Water flow regulation

Reconnecting rivers to floodplains

nst — Mon, 20 Feb 2017 09:27:15 +0000

Reconnecting rivers to floodplains nst Mon, 02/20/2017 - 10:27

Natural flood, runoff, catchment management

River restoration contributes to flood risk management by supporting the natural capacity of rivers to retain water. As flood risk consists of damage times occurrence, flood risk management needs to reduce either the damage, or the likelihood of floods to occur, or both. River restoration reduces the likelihood of high water levels, and improves the natural functions of the river at same time.

Based on kindly provided information by UNEP's "Green Infrastructure Guide for Water Management: Ecosystem-based Management Approaches to Water-related Infrastructure Projects " (UNEP, 2014)

In a natural river system a river spreads water beyond its banks and over extended areas of a floodplain during periods of high water. In order to protect property and contain waters, the classic flood risk management approach is to constrain watercourses with rivers being straightened and building dykes to increase discharge capacity, dredging to deepen channels, and building reservoirs and artificial retention areas to store excess waters.

While this generally reduces the likelihood of flooding, in the event of extremely high waters it increases the amount of damage if the engineered system is overwhelmed or fails. Without natural features such as wetlands and meanders, excess waters cannot be absorbed. Any breach will release an enormous amount of water, with potentially catastrophic consequences. Continuously reinforcing and building higher dykes cannot overcome this weakness, and is a very expensive option. Historically, engineering solutions upstream have created peak flows downstream, leading to more engineering. Moreover, climate change scenarios predict more extreme weather events and higher sea levels. A new approach is needed.

Advantages

By re-connecting brooks, streams and rivers to floodplains, former meanders and other natural storage areas, and enhancing the quality and capacity of wetlands, river restoration increases natural storage capacity and reduces flood risk. Excess water is stored in a timely and natural manner in areas where values such as attractive landscape and biodiversity are improved and opportunities for recreation can be enhanced. In these ways, river restoration directly contributes to climate change strategies aimed at mitigating the effects of increased and erratic peak flows and droughts.

River restoration is increasingly being delivered by flood risk managers to create space for flood water. Reconnecting floodplains to the river and managed realignment in estuaries is an important mechanism of water management.

Future climate change will potentially affect all aspects of the rainfall regime. The precise nature of these changes is uncertain, particularly for those extreme events, whether of short or long-duration, which tend to lead to flooding. Increases in rainfall at all scales will increase the risk of flooding to a greater or lesser extent, depending on how these increases manifest themselves in space and time and of the rainfall-runoff characteristics of the catchment in question.

Benefits

River restoration improves flood protection, but it also brings about co-benefits that are multifold. Firstly, river restoration can improve flood storage capacity of a river and reduce the volume and speed of water. Some of the co-benefitst can include cost reductions by removing the need to maintain hard infrastructure and also improving the quality of water, and thus in turn drinking water costs. Improved biodiversity and the creation of natural wetlands is another adjunct result of river restoration using green infrastructure. Finally it improves resilience to climate change by creating new floodplanes for increased water storage, green networks and more natural space for people and wildlife during higher temperatures.

Costs

There are different kinds and degrees of river restoration. A larger scale project can include an entire floodplane, removing past structures and restoring natural processes and channels of a water course. A smaller project may simply be removing structures in one place, and replacing them with more natural features.

Barriers to Implementation

Lack of funding is often cited as a key reason for failing to restore watercourses and rivers, as well as, consensus in agreement of users of a river. Given that restoration can take place on either a large or small scale, the associated barriers often also relate to how extensive the project is.

Measure category

Mitigation

EXAMPLE: MOSE system of mobile flood barriers, Venice (IT)

nst — Tue, 07 Feb 2017 08:35:19 +0000

EXAMPLE: MOSE system of mobile flood barriers, Venice (IT) nst Tue, 02/07/2017 - 09:35

Flash floods

Coastal floods or storm surges

Reduction

Water flow regulation

Hold the line

Grey infrastructure

Venice, Italy, is a city famous around the world for not only its stunning canals and historic buildings, but also for its high vulnerability to flooding. The MOSE system of mobile flood barriers is a bold initiative intended reduce risk, preserve the cherished cityscape, and protect the entire Venice Lagoon from flooding.

