acre
5 acre
20%
1 acre
2 acre
50%
1 acre
1 acre
50%
0.5 acre
0.5 acre
50%
0.25 acre
The following conditions must be met for exemption from peak rate analysis for small
sites under CG-1:
WORKSHEET 6 . SMALL SITE / SMALL IMPERVIOUS AREA
EXCEPTION FOR PEAK RATE MITIGATION CALCULATIONS
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Appendix D
Peak Rate Calculations for Structural BMP Case
s discussed previously, the residential subdivision was modeled for peak rate
able D-1.
Properties of Infiltration Trenches
able D-2.
Properties of Rain Gardens
d
r for modeling purposes. The properties of the
omposite BMP” are given in Table D-3.
Individual
Total
Individual
Total
Individual
Total
A
mitigation using two techniques: Composite BMP and Travel Time Adjustment with
Volume Diversion. As a comparison, dry detention basins were also simulated for
conventional peak rate control as well as for extended detention. The properties of the
infiltration trenches and rain gardens as shown in tables D-1 and D-2.
T
Stage (ft) Area (SF) Storage
(AF)
Storage
(AF)
Discharge
(cfs)
Discharge
(cfs)
Infiltration
(cfs)
Infiltration
(cfs)
0.00
1,500
0.000
0.000
0.00
0.00
0.00
0.00
0.01
1,500
0.000
0.001
0.02
0.17
0.02
0.17
0.1
1,500
0.001
0.014
0.02
0.17
0.02
0.17
0.2
1,500
0.003
0.028
0.02
0.17
0.02
0.17
0.3
1,500
0.004
0.041
0.02
0.17
0.02
0.17
0.4
1,500
0.006
0.055
0.02
0.17
0.02
0.17
0.5
1,500
0.007
0.069
0.02
0.17
0.02
0.17
0.6
1,500
0.008
0.083
0.02
0.17
0.02
0.17
0.7
1,500
0.010
0.096
0.02
0.17
0.02
0.17
0.8
1,500
0.011
0.110
0.25
2.54
0.02
0.17
0.9
1,500
0.012
0.124
0.69
6.88
0.02
0.17
1
1,500
0.014
0.138
1.25
12.49
0.02
0.17
T
For the Composite BMP method, the infiltration trenches and rain gardens are summe
(AF)
(AF)
(cfs)
(cfs)
(cfs)
(cfs)
0.00
1,820
0.000
0.000
0.00
0.00
0.00
0.00
0.01
1,820
0.005
0.046
0.02
0.21
0.02
0.21
0.1
1,820
0.008
0.084
0.02
0.21
0.02
0.21
0.2
1,820
0.013
0.125
0.02
0.21
0.02
0.21
0.3
1,820
0.017
0.167
0.02
0.21
0.02
0.21
0.4
1,820
0.021
0.209
0.02
0.21
0.02
0.21
0.5
1,820
0.025
0.251
0.02
0.21
0.02
0.21
0.6
1,820
0.029
0.292
0.38
3.81
0.02
0.21
0.7
1,820
0.033
0.334
1.02
10.21
0.02
0.21
0.8
1,820
0.038
0.376
1.78
17.81
0.02
0.21
0.9
1,820
0.042
0.418
2.74
27.41
0.02
0.21
1
1,820
0.046
0.460
3.06
30.61
0.02
0.21
Stage (ft) Area (SF)
Individual
Storage
Total
Storage
Individual
Discharge
Total
Discharge
Individual
Infiltration
Total
Infiltration
into a single combined storage reservoi
“C
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Appendix D
Table D-3.
Properties of Composite Infiltration Trench/Rain Garden
the U.S. Army Corp of Engineers’ Hydrologic
(May 28, 2003). The model schematic for
the impervious and
ervious areas are routed separately to the Composite Storage Reservoir (“Comp.
All scenarios were modeled using
Modeling System (HEC-HMS) Version 2.2.2
0.00
0.000
0.00
0.00
0.01
0.047
0.38
0.38
0.1
0.097
0.38
0.38
0.2
0.153
0.38
0.38
0.3
0.208
0.38
0.38
0.4
0.264
0.38
0.38
0.5
0.320
0.38
0.38
0.6
0.375
3.98
0.38
0.7
0.431
10.38
0.38
0.8
0.486
20.35
0.38
0.9
0.542
34.29
0.38
1
0.597
43.10
0.38
the Composite BMP method is shown in Figure D-1. Notice that
p
RG&Trench”) and then the runoff being infiltrated is removed through a Composite
Infiltration Rate (“Compos. Infilt”) based on the design infiltration rate of the BMPs.
Figure D-1.
Model Schematic for Composite BMP
The model schematic for ‘Travel Time Adjustment with Volume Diversion’ method is
shown in Figure D-2. Figures D-3 and D-4 shown the model setups for conventional
ly.
Stage (ft)
Total
Storage
(AF)
Total
Discharge
(cfs)
Total
Infiltration
(cfs)
peak rate control and extended detention respective
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Appendix D
Figure D-2.
Model Schematic for ‘Travel Time Adjustment with Volume Diversion’
method
Figure D-3.
Model Schematic for conventional peak rate control
Figure D-4.
Model Schematic for extended detention
363-0300-002 / December 30, 2006
Page 17 of 25

