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Physical model tests to determine the roughness of stair shaped revetments
Stair shaped revetments are considered as an attractive alternative for traditional revetments since the surface roughness reduces wave overtopping resulting in a lower crest height. The specific shape enables easy access from and to the watar surface, enhancing the attractiveness for tourists and residents. To implement stepped revetments in the design of a seawall there is a need to quantify the roughness of a stair shaped reventment to predict wave overtopping rates. This paper describes research in which 2D physical model experiments were conducted in Deltares' Scheldt Flume. Based on the obtained data a method to quantify the roughness coefficient of such structures is suggested.
Impact of hydraulic and storage properties on river leakage estimates : a numerical groundwater flow model case study from southern Benin
River discharge loss is known to occur on the Zou and Ouémé rivers in southern Benin since a couple of decades ago. The reason behind this discharge reduction remained so far unclear. In this study, we focus on creating a 3D-numerical model of the system and on evaluating the sensitivity of leakage between the rivers and aquifers to various parameters. Results show that leakages along the Zou river and Ouémé stream are tiny (about 3% of the discharge losses). This implies that the observed water loss from the Zou and Ouémé rivers is not likely caused by the leakage (infiltration) along these rivers into the subsurface. The streambed conductance is found to be among the factors that impact less the computed leakages in the study area. This study has ranked the different hydraulic and storage properties in their order of importance with respect to the computation of river leakages along the concerned rivers. The determined rank of importance of the hydraulic and storage properties can guide river leakage modelling exercises in similar regions elsewhere.
Predicting bird collisions with wind turbines : comparison of the new empirical Flux Collision Model with the SOSS Band model
Collision of birds with wind turbines is an important negative effect of wind energy generation. Assessments of the potential numbers of bird collisions are required prior to the construction of wind farms. Collision rate models (CRMs) are used as a tool to estimate numbers of collision victims for wind farm initiatives. In the past couple of decades various CRMs have been developed. These models are all based on the theoretical calculation of collision probabilities (theoretical models). In this paper we introduce an empirical model, the Flux Collision Model (FCM), in which actual knowledge of species (group)-specific collision probabilities collected in existing wind farms on land is used to calculate collision rates for planned wind farms. An important quality of the FCM is that it provides a means to use empirical information to assess collision rates in Environmental Impact Assessments (EIAs) for wind farm initiatives. In addition, no detailed information on bird behaviour close to the rotor is needed, as this information is already incorporated in the empirical collision probability. In two case studies, one offshore and one on land, we compare and discuss the use and performance of the empirical FCM and the theoretical SOSS Band model for predicting collision rates of birds at wind farm initiatives. To date, no actual collision rates are known for the offshore situation. Accordingly, in the FCM, collision probabilities derived from wind farms on land were used. Nevertheless, in the offshore case study, the results of the FCM were comparable with those of the SOSS Band model. Basic sensitivity analyses for both the FCM and the SOSS Band model showed that purely theoretically both models are equally sensitive to changes in avoidance rates. However, because lower values for avoidance are applied in the FCM (wind farm avoidance) than in the SOSS Band model (overall avoidance), in practice the effect of realistic variation in avoidance rates on the resulting collision rates is much smaller for the FCM than for the SOSS Band model. Our results show that the FCM provides a valuable addition to the existing suite of (theoretical) CRMs. The predictive value of the theoretical SOSS Band model is constrained by the limited availability of knowledge on species (group)-specific (wind turbine) avoidance rates, which is not the case for the FCM. By contrast, the reliability of the empirical FCM is determined only by variation in the availability and quality of information on species (group)-specific collision probabilities.
Solution near of navigation problems at Willemstad Harbour
In the open-air laboratory "De Voorst", sub-division of the Hydraulic Laboratory at Delft, a study of the navigation problems for the harbour of Willemstad in Curaçao has been completed. In St. Anna Bay especially large ships sometimes meet with hindrance from a strong current along the entrance, which have repeatedly caused damage. Delft engineers and their co-operators are engaged in engineering projects which should clear this nuisance to navigation.
Royal visit to Eastern Flevoland and North-East Polder
Royal guests from Luxembourg visited the partially completed Flevo Polder, the new museum at Lelystad and the Laboratory of Watercourse experts at De Voorst (Waterloopkundig Laboratorium). The Zuyderzee works and the plans in connection with the regulation of the Zealand waterways greatly interested the visitors. In this paper most attention is paid to the set up of scale models in De Voorst.
