Reservoirs: managing your reservoir in extreme weather

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Introduction

This guide aims to help you understand how extreme weather events could impact the safety of your reservoir. It suggests how to manage, maintain and improve your reservoir to make it more resilient. All reservoir owners and operators have a legal responsibility for public safety.

This guide does not cover masonry or concrete dams, as there are very few in England.

The guide is designed to be modular. Each section is independent and can be read in isolation, as well as one piece.

Existing guidance

There is a wide range of guidance for reservoir owners and operators available.

Owner and operator requirements outlines the legal requirements for owning and operating a reservoir.

How to manage your large raised reservoir is a good introductory guide for owners. It outlines the principles of reservoir management and contains advice on:

the roles involved in managing reservoirs
your role as an undertaker
common problems, how to spot them and how to reduce the risk of these happening

This guidance on manging your reservoir in extreme weather is designed to complement the How to manage your large raised reservoir guidance.

Climate change and extreme weather

The climate is already changing and the Environment Agency has seen an increase of incidents which relate to weather events. Weather is likely to be more severe in the future. Predictions of how this will develop are also changing with increased understanding. This guide is based on current knowledge. We expect climate change will increase the frequency and/or severity of the following threats, each of which has its own section in this guide:

prolonged heavy and/or extreme rainfall and cumulative flood events
extended ground saturation
drought and fire
storminess and high winds
snow and ice

A review of reservoir safety incidents suggests that floods and heavy rainfall contribute more than any other triggers. The main root causes of incidents relate to:

poor or outdated design principles
construction-related defects
insufficient surveillance and examination
lack of maintenance

A reservoir’s dam might not fail in a single extreme weather event but could fail due to the cumulative effects of long-term repeated damage. Earthwork structures are sensitive to physical and environmental loading over repeated cycles in time. You should monitor your reservoir frequently, so that any damage can be repaired immediately after it occurs.

Using this guide

This guide focuses on 3 main sections of your reservoir site:

embankments
spillways
reservoir outlets and other elements

The outlet and other structures include:

gates and valves to control how much water is released in normal conditions or in an emergency
Mechanical, Electrical, Instrumentation, Control and Automation (MEICA) - this includes the gates and valves, plus other equipment such as motors to operate them, electrical control systems, sensors, read-outs, and signalling
structures such as valve towers, control buildings, inlet and outlet headwalls, screens, access bridges, tunnels, culverts and drainage systems

Terminology

In this guide, we use 3 terms for the levels of activity:

“Management” means:

keeping good records
providing reports to the Environment Agency
arranging engineers’ visits and inspections
arranging frequent walkover checks by operational staff

The level of management needed depends on whether your reservoir is formally designated high risk. This is discussed more at the end of this introduction section.

“Maintenance” means:

the general upkeep of the dam and structures which form the reservoir
restoring them to good condition, so that they work well and safely
preventative routine actions such as completing checks and inspections
managing vegetation
corrective actions when a fault is detected such as repairs, replacing parts or clearing debris and blockages

You should be able to carry out maintenance yourself, or with help from specialist contractors.

“Improving resilience” ranges from minor works to major alterations.

Minor works which can generally be carried out by hand would normally only need discussion with your Supervising Engineer. Alterations are bigger changes which may need construction machinery such as excavators. They could change the nature or shape of a structure and how it performs. These could affect the reservoir’s safety and would need approval from an Inspecting or Construction Engineer.

The term “Qualified Civil Engineer” includes all the specialist reservoir safety Engineers’ roles mentioned above. It is defined in the Reservoirs Act. All reservoirs classed as “high risk” by the Environment Agency must have a Supervising Engineer appointed at all times. They must also be inspected periodically by an Inspecting Engineer. Some reservoirs have been classed as “not high risk.” Although these roles are not required, similar levels of surveillance and inspection are recommended as good practice. Alterations which affect the reservoir’s volume may only be carried out with approval from an Inspection or Construction Engineer.

This guide focusses on problems which are, or are likely to be, common for reservoirs in a small portfolio. It also touches on some problems which are less likely but may have high consequences. There will be problems which are not covered by this guide. This guide is not a substitute for professional advice. If in doubt, you must consult a Qualified Civil Engineer.

