Regional

Rivers setback leeves

nst — Wed, 02 Nov 2016 13:49:55 +0000

Rivers setback leeves nst Wed, 11/02/2016 - 14:49

Riverine or slow rise floods

Estuarine floods

Channel, Coastal and Floodplain Works

Managed retreat

Combined approach (grey + green)

When rivers are denied the space to meander due to levees, rock revetments, or other impediments, many beneficial river services are diminished. Setback levees increase channel capacity for carrying floodwaters. Once a levee is setback, the river may begin to meander and this poses a challenge to implementing riparian restoration on the floodplain.

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)

Along many major rivers, levees have been constructed close to the edge of the river channel, which maximizes the amount of land protected by a levee. By placing levees close to the channel, rivers become more effective conduits for drainage. It can also maximize the use of surrounding lands, even in times of high water levels.

However, levees close to the channel can create a set of problems and challenges. Because they greatly narrow the area available to transport floods, they do work to rapidly flush floodwaters and sediments through the system – but this means that the levees are exposed to high-velocity water along their “wet” side. This can result in erosion and high maintenance costs. In many places, the growing list of sites needing repair has outstripped the maintenance budget, resulting in levees that are more likely to fail during a flood (Leavenworth 2004; American Society of Civil Engineers 2009).

Levees close to a river also dramatically restrict the area of floodplain that benefits from periodic connections with the river and constricts the ability of the river to meander and create new river- floodplain habitats. Because of the vulnerability to erosion mentioned above, these levees often require armouring to prevent erosion and meandering, further diminishing the natural habitat values of the river’s edge, which is generally the most biologically valuable habitat. Also, while levees may prevent flooding at one location, they may increase the risk of flooding upstream and/or downstream of the levees. Moving levees back away from the channel - often called “setback levees” - can alleviate these problems.

Benefits

Setback levees increase channel capacity for carrying floodwaters. By increasing conveyance through a section of river, setback levees can relieve “bottleneck” points on a river where floodwaters would tend to back up and potentially cause flooding.

While levees close to the channel are exposed to deep, high-velocity water during floods, setback levees are less frequently exposed to floodwaters because of the increased channel capacity. Further, because flow over floodplains is generally much shallower and slower than rivers, when setback levees are exposed to floodwaters they are less vulnerable to erosion

Co-benefits

In addition to flood-management benefits, setting levees back increases the area of floodplain exposed to periodic inundation from the river, thus increasing the variety of benefits from river-floodplain connectivity. The expanded area on the “wet side” of the levee provides greater room for the channel to meander and create floodplain habitat features, such as wetlands and forests. During overbank flooding, floodwaters spread out on floodplains and, due to slower water velocities on the floodplain, much of the sediment in transport is deposited there. Because nutrients such as phosphorous are largely adsorbed to sediment particles, this deposition can reduce the loads of sediment and some nutrients in rivers and thus improve water quality for downstream water bodies, such as estuaries and near-shore marine habitats (Noe and Hupp 2005). Biogeochemical processes within floodplain wetlands, such as denitrification, can also reduce nitrogen loads in river water (Burt and Pinay 2005; Valett et al. 2005).

During overbank flooding, a portion of floodwaters can percolate into the shallow groundwater. Portions of the reconnected floodplain can continue to be used for agriculture, with crop selection varying by expected inundation frequency.

Costs

The primary costs for levee setbacks are the removal and construction of levees and, potentially, the purchase of title or easements on the reconnected floodplain. If a levee needs to be replaced or rebuilt anyhow, then the primary costs are for the difference in land area no longer protected by a levee and now prone to periodic flooding. Because the reconnected floodplain can provide habitat and other benefits, conservation funding can be combined with flood-management funding to implement these projects. For example, funds for river restoration were committed to a proposed levee setback project on the Sacramento River in California, USA (Opperman et al. 2011).

American Society of Civil Engineers (2009). Report Card for America’s Infrastructure American Society of Civil Engineers, Washington, D.C.

Burt, T. P. and Pinay, G. (2005). Linking hydrology and biogeochemistry in complex landscapes. Progress in Physical Geography, vol. 29, pp. 297-316.

