Seth Bradley, P.E.

06/20/2013

Understanding Wastewater Collection System Modeling Assumptions and Limitations

Agenda

General Modeling Overview Why model?

Model Detail v. Use

Model Calibration

Model Limitations Model Extents

Flow Meter/Rain Gauge data

Wet Weather I/I

Case Studies – SD1 of Northern Kentucky Maple Tree Avenue

Crestview PS

2nd & Madison

Collection System Model

Runoff

Combined Sewage

Rainfall

Sanitary Sewage

CS flows & water levels

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A Collection System Model is a dynamic skeletonized Mathematical Representation of a Combined or Separate Sewer System

Why do we model?

Analyze large quantities of data quickly and efficiently.

Increase the level of confidence in conceptual design

estimates.

Examine various system conditions and alternatives

(“What-If” scenarios).

Models can be used for all aspects of engineering:

Planning

Design

CSO/SSO Reporting

Demonstrate benefits of projects

Model Detail v. Use Level of D

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Flow to WWTP

System-wide SSO Planning

Trunk sewer planning

CSO Planning

Model Use

Local capacity issues planning and design

The level of detail depends on the questions you

want to answer and where you are looking:

Model Detail v. Use

Highly Skeletonized Model for Planning Purposes

Detailed Model for Detailed Design

Why Calibrate a Model?

Calibration increases confidence in the model and

the resulting decision making

Calibration uncovers unusual conditions in real

systems! (blockages, sewer overflows, etc.)

Calibration detects bad assumptions and errors in

the model data

How do we calibrate?

Adjust model parameters to match a set of

measured data

Flow data and rainfall data and other records

Adjust model parameters such as:

Physical properties (length, diameter, slope, etc.)

Hydraulic properties (pipe roughness, minor losses)

Wastewater loads (dry weather loads)

Groundwater Infiltration

Runoff volume and routing (area, initial abstraction,

routing factor, RTK values, etc.)

○ For combined sewer take from GIS data

○ For separate sewers need to create artificial “area”

Operations (pump stations, diversions, etc.)

Vertical Data accuracy critical !

How do we calibrate?

Subcatchments upstream of a meter receive same calibration parameters

Model Calibration Process

Model Limitations

Models are great tools but they do have

limitations affected by:

Model Extent

○ Lack of Data

○ Model out-of-date

Flow Meter/Rain Gauge Data

○ Data Quality

○ Network Coverage

○ Duration of Data

○ Data resolution

Wet Weather I/I Flows

○ Seasonal groundwater impacts

○ Intense rainfall bursts

Blue pipes not in model

Overview of Limitations – Lack of Data

Overview of Limitations – Model Out-of Date

267 new tap connections since previous calibration in 2008

Overview of Limitations – FM Data Quality

Proper meter location (consider site hydraulics)

Avoid low flow sites as much as possible

Avoid high velocities, hydraulic jumps, turbulence, etc.

Sufficiently downstream of pump station discharge

Meters must be properly calibrated/maintained

Flow meter accuracy

Pipe in model but far from meter

How much flow through pipe with SSO?

Overview of Limitations – Flow Meter Locations

Subcatchment #1

500 acres

Subcatchment #2

500 acres

Subcatchment #3 500 acres

Subcatchment #4 500 acres

Manhole

Legend:

8” Pipe

12” Pipe

24” Pipe

SSO

Flow Meter

4 MGD

1 MGD 1 MGD 1 MGD 1 MGD

U/S area equally calibrated to match D/S

metered flow.

Would be difficult to design new sewer given location of D/S meter

Overview of Limitations – Wet Weather Flows

Wet Weather RDII Response with groundwater:

Wet Weather RDII Response with Intense Rainfall:

Model pipes do not extend back along Maple Tree Lane

At Issue: Size new sewer along Maple Tree Avenue to eliminate basement backups and SSOs in the area.

Case Study #1 – Maple Tree Avenue

Nearest Calibration Meter far away

Closest meter from prior model calibration

Current meters JOS03 and JOS04 could be used to refine the model

Survey confirms

shallow MH 1830024 (depth =

5.37’)

Survey confirms

shallow MH 1830023 (depth =

5.31’) Survey slope =

0.76%

(decrease from 1.25%)

Extend Model with survey information

Revised Model Calibration

JOS04 Before Revised Calibration:

JOS04 After Revised Calibration:

Model shows essentially no response in wet

weather events

Model shows much better response

Proposed Solution – 10YR Profile

DS Sewer has enough capacity for additional

flow

New 10” sewer in existing trench from 1830022 to

1830020

New deeper 8” sewer from 1830024 to 1830022

Model initially lacked enough detail, and

nearest calibration meter was far away.

Additional information (flow monitoring and

survey data) was needed to expand and

update model to accurately represent study

area.

Summary of Key Issues

Case Study #2 – Crestview PS

Extended model pipes, reload subcatchments

At Issue: Refine model calibration and use to size improvements to eliminate SSOs at and upstream of Crestview PS

Refine model calibration with active meters CV01 & CV02

Original Model Calibration

CV01:

CV02:

Over predicting CV01 wet weather flows

Slightly under-predicting CV02 wet weather flows

Refined Model Calibration

Original Model Calibration extended from April 2012 – October 2012

Model Wet Weather Flows well calibrated

CV01:

CV02:

Refined Model Calibration with Additional Data

Refined Model Calibration with data from December 2012 – January 2013

Model significantly under predicting groundwater response

CV01:

CV02:

Revised Model Calibration

Model groundwater I/I response had to be updated to match metered flows

CV01:

CV02:

Model initially lacked some detail, and

additional information was needed to expand

and update the model.

Initial calibration missed groundwater

influence due to duration ending in October.

Revised calibration with groundwater was

needed to properly characterize flows.

EQ required at Crestview increased from

38,000 gallons to 135,000 gallons (2yr LOS).

Summary of Key Issues

Case Study #3 – 2nd & Madison

At Issue: 15” line from parking garage to abandoned 21” outfall (1440074) is constantly submerged and periodically backing up garage.

Want to pipe storm water to parallel storm sewer. No calibration meter on 42” storm sewer.

Nearest calibration flow meter (on interceptor)

Calibrated Model Representation

Single subcatchment to represent all flow to loading point

Subcatchment loads directly to interceptor at MH 1440098

Model didn’t include 15” line from parking garage, or 21” outfall

Need to know if available capacity in 42” storm sewer

Additional Information Needed

Installed Flow meters RC01 & RC02 to determine flows in storm sewer

Obtain inverts for 15” sewer connection and critical garage elevation

2nd & Madison Calibration

Added Abandoned Outfall & 15” Parking Garage Sewer

Increased Storm Flow from Highlighted Subcatchments

Updated Model Calibration – RC01

Original Model :

Updated Model :

Flow in storm pipe much improved, model slightly struggles to predict peak flows from intense storm events

Updated Model Calibration – RC02

Original Model :

Updated Model :

Flow in storm pipe much improved, model slightly struggles to predict peak flows from intense storm events

Original model very coarse and unable to be

used to evaluate potential connection.

Additional information necessary to refine

model and analyze situation.

Revised model showed much less capacity

in 42” storm sewer.

Model predicted proposed connection would

only increase Level of Service at parking

garage from 6-Month to 9-Month return

interval.

Summary of Key Issues

Conclusions

Define model purpose carefully.

Understand varying levels of detail in the

model and when more information is needed.

Understand how model was calibrated:

Extent of flow meter network (Where was nearest

flow meter?)

Duration of monitoring

How long ago?

Seasonal groundwater impacts can have a

large impact on wet weather I/I flows.

Questions???