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 Q ls = C H b 5/2 sin 2  b

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

5. Instrument sled for transport measurement What is C in {Q ls = C H b 5/2 sin 2  b }?

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 VV  V = Q in - Q out

10. Estimating Volume Change From Shoreline Change Rate  V = (H b +D c ) d e [m 3 /m] DcDc HbHb dede Equilibrium profiles

12. East End Sediment Budget South jetty Compartment 1 Compartment 2 2.5 km3 km Q = 0 Q = 41,000 m 3 /yr  V = 41,000 m 3 /yr  V = -35,000 m 3 /yr Q = 6000m 3 /yr

13. Apparent westward longshore transport 180,000 m 3 /yr average

14. YearSelected hurricanes and tropical storms (1956-2001) Maximum storm surge at Galveston Gulf shoreline Number of hours with storm surge above 1.5 m 1957Audrey??? 1961Carla2.7555 1980Allen1.10 1983Alicia2.47 1996Josephine1.00 1998Frances1.40 2001Allison0.90 Storms 1956-2001

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

16. Potential sediment transport based on WIS waves

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

18. Predicted 2011 shoreline as a function of beach nourishment 2001 2011 no nourishment 2011 100,000 m 3 /yr

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

20. 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 m 3 /yr Expect to spend about $3,000,000 to $5,000,000 per year (if 1956-1990 trend returns)

21. Determining offshore sediment transport and sand needs under storm conditions VV Q offshore  = Q in – Q out -  V Q offshore = 500,000 m 3 /y

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

23. Conclusions Sediment budget effective tool for estimating longshore transport and cross-shore transport Modeling (neglecting hurricanes) indicates about 100,000 m 3 /yr needed for hotspot Much more sand (~500,000 m 3 /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

24.  V = 6,000 m 3 /yr 6,000 m 3 /yr  V = -69,000 m 3 /yr 63,000 m 3 /yr  V = -255,000 m 3 /yr 778,000 m 3 /yr  V = -309,000 m 3 /yr 371,000 m 3 /yr (64,000)  - 18,000) (67,000) (20,000)  -155,000)  220,000) (175,000) (-45,000)

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

26. 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 m 3 /yr cross-shore transport

27. 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

28. 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.

29.  V = 6,000 m 3 /yr 6,000 m 3 /yr  V = -69,000 m 3 /yr 63,000 m 3 /yr  V = -255,000 m 3 /yr 778,000 m 3 /yr  V = -309,000 m 3 /yr 371,000 m 3 /yr (64,000)  - 18,000) (67,000) (20,000)  -155,000)  220,000) (175,000) (-45,000)

32. 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.

33. Estimating Volume Change From Shoreline Change Rate  V = (H b +D c ) d e [m 3 /m] DcDc HbHb dede Equilibrium profiles

34. Beach profiles in groin field (Pleasure Pier)

35. Volume Change From Shoreline Change and From Profiles