General principles

You can reduce the problems discussed in this guide by managing your reservoir well and maintaining it continually. There are some scenarios where increased vigilance may be required, such as where:

records or construction details are known to be missing
a defect is identified, or safety improvements are incomplete
temporary works are in place

You should monitor the weather, the water levels and other data for your reservoir. This is to understand how these change over the long term and how you should change what you do. You may need to adopt maintenance practices which are currently used more in other regions. Climate change might mean you need to do more surveys and repairs. You may need an engineer to examine your reservoir more frequently. There may be changes which could make your reservoir more resilient to the changing climate.

The biggest improvement you can make is to increase the frequency and detail of your reservoir checks and observations. You should do these when the weather is or has been more extreme than before.

For example, during:

a storm and/or immediately after, when the reservoir water level is very high
during prolonged, or harsh, dry or cold spells

Focus your attention on the areas of potential impact. Look for signs of damage to the embankment.

When visiting the reservoir during extreme weather, you should be aware of the risks to your own and others’ safety. If water levels rise you could get stranded and if there is a landslip you need to stay clear of it. You may also be at risk from falling trees or other hazards. There is a high risk of:

slips
exposure to cold in freezing conditions
heat-related problems in drought

If you were unable to attend when the storm or flood was at its peak, you can estimate the peak level based on wrack marks. These are the line of debris left on the upstream face of the dam. A knowledge of the peak flood level is often useful in understanding the impact of the event.

For a high risk reservoir, you should review your records and check the inspection report from your Inspecting Engineer. The report will have comments on the flood capacity and when the flood flows were calculated. There have been several advances in calculation methods in recent years and the information may need updating. The last Inspection Report should give you a good understanding of how often the reservoir might overflow and what the risks are when it does. The Inspection Report may also include other requirements or recommendations which are relevant to climate change impacts. You should discuss with your Supervising Engineer whether you need to increase the frequency of monitoring.

You should review your long-term budget to make sure you have access to funds for repairs and improvements in the future.

Rain and flood

Climate change is expected to make winters wetter by up to 30%. Summers may become drier by up to 60%, depending on the region. We have observed changes both in terms of seasonal increases and individual events. Weather extremes are expected to become more extreme and more frequent. Though summers will be drier, predictions suggest rainstorms may be up to 20% more severe.

If rainfall becomes heavier with longer or more frequent storms, the cumulative impact of flood events could cause problems. In winter, reservoirs may be fuller more often and throughout the year they may overflow more often.

When a reservoir is already full and starts to overflow the additional flow should initially be carried safely down the spillway. Some dams’ spillways have capacity for more extreme flood flows than others. There is a high risk of small spillways being overwhelmed. With climate change this may happen more often.

Embankments

Potential impacts of extreme rain and floods

The highest risk from increased flood flows to embankments occurs if the additional flow is more than the spillway can carry. The excess flood flow would overflow at other low points on or near the dam crest. This could erode as they are not armoured against the water’s high energy. This could also happen alongside the spillway if water spills out of the sides of the spillway chute. This could erode the dam and potentially lead to a catastrophic collapse of the dam.

Overland flow from the reservoir, or from intense rainfall, can result in direct damage to the embankment downstream face. In particular where water running off is concentrated due to the shape of the embankment surface. This can happen:

at mitres, where the embankment meets the valley sides
next to structures
at other intrusions on the embankment surface such as fence posts or sharp changes in gradient

Diagram 1: Overflow at low spots in high floods can damage the dam’s downstream face

If the water level in the reservoir is higher than it has been historically, this can trigger seepage through the embankment. Seepage can lead to internal erosion. This is erosion of fine earth particles from inside the embankment fill. Internal erosion may only occur when the water level exceeds a certain threshold but may occur cyclically over a series of flood events.

The top of the saturated zone within an embankment is called the “phreatic surface”. Water level in the reservoir being higher for longer can cause the phreatic surface to rise. This increases the risk of the dam slopes failing in a similar fashion to landslip. These could be deep-seated major failures.