Leavenworth, S. (2004). Rising risk. Page A1, Sacramento Bee, Sacramento, CA.

Noe, G.B. and Hupp, C.R. (2005). Carbon, nitrogen, and phosphorus accumulation in floodplains of Atlantic Coastal Plain Rivers, USA. Ecological Applications, vol. 15, pp. 1178-1190.

Opperman, J. J., Warner, A., Girvetz, E. H., Harrison, D. and Fry, T. (2011). Integrated reservoir-floodplain management as an ecosystem-based adaptation strategy to climate change. Proceedings of American Water Resources Association 2011 Spring Specialty Conference on Climate Change and Water Resources. American Water Resources Association, Baltimore, Maryland.

Valett, H.M., Baker, M.A., Morrice, J.A., Crawford, C.S., Molles, M.C., Dahm, C.N., Moyer, D.L. and Thibault, J.R. (2005). The flood pulse in a semi-arid riparian forest: metabolic and biogeochemical responses to inter-flood interval. Ecology, vol. 86, pp. 220- 234.

Scale

Local

Regional

Measure category

Prevention

Health planning and awareness campaigns

giacomo.cazzola — Thu, 15 Sep 2016 10:31:39 +0000

Health planning and awareness campaigns giacomo.cazzola Thu, 09/15/2016 - 12:31

Riverine or slow rise floods

Flash floods

Estuarine floods

Coastal floods or storm surges

Urban floods

Public Awareness and Preparedness

Non-structural measure

An urban flood event requires immediate measures to ensure that citizens have safe drinking water, including appropriate excreta disposal, disease vector control and waste management. However, during and after a flood event is not necessarily the best time to communicate health messages to individuals and organizations, as they may be dispersed and not have access to the necessary resources. Health Awareness Campaigns are vital ‘soft’ interventions alongside hardware provision (waste water treatment, for example); together they can help preserve public health by increasing preparedness. Health awareness and hygiene promotion campaigns must not be carried out independently from water supply and sanitation, and vice versa.

Based on: Jha, Abhas K., Robin Bloch, and Jessica Lamond. Cities and Flooding: A Guide to Integrated Urban Flood Risk Management for the 21st Century. World Bank Publications, 2012.

Floods can make it difficult to maintain dignity and hygiene, and lead to an increase in the risk of disease in the following ways:

Widespread contamination by fecal material due to destruction, breakage or damage to sewage systems and latrines, and subsequent open defecation
Contamination of drinking water
Thick layers of silt, debris and other materials
Loss or lack of key hygiene item
Standing pools of contaminated water or sewage
Rotting corpses (human and animal) can lead to excessive fly breeding or contamination of water sources from insect feces
Increase in vector breeding
An adverse psychological impact due to loss, and a sense of despair.

An effective health awareness campaign will provide clear timely advice on how best to protect individual and public health during a flood and will facilitate a two–way dialogue such that feedback from the affected persons directly informs priorities and decision making.

Pre-flood campaigns are vital for risk mitigation and preparedness. Campaigns during, or post-flooding, will reinforce messages and mobilize communities into action to preserve public health.

Key Components of Health Awareness Campaigns

Urban residents may have received little or no previous hygiene education or health awareness training and are likely to be ill-prepared to respond to a flood. When this condition is combined with weak local or municipal government and staff who are themselves ill-prepared, a flood event can result in a complete breakdown of basic public services (water, sanitation and solid waste management) alongside a significant increase in the risk of accidents and disease. As a consequence, even a relatively minor flood can result in a dangerous increase in morbidity and mortality.

It is important to plan health awareness campaigns with an understanding of the type of flooding involved, its anticipated effects, probable duration and the likely impact on the urban population. This planning also has to take into account the current status of public understanding and awareness of health issues, as no two urban situations will be the same. Both the messages and modes of communication should be adapted for the particular situation and for different audiences. The messages should take account of beliefs and attitudes that regarding health, disease and hygiene and should appeal to the interests and priorities of different groups.

