Roland Vlijm

Procces-based modelling of shoreline response to submerged breakwaters
Towards design criteria for shoreline repsonse to submerged breakwaters

Background

In the past, emergent coastal structures like groynes or detached offshore breakwaters, have been used commonly as coastal/beach protection measures. These conventional structures are studied extensively and applied successfully many times. Main purpose of a coastal structure near or in the surf zone is to counter shoreline erosion. A frequently adopted structure is an emerged rubble-mound breakwater. Primary function of such a breakwater is to reduce the wave energy in the lee of the breakwater and initiating shoreline accretion by changing the corresponding local currents.Ranasinghe and Turner 2006. Amongst others,Pope and Dean 1986 and Hsu and Silvester 1990, already quantified the shoreline response to (single) emerged breakwaters as a function of the dominant breakwater design parameters.
Despite the successful applications, emergent structures are becoming increasingly less popular, due to their negative impact on beach amenity and aesthetics.Ranasinghe et al. 2006 As a result, alternatives for these conventional protection measures are sought. One alternative is a detached offshore-submerged breakwater. Submerged breakwaters are capable of providing the necessary beach protection while not having the downside of adverse impact on beach amenity and aesthetics. In addition, the recognition that a beach protection measure can be combined with other functions has a wide community appeal Ranasinghe et al. 2006. According to Black and Andrews 2001 artificial sub-tidal and sub-aerial offshore reefs, which are similar to submerged breakwaters, can have different functions, like beach protection, enhancement of marine habitat, surfing, diving and swimming safety. In Ranasinghe et al. 2006 an example is given of such a multi functional design of a submerged breakwater which enhances local surfing conditions. Obviously, a multifunctional beach protection measure, which combines protection of the shoreline with other functions while not impairing aesthetics or beach amenity, will be the preferred choice to coastal management authorities when coastal problems arise.

Problem definition

Despite all the benefits of a proper-designed submerged breakwater, submerged breakwaters have rarely been adopted until now, which is the reason why their efficiency is still largely unknown. An overview of reported submerged breakwaters is given by Ranasinghe and Turner 2006. From this overview, it shows that in spite of all effort, in most cases enhanced shoreline erosion in the lee of the submerged breakwater occurred. Clearly, a better understanding of all the involving processes around submerged breakwaters is required before routinely adopting submerged breakwaters for coastal protection.

As stated above, before routinely adopting submerged breakwaters, more understanding is needed of the effect of a submerged breakwater on hydrodynamic and morphological processes close to the shoreline. In this highly complex zone, waves and currents interact with local conditions like bathymetry etc, enabling morphological changes around the breakwater. Previous studies have been focussing (amongst others) on wave characteristics over submerged breakwaters,Van der Meer et al. 2005, wave set-up Calabrese et al. 2008, scour characteristics , Sumer et al. 2005 and modelling of waves and currents around submerged breakwaters Lesser et al. 2003. Less is known about the morphological shoreline response to submerged breakwaters. A first attempt in quantifying the formation of a salient/tombolo in the lee of submerged breakwaters was done by Black and Andrews 2001. This study focussed on the morphological effect of natural reefs in New Zealand and Australia, which is similar to submerged breakwaters.
Another interesting study on the morphological effect of submerged breakwaters is done by Ranasinghe et al. 2010. This study focussed on the mode of shoreline response, (accretive or erosive) to a single shore-parallel submerged breakwater. Both studies as well as the other studies mentioned show the complexity of the processes around submerged breakwaters. Due to this complexity, computational models are often used. In practise, these models may however be time consuming and 'difficult' to use. In addition, during preliminary stages of the design process, no detailed solutions or outcomes are needed and therefore rules of thumb or (empirical) formulas are commonly preferred instead of computational modelling. However, universal rules of thumb or formulas of the shoreline response to submerged breakwaters, which include all the important hydrodynamic-, structural and environmental parameters are still to be discovered.

Research objective

The main objective of this MSc thesis is the development of universal engineering design criteria that can predict the magnitude of shoreline response to a given design single shore-parallel detached submerged breakwater. These criteria will quantify the formation of a salient/tombolo or erosion as a function of structural, environmental and hydrodynamic parameters.

In order to come up with design criteria for shoreline response to submerged breakwaters this study will be specially focussing on:

• Current literature that is available on submerged breakwaters.
• Modelling of a submerged breakwater with a phase averaging numerical model (Delft3D)
• Sensitivity analysis of the model results to various model characteristics (Morfac, wave-current interaction, mass-flux etc).
• Hydrodynamic and morphologic processes that dominate shoreline response to submerged breakwaters.
• The dependency of the magnitude of shoreline response on principal structural/environmental parameters that govern submerged breakwater induced morphological response.
• Analyses of model results and translation to engineering criteria capable of predicting the magnitude of shoreline response to a design submerged breakwater

Methodology

The proposed study can be divided into the following stages:
Literature review. First, the hydrodynamic and morphological processes that may affect shoreline response to a submerged breakwater are studied from literature. This theoretical study indicates which are the important processes and corresponding parameters that govern submerged breakwater induced morphological response. In addition, previous studies on submerged breakwaters will be summarised and conclusions are taken into account. This literature study will act as a starting point for this thesis.
Modelling with of a submerged breakwater with a phase averaging model (Delft3D). Second the shoreline response to a submerged breakwater is modelled by a numerical phase averaging model, Delft3D, developed by Deltares. A two-dimensional depth averaged model is set-up to study the important driving processes behind submerged breakwater induced shoreline response, without having the large computational times of a full three-dimensional and phase resolving model. Using an initial alongshore uniform profile and avoiding site-specific conditions will lead to a universal approach so that the eventual design criteria will be applicable world wide.
Sensitivity analysis numerical parameters. Applying a numerical model introduces additional (numerical) parameters and assumptions that influence the shoreline response. In order to quantify the influence of these parameters/assumptions and to reduce the computational times, a sensitivity analysis of these parameters is performed.
Analysis of hydrodynamic and morphologic processes. From literature most important processes that govern submerged breakwater induced shoreline changes are known. Analysing model results of Delft3D will show whether this phase-averaged and depth averaged model is capable of modelling all the important processes, ignoring detailed turbulence, while using mild slope equations.
Sunny isle Delft3D calibration case. Next, measurements from an existing submerged breakwater are used to calibrate the Delft3D model. This field case comprises the Sunny Isle breakwater in Florida USA, a coupled breakwater system containing two detached shore normal submerged breakwaters. Fine tuning of the numerical parameters, together with the sensitivity analyses, will lead to more reliable results when comparing the influence of structural/environmental parameters, while not impairing on computational efficiency of the Delft3D model.
Structural and environmental parameters. The next step in this study is the sensitivity analyses on the structural and environmental parameters governing submerged breakwater induced shoreline response. The influence of the most important parameters on shoreline response, followed from the theoretical study, are examined using the model set-up in Delft3D. The results of this analyses, the formation of a salient or erosion, are then quantified and a tendency is searched for.
Design criteria and comparison. Using the tendencies of the different parameters, design criteria for submerged breakwater induced shoreline response are made. The final stage of this study will be a comparison between the proposed design criterion and previous studies on submerged breakwater induced shoreline response, followed by conclusions and recommendations for further research.