villanova urban stormwater partnership chapter 3 stormwater management principles and recommended...

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Villanova Urban Stormwater Partnership

Chapter 3

Stormwater Management Principlesand Recommended Control Guidelines

Robert G. Traver, Ph.D., PE, D. WRERobert G. Traver, Ph.D., PE, D. WREDepartment of Civil and Environmental EngineeringDepartment of Civil and Environmental EngineeringDirector, Villanova Urban Stormwater PartnershipDirector, Villanova Urban Stormwater Partnership



Mission Statement

The mission of the Villanova Urban Stormwater Partnership is to advance the evolving comprehensive stormwater management field and to foster the development of public and private Partnerships through research on innovative SWM Best Management Practices, Directed Studies, Technology Transfer and Education.

Research and directed studies will emphasize comprehensive watershed stormwater management planning, implementation, and evaluation.

Technology transfer will provide tools, guidance and education for the professional.

Partnership Goal is to promote cooperation amongst the private, public and academic sectors.

www. villanova.edu/VUSPwww. villanova.edu/VUSP



Partners:

Members:



Disclaimer

• What follows is my interpretation of Chapter 3 as a member of the BMP Manual Oversight committee

– It is not PaDEP’s official position

– It is not the VUSP’s Corporate Partners position – like other groups, the VUSP made many recommendations during the process. Some were, and some were not included.



BMP Manual Vs Model Ordinance

• BMP Manual

– Guidance Option and Tools

– Living document

– Chapter 3 – relates to Ordinance

• Model Ordinance

– Act 167 start point

– Assistance to municipalities

– Relates to chapter 3



Chapter 3 - Introduction

This Chapter provides guidance for municipalities striving to improve their

stormwater management programs. It presents stormwater

management principles and recommends site control guidelines to

address volume, water quality and flow rate. These guidelines can

serve as the basis for municipal stormwater regulation……. All

municipalities, regardless of their specific setting, are encouraged to

enact the most comprehensive stormwater management ordinances

possible. They should also work with their watershed neighbors to

integrate their individual municipal actions within the watershed

as a whole.



Model Ordinance

POLICY:

The Stormwater Management Program, and other DEP staff, will

recommend to counties that they use this model ordinance as a template

for developing municipal stormwater management ordinances when

preparing Act 167 Stormwater Management Plans, to municipalities without

an otherwise suitable stormwater management ordinance that they adapt

and enact this model ordinance to meet NPDES MS4 permitting

requirements, and that other municipalities may adapt and enact this model

ordinance.



Recommended Site Control Guidelines• Water Volume

– Focus

• Impervious and Compacted soils and smaller storms

– Baseflow - Water Quality - Stream Bank Protection

• Peak Flows

• Flooding (assisted by Volume Reduction)

• Water Quality

• Additional Guidelines



Volume Controls• Goals

– Protect stream channel morphology;

– Maintain groundwater recharge;

– Prevent downstream increases in flooding; and

– Replicate the natural hydrology on site before development to the greatest greatest

extent possible.extent possible.

• Alternatives

– Infiltration;

– Capture and Reuse; and

– Vegetation systems that provide ET, returning rainfall to the atmosphere



Runoff Volume Increase from DevelopmentDifference Between Pervious Woodland (B Soil) and Impervious Surface

0.951.26

2.60

3.043.37

4.044.30

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

1 inchRainfall

1.5 inchRainfall

2-yr Storm(3.27")

5-yr Storm(4.09")

10-yr Storm(4.78")

50-yr Storm(6.61")

100-yrStorm (7.5")

Figure 3-1. Runoff Volume Increase from Impervious Surfaces - B Soils.

Runo

ff (in

ches

)

Runoff Values for the 1" and 1.5" storms generated using the Small Storm Hydrology Methodology (Pitt, 1994) and runoff values for the storms generated using the SCS Runoff Curve Number Method (CN-98 for impervious and CN=60 for woods, B soils, Fair Condition).