The health awareness interventions should be designed with participation and collaboration of all key stakeholders, to ensure that effective messages are developed, and that both clarity and consistency apply to the communication strategy. The relevant government ministries (such as environmental health, social welfare, health, education) should be involved, as well as influential leaders, opinion formers and agencies working in the WASH (the now commonly used acronym used for water supply, sanitation and hygiene promotion) or health cluster. Different sections of the community should participate, including the more vulnerable groups such as low-income groups, women, children, aged and disabled people.

Health awareness is required by three distinct groups of people:

Municipal staff, volunteers and health professionals
The general public and in particular vulnerable groups
Media workers.

When and where to use Health Awareness Campaigns

Any flood-prone city or town would be well advised to invest in a health awareness campaign, both pre-flood and post-flood, irrespective of what type of flood is anticipated. An assessment in advance of the likely public health risks during a flood in that location (including, for example, impacts on water treatment works, or probable disease vectors where appropriate) will guide the design and prioritization of any campaign to make it more cost effective.

Benefits

An effective public health campaign will reduce death and disease caused by flooding. Specifically, pre-flood health awareness campaigns will:

Develop knowledge, understanding and build the capacity of municipal staff and volunteers to work effectively and efficiently post-flood, to preserve public health and reduce mortality and morbidity.
Provide guidelines on key elements of an initial rapid assessment of public health risks and build capacity to institute an appropriate, rapid and coordinated WASH response.
Protect health service capacity from the impacts of flooding.
Post-flood campaigns will help preserve personal and public health by giving the public immediately relevant knowledge and awareness to complement the hardware relief interventions.

The health and hygiene information is also applicable in non-flood situations and, as such, will have a knock-on effect of improving public health in general. Health awareness campaigns to deal with urban floods sit very comfortably alongside other, more traditional, health campaigns such as mother and child health, anti-malaria and HIV/AIDs awareness. The same professional staff and volunteers can and should be involved.

Risks and weaknesses

There are few risks associated with promoting health awareness in urban areas prone to flooding. The challenge is to ensure that this is carried out effectively, as far in advance of floods as possible and in close coordination with the structural mitigation and relief interventions.

Cities may face particular challenges, such as providing sanitary excreta disposal options for low income settlements, or maintaining waste disposal services during the flood. Given the relative poverty of many affected municipal authorities and local governments, there may also be issues around obtaining resources to invest in public health flood preparedness, when structural interventions could appear more politically advantageous.

Essentials and key considerations

The affected population should be made aware of their rights and entitlements to relief and recovery operations; this is particularly relevant to the rights to protection for specific groups of persons (such as internally displaced persons; women, children and adolescents; the aged; people living with HIV/AIDs; persons with disabilities; single parent households; ethnic and religious minority groups; and indigenous peoples). IASC (2008) discusses these issues in more detail.

Public health campaigns can only provide knowledge and understanding, which may do little practical good without tangible interventions such as provision of clean drinking water, chlorine tablets or safe waste disposal sites. However, as stressed in this section, without pre- and post-flood ‘soft’ interventions (specifically aimed at promoting awareness of how to preserve health and hygiene during floods), the ‘hard’ interventions are unlikely to be effectively mobilized or, even if mobilized, will by themselves be of minimal use.

Catastrophic outbreaks of diseases are not inevitable after a disaster: they do not spontaneously occur. However, the keys to preventing disease are to be prepared, to educate and motivate both the appropriate officials and the public, and to promote the meeting of basic sanitary needs.

Literature sources

IASC. 2008. Human rights and Natural Disasters, Operational Guidelines and Field Manual on Human Rights Protection in Situations of Natural Disaster.

Scale

Local

Regional

Measure category

Preparedness

Multi-Criteria Analysis (MCA)

nst — Mon, 12 Sep 2016 09:45:39 +0000

Multi-Criteria Analysis (MCA) nst Mon, 09/12/2016 - 11:45

Public Awareness and Preparedness

Non-structural measure

The Multi-Criteria Analysis (MCA) is one of the five tools used to assess the proposed measures in each of the RISC-KIT case studies with respect to criteria that capture the key dimensions of the decision-making process. The purpose of the MCA is to bridge the disciplinary divide between engineering sciences and social sciences, facilitate the communication and dissemination of project results to a broad audience, and to integrate scientific knowledge with local knowledge with the purpose of improving the assessment of coastal risks.

