Managed retreat

EXAMPLE: Coastal setbacks on the island of Kauai (USA)

nst — Mon, 16 Jan 2017 13:34:18 +0000

EXAMPLE: Coastal setbacks on the island of Kauai (USA) nst Mon, 01/16/2017 - 14:34

Avoidance

Coastal and river setbacks

On the island of Kauai, Hawaii in the USA, the local governing county has implemented flexible and protective coastal setbacks that protect communities from coastal erosion and avoid shoreline armouring in the long term.

Based on J.F. O’Connell et al. (2010): "The island of Kauai, Hawaii's progressive shoreline setback and coastal protection ordinance" In: Shifting Shorelines: Adapting to the Future,The 22nd International Conference of The Coastal Society , June 13-16, 2010 ,Wilmington, North Carolina.

General description

In this particular case, there is a disconnect between state regulations on coastal zone management and local or County regulations. In Hawaii, there are state laws that require setbacks along shorelines that are no less than 20 and not more than 40 feet inland from the shoreline and armouring the shoreline is permitted. The regulations at the state level, have however, led to inappropriate constructions in areas that jeopardize the island’s valuable sandy beaches. Thus in spite of an innovative and flexible Ordinance developed in 2008 called the ‘Shoreline Setback and Coastal Protection Ordinance’, the state still allows armouring.

The Ordinance puts into place procedures establishing minimum construction setbacks based on average lot depth and long-term shoreline erosion rates that are generated by the University of Hawaii.

The objectives of the Ordinance are manifold:

To provide a buffer zone to protect shorefront development from loss due to coastal erosion for a period of time;
To provide protection from storm waves;
To allow the natural dynamic cycles of erosion and accretion of beaches and dunes to occur;
To maintain beach and dune habitat;
And to maintain lateral beach access and open space for the enjoyment of the natural shoreline environment.
To avoid armouring or hardening the shore which along eroding coasts has been documented to ultimately eliminate the fronting beach.
The Island of Kauai is a county within Hawaii and is also the fourth largest of the Hawaiian islands. It is vulnerable to a variety of coastal hazards including inundation, erosion, hurricanes, and tsunamis.

Local Setting

On the island of Kauai the coasts and sandy beaches are important to the economy and the community. Shoreline armoring as a measure for dealing with climate change can benefit coastal infrastructure but it can also threaten coastal marine habitats and beaches. The potential loss of sandy beaches due to coastal hardening is particularly important in a state like Hawaii and specifically on Kauai where the local economy depends on tourism and beach activities.

In Kauai, the county takes the state wide implemented setback of 40 feet as a minimum standard and finds flexible and specific setback lines based on average lot depth and long-term coastal erosion rates that are developed and provided by the University of Kauai. The county, therefore, has taken steps to avoid shoreline armoring and establish safe and environmentally effective setback distances for construction of structures with a 2008 Ordinance. However, the regulations developed at the County level do not match those set by the state.

Political setting

The Kauai case study provides an example of when different levels of government set different regulations that are in conflict with one another. In Kauai, a local level Ordinance has set environmentally protective standards in place that go further than State coastal zone management laws to ensure the integrity of Kauai’s sandy beaches. While the State sets general measures for coastal setbacks and infrastructure development, the County appears to be setting up legislation that is more accurately informed by local circumstances and data and that prioritizes environmental considerations.

The Ordinance puts into place procedures establishing setbacks that go beyond the state-wide laws. The County setbacks also consider lot depth and long-term shoreline erosion rates. In order to determine the erosion rates of different areas around the island, the County has partnered with the University of Hawaii Sea Grant College Program to conduct an assessment on Climate Change and Coastal Hazards in Kauai and to provide data regularly.

Innovative aspect

In Kauai, the county takes the state wide implemented setback of 40 feet as a minimum standard and finds flexible and specific setback lines based on average lot depth and long-term coastal erosion rates that are developed and provided by the University of Kauai. For existing structures 20 feet is the minimum setback area. It also requires lot depths of greater than 160 feet with a proposed building footprint less than or greater than 5000 square feet to calculate the setback by multiplying the erosion rate by 70 or 100, respectfully on top of a forty food safety buffer.

The purpose of the Ordinance is to ensure that structures are not built in areas that are vulnerable to hazards and that shoreline hardening is avoided and not depended on to protect property during its lifetime. There are also specific rules regarding activities and structures that are allowed within the setback are, however, no structure approved within the setback area by variance will be eligible for protection by shoreline hardening.

