Nothing bore-ing about the Severn Estuary

Big tides go in cycles depending on the alignment of the moon and earth, and, to a lesser extent, all the other planets in the solar system. If you want the detail have a look here https://metofficenews.wordpress.com/2015/02/20/super-tides-the-weather-and-coastal-flood-risk/

The cycle is about 18 years and February 2015 saw some of the biggest tides in that cycle. This is big news on the Severn Estuary where the tidal range (the difference in water levels between high and low tide) is huge – up to 15 metres at Avonmouth.

The Severn Estuary is like a huge funnel. Tidal sea water pushes up the Bristol Channel and gets increasingly contrained by the narrowing channel, raising levels dramatically. The bigger the tide, the greater the rise.

The most spectacular result of high tides on the estuary is the world famous Severn Bore.  Large tidal bores (which is what the Severn bore is) are actually very unusual and only happen at a handful of locations across the world where there is a large tidal range. The Severn Estuary has the third largest tidal range in the world (behind the Bay of Fundy in North America and Ungava Bay in Canada).

Bores happen when the front edge of the advancing tide becomes contrained by the sides of the estuary, forcing the water up into a wave. It is pushed along by the millions of tonnes of water in the advancing tide behind.

The Severn Bore can usually be seen between Sharpness and Maisemore Weir in Gloucester, a distance of around 20 miles. It travels at a speed of 8-13 miles an hour.

The height of the wave can be anything from a few inches to over six feet. This is largely dependent on the size of the tide but is also influenced by wind, weather systems over the Atlantic and the amount of fresh water coming down the river. Bore forecasts are available well in advance and give them a star rating, the the best (biggest) being five star bores. You can find the forecasts here: http://www.thesevernbore.co.uk/timetable/4586702434

Watching the bore is a great experience. It makes an amazing rumbling sound which you can hear well before you see the wave. The wave itself and the boiling, tumultuous water pushing it seem completely alien on an estuary that is usually fairly calm and placid.

Not surprisingly the spectacle attracts thousands of sightseers and also large numbers of surfers who travel great distances to ride the wave (with varying degrees of success!). Keen surfers try to catch the wave three times during a single pass (usually at Newnham, then Minsterworth and finally Gloucester).

At the Environment Agency we’re more interested in the hour after the bore has passed. This is the time when water levels rise dramatically and very quickly. The rise can be 15 metres at Avonmouth, 10 metres at Sharpness and 6 metres at Minsterworth. This rise is just as spectacular as the bore itself, although the majority of bore watchers do not stay to see it.The impact of the tide is felt much higher up the river than Gloucester. Amazingly, during the February high tides the river at Upton in Worcestershire (about 60 miles from the estuary) was pushed into reverse twice a day. The graph below shows how normal flows of about 100 tonnes of water per second travelling downstream changed to an upstream flow of up to 40 tonnes per second in a matter of minutes. There was also an accompanying rapid rise in water levels of over half a metre. Incredible!

We have miles of flood defences along the length of the Severn Estuary which protect over a thousand homes and businesses. Few people notice most of them as they look like natural grass banks. However, if they weren’t there high tides would routinely cause extensive flooding across the estuary. When high tides are imminent we check all our defences to ensure they are in good working order. This is no small task given the extent of the defences and the remote nature of parts of the estuary.

After the high tides and storms of 2014 some of our defences were damaged. We spent the autumn and early winter carrying out Over £3 million of repairs so they would be ready for the huge tides of 2015. In February they received a good test (like Minsterworth here) and performed perfectly.

There are more very big tides in 2015 (especially in March and September). We’re ready for them.

Why not get out yourself and experience a truly world class natural phenomenon.

Video courtesy @theocair 

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Describing flood impacts

When we talk about flooding, or issue flood warnings, we always think about what the likely impacts on people, property and essential infrastructure (big roads, railways, water works, power stations) will be. The more severe the impacts the higher the level of warning. The terms we use to describe flood risk are also governed by the level of expected impacts. This blog provides some examples of those different impacts.