Based on the RISC-KIT Results on MCA and RISC KIT Deliverable 4.2 – Evaluation of DRR plans.

General description

MCA methodologies have been widely applied in environmental studies as they have proven useful tools when assessing performance of options against criteria that are difficult to quantify and involve qualitative aspects. In RISC-KIT, MCA is used in three ways: 1) as a way facilitate the communication and presentation of project results in a coherent and contextualized manner to relevant local stakeholders and decision-makers; 2) as a way to capture other types of knowledge, such as local every-day experiences, socio-economic and political factors that might affect how the proposed measures are perceived; and 3) as a way of facilitating interaction between local stakeholders and raising awareness of risks and potential measures.

Results from the implementation of the MCA in RISC-KIT cases highlights several key lessons for future DRR projects with regards to a) the importance of sufficient preparation for participatory sessions; b) stakeholder interaction and inclusion in the DRR projects; c) the way research results are presented to non-research communities; d) and the challenges of implementing single-approaches to diverse contexts.

In RISC-KIT MCAs are used to decide, among many options, which is the most convenient for most stakeholders in terms of a set of criteria (i.e. in flood and coastal risk management decisions can involve the construction of a flood alleviation channel or dredging a river, or harder engineering solution like the construction of barriers or dams). For RISC-KIT, an own MCA methodology is used to evaluate DRR strategies with respect to criteria that capture the key dimensions of the decision-making problem, involving human judgment and preferences (Saarikoski et al. 2015). MCA is about determining the extent to which options create value by achieving objectives, identify the areas of greater and lesser opportunity, prioritize the options, clarify the differences between the options, and help the key players to understand the situation better. Ultimately the use of the MCA in the project would allow each case study to test assumptions on the dynamics between DRR measures, between these measures and the specific social contexts, as reaction and responses from local actors to these measures. Outweighing different DRR measures in different contexts implies that different criteria need to be considered in order to assess which option is the best, for whom, for what, and when.

Selection of criteria

Criteria have been selected based on a literature review of the most important factors when contemplating, planning, financing, and implementing DRR measures. Studies identify factors such as social acceptance, political will, availability of financial resources and technological know-how, as crucial for increased investments in DRR measures (Davis et al. 2015). For the MCAs in RISC-KIT, three main categories of criteria were selected: Feasibility, Acceptability, and Sustainability. Feasibility refers to that (human, technical, time, and financial) resources required to implement the measure are available or can be acquired, whether the proposed measures address underlying concerns in society, whether the proposed location for implementing the measures is suitable for local needs and plans, and whether the proposed measure could have positive or negative impacts (e.g economic) to society at large. Acceptability refers to the expectations of stakeholders and recipients in the case studies sites. These actors may include civil society, interest groups, and influential individuals in society. Sustainability addresses the relevance of the measures in the present and future, its impact upon human activity and ecosystems, and the resilience of the measures to future changes.

MCA Steps in RISC-KIT

The MCA was carried in a workshop format and consisted of five steps:

