Flood Risk Management in Hull - The Real Story

Hull and its heritage are built around water. The city is located where the River Hull meets the Humber Estuary and is built on reclaimed marshland. As Hull is very close to water, it became a major global export hub and its fishing industry thrived in the 19^th and 20^th centuries. Throughout its history, water has shaped Hull’s identity and helped the city grow and develop, although it also has its drawbacks. 20% of England’s land drains via the Humber estuary, and 90% of the city’s land is below the high tide level. 98% of Hull is considered to be at flood risk, giving Hull the second-highest number of properties at risk of flooding in an urban area in the UK, second only to London.

Like in all cities, a below-ground network of combined sewerage infrastructure drains both greywater from sinks, baths, washing machines and other appliances as well as foul water and water falling on hard surfaces such as roofs and roads. 84% of Hull’s surface water drains in this way and surface water from areas surrounding the city connect into this system.

The land around Hull is shaped like a bowl, with Hull in the flat-bottomed basin, and the Yorkshire Wolds and the earth mounds of the historic Humber Bank frontages forming higher sides surrounding the city to the North, West and South. Therefore, there is not enough of a downward gradient for the water to flow through this system due to gravity, so drainage in the city depends entirely on pumping.

Hull’s surface water and sewage flow through combined sewers. The sewers lead to an 8km long tunnel that runs from West to East under the city alongside the Humber leading to Yorkshire Water’s Saltend treatment works. Under normal circumstances, all waste water from Hull is treated at Saltend and returned to the Humber. When there is heavy rainfall, the storm pumps at Saltend operate and can pump up to 22,000 litres of water per second out of the tunnel into the Humber. When the rainfall exceeds the pumping capacity at Saltend, the flows are stored in the 8km tunnel until they can be pumped by the treatment works. During extremely heavy rain, the water in the tunnel can reach a point where it overflows into West Hull pumping station, where it is then pumped out into the Humber. West Hull pumping station only operates when water from the tunnel overflows into it and it cannot be turned on earlier as there would be nothing in the system for it to pump. Sometimes, there can be local surface water flooding without there being enough water in the system to allow the pumps to operate. This is caused by local sewers being inundated with extreme rainfall and isn’t linked to the operation of the pumping station.

As runoff water from a wide area and sewage and waste water from a large population drain into Hull’s sewers, they may not have enough extra capacity to deal with heavy rainfall. In extreme circumstances water may be unable to drain away quickly enough to prevent it ponding at the surface, known as surface water flooding, and may even back up out of gullies, sewers and drains. Surface water flooding can be sudden and difficult to predict, and depends on various factors including the type of ground surface and how saturated it is, how localised the rainfall is and how full the city’s sewers already are. Therefore, although more homes are at risk of surface water flooding in the UK than any other type of flooding, many people do not realise that they are at risk.

Tidal surges along the Humber Estuary have also caused flooding in Hull, most recently in 2013. The Hull Tidal Surge Barrier and tidal flood defences along the estuary frontage have helped to protect the city against this risk since 1980, as well as defences along the Humber frontage and the River Hull. Flooding along the River Hull is dominated by the tides, which can back up the river. As Hull is built on a clay surface over a chalk aquifer, groundwater flooding can also occur when the water table rises through the porous chalk underground and can seep out at the ground surface weeks after heavy rainfall.

Hull can be affected by various sources of floodwater, making it important to understand how to live with this risk to minimise its impact on our lives. We can learn from Hull’s proud maritime history to become resilient to flooding and allow the city and its residents to thrive around water.

Check the immediate risk of flooding from rivers and tides in your area using the Environment Agency’s flood warnings:

https://flood-warning-information.service.gov.uk/warnings

You can learn more about flood risk in Hull and the East Riding, and how you can prepare for flooding, on the Living with Water Be Flood Aware page:

https://livingwithwater.co.uk/be-flood-aware

The Know Your Flood Risk website also provides useful flood advice and guidance:

https://www.knowyourfloodrisk.co.uk

Tidal defence measures

Hull has endured three major tidal flooding events in the last 65 years, the most significant of which flooded 264 properties in December 2013. During high spring tides, water levels can rise by 1-3m in certain parts of the estuary. Recent research also suggests that over the next 100 years, sea levels in the Humber Estuary could rise by as much as 1.0-1.3m due to climate change. As the majority of the city lies around 2m below the high tide level, most of Hull is defended from flood risk by a variety of flood alleviation structures, which considerably reduce its overall risk and help to protect the city’s homes, residents and businesses. It is crucial that these defences remain in good working order to maintain the high standard of protection that Hull relies on.

