Hamilton County, Ohio Wet Weather Initiative Meeting

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Hamilton County, OhioWet Weather Initiative
Meeting

• City of Dublin, Ohio Stormwater
• Master Plan and Other Lessons Learned
• Presented by
• Christopher T. Calpin, P.E.
• Principal Engineer
• Cincinnati, Ohio
• October 25, 2001

AC
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Problem Statement

• Increased frequency of large events
• Increased frequency of overbank flow
• Stormwater pre- and post development runoff
  calculations???
• Appropriate runoff calculation methodologies
• Rational method, TR-55 or SCS runoff curve
  number, SWMM -RUNOFF)

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Dublin, Ohios Stormwater Program Elements

• Stormwater Master Plan
• Stormwater Funding Evaluations
• Administrative Policy and Stormwater Ordinance
• Evaluate BMP and Master Plan Impacts

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Why did Dublin elect to perform a Stormwater
Master Plan?

• Located in northwest Franklin County, and in
  Union Delaware Counties
• Land area - 21 square miles
• Watershed area 35 square miles
• Population - 32,735
• 10 year growth - 200
• Memorial Tournament

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Rainfall data shows the 1 chance storm volume
has increased 20 in the last 30 years.
Note Rainfall volumes are for a 24-hr duration
event.
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Development increases the imperviousness which
impacts the watershed hydrology and hydraulics.
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What are the impacts of more frequent overbank
flooding?
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Erosion and undermining of trees, buildings,
roads, culverts, etc. and adverse impact on
aquatic life.
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Are stormwater engineers accurately calculating
pre-development runoff rates?

• Dublin has determined the pre-development runoff
  rates
• Based on a calibrated SWMM-RUNOFF model
• Dublin dictates allowable post development runoff
  release rates
• Critical storm criteria
• Developers typically use Graphical Peak Discharge
  methodology for post development runoff and
  detention requirements (e.g., TR-55 or SCS Runoff
  Curve Number)

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Dublin Example 16 acre watershed
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What does the example tell us about Dublins
Approach to stormwater management?

• Better predicts predevelopment runoff (calibrated
  model)
• Helps to control erosion and flooding
• Developers are over predicting predevelopment
  runoff for the smaller storms
• Exacerbates erosion
• Equivalent, or slightly more stringent, flood
  control requirements on larger storms

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What does the example tell us about Dublins
Approach to stormwater management?

• TR-55 generally results in more conservative flow
  estimates (pre- and post development)
• SWMM-RUNOFF results in less conservative (more
  realistic) flow estimates
• Result Built In Factor of Safety in Detention
  Basin Sizing

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What are the differences between SWMM RUNOFF and
TR-55?
SWMM RUNOFF
TR-55

• Impervious area
• Directly connected impervious area
• Pervious area
• Initial final soil infiltration capacities
• Maximum soil storage

• Runoff curve number
• Simulate impervious and pervious areas, and soils
• Initial abstractions
• All losses before runoff begins
• Soil storage capacity

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How will Dublins master plan and new ordinance
impact development?

• Release rates/water quality BMPs more stringent
  than MORPC stormwater criteria
• Dublin considered the impacts to be minimal and
  enacted new stormwater management ordinance
• Dublin continues to require these standards on
  all new developments

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What does the future hold for stormwater
management?

• Volume-Time Detention Control
• Control both peak flow and volume over a critical
  time period
• Better for erosion and flood control
• Continuous Simulation Modeling
• Stream bank restoration

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Volume Time Detention Control
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Continuous Simulation Modeling