Interactively present preliminary DRR measures from model results and agree on Strategic Alternatives: The MCA carried out in RISC-KIT was informed by the results produced through the Bayesian Network regarding the effectiveness of DRR measures to coastal risks. Through the use of the interactive materials, stakeholders were able to learn how the different measures behaved in light of different risk scenarios and their effectiveness in preventing coastal hazards like floods and erosion. Hereon, stakeholders had the possibility to collectively agree on the Strategic Alternatives (i.e., combinations of measures) that would be scored in the MCA
Score measures against criteria: Stakeholders assessed the performance of each Strategic Alternatives (SA) against criteria (e.g. how feasible/sustainable/acceptable/suitable are sand dunes as a measure to prevent coastal erosion in your area?) by first assigning a value ranging between -2 and +2, to each criterion per SA, on an individual basis. Once stakeholders had assigned all scores, they used colored post-it’s with pre-assigned values (-2 to +2) to write their individual scores once again, but this time make them public to the other stakeholders by pasting in a MCA flip chart. Once all scores were visible, stakeholders could engage in a facilitated discussion and agree on one score per criteria. In those cases where consensus could not be reached, individual scores were instead averaged.
Weigh criteria: to indicate criteria’s importance relative to the objective of the process (e.g., what criterion is most important to consider if sand dunes were to be implemented to reduce coastal erosion?). This was done through 2 main steps: first, stakeholders were handed out 8 stickers each which needed to be distributed on an individual basis in between the three criteria to indicate their importance. The more stickers a criterion received the heavier its weights. The second step took place once stakeholders had assigned individual weights. Each participant indicated the individual weights on the MCA flip chart so that they would be visible to the group. Thereafter the group engaged in a facilitate discussion to agree on a weight per criteria. Equal weights could be given to more than one criterion; however, it is common in MCA to give different weightings to different options, reflecting their importance in the overall objectives. In those cases were consensus was not reached, weights were averaged. Criteria were only weighted once, as it is assumed that their importance is constant across all SAs.
Calculate weighted scores of criteria: for each measure by multiplying scores times the weight for each criterion for all measures.
Generate sums per measure by adding the weighted scores for all criteria per SA and entering the total value in the row titled “SUMS” at the end of the MCA Matrix. The SA will the highest weighted scores was stakeholders’ preferred alternative. Picture 1 shows an example of a complete MCA.

Scale

Measure category

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

River bank protection and restoration

giacomo.cazzola — Thu, 08 Sep 2016 11:00:06 +0000

River bank protection and restoration giacomo.cazzola Thu, 09/08/2016 - 13:00

Riverine or slow rise floods

Estuarine floods

Erosion

Channel, Coastal and Floodplain Works

Hold the line

Combined approach (grey + green)

Bank protection is needed where there is the risk of erosion of the bank and where this erosion would cause economic or environmental loss. If there is sufficient space available, it may be possible to reduce the need for bank protection by re-profiling the bank to a flatter slope to reduce velocities and encourage good vegetation growth. Even if bank protection is still required, it may be less severe if a flatter slope can be achieved, or may only be required below normal water level.

Based on "A T Pepper, C E Rickard (2009): 8 Works in the river channel. 36p. In: UK Environmental Agency (2009): Fluvial Design Guide (Contains public sector information licensed under the Open Government Licence v3.0.)"

Where it is needed, erosion protection for a bank can range from a good grass cover to heavy concrete slabs, but is broadly categorised as ‘hard’ or ‘soft’. Soft bank protection is generally considered to be vegetation of various types, whereas hard bank protection consists of concrete blockwork, riprap or similar. This is not a universally accepted definition as, in some parts of the country, riprap is termed ‘soft’ (because it is locally sourced and more natural than concrete), with ‘hard’ being reserved for the likes of piling and solid concrete walls.

Bank protection also has an impact on the ecology of a reach, and in this respect the softer the bank protection the more ecologically friendly it is likely to be. Vegetated banks offer little or no restriction on habitat, although animals that dig holes may need to be discouraged. This can be achieved by laying an open geotextile on the bank before adding a layer of topsoil (30–40mm thick) mixed with grass seed over the top. The geotextile assists in binding the grass roots together, at the same time making burrowing more difficult (though not impossible for determined creatures with sharp teeth).

Coir rolls, willow spiling and faggots can all be used to provide erosion protection, and each can also provide habitats for a range of species. However, the softer forms of erosion protection are not appropriate for situations where flow velocities or turbulence are high. Whereas the achievement of an environmentally acceptable protection system is very important, this has to be in the context of achieving the primary aim of bank stabilisation. A revetment that is destroyed in the first major flood because it was not up to the job would fail to achieve both structural and environmental objectives.