Humber: Hull Frontages

The Humber: Hull Frontage Flood Defence Improvements scheme is building new defences in Hull and upgrading the existing tidal defences to reduce tidal flood risk in the Humber Estuary. It is one of the biggest flood defence schemes in the country and will raise the defences to a consistent standard of protection along the Humber frontage. The scheme is expected to reduce tidal flood risk to 113,000 properties and will focus on defences in eight locations: St Andrew’s Quay Retail Park, St Andrew’s Dock, William Wright Dock, Albert Dock, Island Wharf, Humber Dock Basin, Victoria Pier, Victoria Dock Village West and Victoria Dock Village East. More than 7km of tidal defences along the estuary frontage are due to be improved in total.

Who is involved?

The £42 million scheme is being led by the Environment Agency. This is an important part of their work with local partners to develop a new approach to managing flood risk in tidal areas of the River Humber over the next 100 years.

The contract is being delivered by BMM-JV, who specialise in water projects and other major civil engineering projects, and regularly deliver Environment Agency flood alleviation schemes around the UK. BMM-JV is a joint venture between Mott MacDonald, a global engineering consultancy who work on major projects around the world, and BAM Nuttall, one of Europe’s largest construction companies. BAM Nuttall have led many large heavy infrastructure projects including London’s Olympic Park and Crossrail.

The Plans

The designs were based on information from on-the-ground surveys and computer models about estimated future water levels, global warming and sea level rise. Detailed ecological assessments were needed because the Humber Estuary is next to a Special Area of Conservation, a Special Protection Area, a Site of Special Scientific Interest and a RAMSAR site, so extra consideration was taken when planning construction methods and timetables. The scheme also needed to be signed off by the Secretary of State.

The Environment Agency held public consultations during the planning process to ensure that the scheme met the needs of local people. Some of the access ramps, initially proposed as part of the design, were replaced with flood gates to be closed if a flood warning is issued, and glass panels were added to the wall near residential areas and at Victoria Pier to maintain iconic views of the Humber Estuary. Once the planning process was complete, the scheme was approved by Hull City Council in December 2018, on the fifth anniversary of the devastating 5^th December 2013 tidal surge, and was then signed off by the Secretary of State for the Ministry of Housing, Communities and Local Government in early January 2019.

The current scheme is planned to reduce the risk of tidal flooding until the sea levels reach 2040 levels. A second phase is then planned, which will raise the defences to protect against 2115 tidal levels. The foundations for the second phase have already been installed at some sites and the defences are constructed so that they can be raised to accommodate the effects of climate change on sea levels.

Construction works

Construction has been planned to reduce disruption as much as possible. The walls are mainly constructed from pre-cast panels made off-site to minimise the number of people and vehicles needed in the area. Ready-made panels are dropped into place and anchored onto a pre-prepared base, which allows on-site work to progress quickly. Bespoke parts for tidal defence walls are constructed from concrete onsite. Victoria Pier and Victoria Dock Village also feature sheet-piled walls. Landscaping works are being carried out so that the measures fit in visually with the local environment. Construction started in January 2019 at St Andrew’s Quay retail park, where a new concrete flood defence wall has been built along the main frontage. The walls are around 1.1-1.3m high on average across the scheme. The wall has ramp and step access to the Trans-Pennine Trail and space for local artworks.

The Humber: Hull Frontages scheme reduces the tidal risk to various industrial, commercial and residential sites. A new flood defence wall has been built at the eastern end of Albert Dock and pre-cast concrete coping sections have been added to the top of the existing defences at William Wright Dock to raise their height. At the Humber Dock Basin, the existing wall runs south-west of the Mariner View building, extending onto Minerva Pier. Glazed panels are being used next to the Mariner View building to maintain views of the Humber Estuary and Humber Dock Basin. The wall height is being raised and a replacement flood gate is being installed part way across Minerva Pier to allow access through the wall.

