Analysis and numerical modeling of Galveston shoreline change – implications for erosion control

Dr. Tom Ravens and Khairil Sitanggang

Texas A&M University at Galveston

Supported by

Texas Sea Grant, Texas GLO Galveston County, Texas A&M,

Corps of Engineers

Study objectives

• To determine (quantify) the processes responsible for beach change– Longshore sediment transport– Cross-shore sediment transport

• To use that knowledge to design effective and realistic erosion control measures

Longshore sediment transportQls = C Hb

5/2 sin 2b

6

7

8

9

10

11

12

13

14

15

0 5 10 15 20 25

Alongshore distance (km)

Cro

ss

-sh

ore

dis

tan

ce

(x

10

0 m

)

1990 shoreline

2001 shoreline

SEAWALL

GROIN

South Jetty

Galveston Island StatePark

Groin field

Erosionalhotspot

Erosional Hot Spot due to blocked longshore transport

Longshore transport

Instrument sled for transport measurementWhat is C in {Qls = C Hb

5/2 sin 2b}?

Offshore transport due to storms

erosion

deposition

Is offshore transport permanent?

Limitations to direct calculation of beach change from processes

• Available WIS wave data (1990-2001) leads to sediment transport predictions in direction opposite of observed direction.

• No easy way to calculate cross-shore transport

Alternative (indirect) approach

• Analyze shoreline data (1956, 65, 90, and 2001) with a sediment budget and infer longshore and cross-shore transport indirectly

• Identify period (1990-2001) which was dominated by longshore transport

• Use longshore data (from 1990-2001) to screen and select wave data which can then be used for detailed design of shoreline protection measures

Sediment budget to estimate long- and cross-shore transport

compartment i

OCEAN

LAND

QinQout

LAND

V

V = Qin - Qout

Estimating Volume Change From Shoreline Change Rate

-6

-5

-4

-3

-2

-1

0

1

2

-400 -200 0 200 400 600 800 1000

distance offshore of shoreline (m)

ele

vat

ion

(m

)

1990 profile

1970 profile

V = (Hb+Dc) de [m3/m]

Dc

Hb

de

Equilibrium profiles

East End Sediment Budget

South jetty

Compartment 1Compartment 2

2.5 km3 km

Q = 0Q = 41,000 m3/yr

V = 41,000 m3/yrV = -35,000 m3/yr

Q = 6000m3/yr

Apparent westward longshore transport

-100000

0

100000

200000

300000

400000

500000

600000

0 10000 20000 30000 40000 50000

longshore distance (m)

we

stw

ard

lon

gs

ho

re t

ran

sp

ort

(m

3/y

r)

1965-90

1990-2001

1956-65

San Luis Passsouth jetty end of seawall

180,000 m3/yr average

Year Selected hurricanes and tropical storms (1956-2001)

Maximum storm surge at Galveston Gulf shoreline

Number of hours with storm surge above 1.5 m

1957 Audrey ??? ???

1961 Carla 2.75 55

1980 Allen 1.1 0

1983 Alicia 2.4 7

1996 Josephine 1.0 0

1998 Frances 1.4 0

2001 Allison 0.9 0

Storms 1956-2001

* Station 1079

Wave and potential sediment transport calculations on west end

Potential sediment transport based on WIS waves

278

190 21

1

181

208

200

190

161

209

124

149

223

161

186

252

183

229

164

218

211

0

50

100

150

200

250

300

1976

1977

1978

1979

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

Y E A R

Q (

1000

m3 /y

ear)

Predicted and measured 2001 shoreline(based on 1977, 1979,1982,1989, 1991 waves)

0

100

200

300

400

500

600

0 5 10 15 20 25 30 35

Alongshore distance (km)

Y (

m)

DistanceOffshore(m)

1990 2001 measured

2001 calculated

Predicted 2011 shoreline as a function of beach nourishment

0

100

200

300

400

500

600

700

800

0 5 10 15 20 25 30 35

Alongshore distance (km)

Y (

m) 2001

2011 no nourishment

2011 100,000 m3/yr

Offshore breakwater shifts erosion hotspot down drift

0

100

200

300

400

500

600

700

800

0 5 10 15 20 25 30 35

Alongshore distance (km)

Y (

m) 2001

2011

breakwater

Designing erosion control measures for hurricanes

• Approach: use wave data to calculate longshore transport for 1956-65, 1965-90

• Use measured volume change for these periods• Infer offshore transport rates based on sediment

budget concept• Find offshore transport rates of about 500,000

m3/yr• Expect to spend about $3,000,000 to $5,000,000

per year (if 1956-1990 trend returns)

Determining offshore sediment transport and sand needs under storm conditions

Qoffshore

LAND

QinQout

LAND

V

Qoffshore= Qin – Qout - V

Qoffshore = 500,000 m3/y

Who blocks the sand?