Some hard engineering solutions such as riprap can also provide habitats for a different range of species, both above and below the water, as the interstices provide shelter for fish fry and invertebrates below water, and for a range of insects, birds, animals and plants above water. Concrete slabs and solid concrete lining, on the other hand, provide a somewhat barren environment for flora and fauna.

The common approach to protecting the toe of a riverbank from erosion is to provide some form of flexible protection such as a gabion mattress or riprap. Alternatively, the toe can be protected by sheetpiling, with the top level of the piles below normal water level. Protection of the bank toe may be achieved in some cases by the use of in-channel structures such as vanes or groynes to deflect high velocities from the bank, and ideally cause a deposit of material in their lee to help protect the bank toe. These may be especially useful where the bank is well vegetated above the normal water level, and not, in itself, at risk of erosion. In floods these vanes or groynes are fully submerged, so offer minimal reduction in flow capacity.

Restoration techniques

The River Restoration Centre (RRC) collates information on river restoration schemes, and disseminates this information in a number of ways, one of which is the Manual of river restoration techniques (RRC, 2002). This should be among the first references to be consulted when considering a river restoration project.

Careful consideration is needed when proposing the removal of permanent artificial obstructions from a watercourse (such as a redundant weir) in order to restore a river to a previous state. Although there may be an immediate gain in flood conveyance and the removal of an obstruction to fish migration, there are also potential negative impacts. These include the release into the flow of accumulated sediments, which may be contaminated if the site has an industrial history. Removal of a weir may also be seen as a loss of amenity. It can also temporarily destabilise the river by causing bed erosion upstream, and this could undermine infrastructure such as bridges and riverside walls. But in many circumstances, a weir can be removed with no adverse – and many beneficial – impacts.

If a weir is to be removed, consideration must be given to the changes in the pattern of energy dissipation that may be caused. A weir pool may have formed over many years, with a shape and size that allow it to act as an effective energy dissipator. If removal of a weir takes away that function, that energy must be dissipated elsewhere – which is along the bed and banks of the river upstream. Careful assessment is required to determine the mean and maximum velocities likely to be encountered and the shear forces generated at the bed and banks. If these are likely to cause unacceptable erosion, then either the areas at risk should be protected or perhaps the weir should be only partially removed.

An alternative is to replace one large weir with a series of lower weirs or possibly a cascade, which would permit fish passage and improve flood capacity while restricting velocities and potential bed erosion. In less steep rivers, a pool and riffles sequence can be established, often by the introduction of imported gravel to form the riffles. Guidance on the establishment of pools and riffles can be found in the Environment Agency’s Guidebook of applied fluvial geomorphology (Sear et al, 2003).

In some situations, a meandering channel with pools and riffles can replace a shorter straight channel containing a weir, to the benefit of fish passage, an increase in habitat potential and additional flood attenuation. Where restoration involves reintroducing meanders to a long straight channel, it is likely that the new channel will cut across the existing channel in a number of places. If possible, parts of the straight channel that are intersected should be left open rather than being backfilled to provide backwaters. These will provide a still water habitat, as well as offer refuges for fish during periods of high flow. If land constraints permit, purpose-made backwaters or perhaps shallow bays can be excavated.

River diversions

Sometimes it is necessary to divert a river or stream course to make way for major infrastructure such as a motorway. This should be seen as the opportunity to re-create the river corridor, incorporating existing and new environmental features to ensure the new reach of channel adds value to the environmental and ecological status of the stream in question. The Water Framework Directive requires this type of approach. The river restoration techniques described above can be adopted for river diversion schemes (Fisher and Ramsbottom, 2001).

In other cases it may be that the existing course of the river is too constrained to allow enlargement to carry flood flows but is adequate for low flows. In such cases an alternative route for excess flows must be found. This may be a new channel, although this can involve significant land acquisition for something that is rarely used, or it can be a floodway where land is used for its original purpose (except when a major flood occurs) provided that such use is compatible with occasional flooding.

Fisher, K and Ramsbottom, D (2001). River diversions – a design guide. Thomas Telford.

