Travel-Path Ratio as a Measure to Reduce Travel-Path Ratio as a Measure to Reduce Short-Circuit Potential in Stormwater Short-Circuit Potential in Stormwater

Quality BasinsQuality Basins

Theodore G. Cleveland, Ph.D., P.E.Theodore G. Cleveland, Ph.D., P.E.

Samuel OrozcoSamuel Orozco

May 15, 2007May 15, 2007

AcknowledgmentsAcknowledgments U.S. EPA U.S. EPA

BMP Demonstration Grant to Harris BMP Demonstration Grant to Harris CountyCounty

Harris County EngineeringHarris County Engineering Access to permitsAccess to permits

City of HoustonCity of Houston Access to permitsAccess to permits

Carter & Burgess, Inc.Carter & Burgess, Inc. Financial supportFinancial support

BackgroundBackground

Purpose (of the EPA Grant)Purpose (of the EPA Grant) SelectSelect, instrument, monitor the , instrument, monitor the

performance of two (2) pond-type BMPs performance of two (2) pond-type BMPs in Harris County or City of Houston in Harris County or City of Houston jurisdictions to evaluate effectiveness of jurisdictions to evaluate effectiveness of exisitng design criteria (on-going, C&B exisitng design criteria (on-going, C&B is doing monitoring and analysis).is doing monitoring and analysis).

UH role (this presentation)UH role (this presentation) Select the two pond-type BMPsSelect the two pond-type BMPs

BMP SelectionBMP Selection

Select where C&B should instrument Select where C&B should instrument and monitor.and monitor.

Permit Database (Harris Co.; City of Permit Database (Harris Co.; City of Houston)Houston) Includes images of engineering drawings Includes images of engineering drawings

as part of the DBMS.as part of the DBMS. Scoring SystemScoring System

Scoring SystemScoring System Assigns numerical values from various Assigns numerical values from various

measures taken from engineering measures taken from engineering drawings as well as permit application drawings as well as permit application information.information. Hence the need for drawings as part of Hence the need for drawings as part of

DBMSDBMS Scores are ranked.Scores are ranked.

Virtual visit (i.e. terraserver; Google Earth).Virtual visit (i.e. terraserver; Google Earth). Physical visit.Physical visit.

Permits Database(s)Permits Database(s)

Typical MS ACCESS interface:Typical MS ACCESS interface:

Harris Co. Permits Harris Co. Permits SummarySummary

City of Houston Permits City of Houston Permits SummarySummary

Geographical ExtentGeographical Extent

Geographical Extent – Geographical Extent – Basin OnlyBasin Only

Selection ScoringSelection Scoring Selection scoring initially based on:Selection scoring initially based on:

Inlet/Outlet count.Inlet/Outlet count. Age (time-in-service).Age (time-in-service). L:W ratio.L:W ratio. Geographic location; equipment security ; Geographic location; equipment security ;

service type; owner cooperation; etc.service type; owner cooperation; etc. Observed that the 3:1 length-to-width Observed that the 3:1 length-to-width

ratio appears to have been interpreted as ratio appears to have been interpreted as a geometric requirement without a geometric requirement without consideration of flow path. consideration of flow path.

Selection ScoringSelection Scoring Some otherwise acceptable ponds (for Some otherwise acceptable ponds (for

monitoring) looked like short-circuiting monitoring) looked like short-circuiting was quite likely.was quite likely.

Needed to select ponds that followed the Needed to select ponds that followed the intent of the design guidelines – so the intent of the design guidelines – so the travel-path-ratio measure was invented travel-path-ratio measure was invented and added to the scoring system.and added to the scoring system.

TPR CalculationTPR Calculation

Travel Path (TP): a reasonable hydraulic path that Travel Path (TP): a reasonable hydraulic path that the water travels from inlet to outlet.the water travels from inlet to outlet.

TPR: Ratio of TP distance and the geometric path TPR: Ratio of TP distance and the geometric path that follows the basin edge from the inlet to the that follows the basin edge from the inlet to the outlet.outlet. Adjustments for baffles, irregular shapes in 2005 report.Adjustments for baffles, irregular shapes in 2005 report.