Runoff Volume Increase from DevelopmentDifference Between Pervious Woodland (C Soil) and Impervious Surface

0.951.13

2.012.26 2.43

2.77 2.89

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

1 inchRainfall

1.5 inchRainfall

2-yr Storm(3.27")

5-yr Storm(4.09")

10-yr Storm(4.78")

50-yr Storm(6.61")

100-yrStorm (7.5")

Figure 3-2 . Runoff Volume Increase from Impervious Surfaces - C Soils

Runo

ff (in

ches

)

Runoff Values for the 1" and 1.5" storms generated using the Small Storm Hydrology Methodology (Pitt, 1994) and runoff values for the storms generated using the SCS Runoff Curve Number Method (CN-98 for impervious and CN=73 for woods, C soils, Fair Condition).

VOLUME CONTROLS

B C



Volume Control

Design Storm Method

- may require more complex and detailed analyses

- greater opportunity to select stormwater controls that require fewer

resources to construct and operate

Simplified Method

- fewer computations and less design effort

- slightly larger stormwater controls

- for sites with less then an acre of disturbance

- NOT allowed when routing required



Volume Control – Design Storm

• Do not increase the post-development total runoff volume for all storms

equal to or less than the 2-year/24-hour event.

– NOTE – the Model Ordinance uses REAL preexisting

• Existing (pre-development) non-forested pervious areas must be

considered meadow (good condition) or its equivalent.

• Twenty (20) percent of existing impervious area, when present, shall be

considered meadow (good condition) in the model for existing conditions

for redevelopment



Design Storm Method – Scientific Basis

– The 2-year event provides stream channel protection and water quality

protection for the relatively frequent runoff events across the state;

– Volume reduction BMPs based on this standard will provide a storage

capacity to help reduce the increase in peak flow rates for larger runoff

events;

– In a natural stream system in Mid-Atlantic States, the bank full stream flow

occurs with a period of approximately 1.5 years. If the runoff volume from

storms less than the 2-year event are not increased, the fluvial impacts on

streams will be reduced;

– The 2-year storm is well defined and data are readily accessible for use in

stormwater management calculations.



Design Storm Method – Practical Consideration

– The 2-year 24-hour events for precipitation and runoff are well defined.

– Data and software are readily accessible for application of 2-year 24-

hour events in stormwater management calculations.



Volumes of Infiltration - 2 Year Storm

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 0.5 1 1.5 2 2.5 3 3.5 4

Rainfall (in)

Ru

no

ff (

in)

61 - Grass

98 - Impervious

92 - Commercial

85 - 1/8 acre

72 - 1/4 acre

72 - 1/3 acre

70 - 1/2 acre

68- 1 acre

65 - 2 acre

B soils

Impervious

Grass



Design Storm Method – Dr T’s Opinion & FAQ’s• Is this a lot of water?

– Depends on site

• Predevelopment Land Use

• Soil conditions

• Ability to reduce problem

• Is it harder then Detention?

– Yes – But detention doesn’t work.

• Is it replicating nature?

– Depends how much you ET!

– Surface Water – Probably

– Ground Water – Depends on how much

ET you replicate – Rock beds definitely no

• Is it desirable everywhere?

– Lots of places Yes

– Some places No

• Where is this easy?

– Single Family homes (low % Imp)

– Infiltrating soils

• Where is this difficult?

– High % Impervious

– Clay soils (brownfields, etc)

• Is maintenance a concern?

– YES Maintenance is a critical design

component.



Simplified Method

• Stormwater facilities shall be sized to capture at least the first two inches (2”) of runoff

from all contributing impervious surfaces.

• At least the first one inch (1.0”) of runoff from new impervious surfaces shall be

permanently removed from the runoff flow — i.e. it shall not be released into the surface

Waters of this Commonwealth. Removal options include reuse, evaporation,

transpiration, and infiltration.

