@_morphological Modelling Procedure

TU Delft - Morphological modelling procedure

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Civil Engineering & Geosciences Morphological modelling procedure In this chapter we attempt to define a sound morphological modelling procedure, based on the experience in the model studies described in the previous chapters. Obviously, following the full procedure is not always possible, given constraints of time and budget. In such cases explicit choices must be made to leave out certain elements. Data collection and analysis

Bathymetry data The analysis of bathymetric data can comprise the following steps: Conduct a literature review of the problem area. Find historical maps and digitise key features and contours so they can be easily combined and displayed. Interpolate digital depth data to one common grid. Make difference maps. Determine relevant areas for which volume changes must be determined. Compute volume changes for these areas. Draw a number of cross-sections and analyse behaviour of cross-sections. Select morphological units for an analysis of the growth or decay and migration rate of these units. Make animations of evolution of bathymetry. Wave and wind data Analyse wave and wind climate; divide into sectors of 30 degrees, 0.5 s Tp, 0.5 m Hs. Plot wave roses. Determine dominant wave directions. Tidal data Check availability of regional tidal models. Collect water level data from neighbouring stations. Analyse current and discharge measurements. Select time periods for calibration of a tidal model. Longshore current data These are usually not available, since it takes major field campaigns to collect useful data. The only option is to test your model against these datasets and hope that it will be applicable in the specific situation too. Sediment transport data Collect data on sediment properties. Analyse (if available) sediment concentration data and select data for model calibration/validation. Estimate longshore transport rates from local accretion/erosion near structures. Conceptual model Analyse current patterns from regional model, current atlas or previous model studies. Analyse grain size distribution over area. Estimate dominant wave-driven current patterns Estimate transport paths and set up hypotheses about causes of bottom changes. Draw a picture of this estimate, to be updated in the real study. Setting up modelling strategy Define the morphological elements that need to be resolved by the model. Estimate the grid sizes required to represent them. Define model boundaries. Inside tidal inlets, choose boundaries over the tidal shoals as natural boundaries. On the seaward side, choose boundaries perpendicular to or parallel with flow direction. Put them as far away from the problem area as possible. Coarsen grid towards the boundaries. Determine which wave directions will be included in the simulations and sketch wave grids. Estimate runtime for wave run. Estimate flow and morphological time steps. Estimate probable run times for a single tide and a single morphological step. Estimate how long it will take to simulate the desired number of years and lower your standards and expectations if necessary. Take into account that you'll need about five to ten runs in the calibration and validation phase to arrive at a single run you can live with. Define necessary sensitivity runs.

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Setting up model grid and bathymetry

Flow and morphology grid Boundaries and grid resolutions have already been determined. Draw spline grid as a first sketch. In a number of steps, refine grid, globally or locally, orthogonalise, repair local glitches, refine further. Check orthogonality (< 0.05-0.10), smoothness (< 1.3), resolution (conform requirements). Wave grids Define one overall rectangular bottom grid for all coarse-grid wave computations. Try to ensure that the nested wave grids are within the flow model domain, so they can use the bathymetry of the flow model, which is updated in a morphological run. Select the nested grid in such a way that: resolution is better than flow model in most areas grid direction is close to local wave direction the order of nested computations is such that disturbances at the boundaries are avoided For each grid set-up, plot wave heights, dissipation rates and wave forces at least once for a relevant condition. Bathymetry Interpolate digital data to grid. Use triangulation if data points are few, grid cell averaging if you have many points per grid cell. Check important cross-sections. Produce clear and detailed figures of the interpolated bathymetry. Boundary conditions

Wave schematisation Select number of wave conditions by which the full climate is represented. Select criteria (longshore transport rate at some coast sections, stirring of sediment in deeper water at some locations) Group wave conditions. Determine average of the criteria per group. For each group, select the condition for which most of the criteria match the average of the group. Try to avoid using a weight factor for a condition that differs from the probability of occurrence of the group. Make sure that you use the right parameters for input. Don't confuse Hs with Hrms, Tp with Tz, Tm01 or Tm02. Use appropriate relations between these parameters.

Representative tide. Select a month with an average spring/neap amplitude ratio. Run tidal model with time history output in a number of representative locations. At each point, estimate the transport averaged over 59 tidal cycles. Starting at each flow reversal, determine average transport over two consecutive tides, at each point. Select the period for which the transport matches the monthly averaged transport most closely. Generate boundary conditions for this period. Carry out a Fourier analysis over the two selected tidal periods for each of the boundary support points. Take out the diurnal and odd components, in order to get a single representative tide. (Note: the reason for first selecting two consecutive tides and then removing the diurnal and odd components is, that in this way we avoid the situation that the mean component is polluted by the diurnal component.) Sediment transport Usually, equilibrium transport is prescribed at the open boundaries, which is computed based on the local flow and wave conditions. Bottom change For models of tidal inlets, where the boundaries should be chosen reasonably far from the area of interest, the appropriate type of boundary condition is a fixed bed level.

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Calibration Calibration is the process of tuning all parts of the model system based on local data and common sense. Typically, the following checks are carried out:

Calibration of flow model Criterion

Check

Adjustments to make

Smooth flow fields without boundary disturbances

Vector plots of velocity at some points in Type, values of boundary conditions time, vector plot of tide-averaged velocity

Smooth time series, Comparison of time series plots for small enough time step different time steps.

Time step

Periodic solution

Longer initialisation, use of restart file

Time series plot

Matching with overall Comparison of time series of water level model in case of nesting and velocity in collocated points.