River Restoration Centre (2002). Manual of river restoration techniques [online]. RRC. Available from: http://www.therrc.co.uk/manual-river-restoration-techniques

Sear, D A, Newson, M D and Thorne, C R (2003). Guidebook of applied fluvial geomorphology, Defra/Environment Agency Flood and Coastal Defence R&D Programme, R&D Technical Report FD1914. Defra. Available from: http://sciencesearch.defra.gov.uk/Document.aspx?Document=FD1914_1147_TRP.pdf.

Scale

Regional

Measure category

Prevention

River bank relocation – floodplain lowering

giacomo.cazzola — Thu, 08 Sep 2016 10:44:28 +0000

River bank relocation – floodplain lowering giacomo.cazzola Thu, 09/08/2016 - 12:44

Riverine or slow rise floods

Flash floods

Estuarine floods

Channel, Coastal and Floodplain Works

Combined approach (grey + green)

Traditionally, interventions in river channels have been carried out to reduce flood risk at a particular location. This approach has produced artificial river geometries which have often been found, for a variety of reasons, to be unsustainable. A core principle of modern river engineering is that, in general terms, rivers tend to return to their natural ‘regime’ state, in which the main channel has the capacity for a particular flow and no more.

Based on: Jha, Abhas K., Robin Bloch, and Jessica Lamond. Cities and Flooding: A Guide to Integrated Urban Flood Risk Management for the 21st Century. World Bank Publications, 2012. and

Pepper, A.T. and Rickard, C.E. 2009 Fluvial Design Guide. Bristol: Environment Agency. Chapter 8. (Contains public sector information licensed under the Open Government Licence v3.0.)

While major rivers, especially in developing countries, must be treated as being unique, current thinking is that this flow- rate corresponds roughly to the mean annual flood (Pepper and Rickard, 2009). Greater flow-rates, therefore, are not necessarily contained in the main channel. Artificial deepening of a river channel increases cross-sectional area but may reduce slope; the result can be reduced velocity and increased deposition of silt, tending to a reversal of the initial deepening. Artificial widening may cause deposition close to the river banks. Cutting off a meander will shorten the channel between two points and therefore increase the bed slope; this will increase flow- carrying capacity but will also increase velocities. The result may be erosion of the banks or the bed (termed ‘scouring’) together with deposition further downstream; this will also tend to reverse the initial steepening.

In some circumstances, decreasing roughness or straightening the course may solve local flooding problems, by increasing the capacity of the channel, but a reduction in both storage and attenuation is inevitable. In contrast, where channel naturalization or river restoration includes returning a watercourse to a more natural condition (for example, by reinstating meanders) this can increase storage, enhance the amount of attenuation and thereby reduce flood risk downstream.

Making the channel bigger seems the obvious way to increase its capacity and thereby overcome a flooding problem, and many flood alleviation schemes have gone ahead on this basis. Though, as promoters of such schemes have found to their cost, there are drawbacks too. These include:

environmental degradation as of the result of the loss of natural channel features and vegetation during the enlargement process (recovery may take a long time);
channel instability due to the removal of vegetation that helps to prevent erosion and maintains the integrity of the riverbanks;
sedimentation in the engineered channel leading to the formation of shoals and bars, and eventually to the reversion to a smaller, more natural channel (frequent intervention may be required in the form of desilting to keep the desired channel size, again with adverse environmental impacts);
a stark and unnatural appearance, particularly in the usual low-flow conditions (which are much more common than flood flows).

Some of these problems can be overcome by creating a two-stage channel, in which the usual flows are contained within a smaller channel and flood flows occupy a much larger second stage channel. This design emulates the operation of a natural floodplain, but in a more contained and controlled manner.

As a variant of the two-stage channel, a low-flow section can be engineered by the provision of low groynes that confine these flows and result in enhanced flow velocities, but do not interfere significantly with flood flows. Low weirs can also be used to maintain the depth of water in the channel in low-flow conditions without impacting on the flood capacity.

All these options require careful consideration and the involvement of an experienced geomorphologist to ensure they are stable in the long term and do not result in adverse impacts.