Based on information from the project website

General description

The MOSE (short for “Modulo Sperimentale Elettromeccanico” in Italian) system consists of four mobile barriers closing off three inlets in the Venice Lagoon. The barriers themselves are made up of 78 flap gates that are installed at the bottom of the inlets to separate the lagoon from the sea when raised. The system takes approximately 30 minutes to open and can be closed in 15 minutes, but takes on average five hours to close. Once raised, the barriers are able to withstand three meters of high tide. The barrier at the Malamocco inlet even has a lock system installed to allow merchant and industrial ships to cross while the MOSE system is in operation to reduce interference on port activities.

Seasonal high water is a constant threat to Venice, and the city has adapted with raised walkways, waterproofed buildings, and power outlets installed halfway up the wall in businesses and homes. Flooded scenes of a usually picturesque St Mark’s Square can be explained due to the fact that it is the city’s lowest point. However, more extreme high tides that occur roughly every three years and can raise water levels by over a meter present a much greater risk to Venice’s cultural heritage and justify a system such as MOSE.

Governance aspects

Venice and its Lagoon is a UNESCO World Heritage Site, which makes its protection even more important. The MOSE project was implemented by the Italian Ministry of Infrastructure and Transport and managed by the Consorzio Venezia Nuova for the purposes safeguarding Venice and the lagoon. The decision to construct the mobile flood barriers was made after collaboration between all levels of government and consideration of various other coastal defence measures.

Innovative aspects

The MOSE system in its entirety is an impressive and innovation response to the threat of coastal flooding and erosion, both from a construction and coordination standpoint. The hydrological and geophysical profile of the Venice Lagoon needed to be fully considered when designing the barriers and their final locations.

While the MOSE Control Centre uses advanced technology to predict flooding, model the effects of gate manoeuvres, predict port traffic, determine warning levels, and so on, the MOSE project also employs other smaller scale measures to optimise the overall goal of flood risk reduction in the lagoon. These local defences consist of raising quaysides, roads, walkways, and installing smaller gates in the urban canals in the lagoon settlements, known as the “Baby MOSE” gates. This holistic and comprehensive approach to encouraging protection for the entire lagoon, aside from that which is provided by MOSE, is also innovative.

Key lessons learnt

It is not easy to construct such large-scale defences in fragile environments, but, as the MOSE system illustrates, sometimes this approach is necessary to provide significant long-term protection. Venice represents an especially vulnerable coastal city with globally significant heritage sites and a very active tourism industry. With so much at risk, the MOSE system will ensure businesses, residents, and visitors will be able to enjoy the fabled canals, palaces, and plazas without the threat of flooding and building damage. The implementation of local defences diversifies the resilience of the settlements in the lagoon and increases the rate of success for the MOSE project.

Relevant case studies and examples

Flood and storm surge barrier

EXAMPLE: Wallasea Island Wild Coast project (UK)

nst — Mon, 16 Jan 2017 12:39:10 +0000

EXAMPLE: Wallasea Island Wild Coast project (UK) nst Mon, 01/16/2017 - 13:39

Erosion

Channel, Coastal and Floodplain Works

Water flow regulation

Limited intervention

Ecosystem based approach

The aim of the Wallasea Island Wild Coast project is to recreate a natural intertidal coastal marshland to combat the threat of climate-induced coastal flooding. The recreated mudflats, salt and brackish marshes, saline lagoons, and pastures will provide a range of habitats for coastal birds and other wildlife on the Essex coast.

Based on information from the Royal Society for the Protection of Birds (RSPB)

General description

The project uses a technique known as “managed realignment” to recreate an intertidal habitat through the breaching of existing seawalls at strategic locations. These breaches, or holes, allow sea water in, and various kinds of ecosystems can be created depending on the height of the land being flooded. The land of the Wallasea Island will be heightened and extended using the clay, chalk, and gravel excavated from new underground rail line connections in central London. In total, nearly 1500 acres of tidal wildlife habitat will be transformed or created new, including approximately 133ha of mudflat, 276ha of salt marsh, 53ha of saline lagoons, 11ha of brackish marsh, 160ha of grassland, and 15ha of rotational arable fields.

Historically, Wallasea ‘Island’ comprised as many as five individual salt marsh islands. When seawall defences were added to the area to prevent coastal erosion, the landscape eventually evolved into the shape that can be seen today.

