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A Flood of Claims: How Hydraulic Modelling Helps Determine Liability


by Richard Allitt Associates- Expert Witness Services for all Flood & Drainage related issues

Flooding and the impact that it has on property such as homes, businesses and agricultural land continues to grow in importance. Already there are an estimated five million people living in 2 million properties located within flood risk areas in England & Wales. Whilst the winter of 2014/15 was relatively flood free the previous one set new records. The damage for this period has been estimated at over £500m, with insurers receiving more than 174,000 claims between 23 December 2013 and 8 January 2014 alone.

Why is there a rising tide of flooding claims? Historically there was little hope for recovery from flooding claims but the increased incidence and large sums involved has caused the industry to be more inclined towards recovery. Necessarily, as the economic cost of flood events continues to rise there has been an accompanying rise in those seeking to apportion costs. These claims are both from those directly affected as well as from insurance companies and other third parties. Inevitably the legal process has to determine liability in such cases, making it increasingly important to understand how liability for flooding might arise.

What are the legal precedents in respect offlooding?
Companies or individuals can become liable through ownership of their land or perhaps as a result of negligent flood defence works or even due to their contribution to climate change. Flood damage is considered a “natural nuisance” with most flooding claims likely to be brought under the law of nuisance. Those nuisance cases which flooding pertains to can generally be divided into three main areas:
? Land Drainage Rights
? Mining Cases
? Culverting Cases

In addition in some cases Riparian rights may apply such that where there is a natural watercourse or water catchment area e.g. a pond the owners of the land on either side have certain rights which can be enforced against other riparian owners.

The law considers that landowners have a ‘measured duty’ in negligence and nuisance to take reasonable steps to prevent natural occurrences such as flooding on their land from causing damage to neighbouring properties. Equally, landowners have a right to protect their property against a common enemy (such as flooding) but they do not have a right to pass such an enemy on to the land of a neighbour. Notably it is recognised that, the common law does not operate in a vacuum and statutory schemes must be taken into account when considering liabilities in nuisance and negligence. Consequently a discrete body of law has developed concerning the extent of a landowner’s liability for natural nuisances.

Leakey vs National Trust turned the tide for flooding claims
Much of the recent legal precedence for flooding arises from a case called Leakey v. National Trust which was in fact not a flooding case at all but one to do with land slip. However, this case and subsequent others that were deemed to be involving a ‘natural process’ concluded that there is a duty to do what is reasonable in the circumstances to prevent or minimise the known risk of damage or injury to one’s neighbours or to their property. This means that if the condition of your land is such that it could give rise to flooding on your neighbours land then you have a duty to take reasonable steps to abate that risk. What is deemed
reasonable will vary depending on circumstances. Often a public authority is involved and despite potentially having substantial resources consideration will be given as to the competing demand of these resources and the many public purposes for which they are intended vis a vis work that may only benefit a few residents.

Strict Liability may also apply
Where an action is deliberate e.g. obstructing a watercourse there is a duty to see that the works to substitute for the natural flow are adequate to carry off any water brought down by extraordinary rainfall and if damage results due to the works being inadequate there is liability. This also applies to escapes from a reservoir.

Powers and Duties
It might be expected that a substantial number of claims might arise against those involved in flood defences and water & waste water provision.

However, just because a body has the power to act does not necessarily mean it has a duty to do so and cannot therefore be held liable to do so. This applies to bodies such as the Environment Agency. Similarly water authorities have a tendency to resort to a case known as Marcic whereby unless the regulatory body OFWAT has issued an enforcement notice to act they are not held to be liable for not carrying out work which may have prevented flooding. The consequence of such legal precedent is that few cases against such authorities are ever brought successfully.

How can an expert witness help in these cases? The purpose of bringing in an expert witness is to provide not only a subjective opinion based on in-depth flooding specific experience but also to make available some objective science which is based on observable phenomena and is presented factually. Indeed the credibility of an expert witness’ testimony is largely based on their ability to have an objective opinion. The basis of the scientific information used by an expert in these cases is called hydraulic modelling. It is an important tool for any flood & drainage related expert witness work.

So what is hydraulic modelling?
In simple terms hydraulic modelling is the computer simulation of hydraulic processes, in this instance this usually refers to storms and floods. There is a wide diversity of data that can be derived in this way from quite simple models to ones of great complexity with a large number of variables. Essentially the model will look at a set of circumstances incorporating factors such as:
? Rainfall
? Tides
? Ground wetness
? Infrastructure such as screens
? Controls
? Valves

With these variables in place the model takes into consideration a specific location , looking at pipe sizes, channel dimensions and other conditions and it is then able to predict what the outcome would be given a certain set of circumstances. The sophisticated software model can indicate where, when and to what extent flooding is likely to occur given a particular set of circumstances. There are a number of different software tools available but Richard Allitt Associates uses one of the most complex & sophisticated specialist software tools available enabling the most complicated of scenarios to be run.

