peak floods –brays bayou who’s in charge !? 500+ yr. events? · 8/30/2018 6 brays bayou @ rice...

8/30/2018

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Houston Geological Society Presents

Flooding and Floodplains in the Houston Area:Past, Present, and Future: Part 1

An Informational Workshop

Presented August 24, 2018@ Kingwood, TX

Dr. William R. Dupre’Professor Emeritus

Earth & Atmospheric SciencesUniversity of [email protected]

“Civilization exists by geological consent, subject to change without notice. ”

Will Durant

"Those who cannot remember the past are condemned to repeat it.“

George Santayana

“The definition of insanity is doing the same thing over and over again and

expecting a different result.”Albert Einstein

Peak Floods – Brays Bayou 08075000

AllisonHarvey

Memorial Day

Tax Day

Allison

500+ yr. events?

Presentation by W. R. Dupre’ (UH) 2018

Who’s in Charge !?Harris County Flood Control District (HCFCD)?

U.S. Army Corps of Engineers (USACOE)?

San Jacinto River Authority (SJRA)?

City of Houston, Bellaire, West University, Pasadena, etc.?

Municipal Utility Districts (MUDs)?

Levee Improvement District (LID)?

Harris County?

Fort Bend County?

Texas Dept. of Transportation (TXDOT)?


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FEMANFIPFIRM SFHAZone A, Zone AE,Zone X …LiDAREIASIA

HCFCDFASFWSNWSNOAAEPAPMPSSPEEDTSARP

Acronyms LOMC LOMA LOMRLIDAEPARISJRAUSACOEMUDCBA


Brazos

Trinity

ColoradoSan

Jacinto


TexasWatersheds

Brazos

Trinity

ColoradoSan

Jacinto

San Jacinto River Watersheds


* Area drained by a river past a specified point ( )

San Jacinto Watershed(~3940 m i2)

West ForkSan Jacinto

East ForkSan Jacinto

Spring

BrazosRiver

TrinityRiver

Buffalo – San Jacinto

Kingwood

Presentation by W. R. Dupre’ (


Spring

Watersheds consist of a series of smaller sub-basins going upstream

Subbasin

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BrazosRiver

TrinityRiver

Kingwood


Jurisdictional boundaries rarely coincide with

Hydrologic boundaries! Kingwood


East ForkSan Jacinto

Spring

1204101

Buffalo – San JacintoEPA’s

Hydrologic Unit Codes(HUC)

1204102Kingwood


Spring Creek

Cypress Creek

Spring Creek

Cypress Creek

Harris County Drainage Network

Presentation by W. R. Dupre’ (UH) 2018 Presentation by W. R. Dupre’ (UH) 2018

www.harriscountyfemt.orgwww.harriscountyfemt.org

Harris County Drainage Basins

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* only measures stage!

Bubbler-type Gage Station*

http://users.humboldt.edu/finneyb/nws/lesson4.html


https://waterdata.usgs.gov/tx/nwis/rt


@ US‐59

@ H‐99

@ I‐45

Kingwood

LakeHouston

LakeConroe

1/1/1973

1/1/1954

USGS 08069500 W Fork San Jacinto nr Humble, TX

Stage (ft)Hydrograph

Discharge (cfs)Hydrograph

https://waterdata.usgs.gov/nwis/rt

July 4th, 2018


40’

50’

Gage

Hei

ght

(ft)

Dis

char

ge (c

fs)

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Additional Flood Data on USGS’s Gaging Stations

@ 15 minute intervals

Height

http://www.sjra.net/lakeconroe/stormevents/


~260,000?

www.harriscountyfws.org Harris County AreaGaging Stations


USGS Stream Gages

Kingwood


East Fork

Harris County Stream Gages

East Fork

www.harriscountyfws.org

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Brays Bayou @ Rice Ave Three Harvey Flood Hydrographs

Presentation by W. R. Dupre’ (UH) 2018www.harriscountyfws.org

Buffalo Bayou @ West Belt 8

~7‐8 ft. over bank

~3‐5 ft. over bank

~18 ft. over bank

E. Fork San Jacinto@ US 59

Additional Flood Data on HCFCD’s Gaging Stations

https://www.harriscountyfws.org/GageDetail/Index/ Presentation by W. R. Dupre’ (UH) 2018

“What is my risk of flooding?”Risk is a hazard evaluated in the context of the

frequency and magnitude of the event [e.g. flood].