Ad-hoc criterion: TPR of 0.5 is a minimal value for Ad-hoc criterion: TPR of 0.5 is a minimal value for prevention of short circuit.prevention of short circuit.

Selection Scoring Selection Scoring CriteriaCriteria

Pond SelectionPond Selection

Eventually selected two-ponds.Eventually selected two-ponds.

Pond SelectionPond Selection Permit 8-169-8Permit 8-169-8

Pond SelectionPond Selection

Permit 8-077-0Permit 8-077-0

TPR CriterionTPR Criterion

Invented “ad-hoc” to select ponds Invented “ad-hoc” to select ponds without short-circuiting.without short-circuiting. ““Minimum design criteria for Minimum design criteria for

implementation of Certain BMPs” implementation of Certain BMPs” published by Joint Task Force in 2001 published by Joint Task Force in 2001 edition, specifically states that the edition, specifically states that the length to width ratio should be at least length to width ratio should be at least 3:1, but values of 2:1 are still accepted. 3:1, but values of 2:1 are still accepted.

Appears that the criterion was Appears that the criterion was interpreted as a geometric ratio interpreted as a geometric ratio without consideration of path.without consideration of path.

Other LocalesOther Locales L (Length) is defined as the straight L (Length) is defined as the straight

distance from the inlet to the outlet. W distance from the inlet to the outlet. W (Width) is the average width of the pond.(Width) is the average width of the pond.

L:W ratios:L:W ratios: California L/W =1.5California L/W =1.5 Tennessee L/W = 2.0 – 4.0Tennessee L/W = 2.0 – 4.0 Virginia L/W = 2.0Virginia L/W = 2.0 New Jersey L/W = 2.0 – 3.0New Jersey L/W = 2.0 – 3.0 Golden, Colorado L/W= 2.0 – 3.0Golden, Colorado L/W= 2.0 – 3.0 Connecticut L/W = 3.0Connecticut L/W = 3.0

Ideal Flow ModelsIdeal Flow Models Clients intrigued by simplicity of TPR Clients intrigued by simplicity of TPR

computation.computation. Would help permit inspectors rapidly determine if the Would help permit inspectors rapidly determine if the

other part of the JTF guidance “… minimize potential other part of the JTF guidance “… minimize potential for short circuiting”for short circuiting”

Evaluate TPR by Evaluate TPR by Ideal flow model to determine a characteristic Ideal flow model to determine a characteristic

residence time distribution.residence time distribution. Examine if obviously poor (short-circuits) Examine if obviously poor (short-circuits)

designs are identified by TPR.designs are identified by TPR. Evaluation of other states design criteriaEvaluation of other states design criteria Classified designs based on results of Classified designs based on results of

residence time distributionresidence time distribution

Flow Model AssumptionsFlow Model Assumptions

Neglect flow resistance (inviscid fluid).Neglect flow resistance (inviscid fluid). Neglect dispersion (ideal tracers, Neglect dispersion (ideal tracers,

advection only).advection only). No internal sources or sinks of the No internal sources or sinks of the

fluid.fluid. The particles “are placed” when the The particles “are placed” when the

pond is completely full (at SWQ level).pond is completely full (at SWQ level). No change in the outflow during the No change in the outflow during the

particle travel interval (quasi-steady particle travel interval (quasi-steady discharge)discharge)

Flow Model Flow Model ComputationsComputations

The actual computations are The actual computations are implemented in two purpose-written implemented in two purpose-written FORTRAN programs. FORTRAN programs. One is a Stream-Function/Velocity One is a Stream-Function/Velocity

Potential solver with internal baffles Potential solver with internal baffles (Orozco, 2006).(Orozco, 2006).

Second is the particle tracking code Second is the particle tracking code implemented by Cleveland (1991) implemented by Cleveland (1991) adapted from Kinzelbach’s (1987) code.adapted from Kinzelbach’s (1987) code.