What is Hydraulic Modelling normally used for?
Hydraulic modelling is an essential tool for all those involved in the planning and management of urban drainage facilities. Their uses are many and varied with applications including amongst others
? Design of flood defences
? Operational Flood Forecasts
? Maps of Flood Risk
? Reports for Planning Applications
? Sewerage and other infrastructure design
? Investment Planning
? Surface Water Management Plans

Some of the key clients for this type of work are the Environment Agency, Water & Sewerage Companies and Local Authorities. They enable a wide variety of government authorities and developers to provide the right infrastructure and to plan the associated developments suitable for the infrastructure provided.

How can Hydraulic Modelling help in Legal Cases?
A legal case arises when flooding causes damage to property, land or belongings and there is considered to be fault attributable for the associated costs. Hydraulic modelling and a flooding expert witness can help determine who if anyone is liable.

Generally speaking the defendant in flooding liability cases is usually a local authority, Water & Sewerage Company or perhaps the Environment Agency in which case they are quite likely to already have the data they need in the form of an existing calibrated model, as well as having access to the necessary skilled resources.

The claimant however is not so fortunate and is much more likely to be disadvantaged by their lack of detailed data and flooding specific knowledge. They are most unlikely to have access to an existing model with off the shelf models available to purchase being extremely expensive. They will need to create a bespoke model for their particular circumstance and to purchase the necessary data with the easiest and best way of achieving this being via the services of a nominated expert. Solicitors and insurance companies working on behalf of claimants are therefore the most likely to find the services of an expert witness invaluable.

Essentially hydraulic modelling for legal cases needs a very highly skilled modeller who can test alternative hypotheses and can provide clear plans and visualisations.

How do Models work?
In essence a hydraulic model works by taking all the known information and using this to model predicted outcomes. Where all the data is known, this means the model is calibrated.

The Hydraulic Model Process
However, it can also work whereby it is the outcome which is known e.g. flooding occurred and work out what the variables would have been if a screen was
blocked or a control gate left open. Where only a limited number of variable are available, then a series of different options will be run to determine which set of circumstances resulted in the particular flooding incident.

Importantly the models can also be used to predict the pattern of flooding over time, indicating if remedial action could have minimised damage for example.

The models can be used for looking at flooding from a variety of different sources such as ponds, rivers, reservoirs as well as from sewers and culverts. It is possible to observe the implications of blockages in outlet pipes and of screens. Whatever cause the claimant thinks is at the root of the flooding incident can be fully tested and modelled to establish whether this was the case. By using their in depth knowledge of the industry and hydraulic modelling techniques the expert witness can provide the legal team with objective scientific information that can clearly determine what has occurred and who is responsible.

Not only can hydraulic models work out what the cause of flooding was, they can establish how severe the consequences of any particular action were. It can produce comparable scenarios for different sets of circumstances. For example a certain level of flooding may have been inevitable but a direct action such as the installation of railings in a park
for example can alter both the direction and
volume of flooding.

A Case in Point
? Cordin & Others –v- Newport City Council
After prolonged heavy rainfall over many days the flood storage reservoir steadily filled up. At the corner of the reservoir was a control gate which had originally been automated but had fallen into disuse so in this instance the control gate was not adjusted. On this occasion bypass weirs came into operation at the same time as high tides occurred with the resulting consequence being flooding.

What the hydraulic model was able to show was that if Newport City Council had opened the control gate at a previous low tide the flood storage reservoir could have been drained without any flooding. As a consequence they were deemed liable for the damage caused. The model can be run with a fixed set of circumstances showing what happened when the gate was closed and what would have  happened if it had been opened and when it would have needed to be open.

? Proving the case for an inadequate trash screen
In this flooding case a trash screen had been placed at the entry to a riparian ownership culvert. The site had a sub-standard design of screen as there was insufficient room for a full screen. In this case 3 possible scenarios were run through the model to show what the situation would have been had no trash screen be in place, what would have happened had it been in place but had been kept clear and finally as was the case what happened because the trash screen became blocked.

Part of the role played by the expert witness is not only to evaluate the model and know what scenarios to run but also knowing the right questions to ask to
gather essential data that will ensure what is input to the model is correct. Gathering visual data as seen below helps to illustrate the case but quantifying what this observable data suggests is a highly skilled task. In these cases it is not only required to model what actually happened but to quantify what could or would have happened had alternative achievable actions been taken.

The hydraulic modeller can then take this further to show and create designs for schemes whereby future flooding is alleviated. The contribution made by the expert witness therefore goes well beyond the value added to the case itself.

Evaluating the Cost
By being able to look at a range of different scenarios the expert witness is also able to quantify the extent of the liability involved. By using hydraulic modelling it is possible to model what the situation would have been if a particular action which caused the flooding incident such as erecting railings or closing a valve had not been done. In this way it is possible to actually measure the cost difference between what actually happened and what would have been the situation otherwise. This is an extremely useful way of quantifying the incremental costs of damage.