Risk = “Frequency” x “Magnitude”

When is Bad too Bad?It depends on the frequency and magnitude of the event, and how and what you (or society) consider an acceptable level of risk!


“Magnitude” is measured by:

1) Height/ discharge/ extent of flooding?

2) Economic damage due to flooding?

3) Number of structures flooded?

4) Number of people affected?

5) Socio‐economic impact?

6) Environmental impact?

Consequences of the “Event”

The “Event”


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YourLogoHere

Harvey Impacts

52INCHES OF RAIN

DAMAGED HOMES365,000

41,500SQ. MILES IMPACTED ACROSS TEXAS

135,584APPROVED APPLICATIONS IN SHELTERS

37,000

8500+CALLS

AUGUST 2017256,845VOLUNTEER

HOURS

18%COUNTY RESIDENTS SUFFERING SERIOUS PSYCHOLOGICAL DISTRESS POST HARVEY1

Costello, 2018 HGS presentation

+ Environmental Damage


The annual recurrence interval (ARI) of an event is the averagenumber of years between events of equal or greater magnitude. Thus, the 100‐year flood (or rain) is one which occurs, on the average, once every 100 years.

Annual Recurrence Interval (ARI)

Annual Exceedance Probability (AEP)The probability (p) of an event of particular magnitude being equaled or exceeded any given 12‐month period is:

1ARI

Therefore, a 100 year flood (or rain) has a 1 in 100 (1/100=1%) probability of being equaled or exceeded any 12‐month period.

AEP =


GHFMC Fact Sheet 4

“Minimal” Hazard


1% floodplain0.2% floodplain

floodway

BFE = Base Flood Elevation

“Minimal” Hazard

1% floodplainfloodway

0.2% floodplain

1% BFE0.2%


www.harriscountyfemt.org

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FIRM’s (Flood Insurance Rate Maps)What information do they contain?

1. Special Flood Hazard Areas (SFHA) ‐ subject to flooding by 1% eventa) The regulatory floodway

b) The 1% (100 year) floodplain (Zones A & AE, etc.)

c) The 1% coastal floodplain (Zone V & VE)

2. The 0.2% (500 year) floodplain (Zone X ‐ shaded)

3. The area of minimal flood hazard (Zone X – not shaded)

4. Base Flood Elevation (BFE) lines, higher resolution, etc.Presentation by W. R. Dupre’ (UH) 2018

msc.fema.gov/portal

Presentation by W. R. Dupre’ (UH) 2018msc.fema.gov/portal

Floodway

FloodwayFringe

FloodwayFringe

1% (100 yr) Floodplain

Floodway Fringe – A regulatory zone within a 1 percent (100-year) floodplain where development (encroachment) should not result in a cumulative surcharge more than 1 ft (or less) in the floodway. Presentation by W. R. Dupre’ (UH) 2018

Surcharge

1% (100 yr) Floodplain (before)

SurchargeEncroachment Encroachment

Floodway – A regulatory zone within a 1 percent (100-year) floodplain closest to a bayou or creek where land development is regulated by a city or a county.

Used for flood insurance

purposes

RecurrenceInterval

AnnualRecurrenceInterval

AnnualExceedanceProbability

Presentation by W. R. Dupre’ (UH) 2018Harris County Flood Control District: Hydrology and Hydraulics Guidance Manual

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1990 -2007 Change in1% (100-yr) Floodplain

2007

1990

Presentation by W. R. Dupre’ (UH) 2018 Presentation by W. R. Dupre’ (UH) 2018

Increase?

Decrease?

1% AEP

1% AEP

1% AEP

2040?

2021?

GFHMC Strategies Report, 2018

San JacintoBuffalo BayouBrays Bayou

NNN

Important Questions!

•What are the causes for past changes in flooding and the floodplain?

•What are the predictions for the future, and on what assumptions are those predictions made.

•What can we do about it?


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Risk

No Risk

44 ft

“He’s OK – he’s outside the 1% floodplain”

False Binary

Bob Jacobsen: Full‐Spectrum Flood Hazard and Pricing Flood Risk – presentation 2018

“I’m OK – I’m outside the 1% floodplain”BUT!- People outside of mapped high-risk flood areas

file nearly 25% of all National Flood Insurance Program (NFIP) flood insurance claims and receive 1/3 of Federal Disaster Assistance for flooding. https://www.fema.gov/

- Most of the homes flooded in Harvey (~75%), the Memorial Day flood (~55%), and TS Allison (~65%) were outside the mapped 1% floodplains!

