Flooding in the Flooding in the Coastal PlainCoastal Plain

National Flood WorkshopNational Flood Workshop

October 26, 2010October 26, 2010

Houston, TexasHouston, Texas

Thomas Mountz, P.E., D.WRE, CFM

Espey Consultants, Inc.4801 Southwest Parkway

Parkway 2, Suite 150Austin, Texas 78735

Lower Rio Grande ValleyLower Rio Grande Valley NFIP Program losses from ’78-’04 - $9.6 NFIP Program losses from ’78-’04 - $9.6

MillionMillion Fourth fastest growing metropolitan area in Fourth fastest growing metropolitan area in

the State: Growth = Development Pressurethe State: Growth = Development Pressure Federal disasters in 1984, 1991, 2008, 2010Federal disasters in 1984, 1991, 2008, 2010 FLAT - no natural streamsFLAT - no natural streams Arid region with significantArid region with significant flooding potential fromflooding potential from Rio Grande, hurricane/Rio Grande, hurricane/ tropical storm and other tropical storm and other rainfall events.rainfall events.

How Flat is Flat?How Flat is Flat?

Slope = 1 foot per mile or lessSlope = 1 foot per mile or less

Hurricane DollyHurricane Dolly 10-16 inches of 10-16 inches of

rainfall in 24 hoursrainfall in 24 hours Two weeks of heavy Two weeks of heavy

rains followed rains followed Hurricane DollyHurricane Dolly

Some areas were Some areas were flooded for monthsflooded for months

Cameron County Storm Cameron County Storm Drainage StudyDrainage Study

Focus on Focus on UnincorporateUnincorporated Areasd Areas

Focus on Focus on flooding from flooding from Hurricane DollyHurricane Dolly

Lake Tio Cano Lake Tio Cano had extensive had extensive damagedamageProblem Area

Unincorporated Area

Incorporated Area

Tio Cano LakeTio Cano Lake 171 Parcels at or 171 Parcels at or

below elev. 46 ftbelow elev. 46 ft Appraised value in Appraised value in

excess of $8 excess of $8 MillionMillion

More than 2.5 More than 2.5 miles of streets at miles of streets at or below elev. 46 ftor below elev. 46 ft

No Natural OutletNo Natural Outlet FM-506 closed for FM-506 closed for

more than one more than one month (raised by month (raised by TxDOT in 2009)TxDOT in 2009)

Flatland Runoff MethodologyFlatland Runoff Methodology

HEC-HMS Hydrology ModelHEC-HMS Hydrology Model Calculated using Kerby-Kirpich method for Time of Calculated using Kerby-Kirpich method for Time of

Concentration Concentration Kirby-Kirpich recommended by Roussel et al., 2005 Kirby-Kirpich recommended by Roussel et al., 2005

Study of 92 gauged Texas watersheds and 1,600 Study of 92 gauged Texas watersheds and 1,600 storm eventsstorm events

Peak Rate Factor Reduction as suggested by recent Peak Rate Factor Reduction as suggested by recent USACE studies USACE studies

Modified Puls & Reservoir Routing due to extensive Modified Puls & Reservoir Routing due to extensive overbank storageoverbank storage

Source: NEH 4, Fang et al., 2005

Peak Rate Factor : PRF Values

100 Very flat150200300 Flat370 Central Texas Streams400484 Standard SCS500600 Steep mountainous

Kerby – Kirpich MethodKerby – Kirpich Method

Kerby Overland Flow Kerby Overland Flow EquationEquation L = Length in feet L = Length in feet S = Slope (ft/ft)(not to S = Slope (ft/ft)(not to

exceed 1,200 feet)exceed 1,200 feet) N = Retardance Coefficient N = Retardance Coefficient

(dimensionless)(dimensionless)

Kirpich Shallow Kirpich Shallow Concentrated /Channel Concentrated /Channel Flow EquationFlow Equation L = Length in feet L = Length in feet S = Slope (ft/ft)S = Slope (ft/ft)

CalibrationCalibration

0 20000 40000 60000 80000 100000-10

0

10

20

30

40

50

CCDD #5 North Main Plan: Existing Conditions 10/12/2007 9:38:16 AMGeom: Existing Conditions Flow: Base

Main Channel Distance (ft)

Ele

vatio

n (f

t)

Legend

WS May25_SCS_AMC2

WS May25

Ground

OWS May25

FM

142

0

FM

507

FM

508

Brig

gs R

oad

New

Com

bes

Loop

499

Bus

ines

s 77

Exp

ress

way

77

Wils

on R

oad

Che

ster

Par

k

April 5, 2004 Storm Stuart Place Main Drain

Computed Outlet (J-SP01) Hydrograph

0

50

100

150

200

250

300

350

400

450

500

0 6 12 18 24 30 36 42 48

Time (hours)D

isch

arg

e (c

fs)

TR-55, AMC 1

TR-55, AMC 2

Kerby/Kirpich, AMC 1

Kerby/Kirpich, AMC 2

Kerby/Kirpich, PRF 150, AMC 1

Kerby/Kirpich, PRF 150, AMC 2

Proposed Tio Cano Lake Proposed Tio Cano Lake OutletOutlet

20.00

25.00

30.00

35.00

40.00

45.00

50.00

55.00

60.00

0 5000 10000 15000 20000 25000

Length (ft)

Ground

Tio Cano Outlet

Outlet Channel

Approx. 3.8 mile channelApprox. 3.8 mile channel 12-20 ft BW, 2.5:1 SS12-20 ft BW, 2.5:1 SS 10 ft water depth10 ft water depth 70 ft weir length at 70 ft weir length at

channel inletchannel inlet 28 ft max cut28 ft max cut

Tio Cano Lake100-YR, 48 hr Storm

39

40

41

42

43

44

45

46

47

1/1/090:00

1/2/090:00

1/3/090:00

1/4/090:00

1/5/090:00

1/6/090:00

1/7/090:00

Date/Time

Ele

vatio

n (ft

)

No Outlet

70' Weir at elev 41.5 ft

Questions, DiscussionQuestions, Discussion

Thank you for this Thank you for this opportunity!opportunity!

Thomas W. Mountz, P.E., D.WRE, Thomas W. Mountz, P.E., D.WRE, CFMCFM

tmountz@espeyconsultants.comtmountz@espeyconsultants.com

(512) 326-5659(512) 326-5659

Lake Tio CanoLake Tio Cano ““Dry” lake bed located in sump Dry” lake bed located in sump

area between La Feria and area between La Feria and Santa RosaSanta Rosa

Much of the lake area included Much of the lake area included in National wildlife Refuge in National wildlife Refuge

No Natural OutletNo Natural Outlet Pumped outlet to Wilson Canal Pumped outlet to Wilson Canal

& trucks for several months& trucks for several months Hurricane Dolly reached Hurricane Dolly reached

approx. Elevation 46 (100-YR approx. Elevation 46 (100-YR FIRM). FIRM).