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Excellence in Engineering Since 1946
Strand Associates, Inc.® ( )
Innovative Approaches in the Development of Henderson Water Utility’s CSO Long-Term Control Plan
OWEA Collections Systems Workshop
May 9, 2013
Overview of Presentation
• Background and History of HWU’s CSO Control Efforts
• Use of the LTCP EZ for Small Communities
• Alternative Modeling Approach to Predicting CSO
Compliance
• Coordination with State and Federal Officials
• Current Status and the Future
Population ~29,000
HWU’s History of CSO Control Efforts Sets up LTCP Approach • 1995 – First separation projects
• 1997 – Submitted CSO Stormwater Management Plan as an
interim LTCP (never received comments or approval though)
• 1995 – 2006 – Continued separation of the downtown combined
system and redirecting flow away from the combined system
• Ultimate plan is to effectively separate 77% of the
contributing area to the CSS
• 2007 – Entered into joint Federal-State Consent Judgment
• Small system considerations language was included in CJ
• LTCP due February 28, 2009
• LTCP fully implemented by 2018
Minimum Requirements of LTCP-EZ are Significantly Less than Traditional LTCP Traditional LTCP LTCP EZ*
Any community Population < 75,000
Characterization, Monitoring, and
Modeling
Implementation of Nine Minimum
Controls
Public Participation Public Participation
Consideration of Sensitive Areas Consideration of Sensitive Areas
Evaluation of Alternatives Post-Construction Monitoring
Cost/Performance
Considerations
Operational Plan *Subject to Regulatory Approval
Maximization of Treatment at
WWTP
Implementation Schedule
Post-Construction Monitoring
LTCP-EZ Guidance and Documentation is Simpler and Less Burdensome • Basic forms and spreadsheets
• Minimum amount of back-up
documentation required
• Time and technical expertise
necessary to complete is a
fraction of a traditional LTCP
• 66 pages vs. 201 pages for
traditional LTCP instructions
• Demonstrates compliance
through the presumption
approach (4 overflows/year)
Restrictive Options for Compliance Limits LTCP EZ Applicability to HWU • Even with ~80% separation, there was predicted to be several
low-volume, high-frequency CSOs
• The 85% capture criteria was seen as a more likely to be
successful method:
ii. The elimination of or the capture for treatment of no less than 85% by volume of
the combined sewage collected in the CSS during precipitation events on a system-
wide annual average basis
Comparison of Overflow Volumes Yields Percent Capture to Meet Presumptive Approach Criteria
Post-Construction
Overflow Volume (2018)
Pre-Construction
Overflow Volume (1995)
1 -
≥ 85%
USEPA and KDOW Agree to a Modified LTCP-EZ Approach • Completion of LTCP-EZ forms with thorough narrative based on
traditional LTCP guidance
• LTCP to include the following items:
• Executive Summary
• Nine Minimum Controls Documentation
• Sensitive and Priority Areas
• System Characterization
• Public Participation
• Evaluation of Controls
• Affordability
• Recommended Plan
Demonstrating Effectiveness of Controls Meant Proving What Was Already Known Intuitively
1.5
MGD
Assessment of Available Data and Resources Leads to a Non-traditional Model • Limited in-system flow data
• Extensive WWTP flow data
• Extensive GIS and land-use data
• Some overflow volume and duration data
• Verified pipe diameters and slopes
• Verified overflow elevations
• Known pumping capacities and settings
Mass Balance of Flows Calculates Inputs and Outputs
Rational Method Provides a Basic Overland Flow Rate Based on Area, Rainfall Intensity, and Land-Use Coefficient
Q = CIA
Q = Flow
C = Runoff Coefficient
I = Intensity of Rainfall
A = Area
NLCD Provides First Set of Data to Calculate C
GIS Layers Refines C-Factors
Combination of CSO Flow Data and Rain Gauges Provides Final Method to Calculate C-Factor
The Inherent Conservativeness of the Rational Method “Sells” it to Regulators
• Calculates peak flows
• No time of concentration or travel time for flows to enter system –
everything hits all at once
• Assumed “moist” conditions at all times
• Used the highest calculated C-factor for each basin to make
certain that the highest possible inputs were modeled
System Model is Not Hydro-Dynamic but Accounts for System “Drainage” and Storage Through Multiple Hour Long Rain Events
Deciding on What Rainfall Events to Model Results in More Questions than Answers • Guidance requires 85% capture on an “average annual basis”
• What does a City’s average year of rain events look like?
• Number of events
• Duration
• Intensity
• No time to hire a meteorological firm to complete analysis
• How defensible would it be?
Deciding on What Rainfall Events to Model Results in More Questions than Answers • Guidance requires 85% capture on an “average annual basis”
• What does a City’s average year of rain events look like?
• Number of events
• Duration
• Intensity
• Does it include very large events (25-year or greater)?
• Reluctance to hire a meteorological firm to complete analysis
• How defensible would it be?
Substituting Actual Historical Data Sidesteps All of these Concerns • Evansville Airport had 60-years of quality controlled, hourly rainfall
data
• Long enough time span to include large events
• 50-year
• 25-year
• 20-year
• Etc.
• Represents what actually occurred and not a synthesized year
• Run the simulation for all the data and divide by 60 = average
percent capture
Model Results Confirmed What HWU Had Expected All Along • 90% capture
• Complete elimination of Canoe Creek overflows
• Short duration overflows downtown
• LTCP submitted in March 2009 (one month delay due to ice storm)
• LTCP approved in April 2012
Implementation is Resulting in Better than Predicted Performance • Conservative nature of the model
• Additional modifications to the system
• Final construction is expected later this year
Continuous Coordination with Regulators was Key Throughout Entire Process • Meetings leading up to CJ
• Numerous update meetings and phone calls during LTCP
development
• Two addendums
• Two sets of supplemental information
• Maintaining communication with USEPA and KDOW
Lessons Learned
• Communicate with your Regulators so they’re not surprised
• Be prepared to defend/fight for anything outside of the box
• Be willing to compromise to get what you really want
• Take advantage of the information you have
• A plan to make a simple spreadsheet rarely results in a simple
spreadsheet
Questions?
Excellence in Engineering Since 1946
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