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Photograph - River flood plain Winall Moor

2D Flood Modelling

Introduction

We use ISIS software, usually together with LiDAR data, to carry out 2D flood modelling work. The technique is often used in conjunction with flow hydrographs derived from a Flood Estimation Handbook (FEH) hydrological study.

LiDAR stands for Light Detection and Ranging. The data is produced by an aircraft mounted LASER which is directed onto the ground and the reflected light is analysed by computer to produce a rectangular grid of x, y and z ground surface co-ordinates. LiDAR data is typically available in either DTM (Digital Terrain Model) or DSM (Digital Surface Model) forms. DTM excludes trees, vegetation and buildings while these are included within DSM data.

Uses of ISIS 2D software

2D flood modelling can be used to assess the consequences of the failure of river or coastal defences or to model the flooding caused by a dam burst. Further uses are to represent flood flows across river flood plains or to model the flow of water across the ground surface due to flooding from a drainage system manhole. The methodology is therefore often used as part of a detailed Flood Risk Assessment (in England) or Flood Consequence Assessment (in Wales).

Different solvers

ISIS 2D offers the option of using several computer modelling solvers (ADI, TVD and Fast) to deal with the complex mathematics involved in representing the water flow over the ground. Typically the ADI solver is used in most cases although the TVD solver is used where supercritical flows are experienced. The Fast solver is used where an initial estimate of the extent of flooding is required without involving detailed modelling work.

1D versus 2D Flood modelling

ISIS 2D flood modelling software is especially useful in representing flows across a river flood plain. The software can be used to model flows in river channels but there are some limitations due to the spacing of the LiDAR data grid. This means that it may not be satisfactory in all cases especially in modelling structures within the channel such as bridges, weirs and sluices. On the other hand ISIS and HEC-RAS 1D river modelling software are particularly suitable for modelling structures within the watercourse channel. Conversely the 1D modelling software can represent the flood water storage within flood plains but this can be complex and time consuming to model so 2D software might be more applicable in that respect.

1D and 2D flood modelling in conjunction

It is possible to carry out modelling which incorporates both 1D and 2D modelling software. Using this methodology the 1D model represents the river channel while the 2D model represents the flood plain. The two models operate in conjunction with each other with the 1D model passing flood water to the 2D model and visa versa as the storm develops. This methodology allows each program to do what it does best to arrive at the most cost effective modelling solution.

Further Information

Flood Modelling work is site specific so please contact us for further information including a free no obligation quotation.

Subsiduary channel of the River Itchen, north of Winchester showing extensive flood plain.

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