Photo 1 (a): Erosion of embankment dam downstream face due to overtopping. Photo courtesy of the Environment Agency

Photo 1 (b): Erosion of embankment dam downstream face due to overtopping. Photo courtesy of the Environment Agency

How to spot damage

Look for signs of the reservoir overflowing the embankment crest. The first sign would be flattened grass on the downstream slope, followed by patches of exposed earth developing. If earth is being removed, the gullies could erode back up towards the crest. This could also happen where flow paths concentrate at mitres, structures, or other intrusions. The rate of erosion in these types of gullies will differ, but they can easily lead to reservoir safety problems if they continue.

Look for seepage appearing on the downstream slope of the embankment or at its toe (where it meets natural ground level). Some seepage is normal, but increases in flow may be a cause for concern. You need to understand your dam’s normal conditions so you can spot any changes.

Seepage might not be obvious, but there are some signs you can look out for are:

a patch of ground which is wetter than it used to be
changes in vegetation such as reeds
more flow running into the sides of ditches and streams downstream, or the spillway channel or the embankment’s drains
changes in “turbidity” - how cloudy the seepage flow is

Signs of reservoir safety problems where there is probably damage inside the dam are:

if the amount of seepage increases when the water level in the reservoir is at or above a particular height
if the water appears dirty, and is carrying fine material such as sand or silt

Some reservoirs have piezometers, which measure the level of the phreatic surface inside the dam. If the water level in these increases that can be a sign of changes within the embankment. For example, it could indicate leakage through the clay core. It could also mean that the embankment slopes are at risk of a landslip, which could potentially lead to reservoir failure.

Another sign of smaller slips, or early signs of large ones, is cracks appearing at the embankment surface. These can be seen as roughly horizontal cracks developing at the upper edge of a slip and/or as curved cracks along the sides of the slip. There may also be a bulge in the surface at the lower edge. These slips could worsen, leading to reservoir failure.

Maintenance

There are some simple maintenance activities which you can carry out. If you find any significant damage, you should contact your Supervising Engineer before repairing it. They may:

need to see photographs and measurements
examine it in person
advise on appropriate repair techniques

You should:

review (or obtain) survey data of the levels of the dam crest, spillway, slope and other features - this helps you understand accurately where the low points are and where water would flow in a flood. Note that embankments do settle (get lower) over time, so surveys should be repeated at intervals of around 2 years
consider changing the grass seed mix to one which has a more uniform length and a strong laterally-spreading root system. Prevent bare areas from getting worse by cultivating healthy uniform weed-free grass cover. you should also consider drought, which could lead to a different grass seed mix
make the crest and the downstream slope more resilient to overflow, for example by removing fence posts and drops. Prevent burrowing animals causing voids in the embankment. Any holes need to be filled carefully with suitable material and well compacted
repair any damage to the embankment downstream face and crest, as soon as possible after it is identified
prevent trees and bushes from becoming established. Remove any decaying tree stumps or roots
separate drainage flows from the valley sides, away from the dam, to reduce erosive flows down mitres and improve monitoring
install measuring weirs if that helps to detect abnormal seepage

Diagram 3: A cross section of a reservoir embankment with damage caused by trees and animal burrows. Soil deposition and reeds suggest seepage through the bank.

Improving resilience

Depending on the scale of the work, the following could alter the form and function of the dam, so would need input from a Qualified Civil Engineer. Most of these would normally be classed as alterations.

You should:

install hard surfacing on the crest to increase resilience to erosion if it overflows
ensure there are no low points on the crest, where overflow can concentrate - this may be either the main crest, or along crest walls if there are any

It may be appropriate to create a longitudinal camber on the dam crest, so it is higher at the middle than at the ends. Water overflowing will then have greater depth and velocity where the embankment height is lower. This would reduce the size of the breach wave, if the dam failed.

Spillway

There may be gates or valves which affect how the spillway operates. You can read about these in the section ‘reservoir outlet and other elements’.

Potential impacts of extreme rain and floods

Some common spillway issues are the:

weir width may be too narrow
overflow pipe may be too small
weir or pipe level is too close to the dam crest level to discharge an extreme flood

Water levels then rise too high and the dam overflows the embankment crest. This leads to embankment erosion.

