On the modelling of biological effects on morphology in estuaries and seas

Morphological modelling aims to explain and predict the changes in rivers, seas and estuaries due to these interaction. In recent history, a lot of progress has been made, especially with stability analysis approaches. However, so far only the physical interactions have been taken into account. It is known however, that biological factors are important to the dynamics of the water systems. In this paper a first step is made in the inclusion of biology into the morphodynamic models. This inclusion is based on the effect that benthic organisms have on the erodibility of the bed. This can easily be included by a change of the critical bed shear stress. These changes in the critical bed shear stress then influence the morphology. This idea has been applied to two cases. The results of the first case indicate that this approach can reproduce the influence of benthic organisms on the mud content of the bed in estuaries. The second case shows that even low numbers of organisms can influence the characteristics of large bed forms

Sediment modelling for Poole and Christchurch Bay

Poole and Christchurch Bays have narrow beaches which attract many visitors to the area and are therefore essential for the local economy. However, the beaches suffer from ongoing erosion, making regular beach nourishments to maintain them necessary. Sea level rise may cause an increase in the required frequency of these nourishments. If one can determine where the sediment sinks are, i.e. the location where the eroded beach material ends up, it may be possible to recycle the material, by dredging in the area of the sediment sinks and returning this material to the area of the beach. For this purpose, a numerical model is developed that computes the sediment transport pathways in the area between Swanage and the Isle of Wight. This model will be the basis for future projects to determine the changes due to the proposed works, either in large scale engineering or beach nourishments. The model has been calibrated and validated against measurements taken at 9 locations using AWAC systems. The model fits well with the observations and even reproduces most of the observed sediment transport. The model works well as a tool to analyse the sediment transport pathways and assess where material eroded from the beaches is likely to end up. The model run of a whole year shows a clear pathway of sand along the beaches towards the east and back further offshore, south of Dolphin Bank and Dolphin Sands. Only part of the sand leaves the bay and is moved east along the Isle of Wight. The erosion deposition depends strongly on the coarser fractions in the bed composition in the model, but the results are promising for impact assessments of engineering works

Modelling sediment transport with hysteresis effects

This paper details the extension of the sediment transport and morphology model SISYPHE 2D to include a lag term within the bed exchange source term of the, depthaveraged, continuity of sediment concentration equation. This lag term represents the time it takes for a sediment concentration profile to adapt to spatial or temporal changes in the flow. The inclusion of a lag term means that the settling velocity is no longer the only scaling factor for the exchange of sediment between the water and the bed. The newly modified SISYPHE 2D is tested against field data from the Thames estuary (UK), flume experiments on a dredged trench and a hypothetical channel widening. It is illustrated that the lag factor introduced into SISYPHE 2D is essential to model the sediment transport and morphodynamics, especially when considering engineered situations, where the bed is out of equilibrium with the flow conditions. Moreover, with this lag factor included, there is evidence that SISYPHE 2D can be used for (short term)morphodynamic modeling of engineered situations

Morphological acceleration factor: usability, accuracy and run time reductions

Within SISYPHE 6.2, the option is created by BAW to use a morphological acceleration factor (MF) within the coupled TELEMAC2D-SISYPHE model. Multiplying both the evolution and the time by the same factor the model jumps forward in time, reducing the required computation time. This paper presents the usability of this approach, the gains in computation time and the loss of accuracy of this approach. Three different cases were used: a laboratory case of a trench in a constant flow, a river flood case and an estuarine test case. For the river flood case, a single event with varying water discharges, the approach is unsuitable. Using the MF implies that the water levels change too rapidly, altering the hydrodynamics. The same would be the case for tidal flow, but the morphological acceleration factor can still be used due to the repeating nature of the tide [1]. The downside is that strictly the results using a factor N are only valid after exactly N tides. For steady cases the approach can be used flexibly without limitations. Comparisons with the measured data the trench case show that the MF can be used with only limited loss of accuracy. The simulation time reduces rapidly, while the model skills only reduce marginally, up to the MF is 90. The simulation time initially reduces rapidly. For the larger model of an estuary, the gain using a MF of 20 reduced the run time by a factor 20. In this case however, the model does show some significant changes in the prediction

Lag effects in morphodynamic modelling of engineering impacts

This paper details the extension of the sediment transport and morphology model SISYPHE to include a lag term within the bed exchange source term of the, depth-averaged, continuity of sediment concentration equation. This lag term represents the time it takes for a sediment concentration profile to adapt to spatial or temporal changes in the flow. The inclusion of a lag term means that the settling velocity is no longer the only scaling factor for the exchange of sediment between the water and the bed. The modified sediment transport and morphodynamics model is tested against field data from the Thames estuary (UK) and on the morphodynamic development of a dredged trench in flume experiments. It is illustrated that the lag factor introduced is essential to model the sediment transport and morphodynamics, especially when considering engineered situations, where the bed is out of equilibrium with the flow conditions. Moreover, with this lag factor included, there is evidence that SISYPHE can be used for morphodynamic modeling of engineered situations

Probabilistic channel infill approach

The design of access channels aims to achieve a balance between the requirements for navigation and the need for both capital and maintenance dredging; these are essentially the key factors on which decisions are based, whilst considering the constraints posed by the site-specific environmental settings and legislation. Within the constraints of navigation requirements, channel and trench design can be optimized to minimize capital dredge volumes and the expected sedimentation and related costs of maintenance dredging. However, the infill of channels and trenches is highly stochastic, due to the large number of uncertainties in flow and wave forcing, sediment characteristics and the well-known limitations in sediment transport models. This paper describes an approach taking these uncertainties into account

Ecologische landschapsindex (ELI); een nadere uitwerking van de: graadmeter ruimtelijke samenhang & graadmeter hydrologische relaties

Het Meetnet Landschap beoogt een systematische monitoring van de effecten van veranderingen in het landschap op de identiteit en de duurzaamheid in het licht van het streven naar het behoud van landschappelijke kwaliteit. In totaal zijn negen meetdoelen omschreven. Het meetdoel 6 betreft de landschapsecologische kwaliteit van het landschap. Ter operationalisatie van dit meetdoel is in opdracht van het Expertisecentrum LNV het project Ecologische landschapsIndex (ELI) gestart. Het project wordt uitgevoerd door het Alterra en Bureau Vista. Als eerste resultaat heeft het project een theoretische verkenning van aspecten van landschappelijke kwaliteit opgeleverd. In deze verkenning worden vier mogelijke graadmeters voor landschapsecologische kwaliteit onderscheiden, op het gebied van ruimtelijke samenhang, hydrologische relaties, stofstromen en landschappelijke heterogeniteit. Dit rapport is een nadere uitwerking van de graadmeters ruimtelijke samenhang en hydrologische relaties