Pennsylvania Stormwater Best Management Practices Manual
Appendix D
In the ‘Travel Time Adjustment with Volume Diversion’ method the Time of
able D-4.
Time of Concentration Adjustment
ion, the volume-reduction BMPs will also
ignificantly reduce the amount of runoff being discharged by the site. In order to
n Trenches and Rain Gardens
MPs
The volume control provided by infiltration BMPs, even for the 10- and 100-
duced downstream extended detention requirements when using
ed from 45,000 to 25,000 for the “Composite BMP” method
rsion” method
The
Peak Flow
Volume Control
Ave. Residence Time/
Concentration was increased by the average residence time of the volume-reducing
BMPs that were not be routed. The residence time for the 100-year storm was used to
be conservative. The residence time is simply calculated by dividing the storage volume
of the BMPs by the unmitigated post-development 100-year peak flow. As shown in
Table D-4, this results in an average residence time of 9.1 minutes. The post-
development time of concentration was increased by this amount in the model to
account for the slowing effect of the volume-reduction BMPs.
T
Storm Event
without BMPs
(cfs)
BMP Storage
(CF)
Time of Conc.
Increase (min.)
100
47.5
26,020
9.1
In addition to increasing the time of concentrat
s
account for this in the ‘Travel Time Adjustment with Volume Diversion’ method and
“volume abstraction” is incorporated into the model. The runoff simulated in the model is
abstracted or “diverted” until the storage and infiltration volume of the BMPs is full. After
that point, the diversion has no effect on the runoff rate or volume. The total volume
abstracted in the model is calculated in Table D-5.
Table D-5.
Total Volume Abstraction from Infiltratio
The results for the various scenarios are shown in Table D-6. Important results to note
BMP Type
Bottom
Area (SF)
Infiltration
Rate (in./hr)
Period Prior to
Peak Runoff (hr)
Volume
(CF)
Volume
(CF)
Abstraction
(CF)
Infilt. Trench
15,000
0.5
6
3,750
6,000
9,750
Rain Garden
18,200
0.5
6
4,550
20,020
24,570
TOTAL
33,200
---
---
8,300
26,020
34,320
Total
Design
Applied Infiltration
Infiltration
Storage
Total
Volume
include:
The drastic increase in runoff for both cases without volume-reduction
B
year storms
The re
infiltration BMPs:
o
Reduc
o
Reduced from 45,000 to 16,000 for the “Travel Time Adjustment with
Volume Dive
improved peak rate control with volume-reduction BMPs
363-0300-002 / December 30, 2006
Page 18 of 25

