creating a new toolbox for site development under emerging

Creating a New Toolbox for Site Development

under Emerging State/Federal Rules

Presented by:

Gregory P. Kacvinsky, P.E.

OHM Advisors

MWEA Annual Conference

June 24, 2014

OHM 34000 Plymouth Road Livonia, Michigan 48150 T 734.522.6711 F 734.522.6427 www.ohm-advisors.com

MDEQ – New MS4 Rules

New permits phased in between 2013-2017

First permits submitted to MDEQ in early 2013

Permits issued by watershed

2010 Census increases coverage areas

MDEQ – Focus on Frequent Hydrology

MDEQ – BMP Maintenance

• Treatment for first 1 inch of runoff – Not necessarily infiltration

– Could be mechanical separation, filtration

• TSS Reduction (80% reduction or 80 ppm max concentration) – A bit onerous, especially, 80 ppm max

– Typically, treating first inch of rainfall helps to achieve this goal

– 80% reduction probably an easier goal than 80 ppm max

• 2-year volume control – May be most significant requirement in new rule

– Generally requires infiltration unless site conditions prevent infiltration

– Relaxed and clear standard for redevelopment

• What tool(s) can we use to demonstrate site-specific compliance??


• Rules based on measures successfully tested in other states (i.e. Maryland,

Wisconsin, Minnesota, Washington, etc.) through years of regulation and

compliance.

• This is the first set of rules that really addresses pollution resulting from

development. New rules now address runoff volume, which is a key driver of

stormwater quality and stability of receiving streams.


Planning/Designing for Compliance

Small Storm Hydrology

Modeling Tools

Rainfall Statistics

Designing for water quality (‘first flush’ treatment):

• What is a ‘first flush’ event?


Designing for water quality (‘first flush’ treatment):

• Treat runoff resulting from 90% of precipitation events (MDEQ

requirement)

• In Michigan, this is typically 0.75 – 1.0 inch of rainfall, typically

occurring over a 1-2 hour period

• DEQ Rules – applies to entire area, whether new or

redevelopment

• Municipality can provide statistics to substantiate a different

rainfall depth (other than 1 inch)



First Flush Conventional

“Design Event”

HEC-HMS, TR-55, TR-20 SWMM

• We’re accustomed to designing for a flood control event

• Water quality design requires the use of a much smaller storm

0

1

2

3

4

5

6

0 2 4 6 8 10 12 14 16 18 20 22 24

Inch

es

of

Rai

nfa

ll

Hours Into Storm

Large Storm v Small Storm Hydrology

100-yr 24-hrSCS Type II

1-yr 2-hr Huff1st Quartile


Small Storm Hydrology - TOOLBOX

How do we meet the 1-inch ‘first flush’ requirement?

• Some existing design practices may partially or fully address this

• (e.g. wet detention ponds)

• Rain gardens and bioretention are an obvious tool to meet this objective,

especially in redevelopment areas without wet detention option

• Pervious pavement

• Mechanical separation (swirl chamber)

• Applies to entire site (whether greenfield or redevelopment)

How do we meet the 2-year volume control requirement?

• This will be harder to implement, as many stormwater BMPs are designed

for a smaller event

• Only applies to increased runoff volume, which lowers the hurdle for

redevelopment sites

• Requires storage volume and good in-situ soils

• Modeling approach is critical to demonstrate how BMP(s) will function

• Adapting to this rule is going to have a major impact on the

development community, because it:

• Requires a new skill set for site planning, modeling, and design

• Impacts developable footprint

• Impacts economics of development

Small Storm Hydrology - TOOLBOX

Rainfall Statistics

• Updating stormwater rules should include the consideration of

updated rainfall statistics:

• TP40: 1960 (last 50 years of rainfall not represented)

• Bulletin 70: 1992 (last 20+ years of rainfall not represented)

• NOAA Atlas 14: 2013 (most up-to-date and relevant statistics for Michigan)

Rainfall Statistics

Rainfall Statistics – NOAA Atlas 14

1.00

1.50

2.00

2.50

3.00

3.50

4.00

1 2 4 8 16 32

Tota

l Rai

nfa

ll (i

n)