5500

5600

5700

5800

5900

6000

6100

6200

0 5 10 15 20 25 30

longshore distance from south jetty (km)

cro

ss-s

ho

re p

osi

tio

n

1990

2001 no-jetty

2001 no-groin

Southjetty

Galveston

StatePark

Gulf of Mexico

Conclusions• Sediment budget effective tool for estimating longshore

transport and cross-shore transport• Modeling (neglecting hurricanes) indicates about

100,000 m3/yr needed for hotspot• Much more sand (~500,000 m3/yr) would be needed for

west end if hurricanes return• Majority of erosion on west end is due to storm-induced

cross-shore transport • Groin field suffers relatively little storm-induced erosion• Tropical storms do not cause permanent loss of sand

V = 6,000 m3/yr

6,000 m3/yr

V = -69,000 m3/yr

63,000 m3/yr

V = -255,000 m3/yr

778,000 m3/yr

V = -309,000 m3/yr

371,000 m3/yr

(64,000)

(64,000)

-18,000)

(67,000)(20,000)

-155,000)

220,000)(175,000)

(-45,000)

Shoreline Change,1956-1965, 1965-90 and 1990-2001

-20

-10

0

10

20

0 10000 20000 30000 40000 50000

longshore distance (m)

ero

sio

n r

ate

(m

/yr)

Interpretation of “Calculated” Longshore Transport

• Very high longshore transport calculated for 1956-65 and for 1965-90 probably due to neglecting cross-shore transport associated with Hurricanes Carla and Alicia

• Cross-shore transport probably from the beach/nearshore to the offshore– Little evidence of over wash during Alicia– Dellapenna data indicates significant sand deposition

into the mud beyond the depth of closure. • Assume 4 cm/yr deposition, 20% sand, 50 km x 5 km area,• Calculate: 2 million m3/yr cross-shore transport

Conclusions

• Calculating changes in sediment volume based on shoreline change appears to underestimate volume change somewhat.

• Calculations of longshore transport based on offshore wave conditions appears uncertain.

• Sediment budget/flows are a function of time especially at the west end of the island

Future Work

• Account for other flows besides wave-derived longshore transport in the surf zone.

• Account for the build up of sediment at big reef (which suggests transport across the south jetty) and possible cross-shore transport at the East Beach.

• We need to better understand the dynamics of San Luis Pass and the role it plays on the sediment budget.

V = 6,000 m3/yr

6,000 m3/yr

V = -69,000 m3/yr

63,000 m3/yr

V = -255,000 m3/yr

778,000 m3/yr

V = -309,000 m3/yr

371,000 m3/yr

(64,000)

(64,000)

-18,000)

(67,000)(20,000)

-155,000)

220,000)(175,000)

(-45,000)

Analysis of shoreline data from Galveston Island

• Sediment budget based on shoreline data (1956, 1965, 1990, 2001)

• Identify stormy periods (with cross-shore transport) and calm periods

• Quantification of cross-shore and longshore transport during different periods of time

• GENESIS modeling during 1990-2001• Design of beach nourishment 2001-2011.

Estimating Volume Change From Shoreline Change Rate

-6

-5

-4

-3

-2

-1

0

1

2

-400 -200 0 200 400 600 800 1000

distance offshore of shoreline (m)

ele

vat

ion

(m

)

1990 profile

1970 profile

V = (Hb+Dc) de [m3/m]

Dc

Hb

de

Equilibrium profiles

Beach profiles in groin field (Pleasure Pier)

-25

-20

-15

-10

-5

0

5

10

15

20

0 1000 2000 3000 4000

distance offshore (ft)

ele

va

tio

n (

ft)

1977

1994

20011977

1994

2001

Volume Change From Shoreline Change and From Profiles

0

200

400

600

800

0 200 400 600 800

Volume change from shoreline change data [m3/m]

Vo

lum

e c

han

ge

fro

m p

rofi

le d

ata

[m3/m

]