At Victoria Pier and Nelson Street, the old steel sheet pile defences have been replaced by new steel sheet piles which have been embedded approximately 12m into the estuary bed to provide a new retaining wall structure along the frontage. A new flood defence wall is being installed on top of this structure. The walls are clad with brick in keeping with the old town’s historic features and will incorporate small sections of glazed panels within a raised promenade behind the flood wall to maintain the views from Victoria Pier.

Work at Victoria Dock Village started in May 2019, where a flood defence wall has been built. Following public consultation, glass panels have been incorporated within sections of the new flood wall around the Half Tide Basin. The brick cladding and top stones were chosen to match the colour schemes of the surrounding properties. Flood gates are installed within the new flood wall to maintain access to the Trans-Pennine Trail and the promenade.

For more information about the Humber: Hull Defences Scheme and the latest updates on construction, visit the Environment Agency’s public information page for the scheme: https://consult.environment-agency.gov.uk/yorkshire/humber-hull-frontages/#:~:text=The%20Humber%3A%20Hull%20Frontage%20scheme,the%20estuary%20for%20113%2C000%20properties.

Benefits

Once the new defences are complete, they are expected to reduce the flood risk to communities and residential areas, as well as Hull-based businesses, and are therefore likely to bring great social and economic benefits to Hull. Environment Agency analysis has found that the scheme will bring major benefits to homes and businesses both around the defences and in the wider Hull area by reducing their tidal flood risk. Many popular leisure facilities are located in the tidal flood risk area, such as eight leisure and sport facilities (including the iconic ‘The Deep’ aquarium and Hull ice arena), restaurants and a range of retail stores and outlets, and several major cargo businesses are also located at the docks. The scheme could therefore help to preserve business continuity for these local industries and attractions.

Hull Tidal Surge Barrier

The Humber: Hull Frontages scheme has a vital role in reducing tidal flood risk along the frontage of the Humber and inland. However, as the River Hull flows into the Humber, it is dominated by the Humber’s strong tidal influence, which could flow up the River Hull and cause it to flood. The Hull Tidal Surge Barrier protects Hull and the surrounding villages from flooding caused by storm surges flowing back up the River Hull. It protects approximately 17,000 homes and business in the Hull area.

In 1969, Hull experienced severe flooding when a tidal surge peaked 1.2m above predicted levels. As a result, the Yorkshire River Authority prepared a report raising the case to build a barrier, which was followed by an Act of Parliament in 1973 enabling the construction and operation of the Hull Tidal Surge Barrier.

The Hull Tidal Surge Barrier has been an iconic landmark on the Humber Estuary since it was officially opened in 1980. In 2017, the barrier was granted Grade II listed status by Historic England. The Hull Tidal Surge Barrier consists of two reinforced concrete towers supporting a 30m-wide steel gate which spans the River Hull and weighs 212 tonnes. It is operated using a counterweight on either side, each of which weighs 55 tonnes. The concrete towers extend as far below the ground as they stand above it, to operate the counterweight system. The barrier has various failsafe backup mechanisms, which ensures its ongoing operation and the safety of the people of Hull.

The barrier is lowered when tides are expected to reach 4.3m high. The highest tide recorded at the barrier was 5.8 meters in December 2013. Usually, the barrier is used around 30 times a year, although this can change depending on tidal predictions, as well as major river flooding and sluicing events. In 2019, the barrier was lowered 60 times and a further 20 times for testing and training. It takes around 30 minutes to lower and is operated from either tower by experienced operatives.

Over recent years there has been an increase in flooding, as sea levels rise and weather has become more severe. In response, the Hull Tidal Surge Barrier underwent a £10m refurbishment in in 2009/10 to incorporate newer control features and improved computer control.

To find out more about the Hull Tidal Surge Barrier, and to see it in action, watch this video from the Environment Agency:

https://www.youtube.com/watch?v=AZqPH7YdyEk

To learn more about the design and operation of the Hull Tidal Surge Barrier, visit Historic England:

https://historicengland.org.uk/listing/the-list/list-entry/1446522

River Hull Defences

Whilst the Hull Tidal Surge Barrier reduces the risk of extreme tidal surges up the River Hull, it is not lowered for everyday high tides. Therefore, regular high tides can combine with high river flows to increase the risk of flooding in the city centre. The River Hull defences exist to reduce this risk and protect residential areas, businesses and vital infrastructure along the River Hull. However, some of these defences are up to 200 years old, and many have fallen into disrepair over time. Most of the river’s existing defences in the urban area have been constructed over a long period of time by individual landowners, and therefore lack coordination.