Since 2008, the Wallasea Island Wild Coast project has been in partnership with an underground rail line development project called Crossrail. The clay, chalk, and gravel excavated from their tunnelling in central London will be reused to heighten and transform the coastlines of the Wallasea area. The addition of these materials to raise land and extend coastlines is expected to allow approximately 2.1Mm³ of tidal water to enter the area once the sea walls are breached. This would require around 7.5Mm³ of imported fill material. The construction schedule to achieve the objectives of the managed realignment plan is determined by the delivery schedule of the materials from the Crossrail project, and is planned between 2016 and 2019.

The site is located near one of the world’s most important estuaries and one of Europe’s largest economic regeneration zones: the Thames Gateway. The Crouch and Roach Estuaries bordering Wallasea Island have been recognized, under the European Union Directive on the Conservation of Wild Birds, as a Special Protection Area, a Special Area of Conservation, and a Wetland of International Importance through the Ramsar Convention.

In July 2009, the final design of the project received planning approval. Local authorities, yacht clubs, and various organizations were publically consulted and included in developing and designing the project plan.

Innovative Aspects

The Wallasea Island Wild Coast project is a bold initiative to address the alarming amount of coastal change that has happened in this region of Europe. Over the past 400 years, the Essex coast has lost over 91% of its intertidal salt marshes due to accelerating coastal erosion and competition with agriculture for land. The project has set a high standard for 21^st century conservation and engineering efforts, and is at a scale never before attempted in the UK. It jointly considers ecological and economic factors, for the benefit of future visitors, wildlife, and local community members for decades to come.

Perhaps the most innovative aspect of the project is the landmark partnership and collaboration between the project operators, the Royal Society for the Protection of Birds (RSPB), and the underground rail development project Crossrail. By deciding to reuse the excavated materials from London’s new underground connections to achieve the managed realignment objectives of Wallasea Island, the two projects set a global standard for how waste material from large-scale infrastructure projects does not have to be disposed of in a landfill. Instead, excavated soils, clays, and rocks can provide flood protection to coastal communities and refortify coastal ecosystems. Equally, the project cooperation showed that it is possible to transport large amounts of excavation spoil from London to the Essex coast in a safe and reliable way.

Key lessons learnt

The Wallasea Island Wild Coast project showed that despite the challenges, major land realignment can be undertaken in a sustainable way. The use of excavated materials from the London Crossrail project also illustrated a mutually beneficial solution for both stakeholder groups and is an example of cooperation that leads to smart solutions for the benefit of the environment.

Relevant case studies and examples

Marsh vegetation in intertidal and coastal zone

Adaptation or improvement of dikes and dams

giacomo.cazzola — Thu, 08 Sep 2016 14:09:16 +0000

Adaptation or improvement of dikes and dams giacomo.cazzola Thu, 09/08/2016 - 16:09

Riverine or slow rise floods

Coastal floods or storm surges

Channel, Coastal and Floodplain Works

Water flow regulation

Hold the line

Grey infrastructure

Dikes and dams need regular maintenance and strengthening to keep their protection capacities and meet safety requirements. In addition, climate scenarios for sea level rise and extreme weather conditions can lead to reconsidering safety requirements and building new protections on identified weak points or heightening and strengthening existing ones. The design of existing dikes and dams can be modified to fulfill different purposes.

Based on the information available on the ClimateAdapt Platform

Re-enforcing dikes and dams can increase their stability and resistance against dike breaching, e.g. by strengthening the inner core of the dike, or improving characteristics of the dike's surface that contribute to the overall stability of the dike. Overtopping resistant dikes are wide and less steep than traditional dikes, and can be multifunctional (for example, for agriculture, recreation or transport).

Dikes can also be re-enforced by heightening, broadening or by adding spatial components. Heightening is the usual way to re-enforce dikes, but other innovative approaches have recently been developed. Heightening provides coastal and riverside defense, but without integrated development or a combination of functions that a spatial solution may offer. In Schleswig-Holstein, the safety standard for all dikes includes a margin for sea-level rise of 0.5m and is supplemented by building a reserve in case of stronger sea level rise of a further 0.5m. Broadening may offer additional benefits, yet may not be always practicable because space is limited or for socio-economic reasons. Dikes could be widened up to 300m land inward and the dike could be used as a space for agriculture or recreation. Wider dikes are more resistant to “overtopping” by storm waves. If re-enforcing the flood defense system becomes necessary due to climate change, recent studies advocate a three-step approach, considering spacing, broadening and raising consecutively. Dike design can have the aim of allowing water in certain conditions to overtop them without breaching. This is usually achieved by strengthening the inner wall of the dike, by dike broadening, or by developing a parallel dike system with enclosed retention polder The construction of double dike systems, and using the space in between dikes to retain the water that washes over. In Belgium, this approach has been used in the controlled flooding areas for the Sigma Plan in the Scheldt River estuary: one such area is Kruibeke, where the space between the outer, overflow dike and an inner, higher dike has been restored as wetlands and other habitats (the outer dike has sluices to allow water flow between the wetlands and the Scheldt estuary).

