LITTLE LAGOON & LITTLE LAGOON PASS: RESEARCH UPDATES & DIRECTIONS

Transcription

1 LITTLE LAGOON & LITTLE LAGOON PASS: RESEARCH UPDATES & DIRECTIONS Bret M. Webb, PhD, PE, DCE Professor Department of Civil, Coastal, and Environmental Engineering October 19, 2017 LLPS Meeting

2 Acknowledgments Alabama DOT Don Powell USA Collaborators Dr. Alex Beebe, Patrick Hautau, Bryan Groza Dr. Scott Douglass, Ph.D., P.E., D.CE Photos & Figures ALDOT, Groza (2016) 2 Groza, B Modeling changes in hydrodynamics and sediment transport due to inlet modification at Little Lagoon Pass, Alabama. MS Thesis, University of South Alabama, May pp.

3 Navigation I. Background II. New Modeling III. New Work Purpose Fully Coupled CMS SGD Influence Study Area Jetty Modifications WQ Linkages Work to Date Preliminary Results Improved Modeling 3

4 Lagoon Pass I. Background 4

5 I. Background Purpose Monitoring & Evaluation Long Term Monitoring Flood Shoal Volume Bypassing Lagoon WQ 5

6 I. Background Study Area Gulf Shores, Alabama Little Lagoon Lagoon Pass Ebb/Flood Tidal Shoals Channel 3+ km E/W of Pass 6

7 I. Background Monitoring Annual Surveys Ebb/Shoal Volumes Channel Beach Profiles Tides & Currents 7 Figures from Groza (2016)

8 I. Background Work to Date Research & Data Collection 2013 Present Surveys Coupled 2D Model physics only 8

9 Lagoon Pass II. CMS Modeling 9

10 II. CMS Modeling Fully Coupled Model Development Measured Bathymetry Tides Waves Local Winds Sediment Transport 10

11 original widened II. CMS Modeling Deposition With & Without 1-Yr Simulations Full Hindcast Tides, Waves, Wind Same Initial Conditions Channel Only Difference Scour 11

12 II. CMS Modeling Jetty Modifications Hypothetical Modeled worst case scenario leading to deposition Variable tides, winds & waves from SE Scour Results compared relative to baseline conditions Control Simulation 12

13 II. CMS Modeling Single Updrift Jetty Hypothetical 160-ft East Extension 15% Reduction in Volume 3% Reduction in LST Single Jetty Systems Problematic MOD1 13

14 II. CMS Modeling Angled Updrift Jetty Hypothetical 160-ft South of Existing Jetty 21% Increase in Volume 7% Reduction in LST Acts as Breakwater Accretion of West Beach MOD2 14

15 II. CMS Modeling Double Angled Extensions Hypothetical 160-ft South of Existing Jetties 2% Increase in Volume 23% Reduction in LST Major Changes to Ebb Delta MOD3 15

16 II. CMS Modeling Double Straight Extensions Hypothetical 160-ft South of Existing Jetties 36% Increase in Volume 24% Reduction in LST Classic Jetty Design MOD4 16

17 II. CMS Modeling Overall Results Double Straight Extensions Hypothetical 50% 40% 160-ft South of Existing 30% Jetties 20% 10% 36% Increase in 0% Volume -10% 24% Reduction in LST -20% -30% -40% Classic Jetty Design -50% Performance Assessment of Jetty Extensions % Vol Change % LST Change 17

18 II. CMS Modeling Overall Results Deposition General Conclusions Hypothetical Jetty Extensions Results based on limited modeling for purpose of identifying potential feasibility Results suggest that all modifications cause impacts Only one potential option provides more benefit than negative impact, but both are very small No magic bullet! Requires management regardless of solution 18

19 Lagoon Pass III. New Work 19

20 III. New & Ongoing Work Deposition SGD Influence Submarine Groundwater Discharge New instruments Lagoon Pass Shelby Lake Gator Lake Field surveys Linkages to water quality 20

21 III. New & Ongoing Work SGD Influence Submarine Groundwater Discharge New instruments Lagoon Pass Shelby Lake Gator Lake Field surveys Linkages to water quality 21

22 III. New & Ongoing Work WQ Linkages Pass Area & Water Quality Database of pass soundings and area 2012 Present HS student will perform statistical analysis linking pass area with LLPS WQ data 22

23 III. New & Ongoing Work Improved Modeling XBeach Develop new 2D physics-based model Better integration of groundwater inputs Better sediment transport capabilities 23

24 Key Points Overview & Status CMS Modeling New Work Surveying 2012-Present Flood Shoal Volume Steady Jetty Modifications No Magic Bullet! SGD Influence WQ Linkages 24

25 I. Background History Legal Battles! Bridge: 1968 Jetties: 1981 (+90 m seaward) Contentious Legal Battles 1992 Consent Decree Annual Monitoring Figures from Gibson (2006)

26 I. Background History Competing Interests : Bridge 1981: Jetties (+90 m seaward) Contentious Legal Battles 1992: Consent Decree 2015: Widening, Extension ~25,000 CM/yr bypassed 26 Figures from Gibson (2006)

27 II. CMS Modeling Shortcomings Freshwater Input Gator/Shelby Lakes Local Rainfall SGD (aquifer system) Styx River 27

28 III. Channel Stability SGD Influence Sensitivity & Impacts Narrow Range ~3 m 3 /s More SGD -> More Ebb Flow More SGD -> Longer Ebb Flow Flood Flow Decreases More than Ebb Flow Increases 28

29 III. Channel Stability Management Planning & Response Longshore Transport Waves+Tides Freshwater Input Decision Making Magnitude \ Factor Tides Waves Freshwater Weak High Low High Moderate Normal Normal Normal Strong Low High Low 29

30 II. CMS Modeling Validation 6-Week Hindcast Measured Tides Deposition Measured Velocity Measured Bathymetry Correlation & CPRMSE 30 Onshore Bar Migration Scour