Presentation by W.R. Dupre’, Univ. of Houston, 2018

https://www.houstonchronicle.com/news/article/In‐Harvey‐s‐deluge‐most‐damaged‐homes‐were‐12794820.php?source=nlp Presentation by W. R. Dupre’ (UH) 2018

154,170 homes* were flooded in Harris County

1% (100‐yr) floodplain 32% (48,850)

0.2% (500‐yr) floodplain 23% (34,970)

Zone of “Minimal Hazard” 45% (70,370)

* 64% had no flood insurance policy in effectLindner & Fitzgerald, 2018, HCFCD Final Immediate Report; Hurricane Harvey ‐ Storm and Flood Information, June 4, 2018

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Why does it flood outside the mapped 1% floodplain?

1) Statistical variability

2) Hydrologic models could be improved

3) It might be greater than a 1% rain

4) Not all flooding is by overbank flow along mapped channels

5) Dam/Levee problems?

6) The “100-year”floodplain might change w/time!Presentation by W. R. Dupre’ (UH) 2018

How do you Calculate Flood-related Risk?

1) Streamflow Data:‐ Graph‐fitting curves‐ Math‐fitting curves2) Regionalization (Parametric Models):‐ From gaged basins to ungaged basins3) Rainfall Data: ‐ Digital Streamflow Models: from rainfall to runoff4) (Probable Maximum) Flood:‐ especially for high‐risk projects


Assumptions using Streamflow Records

•Events are random and independent

•Only deals with overbank flooding (e.g. no ponding)

•No watershed changes during period of record

•Data are not affected by climatic trends

Modified after U.S. Water Resources Council Presentation by W. R. Dupre’ (UH) 2018

How do we go from this probability?Brays Bayou at Main Street (USGS 08075000)

2015 Memorial Day

2015 Halloween

2016 Tax Day

2017 Harvey

Daily M

ean Gage Heigh

t (Stage) in

feet


R I = N + 1m

N = # yrs of recordm = rank (largest = 1)

https://water.usgs.gov/

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Recurrence Interval Graph-Brazos River

10000

100000

Dis

char

ge (i

n cf

s)

10 100Annual Recurrence Interval (yrs)

Annual Exceedance Probability (%)

50

1%10% 2%

2

50% Extrapolated 1% (100 yr)

Flood[~130,000 cfs]


Note scatter!

Annu

al Pea

k Stream

flow (ft3/sec)

100,000

10,000

R I = N + 1m

https://water.usgs.gov/edu/100yearflood‐basic.html

Note: NO

5% (20 yr)

50% (2 yr)

0.2% (500 yr)

Note: NO confidence limits!

Annual Exceedance Probability (%)

Annu

al Pea

k Stream

flow (ft3/sec)

1% (100 yr)44,300 cfs


~ 50 years of record

https://water.usgs.gov/edu/100yearflood‐basic.htmlAnnual Exceedance Probability (%)

Annu

al Pea

k Stream

flow (ft3/sec)

90% Confidence Interval

36,60056,400 = 44,300!36,60056,400 = 44,300!


1) Statistical Variability Rainfall

‐ Historical rainfall data measured by rain gages or radar (e.g. TS Allison, Hurricane Harvey).

‐ Frequency‐based hypothetical storm (e.g. 1%).

‐ Standard Project Storm (SPS). – rarely used now, largely replaced by the 0.2% storm.


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What do we need to know about rainfall?

•DEPTH: How much fell…

•DURATION: Over different intervals of time…

•FREQUENCY: How common is this rainfall?

Historical Rainfall Data

Presentation by W. R. Dupre’ (UH) 2018 Harris County Flood Control District: Hydrology and Hydraulics Guidance Manual Presentation by W. R. Dupre’ (UH) 2018

Annual Exceedance Probability (%)Presentation by W. R. Dupre’ (UH) 2018

0.2% (500 yr.)event

Precipita

tion Dep

th (inche

s)

Data from: Harris County Flood Control District

18.9 “

Harris County Region 2

Confidence Interval?Harris County Region 2

Precipita

tion (in

ches)

3

6810

20

Lake Charles, LA Airport

24 hr

Average Recurrence Interval (yrs)

500 yr

90% Confidence Band~21”

~29”

~15”

100 yr

http://hdsc.nws.noaa.gov?

