1

Briefing for BPRC onPotomac Interceptor Capacity AnalysisDecember 20, 2007

District of ColumbiaWater and Sewer Authority

2

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

4

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