The river’s defences include a combination of steel sheet piling, concrete and timber wharves, stone walls, buildings and other structures. The height of these defences means that many are unlikely to be overtopped, although there is a significant risk of them being breached in a major flood. Without improvements to the River Hull defences, Hull City Council estimate that there would be a 58% probability of the defences being breached by the river each year, which could cause around £688 million of damage. 23,684 homes and 1992 businesses could be at risk of flooding from a breach of the defences if no action was taken. Improvements to the defences were therefore urgently needed.

Planning

The River Hull Defence Improvements Scheme has been led by the Environment Agency. In 2012, the Environment Agency appointed consultants from the Arup engineering group to survey the River Hull’s defences. Travelling down the river by boat, they found that the construction of the defences along the river changed at different properties, meaning that the walls consist of hundreds of smaller interventions rather than being a consistent defence scheme. The condition of the defences was assessed and used to divide the measures into three groups: those that were likely to fail in 0-5 years, those likely to fail in 5-10 years and those likely to fail in 10-15 years. These three groups make up the three phases of the River Hull Defences scheme, which covers 2700m of defences overall, from the Humber Tidal Barrier to Clough Road on the outskirts of the city centre.

In September 2016, the Environment Agency awarded the £26.9m contract for the design and construction of Phase 1 of the scheme, covering the 39 measures likely to fail over the next 0-5 years, to BMMJV, the joint venture between BAM Nuttall and Mott MacDonald who have also worked on the Humber Hull Frontages Scheme. The design consultancy Mott MacDonald produced the outline and detailed designs and carried out the surveys and investigations required to inform the construction, including structural surveys, ground investigations, topographic information and building stability tests. The designs for the defences included replacement sheet piled walls, repairs to river walls and capping beams, new set-back flood walls and minor works including waterproofing of buildings and ground raising. As the scheme progressed, they also assisted with compiling the specifications and the as-built drawings indicating any changes to the design. These designs were built by BAM Nuttall.

Defence works were carried out across 62 sites in total, 39 during Phase 1 and a further 23 during Phase 2. Some of these sites had 2-3 landowners occupying each site. Engagement began with awareness presentations and drop-ins covering three major flood risk management works in Hull: The River Hull Defences, the Humber: Hull Frontages Scheme and the Holderness Drain Flood Alleviation Scheme. A public consultation was held at the Guildhall in Hull to discuss these measures. Throughout the development and delivery of the River Hull defences, one-to-one meetings were also held with landowners and occupiers along the River Hull, consisting mainly of local businesses and industry, to explain the temporary and permanent effects of the scheme.

Construction

The construction of the first phase began in April 2017 and will be complete as of Spring 2021. Phase 1 covers 2487m and mostly uses combi piles, a combination of tubular and sheet piles. BAM Nuttall have installed piles from land where possible. In other cases, the piles have been installed from a jack-up barge on the river, a large platform with legs on either side that can be jacked up. The combi-piled walls are expected to last around 50 years, as anodes have been installed to induce a small electrical current though the piles, which reduces corrosion by bacteria.

After starting work in 2016, BMM-JV were awarded the £10m contract for the second phase of the defences, which includes 23 interventions. The delivery of Phase 2 was inspired by learning from the first phase and therefore mainly consists of setback solutions including dwarf walls and concrete slabs, to reduce encroachment into the river and associated disruption to river transport. 50m of road along Bankside was also raised near Wilmington Bridge to provide a flood defence running along from a combi-piled wall close to the Bridge. Construction commenced in August 2019 and will be complete as of March 2021. Phases 1 and 2 have been delivered simultaneously to minimise the period of time that road and river transport could be affected. Phase 2 was initially planned to cover 450m of defences, although this was combined with the third and final phase of the scheme. The delivery of Phase 3 of the scheme was brought forwards to minimise disruption to transport and businesses in the area. In total, the extended Phase 2 has improved 1020m of defences, mainly using setback walls and local topography. These setback solutions are expected to last for at least 50 years.