The overflow from the reservoir may be more than the discharge the spillway chute was designed for. Water could overflow the side walls and damage the surrounding ground, leading to erosion and collapse of the side walls.

Water remaining within the chute could be at very high velocity. The energy of this could damage the armour by:

lifting concrete slabs
plucking out masonry blocks
scouring grass

Blockages along the spillway flow route can also restrict the overflow. These blockages would be more likely to cause problems in higher flows.

There may be restrictions at bridge arches or the channel may have sharp bends which limit flow.

Any blockage in the spillway could cause water to overflow the channel and erode the embankment alongside it. It can damage the armour layer within the spillway. If the blockage is near the dam crest, then the water would find another overflow route, leading to erosion elsewhere on the dam. Unintentional blockages may be from vegetation, debris in the spillway or damage to its structure. These could include:

plants, bushes or trees
blocks from the structure which have moved
collapsed sidewall(s), which may fall into the channel and leave unexposed earth
vandalism such as deliberate damage or items thrown into the spillway

Structural damage or changes can lead to other problems, or be signs of developing ones. Higher flows or more frequent floods would place these weak points under more stress. Signs of potential problems can include:

gaps or steps at joints
changes in levels
displaced blocks
bulging sidewalls
water seeping through the walls or base of the spillway chute

Irregularities such as depressions or obstructions would focus the energy of the water, leading to damage.

Problems with the vegetation in grass or reinforced grass spillways such as:

poor grass cover including bare areas
broad-leaved weeds, tall shrubs and bushes can also lead to erosion from flood flows

The bottom of the spillway known as the “toe”, is hidden underwater at some dams. Increased flood flows, or more frequent floods, would mean the toe has to deal with water’s higher energy more often, which can cause major damage. The chute and the toe are most vulnerable where there are changes, such as in:

width
gradient
materials

Damage which occurs here can lead to the spillway “unravelling” or working its way back up the dam and leading to other problems. Sometimes the spillway toe is not protected well enough, or deep enough under the natural ground. The water’s energy undermines it; this can be hard to detect if the toe is under water but can lead to sudden collapse.

Photo 2 (a): Spillway masonry blocks dislodged and displaced by flood flow, revealing bedrock and leading to erosion at the side. Photo courtesy of the Environment Agency.

Photo 2 (b): Spillway masonry blocks dislodged and displaced by flood flow, revealing bedrock and leading to erosion at the side. Photo courtesy of the Environment Agency.

How to spot damage

You should be able to see areas of damage if they are present or developing, allowing you to correct them. Examples include:

patches of erosion beside spillway walls, or locations where flood flows are close to filling the channel on one or both sides
steps or displaced blocks in the spillway chute, which may be easier to see as patches of white water during overflow
blockages or damage in the channel
poor grass cover - for example, bare patches and weeds, exposed soil, or holes where soil has been eroded on grassed spillways
uneven ground, possibly with protruding or exposed mesh or concrete armour beneath
ruts or tracks caused by people or livestock crossing grassed spillways
cracks or settlement around the spillway toe or its sidewalls, because of erosion of foundation material
undermining of the spillway toe - may only be found by checking in-river and probing underwater

Maintenance

If you find damage, you should repair it using similar materials and methods to the original construction. You should do this as soon as possible to avoid problems getting worse. If the damage happens again, then alterations may well be needed.

You should:

re-install blocks which have come loose, and re-point those around them
remove blockages such as trees and debris, if this is from vandals you should try to secure the site boundaries
cut small trees and shrubs off at the base but do not try to remove the roots as this may cause further damage - keep these locations under a close watch in the long-term as the ground may settle when the roots rot away
reseed bare patches on grass spillways and treat weeds to develop healthy grass cover - this should be mown to keep it around 10cm tall
check if mowers or other vehicles cause damage such as ruts or lift up edges of armour mesh, check whether they sank in soft ground - that may be a separate problem, repair the ruts / mesh / armour blocks if needed, seeking specialist advice if you are in any doubt
keep drainage and weepholes clear, which are discussed in the section on saturation