• Wherever possible, infiltration facilities should be designed to accommodate infiltration

of the entire permanently removed runoff; however, in all cases at least the first one-half

inch (0.5”) of the permanently removed runoff should be infiltrated

– Note – detention / retention BMPS must empty in between 24 and 72 hours

Note < 1 acre disturbed area



Simplified Method – RT’s Opinion and FAQ’s

• Use where it makes sense $$$

• Use - retrofits, small disturbed areas

• Where you aren't planning to develop

a hydrologic model

• Use where you have space

– Wetlands / Wet ponds

• More versatile then Design Storm

Method, but lots of times Design

Storm is easier

– Believe “environmentally

equivalent” to Design Storm

Method

• Is it harder then Detention?

– Yes – But detention doesn’t work.

• Is it applicable anywhere?

– NO! < 1 Acre disturbed restriction

• (PaDEP Restraint)

• Is maintenance a concern?

– YES Needs to be designed in.

However if focused on green

designs, (bioretention, bioinfiltration,

green roofs), should have less

maintenance and operational costs



BioInfiltration Traffic Island - Watershed

• 1.3 Ac

• 46% Impervious



Bioinfiltration Traffic Island (PA Growing Greener Grant -2001)• Watershed – 1.3 acres• Bowl Size - .3 Watershed Inches - .6 off impervious.• NEVER any runoff until over 1”• Land Use - The watershed includes a student parking lot, roadway and lawn

areas. It is approximately 50% impervious.• Design….. The island is designed to control smaller storms (1 - 1.5 inches);

infiltrating runoff, reducing downstream stormwater volumes, stream bank erosion, and nonpoint source pollution to the headwaters of the Darby Creek..



William Heasom



Hydrologic Performance

2003 - Traffic Island

0

200

400

600

800

1,000

1,200

1,400

JA

N

FE

B

MA

R

AP

R

MA

Y

JU

N

JU

L

AU

G

SE

P

OC

T

NO

V

DE

C

Ave

rag

e

Vo

lum

e C

ub

ic M

ete

rs

Rainfall Volume

BMP Inflow

BMP Outflow

68 % removal of Runoff88 % removal of Rainfall





0

200

400

600

800

1,000

1,200

1,400

Vo

lum

e C

ub

ic M

ete

rs

Rainfall Volume

BMP Inflow

BMP Outflow

59 % removal of Runoff80 % removal of Rainfall





0200400600800

100012001400

Vol

ume

Cub

ic M

eter

s Rainfall Volume

BMP Inflow

BMP Outflow

71% removal of Runoff86% removal of Rainfall



October 6-8th – BioInfiltration TI

Bill Heasom

Oct 06, 2005 Storm

444.00

444.50

445.00

445.50

446.00

446.50

447.00

10/6/05 0:00 10/7/05 0:00 10/8/05 0:00 10/9/05 0:00 10/10/05 0:00 10/11/05 0:00 10/12/05 0:00

Time

Ele

va

tio

n (

ft)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Ra

infa

ll (

in/5

min

)

WS El (Oct Param) Rainfall

Weir El = 445.92

6.02”



October 6-8th– BioInfiltration Traffic Island

Bill Heasom

Oct 06, 2005 Rainfall Vs Inflow and Outflow

0

1

1

2

10/8/050:00

10/8/052:24

10/8/054:48

10/8/057:12

10/8/059:36

10/8/0512:00

10/8/0514:24

10/8/0516:48

10/8/0519:12

10/8/0521:36

10/9/050:00

Time

Per

vio

us

Infl

ow

( f

t^3/

sec)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

Inflow Surface outflow Rainfall

Rainfall (in / 5 min)



Oct 06, 2005 Rainfall Vs Inflow and Outflow

0.00

0.20

0.40

0.60

0.80

1.00

1.20

10/8/0512:00

10/8/0513:12

10/8/0514:24

10/8/0515:36

10/8/0516:48

10/8/0518:00

10/8/0519:12

10/8/0520:24

10/8/0521:36

10/8/0522:48

10/9/050:00

Time

Per

vio

us

Infl

ow

( f

t^3/

sec)

Inflow Surface outflow

Rainfall (in / 5 min)



