In an Expert Focus article for WaterBriefing, Mark Goodger, Regional Technical Manager for Hydro International, discusses sustainable flood risk management and long-term resilience.

Mark Goodger: The destructive power of flood water, and the human misery it causes, was all too evident during last winter’s relentless and repeated storms in some parts of the UK. 

December 2015 was the warmest for 100 years, and the wettest on record, with almost double the average rainfall and a total bill for flood damage estimated at £5 billion. According to Government figure16,000 properties were flooded in England alone, although a further 20,000 were protected.

The Environment Agency said a "complete rethink" of the UK's flood defences was needed and finding the best long-term, and most sustainable, ways of making communities more resilient to flooding is now firmly back at the top of the agenda. The Government launched a National Flood Resilience Review and provided many millions of pounds in extra funding, targeted at the areas most badly hit during the 2015/6 winter floods.

However, making the right decisions about building sustainable flood protection could be easier said than done, when all around us the goal posts seem to be moving.  

Climate change

Over recent years, the UK has experienced an ever-increasing number of severe flood events.  In the last decade, records for the amount of rainfall have been broken again and again. Is climate change starting to have a demonstrable effect?  Is the historical data on which we base our hydraulic modelling calculations still reliable, or are we less able to predict the likelihood of future events?

No-one can say for sure, but scientists and meteorologists are increasingly in agreement that intense rainfall events are one of the likely consequences of a warming planet; as the earth’s atmosphere warms up it has the capacity to store more moisture – and so the potential for extreme rainfall and flooding increases.                        

Scientists, academics and politicians may all argue over the risks and the policy responses, but for engineers there is an immediate need to respond with reliable, predictable drainage and flood protection.  For them, the demand for robust and pragmatic answers is as pressing as the very next project.

With the uncertainty of climate change and limited public funds, will it be possible to prevent every flood in future?  Probably not. Instead, politicians and regulators will need to concentrate on getting the best ‘bang for their buck’ and working with communities to manage their response to extreme events.  

Priorities and pragmatism – top three priorities for flood defences

In these circumstances, what should our priorities be for designing and building flood defences in future?  Here’s a suggested top three:

• Take flood defences upstream wherever possible, rather than building flood walls higher and higher in the valleys, close to the properties they are protecting.
• Cost for the long-term. Consider the maintenance and operation implications of flood defences to be just as important as the initial capital investment.  
• Consider a full toolbox of natural and manufactured components to achieve the most sustainable outcomes. Ruling out either may limit our ability to develop the most pragmatic and predictable outcomes.

Upstream storage a proven and sustainable alternative

In response to the winter flooding and especially to flash river flooding, many expert commentators have highlighted that upstream flood storage could provide more sustainable long-term resilience. Working with farmers to manage their land in more flood resilient ways, and providing positive rewards for the provision of temporary flood storage areas upstream might be considered more in future.

Our common response to flooding has often been to build hard infrastructure close to the points of flooding.  However just building higher and higher walls, flood gates and levees in the valleys and hoping that the surging river will not overtop them may no longer be sustainable – especially as climate change makes such intense storm events more likely.  The value of holding back the river water upstream, well away from vulnerable homes and businesses, is a proven and sustainable alternative

Well-proven engineering

Environmentalists are increasingly looking at how flood risk can be alleviated by natural defences, such as the replanting of trees, the building of log dams and even the reintroduction of beavers.  However, these are often longer-term solutions that will not work in every case and can only deliver real benefits to the downstream homes and businesses if they are widely adopted across the whole catchment; hard-engineered infrastructure will also be required, which can deliver biodiversity and ecosystems benefits if engineered optimally.

So, what does sustainability in flood risk management and SuDS really look like? It has to be precision-engineered with robust, repeatable performance and demonstrable maintenance regimes. 

Example

Let’s look at one example of a standard engineering device used in flood defence and drainage – the flow control.  Flow control devices regulate the flow in watercourses by flattening out peak flows, thereby protecting the downstream catchment from flash flooding. They are commonplace solutions in our urban environments on residential and commercial developments, highways, sewers and even in wastewater treatment plants.

The same principle of control and attenuation applies from small plot, greenfield-equivalent drainage schemes of just a few litres per second up to large-scale fluvial defences of many thousands of litres per second up and down the country.  Vortex flow control technology has been ‘supersized’ to protect more than 4,000 UK properties in cities, towns and villages. During the 2015/6 flooding the Hydro-Brake® Flood Alleviation projects did their job – no news was good news, as they continued to protect the communities they were designed to serve.

One of the most striking schemes is on the White Cart River, holding back flows in the highlands above Glasgow, where the world’s largest Hydro-Brake® Flood Alleviation flow controls are installed. Their ongoing protection ably demonstrates the benefits of a sustainable, no-power and low maintenance solution to ongoing flood protection – but the same principles apply to even the most prosaic urban surface water scheme.