Duration (hrs)

TP40 5-yr

Bulletin 71 5-yr

NOAA 5-yr

TP40 10-yr

Bulletin 71 10-yr

NOAA 10-yr

TP40 25-yr

Bulletin 71 25-yr

NOAA 25-yr

2h

ou

rs

3 h

ou

rs

6 h

ou

rs

12

ho

urs

24

ho

urs


• 100-yr storm (flood control) statistics have changed significantly

• Comparing TP40 to NOAA Atlas 14 (all storms 24-hr duration):

• Central Oakland County:

• TP40: 4.5 inches

• NOAA14: 5.4 inches (20% increase)

• Central Wayne County:



• Central Washtenaw County:




• Recent analysis of rainfall in Ann Arbor

• Comparing the 1951-1980 and 1981-2010 periods:

• Increase of ~25% in total annual precipitation

• Frequency of the heaviest 1% of storms (as determined for

the 1951-1980 period) has increased by 38%

• From 1958 to 2010:

• Throughout Michigan, both the volume and frequency of

extreme storms have increased

• The magnitude of the largest 1% of storms has

increased by 4%-8%

• The volume of precipitation in the largest 1% of

storms has increased by 37%

Source: (Technical Oversight and Advisory Group (TOAG), June 2014

Rainfall Statistics – Climate Change

Modeling Tools

• The rules are changing, and the old reliable programs may no longer

give us all the tools we need:

• HEC-HMS

• TR-55 / TR-20

• PondPack

• These tools are ok for traditional stormwater design (conveyance and

flood control, but they have limitations):

• No water quality modeling capabilities

• Based on older rainfall statistics

• In most cases, these older tools do not have BMP modeling capabilities

Modeling Tools

• Many free (or inexpensive) tools exist to efficiently size and design

stormwater treatment facilities

• EPA SWMM • ideal for larger developments with complex drainage systems

• EPA National Stormwater Calculator • Ideal for small sites, especially redevelopment areas

• RECARGA • Ideal for sizing and design of rain gardens / bioretention

• WinSLAMM • Ideal when pollutant removal calculations are required

Modeling Tools

• All-in-one modeling solution for hydrology and hydraulics

• Grey Infrastructure (pipes, inlets)

• Flood control (ponds, outlet structures)

• Green Infrastructure

• Can be used to evaluate BMP effectiveness at runoff volume control

and pollutants of concern

• Program is more complex, requiring training and understanding of

hydraulic/hydrologic theory

• LID Module allow for analysis of typical range of structural and non-

structural BMPs

Modeling Tools – EPA SWMM

Modeling Tools – EPA SWMM

• Useful tool to quickly estimate water quality impacts of site-specific

design

• Automatically imports key hydrologic data:

• Soil type(s)

• Slope

• Precipitation

• Evaporation

• User inputs stormwater BMPs

• 10 years of continuous rainfall/evaporation data used to quantify

impacts of on-site controls (uses SWMM runoff routine)

Modeling Tools – National SW Calculator

Modeling Tools – National SW Calculator

• Developed at the University of Wisconsin, used by Wisconsin DNR to

size bioretention facilities

• Single event modeling (i.e. SCS Type II storm)

• Continuous modeling (for annual rainfall series)

• Important to know on-site soil characteristics

• Calculates:

• Ponding times

• Total runoff volume

• Total rainfall volume retained on site

• Evaporation / root uptake

Modeling Tools – RECARGA

Modeling Tools – RECARGA

• Proprietary software (relatively inexpensive)

• Focus on pollutant concentrations (pre vs. post)

• Uses continuous modeling (one year rainfall series)

• Based on decades of field data for pollutant buildup and washoff

• Calculates:

• Pollutant concentrations (TSS, N, P, heavy metals, etc.)