Improving resilience

Some problems may need more work. They may need guidance from an engineer, or a formal design approved by a Qualified Civil Engineer. Alterations or major repairs to the spillway can be difficult and need lots of planning. Any concerns must be followed up promptly. The designs should consider and allow for increased flows due to climate change as far as reasonably practical.

for design issues such as weir width, dam crest level and spillway chute capacity, you would need an engineer to help. They will support you with understanding the dam’s performance in flood conditions. This might lead to major alterations. For example changing the spillway weir or adding another, widening a bridge, or deepening the chute.
if erosion next to the chute is localised, you may be able to strengthen that area simply with a hard surface and return flow to the channel. For example, this may work where water splashes out of the channel next to a bend.
collapsed sidewalls may be a sign of other problems. There may be inadequate drainage behind them, or insufficient wall thickness. Simply rebuilding them is not likely to be enough.
the spillway toe could need underpinning and / or a “scour cut-off” adding. This is a wall which goes down well below the ground level at the toe, to stop undercutting from working back upstream. The toe may need extending, strengthening, or widening to match the river channel and its valley.

Reservoir outlet and other elements

Increased rain or flood flows will not impact all dam elements. This section aims to discuss the more likely or more significant impacts. To prepare for or react to each problem, the text explains:

how to spot problems
what maintenance or resilience improvements could be needed to prepare for or react to each problem

For alterations affecting the reservoir’s safety, you will need input from a Qualified Civil Engineer.

Gates and valves

The main outlet from a reservoir for flood flows is the spillway, as the normal day-to-day flow route is too small. Most gates and valves will not need to be adjusted to deal with flooding, but they can help if you open them. Some spillways also need gates or valves opening for them to work properly too. Check you can access and operate the gates and valves under extreme flood conditions. This includes when the embankment crest is overflowing.

There may be stoplogs (boards) across the spillway to raise the weir crest level for operational or ornamental reasons. They can be removed to increase the outflow but they become hard to remove during high outflow. If they remain, the reservoir level would rise faster. This would increase the risk of the dam overflowing by other routes.

Sluice gates or some types of valves can be hard to open when there is high water upstream. If they cannot be opened it may cause more flood water to flow over the dam.

You should check what types of gates and valves you have, or stoplogs. You must make sure they work properly in normal conditions. You should consider whether you could improve operation in floods. Changing to a different type of control could make the dam more resilient to climate change and so make it safer. Ideally spillways should be “passive”, not requiring any manual operation or intervention in floods.

MEICA equipment

Motors and their controls have a limited lifespan. If they are used more frequently they will wear out sooner and need replacement. As flood frequency increases you may find that valves have to be opened or closed more often. You may be able to adjust the operational settings to improve the lifespan and reliability of these items. Care is needed to make sure they remain effective during floods.

You may find that flood levels rise more often to the point where sensors do not work properly. For example, an ultrasonic level sensor will not work if the water level rises too close to, or above it. You may need to raise the sensor so that it is well above predicted flood levels. Level alarms which use a simple float as a switch, may need changing for similar reasons. You should check regularly that sensors are well calibrated for the full range of conditions. Floods often cause power cuts. You should ensure any electrical elements have back up power if the grid is interrupted.

Structures

You may have control buildings set near the edge of the reservoir, or there may be equipment within a valve tower. This may presently be above the normal flood level but with climate change it could flood more often. You may need to make the doorways and other openings more resilient to flooding. Make sure floods would not damage anything in the building and make sure you can safely access the building.

Increased flood flows could carry more debris through the reservoir. Inlet and outlet headwalls and screens may need more maintenance. You need to check these are clear and working well. You may need to clear them more often.

The reservoir records should be kept where they are easily accessible. If they are kept on site then it may be advisable to keep a backup copy off site in case the records are destroyed by flood.

Ground saturation

Ground saturation is more likely, and more seasonal. Winters will become wetter and summers drier and milder. In winter the ground will be saturated for a longer period. During summer, although ground saturation will generally reduce, heavier rainstorms may result in shorter periods of intense saturation.

Embankment slopes and valley sides could become saturated. When earth is saturated, it is both heavier and weaker, so there is a higher risk of landslips and other problems.