Key lessons learnt

There are several interesting elements of the Kauai case study. Firstly, there is an inter-change between local levels of the island County and established state laws on zoning and management. The Ordinance that determines county rules for setback measures is more protective and exact in determining the rules for building and at the same time was designed in a way that is flexible to specific projects and also informed by local data and research. The partnership between the County and the University of Kauai to establish appropriate setback measurements based on erosion rates on the island illustrates the importance of partnerships between governing entities and institutions with relevant scientific data and knowledge. Finally, the Ordinance is designed to be somewhat flexible but is ultimately environmentally focused in protecting the integrity of sandy beaches and avoiding the hardening of the coast in the future to protect any existing structures.

Relevant case studies and examples

Further Readings

More information from NOAA

Literature sources

O'Connell, James and Aiu, Imaikalani and Milnes, Leslie and Smith, Lisa Ellen (2010) The island of Kauai, Hawaii's progressive shoreline setback and coastal protection ordinance. In: Shifting Shorelines: Adapting to the Future,The 22nd International Conference of The Coastal Society , June 13-16, 2010 ,Wilmington, North Carolina

O'Connell, James et al. (2009): A PROGRESSIVE, BALANCED COASTAL CONSTRUCTION SETBACK ORDINANCE ON THE ISLAND OF KAUAI, HAWAII: IMPLEMENTATION AND LESSONS LEARNED. Proceedings of Coastal Zone 09, Boston, Massachusetts, July 19 to 23, 2009

Measure category

EXAMPLE: Relocation in Criel sur Mer, Normandy (FR)

nst — Mon, 16 Jan 2017 12:49:39 +0000

EXAMPLE: Relocation in Criel sur Mer, Normandy (FR) nst Mon, 01/16/2017 - 13:49

Exposed element relocation and removal

Criel sur Mer is a small town in Normandy in the region of Northern France, known for its stunning coastline of steep chalk cliffs. Erosion of the cliffs in Criel sur Mer is occurring rapidly as a result of climate change but also due to man-made construction works further up the coast. In Criel sur Mer a short piece of land on the coast that is eroding rapidly and several homes built near the sea are threatened by the predicted collapse of the cliff. In particular, a street of homes were faced with immediate danger from erosion. Between 1995 and 2003, the local administration organized the abandonment and demolishment of 14 homes due to imminent risk from natural disaster under the Barnier Law. The adoptive policy was to do nothing against cliff erosion and to demolish and relocate those in immediate threat and compensate them fairly for their lost property.

General description

Coastal erosion is a common challenge along many stretches of the coast in Normandy. The cliffs are extremely steep and the rock material is chalk which is soft and easily erodible. The verticality of the cliffs mean that erosion is especially intense at the base of the cliff leading to significant fractures and collapse of cliff and loss of pebble beaches that would otherwise help mitigate erosion from the sea. Moreover, in Criel sur Mer considerable engineering works carried out along the coast have exacerbated erosion. Specifically, the construction of the ports Le Havre, Fécamp, Saint-Valery-en-Caux, Dieppe, and Le Tréport as well as structures for water and nuclear stations in Paluel and Penly; and the creation of coastal defence structures (sea fronts, groynes etc) at the mouths of all the valleys. These manmade constructions have created disturbances to the transport of sediment (mainly course pebbles) to the shore and resulted in a faster rate of erosion due to lack of protection. Pebbles have also been extracted for gravel purposes. Loss of pebbles leads to a retraction of the beach which protect the mouths of the rivers and the cliffs.

One of the immediate challenges for the community of Criel sur Mer was the actual loss of cliffs where houses existed.Responding to this emergency, the local administration considered both hard and soft measures with for instance the consideration of the implementation of defence works at the base of the cliff. The high cost of defence measures and the low cost of the real estate threatened by erosion led the local administration to evacuate the families faced and to implement a coastal setback plan whereby any new developments must take place 100 m from the cliff top.

The innovative aspect of this relocation measure was the fact that the compensation rate to those individuals that lost their property was calculated against the real market value. It is common that properties known to be at imminent risk lose real market value quickly, however, in the case of Criel sur Mer the French Government ensured that those families losing property were provided for financially based on the ‘riskless’ market value of the homes.

