MIKE by DHI UK and Ireland Blog

Innovative use of MIKE by DHI for morphological evolution of coastal defences

Introduction

For many years numerical models have been able to accurately represent the flood water inundation, and associated flooding risk, resulting from the failure of flood defences. Such failures are often simply described within numerical models as static ‘openings’ in defences, with little or no consideration given to morphological change and the development of a breach over time as a result of various natural processes.

In the UK, funding for new flood and coastal defences is becoming harder to attain. As such, an increasing reliance on natural defences is highly likely, augmented by the optimisation of existing flood defence structures and innovative low impact solutions that work with our natural environment.

DHI is committed to delivering a range of tools that consider the morphological evolution of coastal defenses acted upon by natural processes of storms, waves and tides, exacerbated by our changing climate. The examples below present upcoming new features in MIKE 21/3, as well as recent projects.

MIKE FLOOD (our complete 1D-2D integrated flood modelling package) is already able to represent changes to levees over time and, coupled with the new MIKE FLOOD Dynamic Hazard Mapping module, the new features and technologies presented below will further enhance the MIKE by DHI total solution capability.

Wave Overtopping and Erosion of Sandy Barriers

Work is progressing to adapt DHI's classic Boussinesq type wave model (MIKE 21 BW) to consider the overtopping and erosion of sandy barriers; linking wave run-up, overtopping and lee side down-rush to the morphological evolution and potential breaching of coastal defences.


Overtopping of a uniform alongshore barrier

The present animation (above) shows results from an in-house version of MIKE 21 BW that is currently being developed under the research programme COADAPT funded by the Danish Strategic Research Council.

The animation shows that bathymetric irregularity can have a significant effect on the rates and location of overtopping; in this case, a depression / channel within a submerged off-shore sand bar predicts higher risk of wave overtopping of the barrier just shoreward of the depression.

The model can be used to predict the risk of flooding, as well as the vulnerability of softer defences, to the impacts of climate change and to guide prevention against flooding in a cost effective way.

Dynamic Dam / Dune Break & Morphological Seabed Change

We will soon be introducing a new feature into our MIKE 21/3 Flexible Mesh Series models enabling the dynamic update of a model bathymetry during a simulation (the feature works on element based topography; i.e. dfsu files).

In this example, the new MIKE 21/3 Dynamic Bathymetry Update feature has been employed to consider a breach through an alongshore dune.

Model set-up of dune break through


Flood water inundation resulting from dune break through

The animation above shows a dune break during a 40 hour storm, and the resulting inland flooding as a consequence of the failure.


Section through a breach in the direction of flow

The animation above shows morphological change during the critical erosion phase of a levee failure, and the consequential change in water level.

The model can be used to generate flood inundation and flood hazard maps, to inform emergency planning and evacuation procedures (including early warning systems), and to develop the design of remedial or preventative measures.

Erosion of Soft Cliffs

DHI’s work on the Femern Belt project (a 20 km long connection between Denmark and Germany) includes the design of 2.3km2 artificial landscape with beaches, bays, lagoons and active cliffs; created from material reclaimed from dredging operations for the preferred tunnel option.

The artificial cliffs will be established with the purpose of slowly releasing loose material to the down drift eroding coast line. The first estimates of expected retreat rates have been based on observed retreat rates at similar but natural cliffs.

Femern Belt project (Design group: Rambøll/DK, Arup/UK and TEC/NL, with DHI and Schønherr/DK as sub-consultants)

Cliff erosion is typically connected to events of high water levels and severe wave action. However, depending on the porosity of the cliff, the content of water in the cliff may also provoke collapse. Empirical methods are generally employed for the quantification of cliff erosion. However, the use of MIKE 21/3 for hind casting of waves and water levels could be relevant to assist in an analysis of erosion rates compared to the exposure.

For more information, please visit http://bit.ly/iOyVUr and http://www.femern.com/

Potential Applications for the Above Examples

• River embankment breach and evolution
• Dam embankment breach and evolution
• Reliability of and failure consequences for soft coastal defences
• Application to soft cliff erosion
• Managed realignment schemes and removal of hard defences

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A MIKE 21C Concept Demonstration: Is it Possible to Model the Impact on Flood Risk due to the Morphological and Planform Change of a River over the Lifetime of a Development?

