peak floods –brays bayou who’s in charge !? 500+ yr. events? · 8/30/2018 6 brays bayou @ rice...
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8/30/2018
1
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 Houstonwdupre@uh.edu
“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)?
Presentation by W. R. Dupre’ (UH) 2018
8/30/2018
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FEMANFIPFIRM SFHAZone A, Zone AE,Zone X …LiDAREIASIA
HCFCDFASFWSNWSNOAAEPAPMPSSPEEDTSARP
Acronyms LOMC LOMA LOMRLIDAEPARISJRAUSACOEMUDCBA
Presentation by W. R. Dupre’ (UH) 2018
Brazos
Trinity
ColoradoSan
Jacinto
Presentation by W. R. Dupre’ (UH) 2018
TexasWatersheds
Brazos
Trinity
ColoradoSan
Jacinto
San Jacinto River Watersheds
Presentation by W. R. Dupre’ (UH) 2018
* 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’ (
West ForkSan Jacinto
Spring
Watersheds consist of a series of smaller sub-basins going upstream
Subbasin
8/30/2018
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BrazosRiver
TrinityRiver
Kingwood
Presentation by W. R. Dupre’ (UH) 2018
Jurisdictional boundaries rarely coincide with
Hydrologic boundaries! Kingwood
West ForkSan Jacinto
East ForkSan Jacinto
Spring
1204101
Buffalo – San JacintoEPA’s
Hydrologic Unit Codes(HUC)
1204102Kingwood
Presentation by W. R. Dupre’ (UH) 2018
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
8/30/2018
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* only measures stage!
Bubbler-type Gage Station*
http://users.humboldt.edu/finneyb/nws/lesson4.html
Presentation by W. R. Dupre’ (UH) 2018
https://waterdata.usgs.gov/tx/nwis/rt
Presentation by W. R. Dupre’ (UH) 2018
@ 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
Presentation by W. R. Dupre’ (UH) 2018
40’
50’
Gage
Hei
ght
(ft)
Dis
char
ge (c
fs)
8/30/2018
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Additional Flood Data on USGS’s Gaging Stations
@ 15 minute intervals
Height
http://www.sjra.net/lakeconroe/stormevents/
Presentation by W. R. Dupre’ (UH) 2018
~260,000?
www.harriscountyfws.org Harris County AreaGaging Stations
Presentation by W. R. Dupre’ (UH) 2018
USGS Stream Gages
Kingwood
Presentation by W. R. Dupre’ (UH) 2018
East Fork
Harris County Stream Gages
East Fork
www.harriscountyfws.org
8/30/2018
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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!
Presentation by W. R. Dupre’ (UH) 2018
“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”
Presentation by W. R. Dupre’ (UH) 2018
8/30/2018
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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
Presentation by W. R. Dupre’ (UH) 2018
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 =
Presentation by W. R. Dupre’ (UH) 2018
GHFMC Fact Sheet 4
“Minimal” Hazard
Presentation by W. R. Dupre’ (UH) 2018
1% floodplain0.2% floodplain
floodway
BFE = Base Flood Elevation
“Minimal” Hazard
1% floodplainfloodway
0.2% floodplain
1% BFE0.2%
Presentation by W. R. Dupre’ (UH) 2018
www.harriscountyfemt.org
8/30/2018
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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
8/30/2018
9
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?
Presentation by W. R. Dupre’ (UH) 2018
8/30/2018
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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
8/30/2018
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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
Presentation by W. R. Dupre’ (UH) 2018
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
Presentation by W. R. Dupre’ (UH) 2018
R I = N + 1m
N = # yrs of recordm = rank (largest = 1)
https://water.usgs.gov/
8/30/2018
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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]
Presentation by W. R. Dupre’ (UH) 2018
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
Presentation by W. R. Dupre’ (UH) 2018
~ 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!
Presentation by W. R. Dupre’ (UH) 2018
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.
Presentation by W. R. Dupre’ (UH) 2018
8/30/2018
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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
8/30/2018
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2) Are there better hydrologic models?
Presentation by W. R. Dupre’ (UH) 2018
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
Presentation by W. R. Dupre’ (UH) 2018
8/30/2018
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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
Presentation by W. R. Dupre’ (UH) 2018
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
Presentation by W. R. Dupre’ (UH) 2018
“Leaky” Drainage Divide
8/30/2018
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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
Presentation by W. R. Dupre’ (UH) 2018
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
Presentation by W. R. Dupre’ (UH) 2018
8/30/2018
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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
Presentation by W. R. Dupre’ (UH) 2018
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.
Presentation by W. R. Dupre’ (UH) 2018
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