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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