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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.

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analysis and numerical modeling of galveston shoreline change implications for erosion control

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
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 transport
Longshore sediment transport

Qls = C Hb5/2 sin 2ab

slide4

Erosional Hot Spot due to blocked longshore transport

Longshore transport

Groin field

South

Jetty

Erosional

hotspot

Galveston Island

State

Park

instrument sled for transport measurement
Instrument sled for transport measurement

What is C in {Qls = C Hb5/2 sin 2ab}?

offshore transport due to storms
Offshore transport due to storms

erosion

deposition

Is offshore transport permanent?

limitations to direct calculation of beach change from processes
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
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
estimating volume change from shoreline change rate
Estimating Volume Change From Shoreline Change Rate

de

Hb

Dc

Equilibrium profiles

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

east end sediment budget
East End Sediment Budget

Compartment 1

Compartment 2

3 km

2.5 km

Q = 0

Q = 6000m3/yr

Q = 41,000 m3/yr

DV = 41,000 m3/yr

DV = -35,000 m3/yr

South jetty

slide14

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

slide15

Wave and potential sediment transport

calculations on west end

*Station 1079

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

Distance

Offshore

(m)

1990

2001 measured

2001 calculated

predicted 2011 shoreline as a function of beach nourishment
Predicted 2011 shoreline as a function of beach nourishment

2001

2011 100,000 m3/yr

2011 no nourishment

designing erosion control measures for hurricanes
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
Determining offshore sediment transport and sand needs under storm conditions

Qoffshore = 500,000 m3/y

DV

Qoffshore = = Qin – Qout - DV

who blocks the sand
Who blocks the sand?

South

jetty

Gulf of Mexico

State

Park

Galveston

conclusions
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
slide24

DV = 6,000 m3/yr

(64,000)

6,000 m3/yr

(64,000)

DV = -69,000 m3/yr

(-18,000)

(67,000)

63,000 m3/yr

DV = -309,000 m3/yr

(20,000)

(-155,000)

DV = -255,000 m3/yr

371,000m3/yr

(220,000)

(175,000)

778,000 m3/yr

(-45,000)

interpretation of calculated longshore transport
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
conclusions1
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
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.
slide29

DV = 6,000 m3/yr

(64,000)

6,000 m3/yr

(64,000)

DV = -69,000 m3/yr

(-18,000)

(67,000)

63,000 m3/yr

DV = -309,000 m3/yr

(20,000)

(-155,000)

DV = -255,000 m3/yr

371,000m3/yr

(220,000)

(175,000)

778,000 m3/yr

(-45,000)

analysis of shoreline data from galveston island
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 rate1
Estimating Volume Change From Shoreline Change Rate

de

Hb

Dc

Equilibrium profiles

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