Introduction

In the UK, flood risk and where necessary mitigation must be considered throughout the design life of the development. For the purpose of this example, a design life of 100 years (i.e. minimum lifetime of a residential development from PPS25) has been used.

‘For practical reasons it is difficult to define the lifetime of development as each development will have different characteristics. For guidance, residential development should be considered for a minimum of 100 years, unless there is specific justification for considering a shorter period. An example of this would be if the development was controlled by a time limited planning condition.’ (PPS25 Practice Guide)

The primary long term factor affecting river hydrodynamics, and flood risk, is climate change. However, the consideration of other factors may also be critical.

While it is commonplace to model the physical changes to a watercourse or flood plain as a result of urbanisation (e.g. introduction of culverts, flood defences, etc), morphological change is usually limited to the consideration of scour or silting (e.g. blockages) at structures, and to develop river maintenance schemes. Generally, for flood modelling, river bed and channel form are fixed (excepting the development of breaches).

What is MIKE 21C?

MIKE 21C is a special module of MIKE 21 based on a curvilinear (boundary-fitted) grid, which makes it suitable for detailed simulation of rivers and channels, where an accurate description of bank lines is required. MIKE21C is particular suited for river morphological studies and includes modules to describe: -

• Flow hydrodynamics
• Helical flow (3D secondary currents)
• Sediment transport
• Alluvial resistance due to bed material and bed forms
• Scour and Deposition
• Bank erosion and planform changes

The bank erosion is computed from a formula relating near-bank conditions to bank erosion rates. The accumulated bank erosion can be used for updating the bank lines, and for updating the curvilinear grid (extent of the modelling area) at every time-step. The bank erosion products can be included in the sediment budget for the adjacent riverbed, or it can be disregarded depending on the composition of the bank material.

The modules can run interactively, incorporating feedback from variations in the alluvial resistance, bed topography and bank line geometry to the flow hydrodynamics and sediment transport.

Notes on the Model Set Up

Grid (see right for extract).

HD: Constant representative inflow, constant downstream water level and constant resistance (i.e. the only factor affecting hydrodynamics is morphological change).

ST: Single sediment component (grain size ~ coarse sand). Meyer-Peter and Muller transport theory employed.

Morphological Update: Single layer model (initial bathymetry comprises a uniform bed with sloping banks).

Planform: Erosion = Constant * (V – Vmean). Two eroding banks, the bank erosion model uses Erosion – Accretion = 0 (other models are available).

Model Results


Development of point bars and bend scour is clearly evident (above).

Results can be used to assess the impact on flood inundation modelling. If necessary, it is possible to ‘burn’ the original and updated channel grids into the bathymetry of a MIKE FLOOD / MIKE 21 model, in order to consider changes in flood depths and velocities in the wider floodplain. This is particularly important where water levels in the channel indicate that flood waters may find future overland flow routes that were previously dry (i.e. outflanking of existing defences).

Results can be used to inform the design of any flood defences or bank stability assessments, and to assess the effectiveness and impact of erosion control structures (hard / soft engineered) on future habitats (e.g. fish spawning grounds).


Sudden morphological change (as a result of significant flood event, dambreak, etc) is also possible.

Important considerations

Morphology introduces another level of uncertainty. The undertaking of sensitivity checks / range of scenarios is extremely important; particularly when attempting to predict long term morphological and planform change of a river.

Many more layers (representing varying soil parameters) will likely be required in the morphological model, and detailed geotechnical investigations of the bank and bed materials will be required. Such investigations should, as a minimum, be informed by river / walkover surveys.

Calibration is critical!

• Gauged river flows and meteorological data (for long range flow forecasting and performance monitoring)
• Historic maps & aerial photos (for preliminary assessment of river movement)
• Topographic survey data and spatial monitoring points (ongoing morphological monitoring)

MIKE 21C is generally used in larger rivers (possible scaling issues).

MIKE 11 or MIKE 21 FM will be suitable for most usual riverine sediment transport applications but planform change can only be modelled using MIKE 21C.
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