We issue three levels of flood warning: flood alerts, flood warnings and severe flood warnings. You may well be familiar with the graphics and descriptions we use (below).

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We also publish a daily three day outlook of our assessment of flood risk by English County.

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We use green, yellow, amber and red colours to describe the forecast risk. Green represents a very low risk, yellow low, amber medium and red high. The colours don’t just reflect the likelihood of flooding but also the impacts it will cause. We use this matrix to decide the colour to use.

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So what should you expect to see at these different levels of warning or risk? Here are some examples and pics.

Flood Alert and low (yellow) risk

At our lowest level of warning or risk we’d typically expect rivers to be coming out of their banks with water entering floodplains (rivers don’t burst their banks, they simply come over the top!). Impacts may lead to minor disruption and flooding to low-lying roads and gardens.

You might see large areas of farmland in floodplains flooded, sometimes requiring livestock to be moved to higher ground.

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Minor roads flooded and impassable.

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Fords and minor river crossings impassable.

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Riverside paths flooded.

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Deployment of flood defences preventing access

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Flooding to isolated low-lying gardens and access to homes and businesses

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Flood warning and medium (amber) flood risk

When we issue flood warnings and describe flood risk as medium we expect to see significant impacts. These will include flooding to homes and businesses, possibly affecting whole communities. There are also likely to be impacts on local infrastructure and, in some cases, damage to it. Disruption to travel and business are also probable.

The fact that we have issued one or more flood warnings does not automatically mean flood risk will be described as medium in our outlooks. Our assessment of overall risk will depend on the context of the flooding, its overall impact on a County scale and the likely effects on responding organisations.

In addition to the impacts shown above you should expect:

Flooding to homes and businesses.

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Flooding affecting whole, or large parts of, communities.

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Main roads and railways being significantly affected.

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Disruption to public events

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Severe flood warning and high (red) flood risk

For us to issue the highest level of warning or describe risk as high we will be confident that serious impacts will be probable and there may well be a real risk to life as well as property. It is rare for us to issue these warnings or describe risk as high and we will normally have consulted other responders (local authorities, police and fire and rescue) before we do so.

Impacts that may well be seen at this level of warning or risk include:

Major disruption to critical transport routes (eg: motorways and railways)

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Damage or disruption of critical infrastructure (eg: water treatment plants, electicity sub-stations, roads)

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Flooding to large urban areas

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Large scale evacuations of residents

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Overwhelming or damage of flood defences

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So that’s a quick guide on what to expect at different severities of flooding. Don’t forget you can access our latest warnings (updated every 15 minutes) here:

http://apps.environment-agency.gov.uk/flood/31618.aspx

And our 3 day assessment of flood risk (updated daily) here:

http://apps.environment-agency.gov.uk/flood/3days/125305.aspx

Arc Boat 50 arrives in Shropshire!

ARC-Boat number 50 will be the newest recruit to the Environment Agency Shropshire team, helping us issue quicker and more accurate flood warning information to local communities.

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During times of high river levels, the boat may be seen at sites along the river Severn including Montford, Shrewsbury, Ironbridge and Bridgnorth. The boat will be used to survey the river bed of the Severn, Teme and Tern, sending important information to our teams.

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By measuring river flows at a full range of levels our flood models are massively improved. The flow data allows calibration of the flood models that in the past could never be gained across the full range.

Without actual flow data our flood models are theoretical which is OK, but rivers are dynamic systems that change massively over time and this new flow data allows our people to calibrate the models much more accurately. Better calibrated flood models mean much better flood predictions and therefore more timely and correctly issued flood alerts / warnings.

The boats are manufactured by HR Wallingford in Oxfordshire and have been sold to customers around the world including in Canada, New Zealand, France, Azerbaijan. The Environment Agency already has around 30 of these boats in operation across England, but this one is special as it is the 50th to be produced.

