Development and validation of a three-dimensional morphological model

https://www.leovanrijn-sediment.com/papers/P6-2011A.pdf?origin=publication_detail

Coastal erosion and control

Reversed responses within a segmented detached breakwater, the Tuscany coast Italy—a case study

Maputo Bay, on the coast of Mozambique, is a tidally energetic, subtropical embayment in a region subjected to strong seasonal rainfall. Data from moored current meters, tide gauges and monthly bay-wide surveys were used to characterise the evolution of the density structure on seasonal, fortnightly and semi-diurnal time-scales and its relation to tidal forcing. The bay is subjected to large seasonal variations in freshwater input (10–103 m3 s–1) and pronounced fortnightly variations in tidal amplitude with a spring:neap tide ratio varying between 3.6 and 9.0 with a corresponding variation in tidal stirring power input (10–3–1 W m–3). During the dry season, the water column was continuously fully mixed with weak horizontal density gradients. In contrast, during the wet season, freshwater buoyancy induced marked horizontal salinity gradients and stratification, which was pronounced around the time of neap tides. This stratification was largely eroded at spring tides but semi-diurnal, periodic stratificatio...

Buoyancy-stirring interactions in a subtropical embayment: a synthesis of measurements and model simulations in Maputo Bay, Mozambique

A computational model based on the evolution equation for water waves (Li, 1994b)
derived from the original Berkhoffs (1972) "Mild Slope Equation" is tested against
multidirectional sea data. The model accounts for reflection as well as diffraction-refraction
processes, which is important for applications involving coastal structures. The
accuracy and convergence of the numerical solution, as well as the possibility of the
implementation of an adaptive numerical scheme are investigated and implemented. The
model was firstly tested using laboratory measurements (Briggs et al, 1995) of random
directional wave diffraction around a semi-infinite breakwater on a flat bottom. These tests
confirm the need to use "directional modelling" (using the principle of linear
superposition) for the prediction of wave heights behind the breakwater.
The model was then tested using directional wave data records, which were chosen from
4500 wave records collected in the field campaign, by the University of Plymouth and the
University of Brighton, at Elmer - Sussex, UK from September 1993 - January 1995. The
results showed that the representation of the measured random sea by monochromatic wave
runs can introduce a significant error in wave height predictions shoreward of the
breakwaters in the diffraction region, thus confirming the importance of directional
modelling for random wave simulation. Evidence strongly suggests that non-linear wave
effects have a significant influence (40-60%) on the accuracy of the model. Consequently,
further tests are needed, which should also consider the influence of wave-current
interaction, wave breaking, bed fiiction and transmissive boundaries. Over all the model
predictions are more accurate for the controlled environment in the laboratory (5-13%),
than for field conditions where the directional modelling accuracy varied from 8 - 32%.
A summary of the data, collected by the author as a member of the University of Plymouth
Research Team and a database of spectral and directional parameters is also presented in
this thesis. Field validation of the numerical model required accurate estimates of measured
data. Emphasis was placed on identifying a suitable directional analysis method, which
accurately predicted direction and directional spread in the far-field from structures where
reflection is still present. The non-phase-locked (hfPL) methods developed for a
homogeneous sea are found to be appropriate. The anzdysis of two NPL methods, the
Maximum Likelihood Method (MLM) and Bayesian Directional Method (BDM),
directional estimates for simulated data shows that both methods can predict accurate
incident wave height and direction. Both methods tend to overpredict directional spread
and give non-accurate reflection estimates. The MLM method is easier to implement than
the BDM method, which is sensitive to the chosen starting value of the hyperparameter u.
As the difference between estimates of the two methods for numerical data is small, the
MLM method's estimates were chosen for model testing.

Transformation of multidirectional sea field and computational study

Detached breakwaters alter the nearshore wave climate and hence the wave-driven current and sediment transport patterns. They obstruct a part or all of the longshore sediment transport and because of this they play a role in the large-scale sediment budget of the coasts where they are applied. Their local effect on the coast is to form single or double salients or tombolos. Which of these beach shapes will develop may be important for the attractiveness of the beach, in terms of visual aspects and water quality. Besides having a function as part of a coastal management scheme, detached breakwaters with a jetty to the shore may be designed as a low-maintenance port. Obviously, one hopes that in such cases a salient rather than a tombolo will develop. Clearly, engineers need to have means to assess the bypass-characteristics and the local impact of detached breakwaters if they want to apply them in a responsible manner. Several methods are at their disposal. First, there are rules-of-thumb, based on field and lab experience. These are quite useful to get a first rough idea of the possible impacts, but generally do not give answers that are conclusive enough: a typical result is of the form that two design rules say you'll get a salient and three say you'll get a tombolo. The most common tools applied at present are coastline models. They assume that the coastal profile is more or less constant in shape, and that the longshore sediment transport is related to the local angle of incidence of the waves. Several commercially available packages have special options to assess the local wave climate behind detached breakwaters. Effects that are not included in the current and transport models, such as circulations induced by set-up gradients, can be simulated by adjusting transport parameters locally. Generally, a lot of calibration is required, and the predictive capability is often uncertain. The calibrated models often show nice comparisons with data, especially for the initial stages. The development towards equilibrium is generally not represented at all. Recently, more sophisticated "area models" have matured to the point that fully dynamic wave, current, transport and bed evolution simulations can be carried out over a period which is long enough to approach equilibrium conditions (see for instance Johnson et al., 1994). The advantages are obvious: these models explicitly take into account the most important processes, and therefore the amount of heuristic modelling is reduced substantially. In the light of the ongoing efforts of devising coastal area morphological models, the performance of DELFT HYDRAULICS morphological model is tested on the simple situation of a detached breakwater in a coastal area with parallel iso-bathi. This also furnishes a test for the DELFT3D system and has proven an opportunity of improving the model.

https://icce-ojs-tamu.tdl.org/icce/index.php/icce/article/download/5361/5038

J.A. Roelvink

Papers

Modelling the impact of detached breakwaters on the coast