A rise in the water table during heavy rainfall puts a number of UK properties at risk of flooding. It is reported that around 5 million people in England and Wales live in a flood risk area; and 77,000 properties in Scotland are at risk of inland flooding. There are generally two types of flooding which can occur; groundwater flooding and surface water flooding. Although the two are different they are often interlinked, and one can often cause the other. Flooding can cause irreparable damage and result in massive financial costs for governments and individuals, so we are always looking forways of prevention but first it is important to understand why these things occur and where is most at risk.
Causes of Flooding
Surface Water Flooding
Usually surface water flooding is at risk of occurring after periods of heavy rainfall. Surface water flooding occurs as a result of streams, rivers or other channels of drainage not being able to cope with excess water. This heavy water flow into the drainage channels can lead to the water breaking or over spilling the bank.
Groundwater flooding tends to occur as the result of water rising from underlying rocks or abnormal springs. It usually results from periods of excess rainwater as does surface water flooding. The periods of high rainfall cause the water table to rise above normal levels as the water infiltrates the ground. It is fairly typical for groundwater to flow from areas where ground level is high to areas where the ground level is low which is why flooding is more likely in low lying areas; particularly those underlain by permeable rocks or aquifers such as chalk or sandstone. Groundwater flooding takes longer to disappear than surface water flooding as it tends to move slower in general and then takes time to flow underground.
Groundwater flooding and surface water flooding, although different issues, can often be found to occur at the same time. They can both affect one another; a rise in the water table can result in more water flowing into the rivers which could break the banks and in turn if the water table is higher than usual, drainage networks may not work as well, meaning surface water cannot be drained which may in turn cause groundwater flooding.
The Water Table
What is the Water Table?
National Geographic defines the water table as being the boundary between the unsaturated zone and the saturated zone. Underneath the water table, groundwater fills gaps between and within rock and sediments. Above the water table is considered the zone of aeration as this space is filled with water and oxygen. The water table rises, and falls based on land surface and levels of precipitation.
Measuring the Water Table
The depth of the water table varies depending on seasons, for example in late winter and spring time it tends to rise. This is due to heavy rainfall and the presence of melted snow seeping into the ground. It tends to fall again once we reach late spring and summer as the water-loving plants begin to grow and the temperature increases.
The best way of measuring depth off the water table is generally to measure water level in a shallow well; although this may not always be possible. There are databases available which should be able to give you depth-to-water measurements. The environment agency provides monthly area reports for England which details the water situation in terms of water levels. These reports discuss groundwater concerns in individual areas alongside rainfall and river flows. They analyse a number of other things such as the local reservoirs and soil moisture.
Unfortunately, during heavy rainfall specifically in winter and spring, flooding is fairly common in England. Most recently, during heavy rainfall in early August 2019, the dam wall at Toddbrook Reservoir in Whaley Bridge, Derbyshire was damaged. At the time, the damage resulted in the evacuation of approximately 1,500 residents due to fears of flooding. It is thought that if the dam wall had given way entirely, the entirety of Whaley Bridge would have been underwater. Emergency repairs took place to secure the dam; assisted by the RAF. The water level of the dam is currently being kept low by pumps, but it is not known whether the dam wall will need a large repair or complete rebuild; both of which could take years. Due to fears of surface water flooding across England, the events at Whaley Bridge have triggered an independent review of reservoirs across the country. The events in Whaley Bridge highlight the measures which can be taken in the aftermath of reservoir damage but also show how prevention is better than cure.
Protecting your Property
In general groundwater flooding can be more difficult to prevent than surface water flooding. When it comes to surface water flooding, flood defences can be built by local authorities or environment regulators to prevent river water spilling over however it isn’t as simple when it comes to groundwater. Pumps provide one of the most useful ways in removing groundwater and they do lower the water table. Pumps are great for use in the aftermath of groundwater flooding or to prevent it getting any worse. It is worth remembering that pumps work locally, and you need to ensure there is somewhere to discharge the water.
For property owners, there are precautions which can be taken against groundwater such as sealing your home with a waterproof membrane. This is often a solution when it comes to basement flooding. When the basement is used for storage or as a living room it is essential to protect it against flooding and sealing it is one of the best ways to do so.
It is important to note that areas which are prone to groundwater flooding do also tend to be prone to surface water flooding so ensuring your government or local authority are doing all they can in terms of flood defences is key.
Flood Aftermath and Prevention
As mentioned, it can be hard to prevent flooding, groundwater flooding specifically, but it is important nonetheless to know all your options. Where it may not be possible to fully prevent groundwater flooding there are some things which could help in the aftermath to make it a little easier to handle. Whilst the aftermath of a flood can be difficult, as so many people may have lost their homes or belongings, it is still important to deal with it as practically and efficiently as possible. Pumps are one of the most effective methods of discharging of flood water quickly. They are available for hire in either petrol or diesel and work to effectively pump out flooded areas.