4209 resultaten

  • Composite modelling by applying an inverse technique in analysing interactions between beaches and structures

    Auteurs: Boogaard, H.F.P. van den; Gerritsen, H.; Caires, S.; Gent, M.R.A. van (2009)

    Composite modelling aims to combine the best properties of physical scale modelling and numerical modelling and so provide a more efficient modelling infrastructure. The paper analyses this for the case of wave attack on the sea defence at Petten, the Netherlands, which has a typical foreshore with a bar. An error correction model is created for the results of the numerical simulations. Inverse application of the correction model on the results of a range of efficiently realised numerical simulations provides corresponding “pseudo” scale model experiments of higher accuracy than the uncorrected numerical results. Benefits are enhanced flexibility, efficiency and accuracy.

  • Uncertainty assessment via Bayesian revision of ensemble streamflow predictions in the operational river Rhine forecasting system

    Auteurs: Reggiani, P.; Renner, M.; Weerts, A.H.; Gelder, P.H.A.J.M. van (2009)

    Ensemble streamflow forecasts obtained by using hydrological models with ensemble weather products are becoming more frequent in operational flow forecasting. The uncertainty of the ensemble forecast needs to be assessed for these products to become useful in forecasting operations. A comprehensive framework for Bayesian revision has been recently developed and applied to operational flood forecasting with deterministic weather forecasts. The Bayesian revision yields a posterior density, conditional on all information available to the forecaster at the onset of a forecast run. This conditional density objectively quantifies the uncertainty. Here the Bayesian approach is generalized for use with ensemble weather predictions. An end-to-end application of a Bayesian postprocessor for ensemble streamflow forecasts in the river Rhine forecasting system is presented. A verification of the postprocessor shows good performance when compared in terms of the ranked probability skill score to non-Bayesian uncertainty assessment, such as ranking threshold exceedance probabilities for members of a streamflow ensemble prediction. In this context it is also addressed how the proposed Bayesian processor can serve in supporting rational decision making for flood warning under conditions of uncertainty.

  • Root architecture of six tropical seagrass species, growing in three contrasting habitats in Indonesian waters

    Auteurs: Kiswara, W.; Erftemeijer, P.L.A. (2009)
    Gepubliceerd in: Aquatic botany, volume 90 (2009) nummer 3, pagina 235-245

    Although several recent studies point at the importance of seagrass roots for nutrient acquisition in oligotrophic tropical ecosystems, remarkably little is known about the root architecture of tropical seagrasses. The present study provides a detailed description of the root architecture of six seagrass species that were extracted from three sites differing in sediment type and nutrient availability (i.e., except for one species that was not present at one of the sites). Number of roots per node, order of root branching, length and diameter per root order, root hair density and length were determined and used to calculate a topological index for the different species and habitats. Root architecture differed strongly between species. The relatively long-lived and slowly-growing species Thalassia hemprichii and Enhalus acoroides were characterised by short internodes with relatively few unbranched roots per node and a high root hair density. More fast growing species such as Cymodocea rotundata, Cymodocea serrulata and Syringodium isoetifolium had many roots per node, the majority of which were branched. Surprisingly, differences in sediment type and nutrient availability, as present between locations, had little effect on root architecture. We expect this to be due to a relatively homogeneous nutrient distribution at the small scale within water saturated sediments. Overall, all seagrass species had relatively simple branching (topological index > 0.7), comparable to angiosperms of the low temperate salt marsh. We speculate that relatively simple root architecture of plants in flooded systems reflects the need for a minimal path length for oxygen transport from shoots to roots.

  • Quantification methods of black carbon : comparison of Rock-Eval analysis with traditional methods

    Auteurs: Poot, A.; Quik, J.T.K.; Veld, H.; Koelmans, A.A. (2009)

    Journal of Chromatography A, 1216 (2009) pp. 613-622.