Flow Exposure

0

5

10

15

20

25

30

35

0.00 0.20 0.40 0.60 0.80 1.00

Flow

Tim

e H

ou

rs

Outflow

Inflow



Example 2005 TI

• 77 Events• 48” Rainfall• (not all snow included)• 7 Events Overflowed• Yearly Summary5.5” - Overflow2.5” Pre (Meadow B)

Storm Time Begin Time End Rainfall WF Modeled

(in) (ft^3)59 10/6/05 17:35 10/9/05 5:40 6.01 1170526 4/1/05 22:30 4/3/05 14:20 4.04 429746 7/7/05 21:50 7/8/05 13:55 2.2 127274 12/15/05 19:40 12/16/05 5:55 2.06 2196

6 1/13/05 22:35 1/14/05 11:50 1.95 229115 2/14/05 12:05 2/15/05 1:20 1.95 196725 3/27/05 17:45 3/31/05 12:20 1.62 038 6/3/05 4:25 6/4/05 9:00 1.54 NA62 10/21/05 7:25 10/22/05 21:05 1.48 NA63 10/24/05 17:55 10/26/05 3:45 1.46 NA56 9/15/05 5:15 9/17/05 19:20 1.32 NA71 11/29/05 15:50 11/30/05 0:50 1.13 20224 3/23/05 4:45 3/23/05 22:20 1.06 060 10/10/05 8:10 10/14/05 23:05 1.06 068 11/21/05 15:45 11/22/05 11:50 1.06 0

3 1/5/05 0:50 1/6/05 17:30 1.05 012 2/5/05 14:20 2/7/05 7:00 1.05 047 7/15/05 16:10 7/18/05 4:20 1.03 0



Comparison to BMP Manual

• Design Storm Method

– Would not meet criteria if preconstruction land use was all woods or

meadow. Probably would as there was paved areas on the original site.

• Simplified Method

– Would meet the criteria as we get no runoff with less then 2” or rainfall.

However the site is 1.3 acres so not applies.

• Design Standards

– Site foot print is MUCH smaller then 1:5 ratio

– Ponding depth is well over a foot.



Retention and Detention Considerations

• Infiltration areas should be spread out and located in the sections of

the site that are most suitable for infiltration.

• In all cases, retention and detention facilities should be designed to

completely drain water quality volumes including both the

permanently removed volume and the extended detention volume

over a period of time not less than 24 hours and not more than 72

hours from the end of the design storm.



Peak Flow

• Do not increase the peak rate of discharge for the 1-year through 100-year events (at minimum); as necessary, provide additional peak rate control as required by applicable and approved Act 167 plans.



Water Quality

• Achieve an 85 percent reduction in post-development particulate associated

pollutant load (as represented by Total Suspended Solids), an 85 percent

reduction in post-development total phosphorus loads, and a 50 percent

reduction in post-development solute loads (as represented by NO3-N), all

based on post-development land use.

Generally assumed met with either of the control measures – may require use of swales, or other “Green” practices”



What if I cant do Design Storm Method and my project too big for the Simplified Method?

RT’s opinion

• GET YOUR COUNTY TO DEVELOP A

TAILORED ACT 167 PLAN TO YOUR

REGIONAL CHARACTERISTICS!

• Or…

Meet with PaDep to discuss the

problems, and develop an alternate

design to meet the site conditions.

• ALTERNATIVE DESIGN – Minimize problem– Start with Simplified Method– Use Bio-Retention / Infiltration and

Green Roofs– Use Green Extended Detentio– Take credit for infiltration during the

storm– Calculate Pollutant Loadings– PE License– Design for Maintenance– DON’T IGNORE THE ISSUES!



Infiltration “Risk” Idea

Rock Bed

First 1” Runoff goes to Raingarden“Pretreatment”

Next 1+/-” Runoff goes to Rock Bed

As most rainfall is less then 1” This will extend the life of the rock bed as its use would be less frequent, and the runoff would be cleaner

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