Whether in a river, a Sustainable Drainage System (SuDS) or below ground in a combined sewer, the same rules of precision engineering apply. In every case, to control floodwater by holding back flow means finding space to store the water temporarily upstream, while it can be discharged at a rate that protects the downstream environment. 

Hydrological and hydraulic modelling combined with a range of storm event and duration probabilities will be used to calculate the storage needed and its capital investment implications. The storage volume needed must be carefully balanced against the cost and complexity of the flow control solution selected.

Flow control selection

Choosing the correct flow control solution follows the same principles of precision engineering for any scheme, however large or small. It depends firstly on the physical space available for storage – there may be strict limitations in an urban area, for example – and secondly on the affordable project costs of creating the storage, both in terms of land value and construction. 

The maximum storage water level is often limited by the existing topography, infrastructure or property. Drain down after events should ideally be rapid, to restore land to its primary use and to ensure the flood defences are ready as quickly as possible for the next event.

Figure 1

Best Practice flood alleviation

Based on the Environment Agency’s Fluvial Design Guide, these two diagrams demonstrate the best practice approach to flood alleviation. 

Figure 1 shows the range of measures that can be applied to reduce flood risk to a particular area, including flow rate control both distributed within the defended area and in upstream parts of the catchment.

Figure 2

Figure 2 shows two common types of flood storage reservoir: an online reservoir configuration with a river flowing through the storage area being controlled by a dam wall with a spillway for overtopping protection and a flow control to regulate the flow through the dam.

The alternative is an offline storage area adjacent to the river, with the water entering and exiting the storage reservoir regulated by flow control devices.

Flow controls – the cost balance

Engineering flood protection with flow control technology, whether the watercourse is a river or a local stream, a stormwater culvert or foul sewer, starts with the available temporary storage area upstream.

This dictates the best flow control technology for the job, as each generates a different volume of storage depending on its performance characteristics.

Figure 3

The flood storage hydrograph in Figure 3 shows a peak flow (the blue line) and the ideal flow control response (the red line). The peak above the maximum permissible downstream flow rate is the water volume stored, which is then discharged at a controlled rate to empty the storage; this rate affects the readiness of the reservoir for subsequent rainfall events.

This outflow hydrograph is different for each flow control device. The difference between the performance characteristic of the device and the ‘ideal’ response is the additional storage volume required for that device.  So, the engineer can design a precise, balanced control of flow right across the catchment. As in Figures 1 and 2, there could be several flow control points across the catchment.

Balancing space and cost

Choosing the correct flow control solution is crucial to achieving a cost-effective flood risk management solution. The available physical space for storage - there may be strict limitations in an urban area, for example – must be balanced against the affordable project costs of creating the storage, both in terms of land value and construction. Investment in a high-performance flow control device may pay dividends in terms of a reduction in storage.

To engineer the correct storage, consultants and other agencies such as Local Authorities and the Environment Agency are advised to seek flow control expertise from specialists  such as Hydro International.

Available solutions include:

Orifice Plate

One of the most basic flow control devices is the orifice plate with an opening smaller than the pipe or channel into which it is inserted to restrict the flow.  Its maximum design flow is directly related to the cross-sectional area of the opening and the depth of water in the storage area upstream of the control. It is best suited to site where the area available for storage is not limited.

Vortex Flow Control

Perhaps the most versatile flow control technology is the vortex flow control, which has proved itself in flows from small diameter pipework to major rivers, with CSOs, storm sewers and wastewater systems. The most advanced, such as the Hydro-Brake® Optimum, can be precision engineered to achieve the best-possible hydraulic response with a vortex flow control and the closest to the “ideal” characteristic for a device without any moving parts or external power requirements.  

As the diameter of the vortex flow control is much greater (up to 300%) than for the equivalent flow orifice plate, the device is far less prone to blockage. Vortex flow control devices can operate over an extensive size range with flows from just a few litres per second up to 12 m3/s for large-scale fluvial schemes using Hydro-Brake® Flood Alleviation.

Float-operated flow control

To get closest to the “ideal” characteristic, and therefore the smallest upstream storage volume, a float-operated flow control, such as Hydro-Brake® Agile, provides precise performance by creating a continuously variable orifice that changes in direct relation to the water height and enables rapid drain down to protect against the next peak flow.

Engineering a balance

Placing flow control technology at the heart of a flood prevention scheme provides a sustainable and reliable solution for engineers, planners, developers and water companies. Precision engineering of the flow control devices and growing experience of other successful schemes enables engineers to design practical and predictable solutions and to “future-proof” schemes for changing conditions as a result of climate change.

For further information on flow control selection email  This e-mail address is being protected from spambots. You need JavaScript enabled to view it or call the Hydro-Brake® Flow Control hotline 01275 337955.