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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.

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

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  1. 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

  2. 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

  3. Longshore sediment transport Qls = C Hb5/2 sin 2ab

  4. Erosional Hot Spot due to blocked longshore transport Longshore transport Groin field South Jetty Erosional hotspot Galveston Island State Park

  5. Instrument sled for transport measurement What is C in {Qls = C Hb5/2 sin 2ab}?

  6. Offshore transport due to storms erosion deposition Is offshore transport permanent?

  7. 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

  8. 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

  9. Sediment budget to estimate long- and cross-shore transport DV DV = Qin - Qout

  10. Estimating Volume Change From Shoreline Change Rate de Hb Dc Equilibrium profiles DV = (Hb+Dc) de [m3/m]

  11. 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

  12. Apparent westward longshore transport 180,000 m3/yr average

  13. 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

  14. Wave and potential sediment transport calculations on west end *Station 1079

  15. Potential sediment transport based on WIS waves

  16. Predicted and measured 2001 shoreline(based on 1977, 1979,1982,1989, 1991 waves) Distance Offshore (m) 1990 2001 measured 2001 calculated

  17. Predicted 2011 shoreline as a function of beach nourishment 2001 2011 100,000 m3/yr 2011 no nourishment

  18. Offshore breakwater shifts erosion hotspot down drift breakwater 2001 2011

  19. 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)

  20. Determining offshore sediment transport and sand needs under storm conditions Qoffshore = 500,000 m3/y DV Qoffshore = = Qin – Qout - DV

  21. Who blocks the sand? South jetty Gulf of Mexico State Park Galveston

  22. 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

  23. 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)

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

  25. 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

  26. 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

  27. 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.

  28. 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)

  29. 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.

  30. Estimating Volume Change From Shoreline Change Rate de Hb Dc Equilibrium profiles DV = (Hb+Dc) de [m3/m]

  31. Beach profiles in groin field (Pleasure Pier)

  32. Volume Change From Shoreline Change and From Profiles

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