An overtopping dike can provide more safety against flooding than the typical single-line defenses. The measure reduces flood impacts (population exposed, production affected) by decreasing the sensitivity of people and assets in flood prone areas (i.e. the ability to manage water surplus). As the dike will not breach when overtopped, it prevents the uncontrolled catastrophic dike breaks associated with devastating flooding of the hinterland. The number of potential victims and the resulting damage are therefore much lower than incurred when a traditional dike breaks. The risk, calculated as a product of the probability of occurrence and the resulting damage, is thus reduced. Overtopping can still result in anxiety and minor damages. These can be reduced by spatial planning or subject to compensation and insurance.Another option to reduce flood risk, other than strengthening the primary water defense structures, is to compartmentalize the region to be protected in zones, for example by dike ring areas. Compartmentalization either or both protects critical functions in the flood-prone area and reduces the flooded surface area. It diminishes the flood effects by dividing the area into compartments with the use of dikes.

Stakeholder participation

The choice of the type of dike improvement has important implications not only for safety of the people and assets behind the dikes, but also has visual/landscape implications for the people living close to the dikes. Therefore, stakeholder involvement during the design phase is important.

Costs and Benefits

Experience with dike re-enforcement in the Netherlands has yielded the following indicative estimates of total cost: low river dike: 3 mln €/km; high river dike: 5 mln €/km; estuarine dike: 5 mln €/km; coastal defence: 7.5 mln €/km.

Information is available from the Netherlands on constructing dikes within dunes indicates that this is more complex and costs are significantly higher. The costs to construct a dike within the dunes in Katwijk, over a length of about 900 m, amounts about 45 million €. The advantage is that the water safety against flooding is improved while preserving the existing character of the beach resort. These are costs only for water safety; costs for facilities are not included but an underground parking garage was built afterwards, preserving the existing landscape character. Comparable solutions have been chosen for Scheveningen and Noordwijk.

Success and Limiting Factors

Dike or dam reinforcement has strong supporters and opponents, with concerns and preferences changing over time. Support is typically strong after a flood event. Where reinforcement is planned to pro-actively adapt to climate change it is more likely to meet resistance. Heightening and reinforcement of dikes can affect the landscape in a negative way. In countries like the Netherlands, people started to resist reinforcement programmes. The loss of historic houses and views is perceived as problematic. In addition, raising dike height can increase water levels in the river during high flow. In response, various alternatives to dike reinforcement have been developed, including widening the riparian areas and floodplain, creating overflow channels and lateral diversions do increase the capacity of rivers. “Overtopping resistant dikes” may be more costly, with a typical time horizon of 50 years in economic assessment. For a longer time horizon and including maintenance, the comparison becomes more favourable. At the same time, the overtopping resistant dike can be combined with other functions, raising its multi-functional character and broadening opportunities for financing. Opportunities for this are location specific.

In addition, as a consequence of wave overtopping under extreme conditions, the multifunctional coastal zone should be made adaptive to occasional accommodation of water in that area. Such a spatial adaptation in coastal situations may offer opportunities for salt or brackish ecosystems, recreation, living and wet agriculture.

DG ENV project ClimWatAdapt, DG CLIMA project Adaptation Strategy of European Cities and Ourcoast II

Measure category

Prevention

Flood embankments and Floodwalls

giacomo.cazzola — Thu, 08 Sep 2016 13:37:11 +0000

Flood embankments and Floodwalls giacomo.cazzola Thu, 09/08/2016 - 15:37

Riverine or slow rise floods

Flash floods

Estuarine floods

Channel, Coastal and Floodplain Works

Water flow regulation

Hold the line

Combined approach (grey + green)

The construction of floodwalls and embankments has been the traditional means of protecting lowlying communities and infrastructure against flooding. Although the primary function of a wall or embankment may be flood defence, such structures also frequently have a secondary function – quite often with the aim of enhancing the environment or improving the amenity or both.