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The boats are remote controlled using electric motors and measure the speed and depth of the water to enable it to calculate river flow. They allow us to easily and safely reach places where humans can’t, such as under bridges or during high-flows/floods. Ultra-sound pulses, similar to that used to scan pregnant women, are used to collect important data from the river.

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The boats can operate in river speeds of up to 3 metres a second, meaning they’re capable of dealing with most flows the River Severn has to offer.
As well as collecting flow data, the new boats also scan the river beds. This gives staff information about any debris build up which could cause flooding. It also allows staff to get a much better understanding as to what exactly is happening beneath the river surface.

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Little boats with a serious job to do

Among our many roles, we are responsible for the collection of information about the water cycle. We gather a variety of river level/flow data, weather and climate information and groundwater data.

Across Midlands region we have almost 500 measurement sites along our rivers. These range from our primary sites where a telemetry system automatically takes 96 readings each day, to logger sites where data may be manually downloaded once a month.

Much of this river level information is available online via the environment agency website here:

http://www.environment-agency.gov.uk/homeandleisure/floods/riverlevels/default.aspx

As well as measuring levels of water in rivers, we also need to measure flow. We can do this by using either a structure such as a weir or flume, or by putting equipment in the river. The results we get always have to be calibrated (checked) and this is where our lovely mobile boats and acoustic measurements are used.

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The level and flow data we get is of vital importance as it is used to make key decisions in areas such as flood warning and forecasting, water resource management & abstraction and discharge management.

Our telemetry data also provides the triggers for flood warnings to be issued as well as the building blocks for our flood models which trigger flood defence deployment and the advice we give to our partners during times of flooding and drought.

We measure flow using sounds waves and the Doppler effect. This is the change in frequency of a sound wave for an observer moving relative to its source (so I’m told!).

Sensitive sensors are mounted on the underside of small boats and manoeuvred across the river. This is done either with ropes, cableways (bit boring) or more recently using our remote controlled boat (great fun).

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As the sensor moves across the water surface, it fires high frequency sound waves to the bed of the river. These waves are then bounced back from the bed of the river and the return waves are altered by the Doppler effect of the flowing water. We measure this shift to give an accurate measurement of water speed or velocity.

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The sound beam also measures the river depth and a GPS and built-in compass measures the width.

We then use basic maths to calculate flow – Q (flow) = V (water speed) x A (Area) (Width x depth)

As well as telling us the flow of the river, our boats also give a screenshot of the river channel shape. This is useful to establish if there are any underwater blockages within the channel that need removing or whether future erosion is going to affect the site.

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Our little Q boats have been doing brisk business over the last few weeks’ flooding, helping to ensure our flood information and advice is as accurate as it can be.

Getting ship-shape for Shrewsbury Regatta

Everyone’s been pulling together, ready for the regatta

This weekend is Shrewsbury Regatta. We have been working hard to get a tree out of the River on behalf of the Town Council. It has been caught up on a high voltage electric cable running under the river by Porthill Bridge in the middle of Shrewsbury. This meant it wasn’t just a case of yanking it out!

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Our guys have done a lot of work behind the scenes to safely remove the tree. Firstly they liaised with the power company who agreed to have the cable potted and made safe. They also needed to talk to Severn Trent Water who have two large high pressure mains running under the river here and a sewer which runs under where we needed to put our heavy winching kit.

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All the talking and pre-planning paid off and the tree was removed yesterday without any disruption to power or water supplies, or anyone getting hurt.

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Our teams regularly remove trees and blockages to reduce flood risk in main rivers but it’s good to be able use our specialised kit to help the regatta go on in full swing. If you have time pop along to the rowing regatta this weekend at The Quarry park. For more information go to http://www.pengwernbc.co.uk . They have got a packed schedule of mens, womens, junior and novice races

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