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
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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 – New MS4 Rules
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??
MDEQ – New MS4 Rules
• 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.
MDEQ – New MS4 Rules
Planning/Designing for Compliance
Small Storm Hydrology
Modeling Tools
Rainfall Statistics
Designing for water quality (‘first flush’ treatment):
• What is a ‘first flush’ event?
Small Storm Hydrology
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)
Small Storm Hydrology
Small Storm Hydrology
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
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
Rainfall Statistics – NOAA Atlas 14
• 100-yr storm (flood control) statistics have changed significantly