Follets Island Nearshore Beach Nourishment Project

Transcription

1 Coast & Harbor Engineering A division of Hatch Mott MacDonald Follets Island Nearshore Beach Nourishment Project Arpit Agarwal, P.E. April 1, 2016

2 Project Site!"#$% &'(% &)*+% )#,'-+%

3 Shoreline Erosion Rates!"#$%&'($)"(($ -.$45$-6/$0123$!"#$%&'($)"(($ *+,$-./$0123$ 789:$;<6;$

4 Erosion of West Bay Bird Island

5 Bathymetry ;<6;$ ;<<=$-$;<6>$

6 Flow Control

7 1995

8 Project Need Some of the highest erosion rates in TX due to manmade changes to system Lack of sediment sources! No offshore borrow source available! Upland sources cost prohibitive SLP acts as a sediment sink (approx. 200k cy/yr)

9 Project History SLP Inlet Management Plan performed over 3 phases over the course of 10 years! Phase 1 can we use of material from the SLP flood shoal to restore beaches, dunes and islands?! Phase 2 technical feasibility! Phase 3 environmental feasibility

10 Alternatives Analysis Investigation of potential environmental impacts from proposed dredging alternatives by analyzing:! Hydrodynamics! Salinity! Morphology Investigated 7 different dredge templates! Template selected has least amount of impact

11 Existing 2006 Alt 1 Alt 2 Alt 3 Alt 4 Alt 5 Alt 6 Alt 7

12 Existing 2006 Alt 3 Alt 6 Alt 1 Alt 4 Alt 2 Alt 5 Alt 7

13 Numerical Modeling CMS (USACE)! circulation! waves! sediment transport

14 Numerical Modeling SELFE 3D Salinity Developed numerical model of salinity for Galveston Bay Freshwater inflows, ET from TWDB HF?I$$

15 Salinity Evaluation Almost no change in salinity Trends are consistent for typical, high and low salinity times Typical Salinity

16 Morphology Small changes result from proposed alts Alt 5 (channel) changes less than Alt 6 (wide-area) Areas along Galveston Island experience negligible changes

17 Water Level Comparisons Difference between existing conditions (No Action Alternative 2015) and dredge template very minor More difference between existing conditions and previous bathymetry Range [m] %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%6$$$$$$$$$$$$$$$;$$$$$$$$$$$$$$$$$/$$$$$$$$$$$$$$$>$$$$$$$$$$$$$$$$L$$$$$$$$$$$$$$$6<$

18 Velocity Comparisons Difference between existing conditions (No Action Alternative 2015) and dredge template very minor More difference between existing conditions and previous bathymetry <M.>$ <M.<$ <M/>$ <M/<$ <M;>$ <M;<$ <M6>$ <M6<$ <M<>$ <M<<$ 6$ ;$ /$ >$ L$ 6<$

19 Hydrodynamic Patterns Localized flow rates changed by less than 7% Flow rates around West Bay Bird island decreased No change in water surface elevation, velocity, or tide range observed at sensitive habitat locations NG55K$$ $$$$9OO$$!"#$%&'#(#)*&+,-+'.$/&0/(#'+$1&+,')/.*/&23+(/&!'##(%&'#(#)*&+,-+'.$/&0/(#'+$1&-/')/.*/&

20 Project Proposed

21 Nearshore Beach Nourishment Placement of sand as submerged beach nearshore Widening of beach by reduction in erosive wave forces and onshore movement of material Reduced social impact by not interfering with activities on the beach during construction as no construction equipment on beach. Reduced environmental impact doesn t interfere with turtle nesting or piping plover habitat. Reduced permitting efforts

22 Anticipated Construction Methodology Cutterhead dredger (12-16 dia.) Pipeline corridor Est. 2-3 booster pumps necessary Pipeline diffuser plate at placement location No equipment on beach

23 Monitoring 5 year monitoring plan Bathymetry and topography Sensitive Resources! West Bay Bird Island! Emergent vegetation! Seagrass beds! Oyster reefs! Sand flats

24 Project Impacts No seagrass impacts, wetlands or oyster reef impacts would result from project Dredge template located within piping plover habitat Dredging would impact approximately 55.3 acres of unvegetated bottom Nearshore placement of material would impact approximately 43.5 to 58.9 acres of un-vegetated bottom

25 Project Benefits Local sand source for rapidly eroding area! San Luis Pass has net accretion of 200k cy/year Reduced tidal currents along Treasure Island Shoreline! Reduced erosion at Mud Island, Treasure Island, West Bay Bird Island

26 Contributors Casey Connor, P.E.; Hatch Mott MacDonald Josh Carter, P.E., D.CE.; Hatch Mott MacDonald Thomas Durnin; Texas General Land Office Kevin Frenzel, P.G.; Texas General Land Office

27

28