Radiational tides: their double-counting in storm surge forecasts and contribution to the Highest Astronomical Tide
Tide predictions based on tide-gauge observations are not just the astronomical tides, they also contain radiational tides: periodic sea-level changes due to atmospheric conditions and solar forcing. This poses a problem of double-counting for operational forecasts of total water level during storm surges. In some surge forecasting, a regional model is run in two modes: tide only, with astronomic forcing alone; and tide and surge, forced additionally by surface winds and pressure. The surge residual is defined to be the difference between these configurations and is added to the local harmonic predictions from gauges. Here we use the Global Tide and Surge Model (GTSM) based on Delft-FM to investigate this in the UK and elsewhere, quantifying the weather-related tides that may be double-counted in operational forecasts. We show that the global S2 atmospheric tide is captured by the tide-and-surge model and observe changes in other major constituents, including M2. The Lowest and Highest Astronomical Tide levels, used in navigation datums and design heights, are derived from tide predictions based on observations. We use our findings on radiational tides to quantify the extent to which these levels may contain weather-related components.
Investigating the formation of a filter cake in column experiments, for combinations of filter and fine sand in a coarse sand barrier
The coarse sand barrier (CSB) is a single granular filter used to retrofit an existing structure, making it more stable against backward erosion piping. The barrier material should be chosen carefully to retain particles from the sand layer upstream of the barrier, yet provide optimal resistance against backward erosion piping. This means the particles in the barrier should be large, thus difficult to transport, and the barrier should have a high permeability in order to reduce the local hydraulic gradient inside the barrier. However, transport of the fine sand upstream of the barrier into the barrier, may result in less permeable filter cake just inside the barrier. Therefore, a column set-up was designed and experiments were conducted with various sand types, using the same materials as used in small-scale and medium-scale backward erosion piping experiments with at CSB. The aim was to compare the results of different tests and check if the criterion for the formation of a filter cake is the same as the well-known filter rules. Consequently, this paper presents the results of different column experiments. These materials were carefully selected and fulfilled the selected filter rules but one soil composition caused the filter cake formation. This indicates that to avoid filter cake formation for the conditions tested stricter rules apply than the filter rules considered here.
Analytical groundwater flow calculations for understanding the flow and erosion in a coarse sand barrier
The coarse sand barrier (CSB) is a promising method to avoid ongoing backward erosion piping resulting in increased safety of a dike for this failure mechanism. Experiments are performed at different scales at Deltares, the Netherlands. These experiments show a significant increase in the critical head for structures with a CSB compared to structures without a CSB. The increase of critical head cannot only be ascribed to the lower erodibility of the coarser particles in the barrier, but also to the reduction of hydraulic load on these coarse particles in the barrier, resulting from the permeability contrast of barrier material and surrounding sand. To investigate the influence of a CSB on the flow pattern numerical and analytical calculations have been performed. This paper focusses on the analytical calculations. It will be shown that these can explain the increase in strength and the measured scaling effects.
Databases for backward erosion piping laboratory experiments and field observations
Backward erosion piping is a failure mechanism which involves the formation of shallow pipes in a sandy foundation layer and is considered to be a major risk for levees. For understanding this mechanism and the development of prediction models, laboratory experiments are essential. In addition, due to scale effects and heterogeneity in field conditions, field observations and case histories are indispensable for validation of models and delineation of piping sensitive conditions. However, both experiments and field observations are often not easily utilized for this purpose. Piping experiments have been conducted in various research programmes, countries, and in a variety of configurations making the experiments difficult to compare due to inconsistent observations and differing configurations. Case histories are often poorly documented and like experiments, described in different sources and different levels of detail, due to which their full potential is often not reached. Given the importance of experimental and field data for the prediction of backward erosion piping, a need exists for a centralized organization of data. Two different databases are presented here, for laboratory experiments and field observations respectively, each combined with a web application for viewing and exporting the data. The laboratory experiment database is populated with 332 experiments. The field observation database is currently populated with 3 failure cases and 2840 sand boils located in the Netherlands and the United States. Future work will focus on a more complete population of the databases, user-friendliness of the web viewer, and analysis of the gathered data for improvement of prediction models.
Scale effects in coarse sand barrier experiments
The coarse sand barrier is considered as a promising measure to prevent backward erosion piping from causing failure of embankments. A pipe is allowed to progress backwards until it encounters the coarse sand barrier, which prevents it from progressing unless the head difference over the embankment is significantly increased. A three stage experimental programme supported by groundwater flow modelling is carried out to investigate the feasibility of this method. The hypothesis is that the strength of the barrier is characterised by a local gradient at the interface between the barrier and the pipe. Major questions are: can the horizontal gradient as measured in laboratory tests be used to characterise the strength of the barrier material, over which distance should a horizontal gradient be determined, and is this distance the same for models at different scales? This paper presents the background theory and demonstrates the effects using scale dependent criteria. Preliminary results of small- and medium-scale experiments are used to compare the two approaches.