In the example below (related to the trash screen above) three scenarios are shown depicting the cost of the damage which actually arose (due to a blocked screen being in place), the cost if there had been no screen and the damage level with a screen which was not blocked. The graph shows the connection between the depth of flooding and the costs to both buildings and household contents and highlights the differences arising from different scenarios depicted by the 3 horizontal lines.

It is particularly important to show these differences when it is considered that as the law stands, the only duty owed by a public authority to any member of the public, is not to add to the damages which that person would have suffered had the authority done nothing.

Why employ an expert?
It is a decidedly skilful process demanding considerable experience and training to know which variables to enter into the modelling software and determine which scenarios are the most likely. Hydraulic modelling for legal cases needs a very highly skilled modeller who can test alternative hypotheses and can provide clear plans and visualisations. A skilled modeller will not only be experienced at identifying the causes and mechanisms of the flooding but will also be able to
advise on solutions which can prevent repeat occurrences.

Our experts are able to bring clarity to the complex reality of particular sites with our close attention to detail and practical experience of hydraulic modelling. It is most often the case that the claimant is unlikely to have the necessary specialist modelling information that they need. This usually means that they will either have to do their own surveys and build their own hydraulic models, which is very difficult. So the most straight forward response to this need is to employ an expert such as Richard Allitt Associates as both the information required and its interpretation is highly complex.

What will an expert do?
As well as carrying out sophisticated modelling work, an expert witness will gather essential data and evidence required to inform the model. Importantly the expert will know what data is required and how best to obtain it.

If required they will visit the site in question, observe and photograph conditions and infrastructure to enable them to make visual comparisons with the conditions observable at the time of the incident. They can identify important elements in the analysis process such as the existence of particular infrastructure or newly built structures. Richard Allitt Associates also employs the use of its UAVs or ‘drones’ which can carry out aerial surveys. These aerial surveys are extremely useful in being able to identify things such as ‘urban creep’ where hard landscaping has prevented water soaking away leading to flooding, or identifying ditches and embankments which are not observable from ground level.

How to find an expert with the right skills?
There are a number of experts who offer flood related services but there are few individuals with the experience of Richard Allitt who is one of the foremost hydraulic modellers in the UK who not only advises the flood & drainage industry but has helped to develop its best practises and industry user guidelines. As Managing Director of Richard Allitt Associates Richard is heavily involved in all the company’s Research & Development activities. He is passionate about leading both the company and the industry forward. He has been involved in the industry for over 40 years and has made numerous technical presentations to the Chartered Institute of Water & Environmental Management’s Urban Drainage Group and other forums as well as sitting on a number of the Construction Industry Research & Information Association’s Steering Groups.

As a leading authority in the industry Richard has been instrumental in directing the firm to its current highly regarded status and he has ensured it is at the forefront of research, using all the latest analytical techniques.

He has always worked to attract a diverse range of clients with the main focus of the firm’s work being on hydraulic modelling of sewerage systems and the
preparation of plans for Drainage Areas, Sewerage Management and Surface Water Management. He is recognised as an industry expert, publishing a series of papers on drainage and flood modelling as well as being an influential member of the Urban Drainage Group. He has led a series of research projects for UKWIR (United Kingdom Water Industry Research) furthering industry knowledge on Integrated Urban Drainage Modelling, Urban Creep (the loss of permeable surfaces e.g. the paving over of driveways, creating increased runoff which contributes to flooding and other problems. Richard has also acted as an industry auditor overseeing the technical audits of sewerage models built for catchments across Scotland.

The depth and quality of Richard’s experience has seen his services in high demand having acted as an Expert Witness in over 70 cases including some no table ones such as:

? Bybrook Barn –v- Kent County Council An example of where the Leakey principle was applied where a highway authority was held liable because it was responsible for an inadequate culvert which caused a risk of flooding to neighbouring land.

? Cordin & Others –v- Newport City Council As detailed above this was a case where strict liability applied an escape from a reservoir where flows should have been discharged carefully through a sluice gate.

In the latter case the judge remarked of Richard:

“I found him to be an impressive witness. His reports and evidence were careful, detailed and to my mind displayed impressive technical command and understanding of the issues.”

Do you always need an expert in flooding cases?
In straightforward cases where the cause is clearly observable it may not always be necessary to employ a hydraulic modeller. However where more data is
required and cause is not clear then the modeller should be highly skilled and experienced in order to understand all the possible scenarios. The software helps the modeller to address a wide variety of ‘what if’ questions and can be used to test a variety of alternative hypotheses. There is no doubt that employing a skilled flooding expert will result in clear plans and visualisations which will enable everyone without specific knowledge of the industry to understand clearly what has occurred. The expert witness will in this case not just be a (albeit well informed) subjective opinion but be able to provide objective data to illustrate that opinion.

In flooding related matters the right expert witness can bring facts and clarity to facilitate the process of law.

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