• Pollutant yields (lbs)

• Total runoff volume

• Before/after comparison to demonstrate MS4 compliance

• Cost efficiencies of individual BMPs

Modeling Tools – WinSLAMM

• EPA SWMM, RECARGA, National Stormwater Calculator, and

WinSLAMM are all good options

• Models need to demonstrate:

• Total runoff volume (existing and proposed conditions)

• % of runoff retained (infiltration, evaporation)

• TSS removal (if infiltration cannot be used)

• Models should reference the latest rainfall statistics (NOAA Atlas 14)

• Limiting model options helps to streamline review process and keep

everyone playing by the same rules

Modeling Tools – Overview

• National Stormwater Calculator is best for early design stages only

(‘quick and dirty’ evaluation)

• RECARGA and WinSLAMM are good for BMP sizing

• EPA SWMM is still the best overall option to include both BMP sizing

and system design (pipes, detention ponds, etc.)

• None of these models give you reasonable results if you don’t have:

• Soil conductivity for native soils

• Prevailing groundwater level

Modeling Tools – Overview

• Why are rainfall statistics and

modeling techniques so important?

• Infiltration BMPs can cost in excess

of $15-$25 per square foot (life

cycle costs can exceed $30/sf)

• BMP costs in an urban

environment can approach $50,000

per developed acre

• Developers may bear initial burden,

but permitees are ultimately

responsible for perpetual function

of BMPs

ECONOMICS OF BMPs



Modeling Tools

Rainfall Statistics


Nuts & Bolts of Design

Impact on Flood Control

Footprint for stormwater quality controls

• Infiltration

• Generally, about 3% - 5% of total contributing

impervious area

• Can be <3% if soil conditions are favorable

• Filtration (all flow exits through underdrain)

• 1%-2% of total drainage area (in residential

areas, less than 1% of total area)

Nuts & Bolts – ‘Rules of Thumb’

• Geotechnical Investigation

• Geotech is critical to identify which BMPs will work and those that won’t

• IN-SITU INFILTRATION CAPCITY (3-4 feet below surface)

• Most important factor in models that define runoff volume control

and stormwater quality

• A minimum acceptable infiltration capacity is about 0.4 – 0.5 in/hr


• Groundwater Depth

• Generally, infiltration BMPs are not practical when the groundwater is

within 4-5 of the surface.

• Local rules should require groundwater depth determination

• Alternative BMPs necessary when groundwater is high


• Soil Contamination

• New MDEQ rules prohibit

infiltration BMPs in

contaminated areas

• Other stormwater control

options necessary in

contaminated areas:

• Mechanical separation

(i.e. swirl concentrators)

• Filter media

• 2-year storm volume

control may be impractical

• Need flexibility in local

ordinances


• Underdrain or no underdrain?

• Underdrains help to dewater when soil

conditions do not permit, BUT

• Underdrains flow back into storm sewer

system, thus reducing impact of volume

reduction for 2-year requirement

• In most cases where infiltration

capacity is 0.4-0.5 in/hr, underdrains

aren’t needed


• We’re addressing stormwater quality, and that’s increasing the

footprint necessary for stormwater controls. How does this impact the

economics of site development?

• Zoning flexibility and Low Impact Development

• Relaxation of flood control hurdles

• Current detention pond requirements often cause overdesign

• Peak flow limitations (x cfs per acre) are typically lower than actual runoff

potential

• GOALS

• Mimic pre-development hydrology

• Overdesign (reducing flows relative to existing conditions) can have harmful

downstream impacts


100 Year 24 Hour Storm

0

20

40

60

80

100

120

140

160

180

1/1 1/1 1/2 1/2 1/3 1/3 1/4

Time

Existing Conditions

Proposed Conditions

Flo

w (

cfs

)

Midwest site designed to 0.20

cfs/acre

Peak flow reduced, BUT excess

storage results in prolonged

discharge to receiving stream

Do we need a prescriptive

discharge rate? How about

basing design on actual runoff

potential?


On-site BMPs provide storage volume, even for

big events (i.e. 10-yr, 100-yr storms)

Modeling should account for this volume and it

should be subtracted from the required flood

storage volume

Local ordinances should provide more flexibility on

design (i.e. 100-yr) discharge rate to provide a

balance between development costs and water

quality needs


OHM 34000 Plymouth Road Livonia, Michigan 48150 T 734.522.6711 F 734.522.6427 www.ohm-advisors.com

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