  • Finite element simulation of a slow moving natural slope in the Upper-Austrian Alps using a visco-hypoplastic constitutive model

    Auteur: Ham, G. van den (2009)

    A large-size creeping natural slope was simulated with the Finite Element (FE) method, using a viscohypoplastic material model, which describes the mechanical behaviour of cohesive soils, allowing for viscous effects (i.e. creep, relaxation and rate-dependence of stiffness). First, the required material parameters were determined by means of standard geotechnical laboratory tests using representative soil samples taken from the slope. Then a FE mesh of a slope section was constructed and boundary conditions were established. The slope movements were simulated and compared with inclinometer measurements, which were available for a period of 16 years. The calculation results are in good agreement with the observed velocities. A parameter study was carried out in order to assess the influence of slope angle, initial void ratio, material parameters as well as their spatial distribution.

  • Goede monitoring levert wat op

    Auteurs: Marsman, A.; Geer, F.C. van; Tromp, G. (2009)

    Om hun taak goed uit te kunnen oefenen, hebben waterbeheerders kennis nodig van het watersysteem.

  • Investigation of the influence of a low-tide terrace on wave loads using the Boussinesq-type model TRITON

    Auteurs: Caires, S.; Gent, M.R.A. van (2009)

    Computations of the Boussinesq-type wave model TRITON of wave propagation over a shallow foreshore with a varying low-tide terrace configuration were carried out. The results were compared with Scheldt flume physical experiments, using the same settings and post processing. The comparisons show that TRITON can model the wave propagation along shallow foreshores quite accurately as long as highly nonlinear effects do not play a significant role. Furthermore, the level of the low-tide terrace was found to influence the wave conditions at the toe of a fictive structure as follows: a) As the level of the low-tide terrace increases there is more energy dissipation and the significant wave height at the toe decreases, decreasing the hydraulic load. b) On the other hand, as the waves dissipate low-frequency energy is released and the spectral wave period at the toe increases, increasing the hydraulic load.

  • Weather-induced temporal variations in nitrate concentrations in shallow groundwater

    Auteurs: Rozemeijer, J.C.; Broers, H.P.; Geer, F.C. van; Bierkens, M.F.P. (2009)

    For the evaluation of policy action programs to improve groundwater quality, research institutes and governmentsintensively monitor nitrate concentrations in shallow or near surface groundwater.

  • Coastal bio-geomorphology modeling : linking processes and scales

    Auteurs: Qinghua, Y.E.; Roelvink, J.A.; Jagers, H.R.A.; Postma, L.; Beek, J.K.L. van (2009)

  • Description of the long-term (1991-2005) temporal and spatial distribution of phytoplankton carbon biomass in the Dutch North Sea

    Auteurs: Baretta-Bekker, J.G.; Baretta, J.W.; Latuhihin, M.J.; Desmit, X.; Prins, T.C. (2009)

    Since the beginning of the 1990s phytoplankton species composition and abundance have been monitored at a high frequency (bi-weekly in the growing season and monthly in winter) at a number of fixed stations on the Dutch Continental Shelf, of which 18 are used in this study. Phytoplankton carbon biomass has been calculated from species-specific biovolume/cell data and summed over all species per functional group enumerated in the samples. The species are divided into four functional groups i.e. diatoms, flagellates, autotrophic and mixotrophic dinoflagellates and Phaeocystis spp. The total number of phytoplankton samples analysed up to and including 2005 is almost 4000. The annualmeanphytoplankton biomass over all stations remained stable at around 145mg C m−3. However, the phytoplankton composition has changed significantly, with increases in diatoms and dinoflagellates and compensating decreases in flagellates and Phaeocystis. With increasing distance from the shore, coinciding with a decrease in nutrient availability and increasingwater depth, total phytoplankton biomass as well as the biomass of diatoms, flagellates and Phaeocystis spp. decreased. This pattern was not true for the dinoflagellates, which occurred at more or less the same biomass throughout the region. Stations near river mouths and in the Wadden Sea outlets had much higher phytoplankton biomass than stations further from freshwater discharges. The data, split in two periods (1991–1998) and (1999–2005) and averaged over thewhole Dutch Continental Shelf, had been aggregated into seasonal biomass distributions. The seasonal phytoplankton biomass distribution was unimodal in both periods, with similar spring maxima of around 300 mg C m−3. The spring maximum occurred one month earlier, in April, in the second period. Phaeocystis over the whole study period remained the dominant near-shore species as it has been since the first phytoplankton observations at the end of the 19th century.

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