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Briefing for BPRC onPotomac Interceptor Capacity AnalysisDecember 20, 2007
District of ColumbiaWater and Sewer Authority
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Purpose M&E/COG 1999 work predicted
overflows @ 10-yr storm On PI At connections to PI
Evaluate Fairfax request to add flow to PI: 2 scenarios
Add 5.5 mgd (avg) Add 8.5 mgd (avg)
Was old model overly conservative?
Reasons for concern Model calibrated for smaller
storms/limited period Is there physical evidence of
surcharge?
Great Falls Regression
y = 50.866x - 16.015
R2 = 0.8445
0.00
20.00
40.00
60.00
80.00
100.00
120.00
140.00
160.00
180.00
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00
Rain (in)
Inflo
w V
olum
e (a
c-in
)What happens for larger storms?
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Approach
Decide if model is too conservative – Impact of
additional Fairfax flow
Use WASA meter data on PI
2001-2006
Obtain suburban data for same
period
Evaluate model calibration for large storms
Go look at MH’s with predicted evidence of
surcharge
Meters on PI
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Changes in Model from 1999 Version Changed ground elevations to match PI record drawings: found significant
differences Determined lower section of PI is designed to operate under pressure
1960’s Burns & MacDonald design drawings show it Confirmed by WASA field crews and anecdotal evidence Field visit confirmed it Applies to MH 5-6 and 8-16
Fixed old model stability problems Rock Creek siphon
Fixed apparent data entry error for pipe roughness between MH 17 and 18 Conventional Manning’s n = 0.013 to 0.015 Old value n = 0.065 (we think they were trying create an equivalent pipe with an n= 0.065 due to
SWMM limitation) Effect = no more flooding predicted @ MH 18 Confirmed with field visits
WSSC’s Cabin John Structure Modified to reflect current configuration
Impact of Changes in Model Structure
Peak Flow = 145 mgd
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Old Model
New Model
MH 18
MH 13
MH 9
DC
MH 18DC
MH’s designed for pressure
Ground
HGL
Friction loss @ MH18
Friction loss @ MH18
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Changes to WSSC’s Cabin John Structure
Meter #4
MUPI to DC
PI to DC
Meter Bypass
Valve #2
Valve #3
Meter #9
Valve #8
Valve #
7
From Cabin John sewer
shed
Original Operation: Operate valves to send up to 16 mgd to MUPI, overflow to PI (up to 25 mgd) Observed meter data did not agree with this operation
Current operation: Valves #7, #8 open, flow splits based on pipe friction About 19 mgd goes to MUPI, and 17 mgd goes to PI
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Obtain New Flow Data
Correlate rain to flow increases in areas delivering flow to PI
New flow data obtained: WASA: 1999-2007 Fairfax: 1999, 2004 – 2007 WSSC: 2003 – 2007 LCSA: 1999, 2004-2007 Dulles: 1999
Used about 198 storms for Fairfax regressions and 47 for LCSA (vs 4 to 11 storms for prior work)
Received in Dec., needs to be
incorporated
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Simulation Approach
Separate sanitary sewer shed
Diurnal DWF
Impervious area Wet Wea
ther Flow
Total Flow
Find equivalent impervious area for
each sewer shed
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Example Regression:Fairfax’s Sugarland Run
Old Regression
New Regression
15-y
r 24-
hr s
torm
= 5
.3”
10-y
r 24-
hr s
torm
= 5
.04”
5-yr
24-
hr s
torm
= 4
.32”
2-yr
24-
hr s
torm
= 3
.12
Most regressions showed less wet
weather flow for larger storms than old
relationships
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Re-calibration:
Calibrated to Meters along PI Meters at points of
connection to PI Parameters:
Wet weather volume
peak flow rate shape and lag of
flow vs time curve
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Example Calibration:Fairfax’s Great Falls
WW Peak Comparison Great Falls
0
10
20
30
40
50
0 10 20 30 40 50
meter
mo
del
WW Volume Comparison Great Falls
0
5
10
15
20
25
30
0 5 10 15 20 25 30
meter
mo
del
Flow Rate in mgdWet Weather Volume in mg
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Results of Modeling
PI capacity at DC line (MH #2) Model: 144-150 mgd Burns & MacDonald 1961 design basis: 151 mgd
Not much data to calibrate to in the range of 15 year storm (1 storm)
With this modeling approach, more rain = more flow Is this realistic? Is there a physical limit to wet weather flow that can enter pipe defects and
illicit connections
Need to look at real world data