Based on "C E Rickard (2009): 9 Floodwalls and flood embankments 29p. In: UK Environmental Agency (2009): Fluvial Design Guide (Contains public sector information licensed under the Open Government Licence v3.0.)"

Flood embankments

Flood embankments are earthfill structures designed to contain high river levels. They are commonly grass-covered, but may need additional protection against erosion by swiftly flowing water, waves or overtopping. Protection may take many forms, but options include: stone riprap; gabions and gabion mattresses; open-stone asphalt; concrete bagwork; concrete blockwork (which can either be individual blocks or linked to form a mattress); various products that may be categorised as bioengineering such as coir rolls, faggots and fascine mattresses. Geogrids and geotextiles can also be used to reinforce grass on flood embankments.

The basic form of a flood embankment is trapezoidal in cross section, with a horizontal crest and sloping inner and outer faces.

The width of the crest is normally determined by asset management requirements, with widths of 2m to 5m being the normal range. In the absence of more specific guidance, designers are advised to adopt a crest width which is two metres wider than the maximum width of plant that will be used on the crest (allowing one metre safety margin on each side).

The slopes of the inner and outer faces are a function of:

the strength characteristics of the earthfill material used;
the type of maintenance equipment used (for grass cutting, for example);
any landscaping requirements.

Normally the embankment side slopes are between 1:2 (vertical to horizontal) and 1:3. Steeper slopes are very difficult to maintain (grass cutting), while flatter slopes tend to add unnecessarily to the footprint of the embankment and the quantity of fill material required.

An embankment with relatively steep face slopes has a smaller footprint and lower earthfill require-ment than one with more gentle slopes; it may therefore cost less and have a lower environmental impact. Steeper slopes can be achieved by using earthfill with a higher clay content or by a range of soil strengthening techniques, but designers must always take into account the asset management needs and ensure that these can be carried out safely (for example, avoiding the risk of maintenance plant overturning on a steep slope). The designer must be certain that the profile of the embankment selected meets all the service requirements and, in particular, is stable throughout the full range of loading conditions.

Embankments are normally set back from the edge of the river to:

allow for some flood storage on the floodplain;
reduce the risk of undermining caused by riverbank erosion.

Set-back embankments are also less prone to erosion of the riverward face due to high velocity flow, but may be more prone to wave damage.

Flood embankments can be constructed from a variety of earth materials. Wherever possible, locally won material should be used, to reduce costs and lessen the environmental impact. The strength of the material used to construct the embankment is increased by compaction, which is a fundamental part of the construction process. The required strength is achieved by constructing the embankment in layers and compacting each layer using mechanical plant appropriate to the type of soil. It may be necessary to add water to each layer to improve the degree of compaction required; this depends on the nature of the soil and its moisture content. The advice of a geotechnical engineer should be sought regarding the appropriate layer thickness and the type of compaction plant required.

Soils with high clay content are best avoided because these crack when they dry out, and such cracks can extend a metre or more into the bank, compromising its function as a flood defence. Soils with a high sand or gravel content can be used, but may have to incorporate some form of cutoff to reduce seepage in flood conditions. Granular soils are less resistant to erosion than cohesive soils once the topsoil layer has been eroded.

Because of the shortage of suitable fill and the adverse environmental consequences of importing large quantities of fill from afar, various alternatives to conventional fill material have been explored. These include the use of recycled tyres compressed into bales to form a central core to a flood embankment. Options such as this need careful investigation before being adopted, with particular emphasis being given to long term durability and stability, environmental risks (such as contamination) and the overall environmental impact.

It is normal to strip topsoil from the foundation of an embankment before construction starts. This helps to key the embankment to its foundation and reduces settlement. It also provides a source of topsoil to encase the embankment and allow the establishment of a suitable grass cover.

Where the foundation soils are weak (for example, a layer of peat), the options are:

remove the weak layer (if it is near the surface and relatively thin);
strengthen the foundation (potentially an expensive option);
accept and allow for the resulting long-term settlement;
pre-load the foundation to accelerate settlement.

If the foundation is highly permeable (for example, a thick layer of gravel), it may be necessary to take steps to cut off the seepage path through the foundation.

Embankment foundations should always be checked for the presence of buried (agricultural) land drains prior to construction, as any that are left in place could result in excessive seepage and even embankment failure.