Pennsylvania Stormwater Best Management Practices Manual
Appendix D
Table D-6.
Modeling Results for all scenarios
Runoff Volume Results
Storm
Event
Pre-Dev.
Runoff
(in.)
Post-Dev.
Runoff
(in.)
Change
(%)
Post-Dev.
Runoff
(in.)
Change
(%)
1
0.43
1.09
153%
0.23
-47%
2
0.64
1.39
117%
0.47
-27%
10
1.57
2.62
67%
1.58
1%
100
2.71
3.96
46%
2.86
6%
Peak Rate for Detention - 40,000 CF Convential Basin & 45,000 CF E.D. Basin*
Storm
Event
Pre-Dev.
Peak (cfs)
Post-Dev.
Peak (cfs)
Post-Dev.
Peak w/
Basin
(cfs)
Change
(%)
Post-Dev.
Peak w/ ED
(cfs)
Change
(%)
1
3.9
12.2
3.0
-23.1%
1.14
-62.0%
2
6.3
16.0
5.0
-20.6%
2.6
-48.0%
10
17.6
31.2
15.4
-12.6%
14.2
-7.8%
100
30.9
47.5
29.9
-3.2%
29.2
-2.3%
* Extended detention flow target for 1-year storm is 1.15 cfs from WS 9
Peak Rate for Volume Control Approaches (Trenches/RGs & Reduced Detention)
Storm
Event
Pre-Dev.
Peak (cfs)
Post-Dev.
Peak (cfs)
Post-Dev.
Peak w/
Volume
Control
(cfs)
Change (%)
Post-Dev.
Peak w/
Volume
Control
(cfs)
Change
(%)
1
3.9
12.2
1.2
-69.2%
0.22
-81.7%
2
6.3
16.0
3.0
-52.4%
0.7
-78.3%
10
17.6
31.2
14.9
-15.5%
11.8
-20.8%
100
30.9
47.5
30.8
-0.3%
30.4
-1.3%
Conv. Basin (40,000 CF) Infilt. BMPs (26,000 CF)
Composite Volume BMPs
& 25,000 Det.
TOC Adj./ Vol. Abstract.
& 16,000 CF Det.
Conventional Basin
Extended Det. Basin
363-0300-002 / December 30, 2006
Page 19 of 25

Pennsylvania Stormwater Best Management Practices Manual
Appendix D
Part 2 – Structural and Non-structural BMP Design
Date:
Project Name:
10 Lot Residential Subdivision
Municipality:
Smith Township
County:
Blair County
Total Area (acres):
10
Major River Basin:
http://www.dep.state.pa.us/dep/deputate/watermgt/wc/default.htm#newtopics
Watershed:
Purdy Creek
Sub-Basin:
Nearest Surface Water(s) to Receive Runoff:
Tributary to Purdy Creek
Chapter 93 - Designated Water Use:
HQ
http://www.pacode.com/secure/data/025/chapter93/chap93toc.html
Impaired according to Chapter 303(d) List?
Yes
http://www.dep.state.pa.us/dep/deputate/watermgt/wqp/wqstandards/303d-Report.htm
No
X
List Causes of Impairment:
Is project subject to, or part of:
Municipal Separate Storm Sewer System (MS4) Requirements?
Yes
No
X
Existing or planned drinking water supply?
Yes
No
X
If yes, distance from proposed discharge (miles):
Approved Act 167 Plan?
Yes
No
X
Existing River Conservation Plan?
Yes
http://www.dcnr.state.pa.us/brc/rivers/riversconservation/planningprojects/
No
X
Worksheet 1. General Site Information
INSTRUCTIONS: Fill out Worksheet 1 for each watershed
http://www.dep.state.pa.us/dep/deputate/watermgt/wc/Subjects/StormwaterManagement
/Approved_1.html
http://www.dep.state.pa.us/dep/deputate/watermgt/wc/Subjects/StormwaterManagement
/GeneralPermits/default.htm
363-0300-002 / December 30, 2006
Page 20 of 25

Pennsylvania Stormwater Best Management Practices Manual
Appendix D
363-0300-002 / December 30, 2006
Page 21 of 25
INSTRUCTIONS:
YES
3
3
33
PROTECTED
AREA (Ac.)
EXISTING NATURAL
SENSITIVE RESOURCE
Worksheet 2. Sensitive Natural Resources
Waterbodies
1. Provide Sensitive Resources Map according to non-structural BMP 1.1 in
Section 5.0 Non-Structural BMPs. This map should identify waterbodies,
floodplains, riparian areas, wetlands, woodlands, natural drainage ways, steep
slopes, and other sensitive natural features.
2. Summarize the existing extent of each sensitive resource in the Existing
Sensitive Resources Table (below, using Acres).
3. Summarize Total Protected Area as defined under BMPs in Section 5.0.
4. Do not count any area twice. For example, an area that is both a floodplain
and a wetland may only be considered once.
Natural Drainage Ways
Steep Slopes, 15% - 25%
TOTAL AREA
(Ac.)
MAPPED?
yes/no/n/a
Floodplains
Riparian Areas
Wetlands
Woodlands
Steep Slopes, over 25%
Other:
Other:
TOTAL EXISTING:

