Tuesday, 22 March 2011
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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Sunday, 6 March 2011
The 12th MIKE by DHI UK User Group Meeting at the Lion Quays Conference Centre
Tuesday 8th and Wednesday 9th March 2011

The User Group Meeting is now just a few days away. It is NOT too late to join us for this exciting FREE event!

The UGM is free of charge and open to both users and potential users alike. If you would like to find out what the DHI e-Community are doing, and what DHI are planning for the future, please join us on the 8th March.

There is 'something for everyone' in the Agenda.

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Welcome to DHI UK

DHI is an independent, international consulting and research organisation with the global objective of advancing technological development and competence with respect to water, in all of its environments.

Worldwide, we offer a wide range of consulting services and leading edge technologies, software tools, environmental laboratories, and physical model test facilities, as well as field surveys and monitoring programmes. Designated as a not-for-profit organisation, DHI is able to invest a considerable portion of its resources in research and development. Today we co-operate with many Universities, and research organisations, and are recognised globally for our innovation and expertise.

In the UK, DHI offers niche or specialist consultancy services in the water and environment market to government agencies, commercial entities and selected research organisations. We fulfil a research based specialist advisor role; a ‘Consultant to the Consultants’. We also supply and support the renowned MIKE by DHI suite of integrated water modelling tools.

MIKE by DHI software is the result of years of experience and dedicated development and has, in many regions, become the standard modelling tool. It transforms our science into practice and gives you the competitive edge and, through the DHI Academy, you can rest assured that there is a local team of highly skilled experts committed to train and support you every step of the way.

MIKE by DHI truly models the world of water - from mountain streams to the ocean and from drinking water to treatment plant and beyond.

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