Key lessons learnt

The lesson of the Criel sur Mer illustrates is the inevitability of managed retreat in the face of climate change and the fact that multiple variables affect the situation and decision taken. For example, the cause of erosion was not only climate change but also a result of manmade constructions and attempts to mitigate against erosion. Moreover, the possibility of implementing a hard defense was considered but was economically disadvantageous. Thus, the Criel sur Mer provides an example of an extreme case of communities being threatened by climate change and provides an example of how governments and administration can more fairly compensate them economically.

Relevant case studies and examples

Further Readings

PDF: Geotechnical study

Scale

Measure category

EXAMPLE: Managed Retreat at Surfer’s Point, California (USA)

nst — Mon, 16 Jan 2017 10:38:51 +0000

EXAMPLE: Managed Retreat at Surfer’s Point, California (USA) nst Mon, 01/16/2017 - 11:38

Based on a case study from the Climate Adaptation Knowledge Exchange (CAKE)

The Ventura County Chapter of the Surfrider Foundation in California, USA decided against traditional coastal defence measures to reduce beach erosion at a popular beach spot called Surfer’s Point. Along with other stakeholders, the County instead designed a two-phase plan to strategically relocate a parking lot, pedestrian path, and bike path away from erosion zones.

General description

Surfer’s Point is a popular beach and surfing location in the City of Ventura, USA which has suffered from coastal erosion for decades. Since the 1980’s, concern has grown amongst tourists and local residents over the eroding shoreline which has threatened a main bike path and parking lot.

The project involved the relocation of the bike path and parking lot approximately 60 feet inland, completely removing rip-rap (shoreline rock amour), undertaking natural beach restoration and beach nourishment, and potentially later inland waterway work to restore natural sediment supply. The project cost is estimated at approximately $3.8 million (USD).

The City of Ventura had taken the traditional approach to coastal protection in the 1980’s and 1990’s. This “shoreline hardening” had the effect of causing increased erosion both onsite and elsewhere along the coast. By 1995, the California Coastal Commission stepped in to prevent further shoreline hardening, and plans were in place by 2001 to address erosion risk through managed retreat.

The project is a partnership of the City of Ventura, the Ventura County Fairgrounds, the California Coastal Conservancy, California State Parks, the California Coastal Commission and the Surfrider Foundation. Project leaders have identified the importance of good communication and participation of all players in the planning process to ensure successful project execution.

Ecosystem-based aspects

During the first phase of the project, in addition to the relocation and removal of grey infrastructure, volunteers planted native vegetation on sand dunes and around bioswales. Increased vegetation cover provides slope stability so the dunes and bioswales can in turn protect the beach and ocean from intense storm water runoff while protecting the new grey infrastructure from wave action.

The project was specifically intended to address all natural and human factors that exist at Surfer’s Point, including the sand, stones, wind, vegetation, and human activities and infrastructure. By doing so, the functionality of the beach can be improved for those communities and people who use it while strengthening the resilience of the natural environment to erosion and threats such as storms and sea level rise.

Key lessons learnt

Surfer’s Point has remained stable, functional, and aesthetically pleasing since the implementation of the first phase of the project and as a result of ongoing efforts. While much of the City of Ventura Promenade suffered damage from erosion during the winter of 2015, the beach at Surfer’s Point did not experience the same level of damage.

While managed retreat, involving relocation and removal of exposed elements, is not the most popular shoreline management technique, it can sometimes be the most economically and environmentally appropriate. In the case of Surfer’s Point, the failure of historical strategies and the notable effectiveness of relocating and removing at-risk infrastructure both illustrate this clearly.

Relevant case studies and examples

Exposed element relocation and removal

Further Readings

More information on the project website

Measure category

Rivers setback leeves

nst — Wed, 02 Nov 2016 13:49:55 +0000

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

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)

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.

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).

Further Readings

Dierauer et al. (2012): Evaluation of levee setbacks for flood-loss reduction, …

Literature sources

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

Regional

Measure category

Managed realignment

giacomo.cazzola — Sun, 11 Sep 2016 21:33:51 +0000

Managed realignment giacomo.cazzola Sun, 09/11/2016 - 23:33

EXAMPLE: Titchwell Marsh (UK) seawalls and managed realignment

Managed realignment is a measure that usually results in the creation of a salt marsh by removing costal protection an allowing for an area previously protected from flooding to become flooded. Managed realignment is a measure dealing with sea level rise and coastal erosion. It is also often a method that replaces hard coastal defense measures with soft coastal landforms. Rather than relying on hard structures for defense, managed realignment depends on natural defenses to absorb or dissipate the force of waves.

