Cobble Sea Defence: Hydraulic Interface Stability of Sand underlying a Single Filter Layer

A cobble sea defence appears to be an easy constructible protection, with relatively low total costs of ownership. Flume tests show that sand under such single layered constructions is stable. However, it is not exactly known how the hydraulic loading of a breaking wave is reducing in the filter. Two datasets containing pressure measurements in a revetment were available for identification of this reduction: 1) A dataset of a test performed in the Großer Wellenkanal (Hannover), for improvement of the understanding of all relevant processes in Elastocoast revetments (obtained from the Braunschweig University of Technology). 2) A dataset of tests in the Delta Flume, for the verification and optimization of the cobble shore design of the ‘Maasvlakte 2’ (obtained from PUMA). The dataset of the Großer Wellenkanal test was analysed to increase the insight in the behaviour of pressures in a filter resulting from (breaking) waves and to explain processes and test results of the Delta Flume model tests. Hydraulic loading at the top of a revetment exist out of two types of loads; impact and non-impact loads. The impact load, resulting from plunging waves, can be distinguished in an impact peak and a quasi-static part. The impact peak is very high (>10 kPa), last for only a fraction of time (<0.2s) and reduces completely in the filter. The non-impact load and the quasi-static part of the impact load reduce less and are responsible for the hydraulic loading at the interface of sand underlying a thick, single filter layer. Although large influence was expected for turbulence generated by breaking waves it does not have an important role in the hydraulic loading that eventually reaches the interface. The parallel gradients at the interface are only the result of pressures in- and decreasing with the wave period. The maximal destabilizing perpendicular gradients are frequently accompanied by a maximal parallel gradient. The main loading mechanism at the interface results from the run-down; during run-down the largest parallel and perpendicular gradients are generated. Predicted gradients acting on the interface between gravel and sand in the Delta Flume models are higher than traditional stability criteria. Therefore, an erosion process of the sandy embankment would be expected for the tested Delta Flume models. However, erosion was not observed. The stable interface in the Delta Flume tests can thus not be explained from the pressure reduction resulting from the analysis of the measurements in the Delta Flume models itself. The damping observed in the Großer Wellenkanal and the non-distorted interface in the Delta Flume models give strong indications that the reduction observed in the gravel material of the Delta Flume models is not representative for the performance of the gravel layer, resulting in a conservative prediction of gradients at the interface of gravel and sand. Several factors could have been influencing the pressures in the revetments of the Delta Flume models and the reduction found in the analysis of the measurements.Hydraulic EngineeringCivil Engineering and Geoscience

Case study Piçarras Beach: Erosion and nourishment of a headland bay beach

Master project report. Piçarras is one of the touristic beaches of Santa Catarina state in Brazil. Piçarras beach is a headland bay beach. In the bay irregular features like an island, rocky outcrops and shoals are present influencing wave propagation. In the south Piçarras is bounded by Piçarras river. The river mouth has been fixated in 1970, after which erosion started. The part just a few hundred meters north of the river jetty has the most severe erosion. The erosion gradually decreases towards the north, where even some accretion has been measured. When the situation became critical a nourishment was executed in 1999, which has disappeared totally on some places. The decrease in beach width causes a devaluation of the houses and a decrease in tourism which consequently leads to a decrease of employment. It is therefore necessary to investigate the causes and the amount of the erosion and to generate measures to counteract the negative impact of the erosion. Prosul, a Brazilian engineering company, has designed a nourishment of which execution started in July 2008. The main goal of this study is investigate erosion at Piçarras beach and to design a nourishment to counteract the effects of the current erosion. A model has been built to represent the situation at Piçarras beach. With the model the evolution of the nourishment and the evolution of the existing plan of Prosul could be evaluated. The bathymetry has been composed of recent profile measurements and old nautical maps. They have all been related to the reference level of IBGE. To investigate the erosion at Piçarras beach the wave climate has been schematised. The available wave data was given for four direction (NE, E, SE, S) in the form of wave heights and periods. To be able to compare what the results of the incoming wave energy from these four directions were on the erosion and accretion on the beach, a schematisation has been made. A representative average wave per direction has been determined, that supplied the same energy input from that direction as did all the different waves from that direction. Headland bay beaches are historically formed in such a way that the incoming waves and thereby the wave energy, arrive perpendicular at the beach, thus absorbing the incoming wave energy in the most efficient way. This theory formed the basis of this schematisation. The mean tidal variation is 0.6m, at spring tide this is 0.9m. Storm surges lead to a set-up in water level of approximately 1.0m. Currents and wind are not taken into account. The sediment present at the beach has a D50 of 0.285mm. The sand used in the nourishment of 1999 was coarser than the native sand, which had a D50 of 0.260mm. Erosion processes can take the sand either in cross-shore direction or in longshore direction. Without looking at the underlying process, just to get a realistic idea of the erosion and accretion patterns, the amount of eroded sediment has been calculated with shoreline changes and deduced erosion rates [2]. The erosion of the past nine years is calculated to be 395,000 m3. To find out where the sediment is transported to at Piçarras beach a model has been build. First the nearshore wave conditions have been modelled with Delft3D (D3D) for the four wave scenarios. These conditions serve as input for Unibest (UB). This program is applied to model the shoreline changes.Hydraulic EngineeringCivil Engineering and Geoscience

OpenFOAM design sensitivity analysis on a homogeneous low-crested structure with concrete elements seaward of a vertical seawall to reduce overtopping

This study treats a detached homogenous low-crested structure (HLCS) made of Cubipod concrete elements placed seaward of a vertical wall (forming a basin in between) to reduce overtopping. Assessing the complex hydrodynamics and effects of changing the geometry of such a system in relation to overtopping reduction is challenging. The numerical model OpenFOAM was applied to this end. Forchheimer coefficients for wave transmission and the flow through the HLCS were calibrated and validated using existing physical modeling data (α = 500 and β = 1.0, with varying porosity based on the Cubipod shape), while the effect of the basin and vertical seawall was determined fully numerically. The crest freeboard (Rc), crest width (B), and basin length (LB) were the main geometrical parameters that influenced the performance of the HLCS in reducing overtopping. An exponential decay was observed in the overtopping discharge when the values of these geometrical parameters increased. As LB increased, this decay was primarily due to the dissipation of the broken-wave bores. The largest gradient in the predicted overtopping discharge was noted at Rc/Hs,i ≈ 0, B/Hs,i ≈ 4.5, and LB/Lp ≈ 1.2, where Hs,i is the incident significant wave height and Lp is the peak wavelength in the basin.Hydraulic Structures and Flood RiskCoastal Engineerin