Hydrologic and Hydraulic Guidelines for Dams in Texas · 1 New PMP Temporal Distributions and a Simplified Breach Method for Dams in Texas US Society of Dams April 29, 2008 John Rutledge

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New PMP Temporal Distributions New PMP Temporal Distributions and a Simplified Breach Method and a Simplified Breach Method

for Dams in Texasfor Dams in Texas

US Society of DamsUS Society of DamsApril 29, 2008April 29, 2008

John Rutledge – Freese and Nichols Warren Samuelson – TCEQ

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New GuidelinesNew Guidelines in Texasin TexasDeveloping Design Floods Developing Design Floods and Performing Breach and Performing Breach AnalysesAnalysesHydrologic and Hydraulic Hydrologic and Hydraulic Guidelines for Dams in Guidelines for Dams in TexasTexasWebsite for downloading Website for downloading Guidelines listed in paperGuidelines listed in paper

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Design Flood Design Flood GuidelinesGuidelinesThings that are Different:Things that are Different:–– PMP Temporal DistributionPMP Temporal Distribution–– PMP Critical DurationPMP Critical Duration–– Simplified Breach MethodSimplified Breach Method

Each will be different under Each will be different under Federal guidelinesFederal guidelines

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Design Flood GuidelinesDesign Flood Guidelines

Things that are Things that are NotNotDifferent:Different:–– PMP Totals (HMRPMP Totals (HMR––51)51)–– PMP Spatial Distribution PMP Spatial Distribution

(HMR(HMR--52)52)–– Runoff MethodologiesRunoff Methodologies–– Routing MethodologiesRouting Methodologies

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Background on Temporal Background on Temporal DistributionDistribution

““Critically stackedCritically stacked”” distribution assumes distribution assumes that the PMP 1 hr, 2hr, that the PMP 1 hr, 2hr, …… all occur in the all occur in the same event as the 72 hr.same event as the 72 hr.Extreme rainfall events follow one of two Extreme rainfall events follow one of two patterns:patterns:–– Short highly intense eventShort highly intense event–– Long, steady, heavy rainfall eventLong, steady, heavy rainfall event

Research shows that these two distinct Research shows that these two distinct hydrologic patterns do not happen in the hydrologic patterns do not happen in the same event same event Records show most near PMP events are Records show most near PMP events are front end loadedfront end loaded–– The closer to the PMP, the more likely to be front The closer to the PMP, the more likely to be front

end loadedend loaded

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Design Flood PrecipitationDesign Flood Precipitation

Design Flood based on PMP Design Flood based on PMP –– PMFPMF–– Percentages based on full PMFPercentages based on full PMF

Avoid critically stacked PMP amountsAvoid critically stacked PMP amounts–– Still use HMR 51 & 52 for totals and Still use HMR 51 & 52 for totals and

spatial distributionspatial distribution–– Different temporal distribution, based on Different temporal distribution, based on

durationduration

Critical Duration for PMPCritical Duration for PMP–– 1, 2, 3, 6, 12, 24, 48, 72 hr1, 2, 3, 6, 12, 24, 48, 72 hr

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Temporal Distributions

0%

10%

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100%

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

% of Total Time

% o

f Tot

al R

ainf

all

1 hr2 hr3 hr6 hr12 hr24 hr48 to 72 hr

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Temporal DistributionTemporal Distribution

Curves do not Curves do not represent an represent an ““envelopeenvelope”” approach, but are approach, but are reasonably reasonably conservative conservative estimates of the estimates of the temporal temporal distribution that distribution that would be expected would be expected for each specific for each specific durationduration

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Synthetic Rainfall Distributions Used for PMF Development

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0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

% of Total Time

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ainf

all

HMR52NRCS 5-PointNRCS Type IINRCS Fig 2-4, TR60Texas PMP

1010

3-hour Actual

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0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

% of Total Time

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all

Proposed

D'Hannis (103)

North Palm Beach (99)

1111

6-hour Actual

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Proposed

Elbert Actual (102)

North Palm Beach (97)

Simpson, KY - (71)

Smethport Actual (81)

1212

24-hour Actual

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Proposed

Alvin Actual (91)

Thrall - (86)

Vic Pierce - (73)

Yankeetown, FL - (82)

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48 to 72-Hour Actual

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Proposed

Alvin Actual (87)

Yankeetown, FL - 48 Hr (83)

Yankeetown, FL - 72 Hr (81)

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Temporal DistributionTemporal DistributionCAUTIONCAUTION–– Do NotDo Not Compare old 24 hr or old 72 hr Compare old 24 hr or old 72 hr

with new 24 hr or 72 hr.with new 24 hr or 72 hr.Particularly for small drainage areasParticularly for small drainage areas

–– DoDo compare old PMF level with new compare old PMF level with new PMF level using the critical durationPMF level using the critical duration

New distributions must use critical New distributions must use critical durationdurationSingle duration for all sizes must use Single duration for all sizes must use critically stacked distributioncritically stacked distribution

–– Inflow rates may be significantly less, Inflow rates may be significantly less, while runoff volumes should be similar.while runoff volumes should be similar.