Presentation by W. R. Dupre’ (UH) 2018Diagram from HCFCD

“1% rain event”

Used to estimate 1% flood

Used to estimate 1% floodplain

Hydrologic Modeling Programs

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2) Are there better hydrologic models?


Published in: Natural Hazards Review, 2017

“…spatially distributed hydrologic models greatly improve floodplain delineation, provide important insights on the drivers of flood damage outside of the floodplain, and offer alternative ways to more effectively communicate flood risk.”

Blessing et al., 2017 Lindner & Fitzgerald, 2018, HCFCD Final Immediate Report; Hurricane Harvey ‐ Storm and Flood Information, June 4, 2018

3) It might be greater than a 1% Rain/Flood

Four‐Day Peak Rainfall Frequency

Hurricane HarveyAugust 25‐29, 2017

Ponding & Overland Flow?

4) Not all flooding is by overbank flow along mapped channels

•Ponding occurs when rain water cannot reach the nearby streams, channels, and rivers, and the ground has exceeded its capacity to store water.

•It is NOT identified on floodplain maps, and can occur anywhere!

Presentation by W. R. Dupre’ (UH) 2018http://www.houstonconsortium.org/p/report

FEMA Floodplain Map TIRZ17 Drainage Study

Ponding Modeled


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5) Problems with Dams and Levees

Dam built in 1961U.S. GEOLOGICAL SURVEYFact Sheet 229-96 Presentation by W. R. Dupre’ (UH) 2018

LeveeDam

Diversion Canal

Diversion Canal

Dam

Dam


Buffalo Bayou Watershed (before)

Buffalo Bayou Watershed (before)

Brays Bayou Watershed

http://www.harriscountyfemt.org/ Presentation by W. R. Dupre’ (UH) 2018

Cypress Creek Cypress Creek

Buffalo Bayou

AddicksWatershed

Barker Watershed


“Leaky” Drainage Divide

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Cypress Creek Overflow During the 1% (100‐year) event, 83% of the stormwater

volume flowing through upstream Cypress Creek overflows into the Addicks

watershed.

Presentation by W. R. Dupre’ (UH) 2018Texas Water Development Board Contract Report Number 1248321466

Reservoir Pools


GHFMC Briefing Document 2: How do Addicks & Barker reservoirs work?

What Reservoir Pools?i.e. water that lies above the conservation pool in

reservoirs used to regulate floodwaters.

Reservoir Pool

Conservation Pool Presentation by W. R. Dupre’ (UH) 2018

v

Flooded by

Harvey

Flooded by Harvey

1% Floodplain0.2%

GHFMC Briefing Document 2: How do Addicks & Barker reservoirs work?

Flooded by Harvey

0.2%

0.2% Floodplain

1% floodplain

Floodway

Government property boundary

Harvey flood pool


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“In December 2010 Addicks and Barker Dams were classified as a DSAC I by HQUSACE (Headquarters, U.S. Army Corps of Engineers).

http://www.swg.usace.army.mil/Portals/26/docs/Planning/Addicks%20and%20Barker%20Decision%20Document%20Review%20Plan.pdf

www.spl.usace.army.mil/Portals/17/docs/DamSafety/damsafety_actionsclassifaction.pdf

These dams were classified as two of the six most dangerous dams in the United States operated by the Corps of Engineers due to the risk of structural failure and its potential impact on people or property.

Dam Safety Issues


Actual Buffalo Bayou Inundation* Map:August 30, 2017 *Following dam release

https://www.harriscountyfws.org/

Presentation by W. R. Dupre’ (UH) 2018Flooded outside 1% floodplain

Levees*

http://www.cnrfc.noaa.gov/wfo_hydro.php Presentation by W. R. Dupre’ (UH) 2018*Remember Katrina! www.swg.usace.army.mil/Portals/26/(07)%20FortBendUSACE080416Presentation.pdf?ver=2016‐

08‐03‐145835‐400

Harvey flooded at least 900 levee-protected homes in Fort Bend, including about 580 in Riverstone and others in First Colony, New

Territory and Sienna Plantation.


peak floods –brays bayou who’s in charge !? 500+ yr. events? · 8/30/2018 6 brays bayou @ rice...

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