The completed defences cover 3507m of the banks of the River Hull. They will be inspected approximately every 6 months for the first 2-3 years to ensure that no issues arise. After this, they will be inspected annually. The Environment Agency will also use a remote monitoring system to check the condition of the defences and alert staff to any areas where they may require maintenance work.

Benefits

The upgraded River Hull defences could provide up to £740 million in benefits, including damage costs avoided. The scheme is expected to reduce the risk to over 60,000 properties, which could be flooded by a breach of the defences if no action was taken. The completion of the defences will enable growth in housing and could boost economic growth in 11 sites along the River Hull. As well as providing greater peace of mind for Hull residents, there are also hopes that the scheme will increase economic development along the River Hull corridor.

As sea levels rise due to climate change, the creation and maintenance of Hull’s tidal defences will play an important part in enabling the people of Hull to live with water over the years to come. The pioneering approach to surface water management being delivered via the Haltemprice Surface Water Schemes is also minimising the risk of flooding due to heavy rainfall. As the city with the second-highest flood risk in the UK, Hull is actively adapting to meet this challenge, as local authorities and the Environment Agency strive to deliver new approaches to protect homes and businesses today and in a future of increasing flood risk.

Surface water management measures

In June 2007, many parts of East Yorkshire experienced widespread flooding. Over 11,330 properties in Hull and Haltemprice were damaged by surface water flooding after heavy rain fell on already-saturated land in the Hull and Haltemprice catchment. Runoff ran quickly over the land and into the drainage ditches and sewers, which became overwhelmed, causing widespread flooding. After the flood event, East Riding of Yorkshire Council and Hull City Council began to investigate a range of solutions to reduce the surface water flood risk in the area. Some of the most complex hydrological modelling in the country at the time was undertaken to determine the runoff sources and where runoff could be stored to prevent it overwhelming the sewers. It was found that a series of surface water storage schemes in the Haltemprice area of the East Riding would benefit both West Hull and Haltemprice. The result was the Haltemprice surface water storage schemes. These versatile green spaces hold excess water after heavy rainfall and release it gradually to slow the flow into and relieve pressure on the local drainage system.

Hull surface water storage schemes: How do they work?

Each scheme involves a series of connected storage lagoons which run from higher to lower ground. When not in use during severe wet weather, the flat green spaces look like pastureland. During heavy rainfall, water is stored in the lagoons, which lie below road level so that water flows into them. This water would otherwise run quickly off the land into the drainage system, which struggles with large volumes of water due to its reliance on pumping. Once the storm has passed and the flow levels in the drainage system are declining, the water can be gradually released. It moves down from the highest basin to the lowest, which provides additional storage capacity for more water in the highest lagoon. The water from the lowest lagoon flows into the sewers. Most of the water ultimately reaches the Northern Trunk Sewer, although this happens at a much slower rate than before the lagoons were built as the water must pass through each one before reaching the sewer, and therefore reduces the peak flow that the system needs to accommodate.

The Haltemprice schemes

There are three major surface water storage schemes in the Haltemprice area. They were designed and delivered by East Riding of Yorkshire Council in partnership with Hull City Council and the Environment Agency. These schemes aim to slow the rate of surface water flow into the Northern Trunk Sewer and have already operated in the storms of November 2019, protecting thousands of properties.

The Willerby and Derringham Flood Alleviation Scheme (WADFAS) was completed in 2017, and is the first of the three sister schemes. The scheme was funded by European Regional Development funding and Flood Defence Grant in Aid. This series of storage lagoons reduces the surface water flood risk for around 8000 homes and 200 businesses, and stores 235,000m³ of water. This slows the rate at which water enters the Northern Trunk Sewer, as sewers replaced the area’s watercourses in the 1940s, but have since struggled to accommodate the volume of runoff from the built up areas, agricultural land and drainage ditches. The lagoons are situated along the Great Gutter Valley.