Flow Model Flow Model ComputationsComputations

Particle Tracking Particle Tracking Based on the stream-function/velocity-Based on the stream-function/velocity-

potential results, the position of tracer potential results, the position of tracer particles are tracked over time, their particles are tracked over time, their arrival times are interpreted in the arrival times are interpreted in the context of a residence time for a given context of a residence time for a given pond geometry.pond geometry.

Basin TemplatesBasin Templates

Basin_001 Basin_002 Basin_003 Basin_004Basin_001 Basin_002 Basin_003 Basin_004

Basin_002 GeometryBasin_002 Geometry Cloud of 10 markers Cloud of 10 markers

at different elapsed at different elapsed times.times.

Times are non-Times are non-dimensional.dimensional.

All model basins are All model basins are adjusted to have adjusted to have same volume.same volume.

123456789

101112131415161718192021222324252627282930313233

1 2 3 4 5 6 7 8 9 10 11

Distance (x)

Distance (y)

initial t=1000 t=2000 t=3000

t=4000 t=5000 t=6000

Flow NetsFlow Nets

L/W = 3.0TPR = 0.75T_mean = 2909

L/W = 3.0TPR = 0.25T_mean = 137

L/W = 3.0TPR = 1.15T_mean = 9369

Basin_001 : Reference

Basin_003 : Short-Circuit

Basin_004 : Baffle

Cumulative Arrival Time -- 4 BasinsCumulative Arrival Time -- 4 Basins

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1 10 100 1000 10000 100000

Time (units unspecified)

Tracer Arrival Fraction

Basin-4 Basin-1 Basin-2 Basin-3

Based on these simulations we conclude that the flow-path ratio criterion used in the site selection procedure is a plausible indicator of short-circuiting

Actual DesignsActual Designs

Several categories identifiedSeveral categories identified Based on model results that assume Based on model results that assume

equal area (volume) ponds.equal area (volume) ponds. Low performing pondsLow performing ponds

TPR < 0.5; T_mean < 2500TPR < 0.5; T_mean < 2500 Acceptable performing pondsAcceptable performing ponds

TPR > 0.5; T_mean >2500TPR > 0.5; T_mean >2500

Low Performing Designs Low Performing Designs

Permit 2003-0070Without BaffleL/W = 2.0TPR =0.33Res. time = 651

Permit 8-279-9L/W = 1.6TPR =0.42Res. Time= 600

Permit 2003-0080L/W = 21.0TPR =0.48Res. Time=1172 Permit 2003-

0117L/W= 1TPR= 0.5Rest time=

2413

Acceptable Performing DesignsAcceptable Performing Designs

Permit 8-108-2

L/W=3

TPR=0.67

Res. Time=2558

Permit 8-061-7

L/W=2.5

TPR=0.76

Res. Time=2860

2003-0070

L/W=2.0

TPR=1.07

Res. Time= 4597

Connecticut design

L/W=2.82

TPR=0.78

Res. Time=2860

Excellent DesignsExcellent Designs

Basin_001

Two Baffle design L/W = 1.0 TPR=1.3 Res. Time =5507

Basin_002

Three Baffle design L/W = 1.0 TPR=1.5 Res. Time =7357

Permit 2004-0040

L/W = 11.0 TPR=0.92 Res. Time

=6863

Pond Design Groups IdentifiedPond Design Groups IdentifiedCumulative residence time for the Stormwater Quality Ponds studied

0

1

2

3

4

5

6

7

8

9

10

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 8500

Cumulative residence time (time units)

8-077-0 8-061-7 8-108-2 8-169-8 8-279-9

2003-0070 2003-0070 no baffle 2003-0117 2004-0040 2005-0080

Other states design Connecticut design 8-262-4 1 Baffle design 2 Baffle Design

Section 1

Section 2

Section 3

Section 4

A

B

Baffle Improvements Baffle Improvements AnalysisAnalysis

Based on permit 2003-0117 design: L/W =1.0 and Based on permit 2003-0117 design: L/W =1.0 and TPR = 0.5TPR = 0.5 1 baffle increased the average residence 1.4 times and the 1 baffle increased the average residence 1.4 times and the

