understanding wastewater collection system modeling assumptions … · 2013. 6. 26. ·...
TRANSCRIPT
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Seth Bradley, P.E.
06/20/2013
Understanding Wastewater Collection System Modeling Assumptions and Limitations
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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
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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
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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
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Model Detail v. Use Level of D
eta
il
(nu
mb
er
of p
ipe
s a
nd
flo
w m
on
ito
rs)
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:
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Model Detail v. Use
Highly Skeletonized Model for Planning Purposes
Detailed Model for Detailed Design
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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
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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 !
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How do we calibrate?
Subcatchments upstream of a meter receive same calibration parameters
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Model Calibration Process
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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
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Blue pipes not in model
Overview of Limitations – Lack of Data
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Overview of Limitations – Model Out-of Date
267 new tap connections since previous calibration in 2008
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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
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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
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Overview of Limitations – Wet Weather Flows
Wet Weather RDII Response with groundwater:
Wet Weather RDII Response with Intense Rainfall:
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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
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Nearest Calibration Meter far away
Closest meter from prior model calibration
Current meters JOS03 and JOS04 could be used to refine the model
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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
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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
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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
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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
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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
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Original Model Calibration
CV01:
CV02:
Over predicting CV01 wet weather flows
Slightly under-predicting CV02 wet weather flows
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Refined Model Calibration
Original Model Calibration extended from April 2012 – October 2012
Model Wet Weather Flows well calibrated
CV01:
CV02:
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Refined Model Calibration with Additional Data
Refined Model Calibration with data from December 2012 – January 2013
Model significantly under predicting groundwater response
CV01:
CV02:
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Revised Model Calibration
Model groundwater I/I response had to be updated to match metered flows
CV01:
CV02:
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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
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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)
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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
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Additional Information Needed
Installed Flow meters RC01 & RC02 to determine flows in storm sewer
Obtain inverts for 15” sewer connection and critical garage elevation
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2nd & Madison Calibration
Added Abandoned Outfall & 15” Parking Garage Sewer
Increased Storm Flow from Highlighted Subcatchments
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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
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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
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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
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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.
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Questions???