The Cottingham and Orchard Park Flood Alleviation Scheme (COPFAS) stores runoff from the Raywell Valley and Northmoor and minimises the surface water flood risk to around 4000 properties by storing 330,000m³ of water. The first stage of COPFAS involved improvements to Millhouse Beck and nearby watercourses, leading into a flow control structure at Creyke Beck. This connects to a lagoon excavated near Danepark Road on Orchard Park. In Winter 2019/2020, the new lagoon was filled by heavy rainfall for the first time. The second stage involves creating eight lagoons in the Raywell Valley linked by culverts to allow water to flow from one to the next to capture as much water as possible. The scheme has a budget of £22 million, funded by £17 million of Flood Defence Grant in Aid, £5 million from the Humber Local Enterprise Partnership’s Local Growth Fund, which invests in projects that help to establish an infrastructure that supports growth, and contributions from both councils.

The Anlaby and East Ella Flood Alleviation Scheme (AEEFAS) was designed to store runoff from the West of the Hull and Haltemprice Catchment. The scheme links an existing lagoon at Anlaby Common to a new lagoon on the site of the former Sydney Smith School. This holds up to 115,000m³ of water and can be used as sports pitches when dry. Phase 1 of the works improved the New Marr Bride over the Western Drain in Anlaby and was completed in 2018. During Phase 2, a new spillway structure was built alongside the existing lagoon and channels were dug to link it to the new lagoon. Phase 3, the final phase, involves the construction of a 1.5km-long tunnel that is 1.8m in diameter and up to 14m deep to link the North-Eastern Tranby catchment to the two lagoons. An energy dissipation unit at the outfall reduces scouring as the water leaves the tunnel. Runoff from the West Ella Valley and Tranby Hill, covering around 1200ha., is stored in the two lagoons and released gradually into the Newington Trunk Sewer, which joins the Northern Trunk Sewer. The scheme is expected to reduce the likelihood of surface water flooding for over 4000 properties. AEEFAS has a budget of £20m, of which £15 million came from Flood Defence Grant in Aid and £5 million from the Humber Local Enterprise Partnership’s Local Growth Fund and Council contributions.

Benefits

These three surface water schemes bring combined benefits to the Hull and Haltemprice area’s drainage system. Between them, they can hold as much water as 272 Olympic swimming pools, and are expected to reduce the likelihood of severe surface water flooding for around 20,000 properties in Hull and the East Riding. During heavy rainfall in November 2019, all three schemes stored significant volumes of rainwater, despite two of them not being fully complete at the time.

By minimising the potential impacts of surface water flooding, the Haltemprice surface water schemes also help to reduce its effects on communities, social welfare and the local economy. In Hull, the social and recovery costs after flooding are approximately double the national average. The 2007 floods cost £650m (in 2007 prices) and indirect health impacts cost £2.5m. There were also many cases of uninsured business losses. Storing and delaying damaging surface water flows reduces the risk of such detrimental impacts. The Environment Agency therefore identified that WADFAS’s economic benefits would be almost 40 times its cost.

The Haltemprice surface water storage schemes therefore provide a crucial line of defence in response to Hull’s complex flood risk. As one of the country’s largest surface water storage schemes, this innovative development sets a positive national example of how to manage surface water sustainably.

Development

Many cities across the UK are growing and developing as more residents and businesses are drawn to them. Although this brings benefits to the local economy, it also raises another question: where will the runoff water from all of these new developments go? Open green spaces are widely recognised as a valuable asset for soaking up rainfall and surface water, whereas hard urban surfaces like concrete tend to increase runoff. As the need for housing and business premises grows in cities like Hull, so do concerns about whether this will increase the risk of flooding.

Historically, new developments have had inadequate drainage. From the 1940s, due to the massive demand for housing in the post-war period, drainage was not considered in detail in the rush to provide new homes. This persisted for decades, although severe floods in the year 2000 and a number of major events following this led to flood risk becoming a key consideration in the planning process.