Other services may also be present along the route of the flood embankment, and these may need to be diverted or protected to avoid damage. The cost of diverting a gas or water main can be significant, but is normally much less than the costs from accidental damage during construction of the embankment!

Embankments in rural settings are often accessible by livestock and agricultural machinery. Both can cause significant damage, degrading the bank crest where they regularly congregate or cross the defence. Fencing can be used to control livestock movement, and pathways and machine access routes can be surfaced to reduce the likelihood and amount of damage. Cattle can be prevented from grazing flood embankments by providing two strands of barbed wire at the top of fence posts. The height of the lower strand can be high enough to allow sheep to pass under, as sheep do not cause damage to the embankment surface. Stock-proof fencing may be required at field boundaries. Gates or stiles may be required to maintain pedestrian access.

If a high level of burrowing damage is expected, it may be appropriate to incorporate a deterrent (such as wire netting) into the surfaces of the embankment.

Cracks in embankments can create seepage paths. Cracking occurs in clay soils during dry conditions and is best avoided by not using highly plastic clay soils for fill in the top metre of the crest.

Seepage can also occur where structures pass through the embankment (for example, a drainage culvert). The soil–structure interface requires careful attention during construction to minimise this risk, most notably by ensuring good compaction of the embankment fill around the structure. The likelihood of seepage can also be reduced by lengthening the seepage path (for example, by constructing a concrete collar round a pipe passing through the embankment)

Floodwalls

There are two basic types of floodwall:

those that also form part of the river frontage, such as a wharf, retaining wall, or quay;
those that are remote from the river, generally with the sole purpose of providing a flood defence.

Defences that form part of the river frontage usually have deep foundations and considerable overall height. Often such walls have been in existence for many years and their flood defence function has increased with time, with progressive heightening of the crest level. Such defences need careful investigation if they are to be upgraded or refurbished to provide an acceptable service life.

The form of construction of such walls includes brick, masonry, timber, sheetpiling and concrete.

The main factors to consider include:

the type, condition and stability of any existing foundations;
the presence of historic wall elements that might make driving of new sheetpiles very difficult (old timber piles that have rotted away often leave embedded parts in surprisingly good condition – these can present significant obstructions to the driving of new piles);
there may be a requirement to conserve historic elements of a wall;
the need for tie rods or ground anchors to restrain the wall against overturning (commonly used with steel sheetpile walls);
the need for access ways in the defence to allow the continuation of business and leisure activities on the river frontage;
traffic loading surcharge on the landward side (these can be particularly onerous at an operating wharf or quayside);
additional loadings on the wall from mooring or boat impact;
the need to accommodate diurnal variation in river level for tidal rivers (which may result in daily changes in the hydrostatic pressure direction on the wall).

Should the existing river frontage not be suitable for upgrading or rehabilitation (having reached the end of its service life), the option of setting the floodwall back from the frontage should be considered. This has implications for the flood defence of the land between the river and the floodwall, but may be the only acceptable option if the flood defence is to remain independent of the frontage and thereby not dependent on its stability. Such a situation is likely to arise when the party responsible for constructing and maintaining the flood defence does not have (and does not want to take on) any responsibility for the existing river frontage structures.

For defences remote from the river, construction tends to be more straightforward. Concrete (both precast and insitu) is the most common form of construction, often with some form of cladding or decorative finish. Brick and masonry can be used, but these either have to be massive structures (unless very low in height) or be reinforced with steel bars. Low brick walls can be formed by constructing a tied cavity wall on a concrete foundation, with reinforcing bars extending from the foundation up the cavity. The cavity can then be filled with concrete, during which the brick skins may need external support while the concrete in the cavity hardens.

Where a cutoff is required, a sheetpiling wall offers the advantage of providing both the cutoff and the wall – though it is normal to clad the wall with brick or masonry to improve its appearance. Where space permits, one side of a sheetpile wall can be given a ‘half-bank’, so that it appears to be a flood embankment from that side.

Standard precast wall concrete units offer the advantage of speed of construction, but may lead to wastage if the ground level along the wall alignment is very variable, requiring the wall height to vary. (The advantage of using precast units is reduced if many different sizes are needed or if the largest size required is used throughout.) Cast insitu walling is more often used where there are frequent changes of direction or wall height.

Where a floodwall passes through private land, there may be a need for an easement to ensure the right of access for inspection and maintenance is provided for ever.

Scale

Local

Regional

Measure category

Prevention