Feasibility Investigation Report Restoration of Hydrology along Mobile Bay Causeway December 2015 APPENDIX E Hydrodynamic Modeling Report Weston Solutions, Inc. SOUTH COAST ENGINEERS Hydrodynamic Modeling Report for: Alabama Department of Conservation & Weston Solutions Natural Resources {Justin's Bay} Hydrodynamic Modeling Report Restoration of Hydrology of Mobile Bay Causeway, Alabama {CIAP AL-12} Bret M. Webb, Ph.D., P.E. Scott Douglass, Ph.D., P.E., D.CE Beau Buhring, Chris Blackwood Report Date | October 7, 2014 S O U T H C O A S T E N G I N E E R S | PO Box 72 | Fairhope, AL 36533 SOUTH COAST ENGINEERS Hydrodynamic Modeling Report EXECUTIVE SUMMARY This report, prepared by Dr. Bret Webb of South Coast Engineers (SCE), describes the hydrodynamic modeling activities completed as part of the CIAP AL-12 project aimed at investigating hypothetical restoration strategies for the Mobile Bay Causeway in Alabama. The report documents the steps taken to develop and validate a tidal circulation model for the study area, including field data collection and model hindcasting of the data collection period. The results of fourteen (14) unique model simulations of the hypothetical restoration alternatives, their forcing conditions and parameters, and their pertinent results, are described in this report. Simulation results are presented in a manner that addresses project goals, objectives, and performance measures identified in the project plan formulation. The primary goal of this hydrodynamic model study is to evaluate the effects of constructed openings through the Mobile Causeway on tidal exchange between Mobile Bay and water bodies north of the Causeway. The hypothetical scenarios include openings through the Causeway at Choccolatta Bay, Justin's Bay, and Shellbank River, herein referred to as "Pass Choccolatta," "Pass Justin," and "Shellbank Cut." Specific areas of interest include Choccolatta Bay, Justin's Bay, Sardine Pass, John's Bend, Ducker Bay, and Shellbank River. Four specific objectives are used to frame simulation results and include assessments of: 1) increased tidal communication; 2) increased tidal prisms; 3) decreased tidal phase lags with Mobile Bay; and 4) increased flushing within each system. Specific performance measures for each objective are used to quantify the degree to which an objective is met. Field data collection was completed over the period March 27, 2014 to April 9, 2014, with ship-based surveys of velocity and bathymetry conducted on April 3, 2014. Data collection included the measurement of water levels (i.e., tides) in Choccolatta Bay, Ducker Bay, and Sardine Pass over the two-week period; as well as mapping of velocity, discharge, bathymetry, and standard water characteristics (e.g., temperature and salinity) at I-10 Cut, the box culverts, Pass Picada, Apalachee River, Sardine Pass, Duck Skiff Pass, and Blakeley River. These data were used to develop the unstructured mesh for the hydrodynamic model and to validate the model through comparisons of predicted and measured water levels and velocities. The Advanced Circulation (ADCIRC) model was applied to a hindcast simulation of the period March 27, 2014 to April 4, 2014. Forcing included predicted tides, observed discharge for the Mobile and Tensaw Rivers, and observed meteorology (i.e., winds and pressure). Model-data comparisons were generally good within the study area, capturing the range and phase of tides as well as the magnitudes and directions of flows. Predictive errors for water levels were 20% (~10 cm) or less over the entire simulation. Predictive velocity errors were 30% (~5 cm/s) or less over the entire simulation. The ADCIRC model was used to simulate unique restoration alternative scenarios under representative tidal forcing and river discharge for present and future sea levels. Five restoration scenarios were simulated with typical summer (July) river discharge (~470 m3/s) on present-day sea levels. Those same forcing conditions were used to simulate the five restoration scenarios with an elevated sea level that was 30 cm higher than present-day levels. The restoration alternative with openings at Choccolatta Bay, Justin's Bay and Shellbank River was simulated with high (wet season) river discharge (~1950 m3/s) on ii SOUTH COAST ENGINEERS Hydrodynamic Modeling Report present-day sea levels. A corresponding simulation of existing conditions within the study area (i.e., no openings) was performed for each of the three forcing conditions, resulting in fourteen (14) total model simulations. Restoration scenario results are generally expressed in terms of changes, increases or decreases, relative to existing conditions. A summary of the major conclusions, relative to the objectives stated above, are listed below: The