Pennsylvania Stormwater Best Management Practices Manual
Appendix D
363-0300-002 / December 30, 2006
Page 22 of 25
5.1
0Ac.
5.2
0Ac.
5.6
3Ac.
TOTAL
3
Ac.
Site Area
minus
Protected
Area
=
10
-
3
=
5.3 Protect/Utilize Natural Flow Paths
Flow Path/Depression NA
ft
2
x 1/4" x 1/12
=
0
ft
3
5.7 Minimum Soil Compaction
Lawn
105,006
ft
2
x 1/4" x 1/12
=
2,188
ft
3
Meadow
25,240
ft
2
x 1/3" x 1/12
=
701
ft
3
3.3 Protect Existing Trees
For Trees within 100 feet of impervious area:
Tree Canopy
NA
ft
2
x 1/2" x 1/12
=
0
ft
3
For Trees within 20 feet of impervious area:
Tree Canopy
NA
ft
2
x 1"
x 1/12
=
0
ft
3
5.1 Disconnect Roof Leaders to Vegetated Areas
For Runoff directed to areas protected under 3.1 and 3.2
Roof Area
NA
ft
2
x 1/3" x 1/12
=
0
ft
3
For all other disconnected roof areas
Roof Area
NA
ft
2
x 1/4" x 1/12
=
0
ft
3
5.2 Disconnect Non-Roof impervious to Vegetated Areas
For Runoff directed to areas protected under 3.1 and 3.2
Impervious Area NA
ft
2
x 1/3" x 1/12
=
0
ft
3
For all other disconnected roof areas
Impervious Area NA
ft
2
x 1/4" x 1/12
=
0
ft
3
2,889
ft
3
* For use on Worksheet 5
Stormwater Management Area
This is the area that requires
stormwater management
TOTAL NON-STRUCTURAL VOLUME CREDIT*
VOLUME CREDITS
7
Worksheet 3. Nonstructural BMP Credits
Area of Protected Sensitive/Special Value Features (see WS 2)
Area of Riparian Forest Buffer Protection
Area of Minimum Disturbance/Reduced Grading
PROTECTED AREA

Pennsylvania Stormwater Best Management Practices Manual
Appendix D
363-0300-002 / December 30, 2006
Page 23 of 25
PROJECT:
10 Lot Subdivision
Drainage Area:
1
(acres)
2-Year Rainfall:
2.8 in
Total Site Area:
10
acres
Protected Site Area:
3
acres
Stormwater Management Area:
7
acres
(From Worksheet 3)
Existing Conditions:
Cover Type
Soil
Area
Area
CN
S
Q
Runoff
1
Runoff
Volume
2
Type
(sf)
(ac)
(in)
(ft
3
)
Woodland Not Included
Meadow
C
304,920
7.0
71
4.08
0.65
16,469
Impervious
C
-
0.0
TOTAL:
7
16,469
Developed Conditions:
Cover Type
Soil
Area
Area
CN
S
Q
Runoff
1
Runoff
Volume
2
Type
(sf)
(ac)
(in)
(ft
3
)
Buildings
C
45050
1.0
98
0.20
2.57
9,645
Roads, Driveways, walks
C
24619
0.6
98
0.20
2.57
5,271
Lawn
C
90006
2.1
79
2.66
1.04
7,834
Detention Basin
C
15,000
0.3
79
2.66
1.04
1,306
Lawn with Minimal Comp
C
105,005
2.4
79
2.66
1.04
9,139
Meadow
C
25,240
0.6
71
4.08
0.65
1,363
Woods Not Included
C
TOTAL:
7
34,557
2-Year Volume Increase (ft3):
18,088
2-Year Volume Increase = Developed Conditions Runoff Volume - Existing Conditions Runoff Volume
= 34,5577 - 16,469 = 18,088 ft3
1. Runoff (in) = Q = (P - 0.2S)
2
/ (P+ 0.8S) where
P = 2-Year Rainfall (in)
S = 1000/ CN
2. Runoff Volume (CF) = Q x Area x 1/12 x 43,560 ft
2
/acre
Q = Runoff (in)
Area =
Stormwater Management Area (ac)
from Worksheet 3
Note: Runoff Volume must be calculated for EACH land use type and soil.
he use of a weighted CN value for volume calculations is not acceptable.
WORKSHEET 4 . CHANGE IN RUNOFF VOLUME FOR 2-YR STORM EVENT
S = (1000/CN)-10
T