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Observed Peak Flows in PI:1999-2006
Peak Flow @ MH 2 on PI
No. DatesTotal Flow
Dry Weather Flow Wet Weather Flow Average Rain (in)
Approx Return Frequency
1 6/7/2003 129 75 54 1.53
2 2/22/2003 127 50 77 2.2 Rain on top of snow melt
3 6/19/2003 125 80 45 2.28
4 6/25/2006 124 51 73 6.23 25 to 50 year
5 9/16/1999 119 50 69 3.48 Hurricane Floyd
6 2/6/2004 116 59 57 1.47
7 3/21/2003 115 70 45 1.88
8 1/2/2003 115 75 40 1.41
9 5/26/2003 114 75 39 1.29
10 5/16/2003 114 65 49 2.15
11 10/8/2005 112 48 48 4.9 ~ 5 year
12 4/2/2005 112 55 57 2.33
13 6/13/2003 112 75 37 0.89
14 8/12/2001 109 50 59 1.44
15 3/3/2003 106 65 41 0.28
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Observed Peak Flows in PI
Peak Flow at MH#2 -- WET WEATHER COMPONENT ONLY
0
20
40
60
80
100
120
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00
Rainfall (in)
Flo
w R
ate
(mg
d)
Model
Meter Data
Peak Flow at MH #2 -- TOTAL FLOW
0
20
40
60
80
100
120
140
160
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00
Rainfall (in)
Flo
w R
ate
(mg
d)
Model
Meter Data
PI Capacity
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Observed Peak Flows in Catchments
Ann. Avg. Flow (mgd)
Observed Peak IMA / LCSA Flow (mgd)
Jurisdiction Meter Name Flow Rate (mgd) Date Ann. Avg Peak
WSSC
CJ Dulles (to PI) - 47.2 2/22/2003 0.7 25.0Muddy Branch 6.14 N/A N/A 15.5 40.3Watts Branch 6.49 16 6/25/2006 4.5 14.2Rock Run 0.96 2.7 6/25/2006 0.9 3.7Unmetered 0.47 - - - -
LCSALCSA 18.14 Just received data Just received data
Broad Run Offload 4.34 Just received data Just received data
Fairfax
Sully Road #1 4.02 12.4 6/25/2006 4.0 9.2Sully Road #2 1.44 9 6/25/2006 1.1 2.1Rock Hill Road 0.62 2 6/25/2006 0.9 2.3Sugarland Run 6.81 19 6/25/2006 4.0 12.0Great Falls 12.17 45 6/25/2006 8.7 22.5Scotts Run 4.74 5.5 6/25/2006 2.9 9.4Pimmit Run 8.36 40.2 6/25/2006 9.4 23.6
x
x
xx
xx
x
Exceed IMA?
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Conclusions from Real World Data
Peak flow in PI @ MH 2 limited to 120-130 mgd
Wet weather increase limited to about 75-80 mgd
Reported data and physical inspections do not indicate evidence of overflows
Absent known overflows, appears to be a physical limit to what flow rate can enter pipe defects & illicit connections
Base flow in PI (wet vs. dry year) has a large impact
Municipal flows delivered to PI exceed current IMA transmission limits
True for 25-50 yr storm
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Evaluations Requested by Fairfax
IMA Allocation (mgd) Average Flow (mgd)
Connection Point Meter Name Avg. Peak
Scenario 1 – No Diversion to Noman Cole
Scenario 2 –3 mgd Diversion from Difficult
Run to Noman Cole
Horsepen A1 Sully Road #1 4 9.2 4 4
Horsepen A2 Sully Road #2 1.1 2.1 2 2
Horsepen A3 Rock Hill Road 0.9 2.3 0.5 0.5
Sugerland RunSugerland Run 4 12 7 7
Difficult Run Great Falls 8.7 22.5 15 12
Scotts Run Scotts Run 2.9 9.4 5 5
Pimmit Pimmit 9.4 23.6 7 7
Total 31 N/A 40.5 37.5
Amount above IMA Allocation 9.5 6.5
Amount above IMA Allocation after 1 mgd flow swap with LCSA under negotiation 8.5 5.5
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Fairfax Request - Findings
Adequate capacity for additional 5.5 to 8.5 mgd DWF
Adequate capacity for some additional wet weather peak:
Cannot tolerate increase in wet weather peak proportional to DWF increase
Current IMA Allocation (mgd) Current (mgd) Proposed (mgd)
Meter Name Avg. PeakObserved
Peak Avg Peak
Sugerland Run 4 12 19 7 ??
Great Falls 8.7 22.5 45 15 ??
Scotts Run 2.9 9.4 5.5 5 ??
What wet weather peak goes along
with increase flow?
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Proposed Work – Near Term
Use Loudoun data (received in Dec)
Develop method to cap model peaks based on real-world data
Determine what Fairfax peaks can be tolerated with additional DWF. Example: New Great Falls DWF = 15 mgd Try peaking factors ranging from 1.5 to 3.0 to determine what is achievable in
PI
Use area vs. rainfall relationship to moderate 5, 10, 15-year storms 15-year storm applied uniformly over 340+ sq miles of PI sewer shed should be
smaller than one applied over small area
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Look Ahead for Long Term(New IMA) Real world peaks exceed
IMA limits
What peaks should be included in new IMA Select design storm for PI 5, 10, 15 25 yr?
Determine allowable PI-specific rating curves based on drainage area size
Equitable basis for peak flow allowance
PI- Specific Peaking Factor Curve
0
2
4
6
8
10
0 100 200 300 400 500
Drainage Area Size, DWF or Other Characteristic
Pe
ak
ing
Fa
cto
r