restoration scenarios at Choccolatta and Justin's Bay would measurably increase all aspects of tidal communication between those bays and Mobile Bay Model predictions suggest that the proposed restoration alternatives would, overall, experience an 80% increase in tidal exchange (volume of water entering the water body) for Choccolatta Bay, and a 120% increase for Justin's Bay. Constructed openings would generally eliminate all existing tidal phase lags in Choccolatta and Justin's Bays. In other words, the high tide would occur at the same time as it does in northern Mobile Bay. Tidal prisms in Choccolatta and Justin's Bays would increase by 8% and 64%, respectively. Flushing of Choccolatta and Justin's Bays would be improved under the restoration alternatives considered. The existing man-made tidal channels that were built north of the Causeway (Pass Picada and the I-10 Cut) that govern the tidal exchange of Choccolatta Bay under existing conditions would experience 90% reductions in tidal exchange as a result of the constructed openings evaluated here as restoration alternatives. Optimizing the size of the hypothetical opening through the Causeway could moderate such reductions. Reductions in tidal exchange in Pass Picada, I-10 Cut, and Sardine Pass may alter the characteristics of those systems, including changes to water quality and possible sediment deposition over time. These uncertainties could be addressed in future studies. The restoration alternatives mostly act independent of one another with only small changes (<1%) noted between scenarios. Tidal exchange would be reduced at higher river discharge due to a general reduction of tidal forcing. Most effects of the constructed openings evaluated as restoration alternatives would be within the immediate vicinity of the Causeway, Choccolatta Bay, Justin's Bay, and Shellbank River. The effects of these hypothetical openings on wave action in Choccolatta and Justin's Bays was not considered here, but could be evaluated in future studies. iii SOUTH COAST ENGINEERS Hydrodynamic Modeling Report Table of Contents EXECUTIVE SUMMARY II LIST OF TABLES VII LIST OF FIGURES IX LIST OF ABBREVIATIONS XIII INTRODUCTION 15 BACKGROUND 15 STUDY AREA 16 GOALS, OBJECTIVES & PERFORMANCE MEASURES 16 FIELD DATA COLLECTION 18 OBJECTIVES 18 SAMPLING LOCATIONS 18 CONDITIONS 18 TIDES & WATER LEVELS 20 VELOCITY & DISCHARGE 23 BATHYMETRIC MAPPING 24 MODEL VALIDATION 28 MODEL DESCRIPTION 28 MODEL MESH 28 MESH BOUNDARY CONDITIONS 31 VALIDATION PERIOD & CONDITIONS 32 VALIDATION RESULTS 33 WATER LEVELS 34 VELOCITY 37 RESTORATION ALTERNATIVE MODEL SETUP 41 OVERVIEW 41 NAMING CONVENTIONS 41 MODEL SETUP 42 SIMULATION CONDITIONS 44 ANALYSIS METHODOLOGY 47 SIMULATION RESULTS | TYPICAL CONDITIONS 50 EXISTING CONDITIONS - CASE 002 50 WATER LEVELS 50 FLOWS 51 SEDIMENT TRANSPORT POTENTIAL 53 FLUSHING 56 iv SOUTH COAST ENGINEERS Hydrodynamic Modeling Report CHOCCOLATTA BAY - CASE 102 57 WATER LEVELS 57 FLOWS 60 SEDIMENT TRANSPORT POTENTIAL 63 FLUSHING 65 JUSTIN'S BAY - CASE 202 66 WATER LEVELS 66 FLOWS 68 SEDIMENT TRANSPORT POTENTIAL 71 FLUSHING 74 SHELLBANK RIVER - CASE 302 74 WATER LEVELS 75 FLOWS 76 SEDIMENT TRANSPORT POTENTIAL 78 ALL OPEN - CASE 402 80 WATER LEVELS 80 FLOWS 81 SEDIMENT TRANSPORT POTENTIAL 83 FLUSHING 86 CHOCCOLATTA + JUSTIN'S - CASE 502 87 WATER LEVELS 87 FLOWS 88 SEDIMENT TRANSPORT POTENTIAL 89 FLUSHING 91 ALTERNATIVE SIMULATION RESULTS | HIGH FLOWS 93 EXISTING CONDITIONS - CASE 003 93 WATER LEVELS 93 FLOWS 94 SEDIMENT TRANSPORT POTENTIAL 96 FLUSHING 98 ALL OPEN - CASE 403 99 WATER LEVELS 100 FLOWS 101 SEDIMENT TRANSPORT POTENTIAL 102 FLUSHING 105 ALTERNATIVE SIMULATION RESULTS | SEA LEVEL RISE SCENARIO 106 EXISTING CONDITIONS - CASE 012 106 WATER LEVELS 106 FLOWS 108 SEDIMENT TRANSPORT POTENTIAL 110 FLUSHING 112 CHOCCOLATTA BAY - CASE 112 113 WATER LEVELS 113 FLOWS 116 SEDIMENT TRANSPORT POTENTIAL 118 FLUSHING 120 JUSTIN'S BAY - CASE 212 121 WATER LEVELS 121 v SOUTH COAST ENGINEERS Hydrodynamic Modeling Report FLOWS 124 SEDIMENT TRANSPORT POTENTIAL 126 FLUSHING 128 SHELLBANK RIVER - CASE 312 129 WATER LEVELS 129 FLOWS 131 SEDIMENT TRANSPORT POTENTIAL 133 ALL OPEN - CASE 412 136 WATER LEVELS 136 FLOWS 139 SEDIMENT TRANSPORT POTENTIAL 141 FLUSHING 143 CHOCCOLATTA + JUSTIN'S - CASE 512 144 WATER LEVELS 144 FLOWS 147 SEDIMENT TRANSPORT POTENTIAL 148 FLUSHING 150 CONCLUSIONS 152 MODEL STUDY OVERVIEW 152 SUMMARY OF OBJECTIVES & PERFORMANCE MEASURES 152 OBJECTIVE 1: INCREASE TIDAL COMMUNICATION 152 OBJECTIVE 2: INCREASE TIDAL PRISM 154 OBJECTIVE 3: DECREASE TIDAL PHASE LAG 154 OBJECTIVE 4: INCREASE FLUSHING 155 SUGGESTIONS FOR FUTURE WORK 156 WORKS CITED 157 vi SOUTH COAST ENGINEERS Hydrodynamic Modeling Report List of Tables Table 1. Root mean square difference (error) assessment