The National Planning Policy Framework now sets clear restrictions on development in flood risk areas. Construction projects must pass a number of tests to be approved. The first is called the sequential test and makes sure that that the most vulnerable developments are in the lowest flood-risk areas. Development should not be permitted if there is a reasonably available site with a lower flood risk, so areas with low or no flood risk need to be chosen to pass this test. However, if it is not possible for the development to be built elsewhere, the exception test can be used to ensure that a site could be safely developed. In order to pass this, developers need to show that the development will bring other sustainability benefits to the area that outweigh its flood risk and that it will not increase flood risk in the area or elsewhere; ideally, it should reduce it. Therefore, development on floodplains is discouraged where possible. Where it is it not, planning permission is only granted when developers can ensure that the flood risk at the site will be managed sustainably and not increased in the surrounding area.

These national policies state that planners should promote sustainable drainage systems (SuDS) to manage surface water runoff from developments. These measures mimic natural processes that soak up rainwater in green spaces by enabling water to soak into the ground through permeable surfaces, using plants and trees to slow down runoff and storing excess surface water in basins and ponds. These SuDS measures should be based on advice from local authorities, and each Lead Local Flood Authority will have their own advice and guidance for sustainable drainage. Due to recent developments in SuDS technology, development on green spaces no longer has to mean that their valuable water storage capacity is lost.

To find out more about how SuDS is included when planning new developments around the UK, visit the Susdrain website: https://www.susdrain.org/delivering-suds/using-suds/delivery/planning.html

Hull City Council has taken surface water management a step further. In June 2019, they became the first UK council to partner with a water company to create a flood risk planning policy. Under the Living with Water partnership, Hull City Council and Yorkshire Water worked together to produce a supplementary planning document, a document that can be used to provide further guidance for development on specific sites or on particular development issues. This contains guidance that aims to ensure that new developments in Hull do not increase flood risk to the area’s residents and businesses. Under this local guidance, construction projects on previously developed, or brownfield, sites need to reduce the volume of surface water running off the site by at least half. Developments on green-field land, which has not been built on before, must not increase the volume of water entering the city’s drainage system. Watercourses should not be channelled into culverts, which restrict flow and risk becoming blocked, and where possible, existing culverts should be opened up. Developers need to plan how much floodwater storage they will create and how their designs will accommodate this for planning permission to be granted. Water should be stored above ground where possible, such as in:

• Swales – broad, shallow channels with plants growing in them that help to store and transport surface water.
• Storage basins and ponds – depressions or specially-designed lagoons that store surface water runoff and release it gradually after the risk of flooding has passed to slow the flow into local drainage systems.
• Tree pits – pits inserted into pavements that allow trees to be planted in paved areas by giving them to access soil below.
• Rain gardens – small dips in the ground that water running off the roof of a property, and other sources of surface water, can soak into. They are planted with vegetation that likes wet conditions, and the downpipe of a property can be diverted into them.

These can help to remove pollution from runoff water by trapping it in vegetation, and can improve local biodiversity as well as reducing the surface water flood risk.

New developments can only happen if they meet these strict guidelines. In Hull, greenfield sites now need to be carefully planned to make sure that no more water leaves the site than existing runoff rates or less, whilst development on brownfield land can only be permitted if it reduces runoff overall. New national planning policies are breaking the link between construction and surface water flooding by ensuring that drainage is provided through sustainable means, based on similar processes to those that would occur naturally. By carefully planning developments and using SuDS, local and national policies aim to ensure that new developments are a true asset to cities such as Hull, helping to manage flood risk sustainably as well as bringing benefits to the local community and economy.

Many other local authorities across Yorkshire and beyond have far reaching and ambitious planning policies or guidelines to ensure that developers deliver sustainable solutions that work with local flood risk or drainage issues in a considered way so as to not increase risks to themselves or their neighbours.

To find out more about how you can manage flood risk sustainably in your home, and how developments near you can become more flood-resilient, tune into the Hazard + Hope series.

Maintenance

Throughout Hull’s history, the city has been flooded on several occasions by both tides and surface water. With people concerned about the risk of further damage and disruption, many questions are asked about what more can be done.

After heavy rainfall, it isn’t uncommon to see water pooling on the road around highway gullies. These roadside drainage pits aim to prevent surface water flooding on the carriageway. Water flows through the grate into a gully pot where mud, gravel, litter and other sediment and debris settles and is stored, and through an outlet pipe into the sewers. Sediment that washes off the road cannot enter the sewage system through the outflow pipe, which helps to reduce blockages. However, flooding around gullies, on roads and sometimes into nearby houses, is often seen during surface water flooding and has been observed in Hull during over the last decade, leading to calls for more regular gully cleaning.