Relevant case studies and examples

EXAMPLE: Lowering the floodplain in Emilia–Romagna area (IT)

Literature sources

Pepper, A.T. and Rickard, C.E. 2009 Fluvial Design Guide. Bristol: Environment Agency. Chapter 8.

Scale

Local

Regional

Measure category

Prevention

Adaptive management

nst — Tue, 14 Jun 2016 14:50:07 +0000

Adaptive management nst Tue, 06/14/2016 - 16:50

Riverine or slow rise floods

Estuarine floods

Coastal floods or storm surges

Erosion

Avoidance

Deal with the effects

Ecosystem based approach

Highly dynamic coastal systems (like sandy beaches, dunes or estuaries) might be best managed by not interfering with the natural processes, but instead accepting that change will occur and adapting backshore management accordingly. Key in this approach is a proper monitoring of the processes to analyze and evaluate the changes (for examples at eroding cliffs or dunes). With a proper planning horizon, these changes can be anticipated and with enough room for the environment to involve this can be a very cost-extensive approach.

Based on kindly provided information by the Scottish Natural Heritage

Technical feasibility

Adaptive management should be considered at all sites before considering any of the other options of hard engineering coastal defence measures. In some cases this can mean loss of land or other values, so an assessment of these impacts has to be carefully undertaken with the integration of relevant stakeholders.

In contrast to managed realignment, this measure is not a planned retreat (e.g. with opening of dykes or removal of groynes). Instead it is the allowance of natural processes that could lead to relocation of the coastline but does not necessarily have to.

If management of marine erosion is being considered it is assumed that assets of value are being threatened. These assets may be natural, such as a rare habitat or a particularly interesting geomorphological feature. In general erosion of such features would be considered part of their natural evolution and therefore preferable to management interference. More often the assets will have socio-economic importance, ranging from amenity access to a beach up to an industrial complex or power station. The shoreline manager must start by considering the value of the assets that may be at risk, then try to establish an understanding of the likely future evolution of the beach/dune system. Finally the manager must determine whether it is better to lose/move the assets or attempt to prevent or reduce the erosion.

Consideration from Italy: Retreat is expensive - staying too!

Political & social feasibility

Monitoring, consultation and education are at the heart of adaptive management. The shoreline should be continuously assessed using data collected from site, combined with any available historic or published data. The monitoring will allow the management policy to be reviewed from time to time. Impacts of the policy on recreation, land use and habitats should also be monitored.

Consultation is required to assess the values associated with the backshore, and to develop a consensus view on how to deal with the assets. This process is firmly linked with education, requiring the manager to set out the background issues in a language that can be readily appreciated by those who are affected.

Those responsible for the management of eroding dunes and cliffs should be aware of the potential danger to the public of a collapsing dune or cliff face. Dangers exist both from falling down the face and from being buried at the base. Warning signs set up along the crest and at public access points should be the minimum response to these dangers.

Cost of implementation & maintenance

The value of the backshore may be assessed in economic terms, based on the present replacement cost of buildings, infrastructure or land. The assessment should also consider the wider values such as potential loss of jobs, transport routes, rare habitats, recreation or cultural heritage (i.e. archaeological sites). This assessment therefore considers costs and benefits (SET LINK).

Costs associated with adaptive management are site specific and cannot be generalised. Accepting the gradual loss of a site valued as an undeveloped public recreation area may incur no actual cost at all apart from monitoring and minor works to delay erosion or encourage. At the other end of the scale the demolition and replacement of threatened shoreline buildings or recreational facilities may be very cost intensive.

Ecological feasibility

Adaptive management minimises interference with the natural processes and ecosystem of an evolving dune system. The approach allows for the sustainable, long term management of the shore, with no environmentally disruptive engineered schemes.

Key lessons learnt

Adaptive management can result in the controlled loss of backshore assets and the continued evolution of dune habitat and land form. This approach can be highly emotive, with local interest groups protesting vigorously and demanding that more positive actions be taken. However, it must be accepted that both erosion and accretion are natural elements of coastal evolution, and that maintenance of natural evolution is, wherever possible, preferable to costly and environmentally disruptive intervention.

Scale

Local

Regional

Measure category

Prevention