TPR from 0.5 to 0.79TPR from 0.5 to 0.79 2 baffles increased the average residence time 2.3 times 2 baffles increased the average residence time 2.3 times

and the TPR value is increased from 0.5 to 1.3. and the TPR value is increased from 0.5 to 1.3. 3 baffles increased 3 times the residence time of the pond 3 baffles increased 3 times the residence time of the pond

without baffles as well as the TPR value from 0.5 to 1.5.without baffles as well as the TPR value from 0.5 to 1.5. Baffle design will:Baffle design will:

Take advantage of the area designated for the stormwater Take advantage of the area designated for the stormwater quality pondquality pond

Increase the pollutant removal considerably Increase the pollutant removal considerably Not be too expensive to buildNot be too expensive to build Not change the hydraulic retention time but the residence Not change the hydraulic retention time but the residence

timetime

Baffle Design Analisys

0

1

2

3

4

5

6

7

8

9

10

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 8500

Residence time (Time units)

Number of Particles

No Baffle 2 Baffle design 3 baffle design 1 Baffle design

Based on permit 2003-0117 design: L/W =1.0 and TPR = 0.5

Permit 2003-0070Permit 2003-0070

5 10 15

5

10

15

20

25

30

5 10 15

5

10

15

20

25

30

L/W = 2.0

TPR =0.33

Res. time = 651

L/W = 2.0

TPR =1.07

Res. time = 4957

1 baffle increased the average residence time by about 7 on permit 2003-0070

SummarySummary

Harris County and the City of Houston Harris County and the City of Houston have similar the types of systems have similar the types of systems permitted. permitted. Basins are the most common type in the Harris Basins are the most common type in the Harris

County jurisdiction, with nearly three-quarters County jurisdiction, with nearly three-quarters of the permits issued in Harris County being of the permits issued in Harris County being this type of BMP. About half the City of this type of BMP. About half the City of Houston permits issued are basins.Houston permits issued are basins.

Total drainage area served is 35 square miles; Total drainage area served is 35 square miles; about 25 square miles are served by basins.about 25 square miles are served by basins.

SummarySummary

Examples exist in both jurisdictions of Examples exist in both jurisdictions of permitted ponds that neglect portions of permitted ponds that neglect portions of the guidelines, yet were permitted after the guidelines, yet were permitted after guidelines were available.guidelines were available. The L:W ratio guideline appears to have been The L:W ratio guideline appears to have been

interpreted as a geometric condition without interpreted as a geometric condition without regard to inlet-outlet location.regard to inlet-outlet location.

The short circuiting guideline appears to be The short circuiting guideline appears to be neglected (in some cases) without explanation.neglected (in some cases) without explanation.

SummarySummary

A metric TPR is employed to quantify the A metric TPR is employed to quantify the potential for short circuiting for ponds potential for short circuiting for ponds meeting the geometric conditions.meeting the geometric conditions. The TPR was created for a selection scoring The TPR was created for a selection scoring

procedure..procedure.. Ideal flow modeling was used to determine if Ideal flow modeling was used to determine if

the TPR was sensible.the TPR was sensible.

SummarySummary Using the TPR and an ad-hoc residence time value, Using the TPR and an ad-hoc residence time value,

several hypothetical and real basins were examined. several hypothetical and real basins were examined. modeling was used to determine if the TPR was modeling was used to determine if the TPR was sensible.sensible. TPR appears to be able to detect potential for short circuiting.TPR appears to be able to detect potential for short circuiting. Most basins in service are adequate, several are inadequate Most basins in service are adequate, several are inadequate

as per TPR.as per TPR. Retrofits using baffles are examined that increase the Retrofits using baffles are examined that increase the

TPR to the ad-hoc criterion and greatly improve TPR to the ad-hoc criterion and greatly improve residence time.residence time.

The TPR would help Harris Co., and COH quickly The TPR would help Harris Co., and COH quickly evaluate if JTF guidelines are satisfied in the context evaluate if JTF guidelines are satisfied in the context of basin dimensions and inlet/outlet location.of basin dimensions and inlet/outlet location.