Unfortunately, due to the complexity of urban drainage systems, the answer is not so simple. For example, in Hull, almost all of the city’s surface water flows into the same combined sewer system so its capacity can easily become overwhelmed when large volumes of water flow into it, such as after heavy rain. The signs of a blocked gully look very similar to gullies where water is unable to be transferred downstream by the sewerage system, with water ponding on the roads.

When sewers become overwhelmed by high inputs, no more water draining in from the roads can fit into them as they are already full to capacity. Water cannot drain through the gullies, not because they are blocked, but because there is no room for the water in the sewer below. In severe floods, water from the sewers can back up the outlet pipe and rise up through the grate onto the road. Therefore, flooding out of gullies onto the roads is not always caused by gully blockages, as water can also back up out of gullies and onto the road if sewers are overwhelmed in flood conditions.

Surface water outfalls into rivers and ditches allow rainfall to drain from the sewage systems. Throughout history, rivers have been straightened and widened through dredging in an attempt to increase flows. In some cases, river channels have been straightened and deepened to support navigation, such as the River Hull. As deeper channels can theoretically hold more water, would dredging rivers help to reduce their risk of flooding?

In normal flow conditions, rivers are able to drain their catchments efficiently with the water being held within the main river channel. During flood conditions river levels rise as flood water drains into the channel from upstream. In extreme events, river levels can be higher than the level of the riverbanks, causing the water to spill out onto surrounding floodplains. In urban areas, this can flood homes, businesses and infrastructure.

Historically, dredging has been carried out to deepen and widen channels to contain flood flows. However, in most of situations, including the River Hull, major engineering works would be needed, including digging the channel several metres deeper and wider, to accommodate this huge volume of water. Dredging to accommodate large floods would involve digging the River Hull’s channel deeper than the Humber’s, which would prevent the water flowing out of the river into the estuary and cause it to back up into the city. Dredging therefore cannot be carried out on such a massive scale. Due to the huge volumes of water involved in flooding, the impact of dredging on a river’s capacity is small and would not be enough to prevent a major flood.

After dredging, rivers quickly refill with mud and silt and need to be re-dredged regularly. Each flood washes more silt into the channel. Dredging involves regularly hiring heavy machinery, which is hugely expensive. Dredged material is often contaminated by pollutants, especially in watercourses surrounded by agricultural areas or by cities with riverside industry, so it needs to be disposed of in landfill, which incurs further costs. Dredging is unlikely to be a useful long-term solution as major investment would be needed to maintain it. Disturbing contaminated sediment could also wash pollutants downstream. For example, dredging the River Hull could wash pollutants into the Humber Estuary, which is an internationally important nature conservation site.

Dredging removes vegetation from the riverbanks, which can destabilise them and cause them to collapse and dump huge quantities of sediment into the channel. It can also undermine the foundations of structures built around the channel, and could therefore weaken bridges and existing flood defences around river banks. If structures are not altered to line up with the new banks after dredging, this creates narrow points that locally increase flood risk by forcing large quantities of water to flow through a small gap, called ‘Pinch points’. Also, removing gravel from the riverbed can destroy spawning grounds for fish, whilst removing soil from river banks can disturb wildlife such as water voles and otters. Although some local councils, such as Hull City Council, carry out minor de-silting works on smaller waterways to locally improve flows, major dredging to accommodate large floods could be damaging to built and natural environments, as well as the local economy.

Learn more about how dredging works and its impacts on the Newground website: https://newground.co.uk/blog/how-can-dredging-and-gravel-removal-reduce-the-risk-of-flooding/

To find out more about examples of dredging and more sustainable alternatives, read this guest blog on the Environment Agency’s website: https://environmentagency.blog.gov.uk/2020/02/21/floods-and-dredging/

Sustainable flood risk management needs to look at the bigger picture. Due to the complex nature of flood risk, simply altering maintenance features such as highway gully cleaning and dredging rivers is unlikely to be enough to accommodate the volume of floodwater that has flooded residential areas in the past. Instead, a much wider-reaching approach is needed, that looks at the whole catchment to manage its flood risk sustainably and help its residents to live with water.