DamDam Drainage Drainage AreaArea((smsm))

Old Peak Old Peak InflowInflow

(1,000 cfs)(1,000 cfs)

New Peak New Peak InflowInflow

(1,000 cfs)(1,000 cfs)

Change Change in Peak in Peak Elev.Elev.(ft)(ft)

Change Change in in

DurationDuration(Hr)(Hr)

LowerLowerBois Bois dd’’ArcArc

328328 464464 242242 (2.2)(2.2) 72/7272/72

Lake Lake HoustonHouston

2,8602,860 615615 582582 (0.5)(0.5) 72/7272/72

Lake Lake CleburneCleburne

100100 220220 196196 (3.0)(3.0) 72/2472/24

DamDam Drainage Drainage AreaArea((smsm))

Old Peak Old Peak InflowInflow

(1,000 cfs)(1,000 cfs)

New Peak New Peak InflowInflow

(1,000 cfs)(1,000 cfs)

Change Change in Peak in Peak Elev.Elev.(ft)(ft)

Change Change in in

DurationDuration(Hr)(Hr)

Brushy Brushy CreekCreekSite 6Site 6

5.95.9 4141 2424 (2.2)(2.2) 24/1224/12


3.43.4 3232 1414 (1.5)(1.5) 24/1224/12


5.05.0 4343 2121 (1.9)(1.9) 24/1224/12

Lake Lake TawakoniTawakoni

756756 374374 365365 (0.5)(0.5) 72/7272/72

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Construction Cost SavingsConstruction Cost Savings

DamDam Cost SavingsCost Savings

Brushy creek Brushy creek Dams (3)Dams (3)

$1,200,000$1,200,000

Lower Bois dLower Bois d’’ArcArc $2,000,000$2,000,000

Lake CleburneLake Cleburne $1,000,000$1,000,000

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Applicability to Other AreasApplicability to Other Areas

Rainfall examples primarily from area Rainfall examples primarily from area covered by HMR51 and HMR52covered by HMR51 and HMR52–– Mostly gulf statesMostly gulf states

Temporal distribution concept should Temporal distribution concept should apply over same areaapply over same area

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Breach AnalysesBreach Analyses

Breach Analyses performed Breach Analyses performed for:for:–– Hazard ClassificationsHazard Classifications–– Inundation maps for EAPsInundation maps for EAPs

Two types:Two types:–– Full Breach analysisFull Breach analysis–– Simplified breach analysisSimplified breach analysis

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Simplified Breach MethodSimplified Breach Method

Small and intermediate size dams onlySmall and intermediate size dams onlyQbQb = 3.1 * B * H^3/2, where= 3.1 * B * H^3/2, where–– QbQb = peak total discharge from the breach, = peak total discharge from the breach,

in cfs.in cfs.–– B = bottom width of breach (ft) [3*H]B = bottom width of breach (ft) [3*H]–– H = maximum height of the dam (ft) [H/3]H = maximum height of the dam (ft) [H/3]

Qt = Qt = QbQb + Qs, where + Qs, where –– Qs = peak discharge capacity from the Qs = peak discharge capacity from the

spillway(sspillway(s) with the reservoir at the top of ) with the reservoir at the top of the dam, in cfs the dam, in cfs

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Length (Lu) = 0.012 * Ks*SQRT(2*C * H)Length (Lu) = 0.012 * Ks*SQRT(2*C * H)–– Lu = Inundation length (miles)Lu = Inundation length (miles)–– C = capacity at top of dam (acC = capacity at top of dam (ac--ft)ft)–– H = Maximum Height of damH = Maximum Height of dam

Ks= Ks= QbQb / Qs/ Qs [0.5 < Ks < 2.0][0.5 < Ks < 2.0]Interpolate flows from Qt at the dam to Interpolate flows from Qt at the dam to Qs at Lu.Qs at Lu.Determine Stage by normal flow or Determine Stage by normal flow or steady state backwatersteady state backwater–– Increase n values by 25%Increase n values by 25%

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Empirical Equation based on best fitEmpirical Equation based on best fitBased only on dams in TexasBased only on dams in TexasExamples: Examples: –– Lake Weatherford (Intermediate Size)Lake Weatherford (Intermediate Size)

H= 54 ft; C = 49,000 H= 54 ft; C = 49,000 afafDrainage Area = 108 Drainage Area = 108 smsm

–– Daniel Lake Dam (Small Size)Daniel Lake Dam (Small Size)H= 16 ft; C = 465 H= 16 ft; C = 465 afafDrainage Area = 5.0 Drainage Area = 5.0 smsm

23230 20000 40000 60000 80000 100000 120000

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Lake Weatherford Simplified Breach Plan: 1) PMFBR2 1/22/2007 2) SimPMFmaxW 1/22/2007

Main Channel Distance (ft)

Elev

atio

n (ft

)

Legend

WS Max WS - PMFBR2

WS PF 1 - SimPMFmaxW

Ground

Pilot Channel

ClearFork Lower ClearFork Upper

2424

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Inundation Mapping for EAPsInundation Mapping for EAPs

Simplified Breach AnalysisSimplified Breach Analysis–– Pick sufficient points to map, orPick sufficient points to map, or–– Show limits at all downstream structures Show limits at all downstream structures

within inundation lengthwithin inundation length–– Can do steady state backwater, if desiredCan do steady state backwater, if desired–– Procedure does not allow for time to flood Procedure does not allow for time to flood

estimatesestimates

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Applicability to Other AreasApplicability to Other Areas

Simplified Procedure should apply Simplified Procedure should apply almost anywherealmost anywhereExamples all from Texas, so equation Examples all from Texas, so equation for Inundation length would have to for Inundation length would have to be derived for new areas. Should be be derived for new areas. Should be similar for similar topography.similar for similar topography.

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Questions?Questions?

Documents

Hydrologic and Hydraulic Guidelines for Dams in Texas · 1 New PMP Temporal Distributions and a Simplified Breach Method for Dams in Texas US Society of Dams April 29, 2008 John Rutledge