Type of boundary conditions, location of support points

Water levels, flow Comparison of time series, determining velocity, total flow rates rms error and phase errors match measurements

Roughness, viscosity

Wave-driven currents represented well

Grid resolution

Vector plots showing at least five grid rows in surf zone

Calibration of wave model Criterion

Check

Adjustments to make

No disturbances at grid Contour plot of wave heights, plot of boundaries dissipation and wave forces

Changes nested grids and order of nested computations

Water level and velocity Check disturbances at boundaries flow inside flow grid match grid in contour plot of wave heights on overall values outside overall wave grid flow grid.

Change overall values of water level and velocity per time point

Wave heights match measurements

Wave breaking parameters, bottom friction.

Compare wave heights as function of time for measurement locations

Calibration of transport model Criterion

Check

Adjustments to make

Smooth time series, small enough time step suspended transport

Time series of concentration and Time step, time interval, number of transport at some locations; initial steps comparison of different time steps

Smooth, consistent transport fields

Vector plots at some points in the Remove errors in bathymetry or wave tidal cycle and residual transport grids

Overall transports through some cross-sections in accordance with observations or conceptual model

Integral of residual transport through cross-sections

Change coefficients in transport formulation

Calibration of morphological model Criterion

Check

Adjustments to make

Sedimentation-erosion pattern in agreement with measurements

Contour plots of measured and computed sedimentation and erosion

Transport coefficients or formulation, wave climate, sediment parameters

Volume changes over control areas in accordance with soundings or dredging figures

Area integrals of bed level changes

As above

Cross-section shape

Compare shape, migration and area change of measured and computed cross-sections

Side slope effect, (spiral flow), transport formulation

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TU Delft - Morphological modelling procedure Plan view of morphology after some years

Contour and isoline plots of measured and computed bathymetry

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Validation In the validation phase, no further adjustments are made to the model. First we look at the results at the end of the calibration phase and assess how well the model is capable of simulating a number of morphological processes simultaneously . Even in a model that has been calibrated to its limit, it is still very well possible that some features are not represented well. The number of degrees of freedom in the morphological model is limited, and the calibration parameters and coefficients are global parameters, which are not allowed to vary over the domain. When such data is available, a more strict validation may consist of simulating a different time period in the same area, preferably one in which different things happen. The criteria against which the model performance is judged are the same as in the calibration phase of the morphological model. Preparing scenarios Once the model has been calibrated and validated to satisfaction, it can be applied to evaluate relative effects of various scenarios, such as different port layouts, dredging scenarios, nourishment schemes or structures aimed at mitigating coastal erosion or improving navigability. Since these schemes are usually yet to be built, the starting bathymetry is usually the most recent one. In order to enable a clean evaluation of the effect of the scheme, a so-called T0 or reference simulation must be carried out first: a simulation starting from the latest bathymetry and covering the desired number of years. Next, for each of the schemes the bathymetry must be adapted and if necessary structures have to be added to the schematisation. The simulations are then carried out sing exactly the same settings as in the reference computation. Defining output By this time, the amount of output can be reduced considerably and can be limited to outputs directly relevant to the engineering questions, and to a minimum set of standard plots which allow the modeller to ascertain that the process is running correctly. It is important to realise at this point all the steps necessary to translate the model results to the design criteria that are important to the client. He or she may not be interested at all in nice colour pictures, but mainly in the effect of various layouts on the annual dredging volume. Make sure that you give that type of information in a clear way, besides the necessary information to substantiate it. Running and postprocessing Since the running of various scenarios is vary similar to the reference simulation, it pays off to automate this process by using shell scripts or batch files. Preferably, in these scripts everything is arranged from running the scenarios to making graphs and doing volume computations and integration of transports along sections. This is even useful if not too many scenarios are run, because very often errors are found at a late stage, and sometimes all runs must be redone. When this has been well organised, it can be done quickly. Additionally, these scripts offer a clear insight to the experienced user into how results were obtained. This is vital for quality checking. It is advised to use one machine and disk section for a morphological project, and to use separate directories for separate activities, and for each run. The directory for each run can be further subdivided into input and output directories per simulated condition. Interpretation The end results must be interpreted carefully, with the deficiencies encountered in the validation phase in mind. Part of the interpretation consists of combining and reducing data, for instance from erosion and sedimentation patterns to volume changes over control areas, and from transport vector fields to integrated transports over a selection of cross-sections. A useful way of showing the effects of a certain scheme is by plotting on one page the bathymetry, the bottom changes in the reference run, the bottom changes for the scheme concerned, and the difference in bottom change between the reference run and the scheme concerned. At the end, the main findings related to the various schemes to be compared must be summarised in a few

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tables and / or figures showing the effect of the schemes in terms of parameters relevant to the client, such as overall dredging volumes, nourishment needs depth of scour. Reporting Typically, the kind of reporting carried out in these studies falls into the following categories: A background report or study report in which the full model set-up is given and an extensive description is given of the whole modelling process, the choices made in schematisations and detailed explanations of the results. Such a report should explain in enough detail how a model was built, so that the results may be reproduced by someone else using the same model. It must explain why the inevitable choices were made. An executive summary (sometimes written by the client) in which the main findings are given in layman’s terms and only those figures are given that are necessary to illustrate the conclusions. Sometimes a CD-ROM is provided with a large number of graphs and animations, which can be browsed using an Internet browser. Archiving The contents of the project directory and all subdirectories can be stored on tape and on CD-ROM’s. The latter is preferable since CD’s follow a very clear world standard, contrary to tape devices.

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