Pennsylvania Stormwater Best Management Practices Manual
Appendix D
PROJECT:
10 Lot Subdivision
SUB-BASIN:
1
18,088
-
2,889
15,199
Proposed BMP*
Area
Storage
Volume
(ft
2
)
(ft
3
)
6.4.1
Porous Pavement
6.4.2
Infiltration Basin
6.4.3
Infiltration Bed
6.4.4
Infiltration Trench
10
3,500
6.4.5
Rain Garden/Bioretention
10
11,770
6.4.6
Dry Well / Seepage Pit
6.4.7
Constructed Filter
6.4.8
Vegetated Swale
6.4.9
Vegetated Filter Strip
6.4.10
Berm
6.5.1
Vegetated Roof
6.5.2
Capture and Re-use
6.6.1
Constructed Wetlands
6.6.2
Wet Pond / Retention Basin
6.6.3
Dry Extended Detention Basin
6.6.4
Water Quality Filters
6.7.1
Riparian Buffer Restoration
6.7.2
Landscape Restoration / Reforestation
6.7.3
Soil Amendment
6.8.1
Level Spreader
6.8.2
Special Storage Areas
Other
15,270
Total Structural Volume (ft
3
):
15,270
Structural Volume Requirement (ft
3
):
15,199
DIFFERENCE
71
* Complete BMP Design Checklist for each measure proposed BMP
NOTE: Provide supporting Volume Calculations for each Structural BMP
(Required Control Volume minus Non-structural Credit)
WORKSHEET 5 . STRUCTURAL BMP VOLUME CREDITS
Non-structural Volume Credit (ft
3
) -
from Worksheet 3
:
Required Control Volume (ft
3
) -
from Worksheet 4
:
Structural Volume Reqmt (ft
3
)
363-0300-002 / December 30, 2006
Page 24 of 25

Pennsylvania Stormwater Best Management Practices Manual
Appendix D
363-0300-002 / December 30, 2006
Page 25 of 25
Supporting Calculations for Worksheet 5: Part 2 – Structural and Non-Structural
BMP Design
Volume Credits for Structural BMPs
1. Infiltration Trench
:
Storage Volume
= Area x Depth to overflow x Void Space in Stone
= 875 ft
2
x 1.0 ft x 40%
= 350 ft
3
Infiltration Volume for “Volume Abstraction” in Routing Process:
= Infiltration Rate x Infiltration Area x Infiltration Period (assume 6 hours)
= 1/2 in/hour x 875 ft
2
x 6 hr x (1/12) ft/in
= 219 ft
3
Total “Volume Abstraction”
= Storage Volume + Infiltration Volume
= 350 ft
3
+ 219 ft
3
=
569 ft
3
2. Rain Garden
:
Storage Volume
= Surface Storage + Soil Storage*
= (Area x Depth to Overflow) + (Area x Soil Depth x 10%)
= (1,070 ft
2
x 1.0 ft) + (1,070 x 1 ft x 10%)
= 1,177 ft
3
Infiltration Volume for “Volume Abstraction” in Routing Process:
= Infiltration Rate x Infiltration Area x Infiltration Period (assume 6 hours)
= 1/2 in/hour x 1,070 ft
2
x 6 hr x (1/12) ft/in
= 268 ft
3
Total “Volume Abstraction
= Storage Volume + Infiltration Volume
= 1,177 ft
3
+ 268 ft
3
=
1,445 ft
3
Structural Volume Storage per Lot = Infiltration Trench + Rain Garden = 1,527 ft
3
* Assume 1 ft depth modified soil with 10% void space for water retention.

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