Evaluation of Cost-Effective Shoaling Reduction Alternatives for Navigation Channels Near Inlets

Abstract: The Intracoastal Waterway (IWW) passes 0.4 mile west of Bakers Haulover Inlet in Miami, FL. At this location, wave- and tide-generated currents entrain and transport littoral sediments inshore to require IWW maintenance dredging once every 4–5 years. Experience at several Florida inlets (e.g., Sebastian Inlet and Jupiter Inlet) and waterways shows an effective deposition basin on, or near, a waterway can capture sediments, reduce sediment deposition in navigable waterways, reduce maintenance dredging requirements within navigation channels, and serve as a sand source for inlet sand bypassing operations. The prospect of these cost-saving outcomes encouraged the Florida Inland Navigation District to request Taylor Engineering to perform a sediment deposition basin and Cut-DA9 rerouting study at Bakers Haulover Inlet and adjacent waterways.

To analyze the effect of sediment impoundment basins and IWW channel rerouting, this study developed and applied MIKE21 Flexible Mesh (FM) hydrodynamic (HD), wave (SW), sediment transport and morphology (ST), and particle tracking (PT) models. HD model calibration and verification provided good agreement with measured August – September 2020 inshore and May – June 2018 nearshore data. HD and SW model calibrations/verifications also provided good agreement with measured nearshore data in May – June 2018. The ST model sedimentation is generally consistent with the observed sedimentation pattern at the area of interest.

Littoral transport magnitude and direction can vary by season, day, or hour because waves can vary with the same time scale. Such long-term simulations require lengthy computational time on huge computation resources. Alternatively, this study simulated a month-long representative period from a representative year to estimate short-term shoaling rates and then prorated the computed shoaling rates for an average year (i.e., a year with average gross littoral transport). The prorated shoaling rate offers a good approximation of the shoaling rates of alternatives to reduce in-channel shoaling relative to the shoaling rate for baseline conditions (i.e., no basins and no IWW Cut-DA9 rerouting). Moreover, the prorated shoaling rate provides an effective way of determining which alternative performs best to reduce in-channel shoaling. Comparison of the shoaling rates for baseline and shoaling reduction alternatives provides the means to estimate shoaling rates at IWW Cut-DA9.

Model results show the best performing alternative — the IWW Cut-DA9 reroute alternative requires an additional initial dredging of 86,570 cy for the new channel and each succeeding maintenance dredging operation would remove approximately 38,500 cy once every eight years or more at an equivalent uniform annual cost of $435,000. Compared to the baseline equivalent uniform annual cost, the estimated annual savings of $249,000 is the largest among the analyzed alternatives.

The study recommends a geotechnical/environmental analysis and a boat navigation analysis of the channel routes for the channel reroute alternative to evaluate the bottom resources and sub-bottom strata that might be affected by the IWW re-routing. The Engineering design for the final channel modifications should evaluate the long-term shoaling rate for better estimation of the variation of the sediment impoundment performance and to better account for more potential long-term variations of waves and water levels.

How does this project / topic contribute to the advancement of the industry and profession?: The study presents a method to compare alternatives for reducing navigation channel shoaling, evaluate the impacts of navigation channel re-routing, and approaches to reduce maintenance dredging costs. The study provides navigation channel managers and engineers/scientists the means to consider the alternative impacts relative to hydraulic conditions, environment, and maintenance dredging cost. Knowing which navigation channel shoaling reduction alternative provides the most maintenance dredging cost savings and least environmental impacts allows the federal and local agencies and engineers to better plan for future development, operation, and management of navigation channels.

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: Yes, the study implemented state-of-the-art and fully integrated numerical modeling technologies to assess the impacts of channel re-routing on shoaling rates and dredging maintenance cost. The study employs an alternative method to reduce the computation time and resources needed to evaluate several shoaling reduction and cost saving alternatives and select the best performing alternative.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: The most challenging aspect of the study is the large number of models and computations needed to analyze and estimate shoaling rates for the various alternatives and evaluate their effects on the frequency and volume of navigation channel maintenance dredging. The utilization of a representative period for the evaluation and high-performance computers provided the means to complete the computational needs of the various models and achieve success in this study.

Who is the target audience for this paper?: The target audience is navigation channel managers, designers, operators; hydraulic and environmental engineers and scientists; dredging engineers; and channel/inlet planners.