Based on kindly provided information by the ClimateTechWiki and the TNA Guidebook on 'Technologies for Climate Change Adaptation' by Matthew M. Linham & Robert J. Nicholls

Managed realignment effectively forms intertidal habitats. Intertidal habitats are effective in absorbing wave energy and reducing offshore sediment transport or erosion. Extensive root networks created by saltmarshes also help to prevent erosion.

Managed realignment sometimes requires the complete removal of existing coastal defense structures. In some cases managed realignment is an introduced alternative measure when existing coastal defenses can no longer be maintained and are abandoned.

Conditions to consider in undertaking managed realignment according to Gardiner et al., 2007 are:

1) presence of coastal defences

2) availability of low-lying land

3) desire or need to improve flood or coastal defence systems

4) presence of a sustainability-oriented coastal management attitude

5) desire or need to create intertidal habitats

6) societal awareness about the benefits of managed realignment

Advantages of the technology

Realignment is considered a soft measure and can significantly reduce the cost of protection against coastal flooding and erosion. Intertidal habitats such as saltmarshes, or mangroves, are effective in reducing the energy and inertia of incoming waves and thus the damage that they do. With intertidal habitats the need for intrusive hard defenses is reduced and often avoided. In addition to protecting against floods and buffering storms, rich intertidal zones can improve the resilience of the local ecosystem and provide environmental benefits such as absorbing carbon dioxide and methane emissions which are stored in the sediment deposits.

Increasingly intertidal habitats are implemented in areas that have been surrendered or abandoned. In these cases, degraded land or land that would otherwise be used for development is used to create an intertidal habitat. Intertidal habitats are important ecosystems for a variety of flora and fauna. Realignment in the UK for instance has created new opportunities for ecotourism with activities such as walking and bird watching.

Intertidal habitats can also improve water quality in nearby urban areas. In situations where drinking water is threatened by sea level rise, intertidal habitats can help avoid saltwater intrusion from inappropriate land use and reduce the effects of eutrophication.

Disadvantages of the technology

When managed realignment is implemented, land is effectively relinquished to the sea. One of the greatest disadvantages or barriers is the loss of the land itself and the potential necessity of relocating important coastal infrastructure which can be expensive.

Although experience in the application of managed realignment is increasing, the approach is still relatively new and uncertainties still exist. For example, it is not fully understood how long it will take to create typical intertidal habitats that deliver the full benefits of naturally occurring systems. Moreover, managed realignment is not well monitored which has not built up an evidence base for future projects.

The approach is also not appropriate for all environments as wetlands and saltmarshes tend to occur in locations where wave energy is low and where high volumes of sediment are available. It is therefore important to carefully evaluate the feasibility and effects of this approach in specific locations.

Barriers to implementation

Relinquishment of land by the coastline is also often met by a lack of public acceptance and conflicts among those that own the land. In densely populated coastal areas this may be very difficult. Conflicts between users and landowners bring about legal and financial difficulties. For example the creation of intertidal habitats is often at sectoral odds with social and economic factors when the proposed land area is used for agriculture production. One way to navigate conflicts of interest is to ensure a participatory process in decision making and planning.

The potentially high cost of managed realignment also poses a barrier. The relocation of infrastructure and/or compensation to landowners in the managed realignment zone is potentially costly.

Managed realignment can only take place in certain areas and contexts. It is especially beneficial to have low-lying land that is sheltered by existing coastal defences as well as societal willingness.

Opportunities for implementation

Managed realignment is an extremely effective soft measure that replaces or reduces the need for constructing hard measures. When implemented managed realignment creates space for new habitats that are beneficial for flora and fauna and that preserve land. They are an effective soft measure to the coastline dealing with sea level rise and extreme weather events and also reduce the need for additional climate change measures to be implemented behind them.

Managed realignment usually requires significant amounts of coastal land. The beneficial environmental impacts of this are complimented also open up new opportunities for the creation of a new nature reserve and ecostourism.

Relevant case studies and examples

Literature sources

Main source: Matthew M. Linham & Robert J. Nicholls (2010): TNA Guidebook on 'Technologies for Climate Change Adaptation' UNEP , 166p.

Gardiner, S., Hanson, S., Nicholls, R., Zhang, Z., Jude, S., Jones, A., Richards, J., Williams, A., Spencer, T., Cope, S., Gorczynska, M., Bradbury, A., McInnes, R., Ingleby, A. and Dalton, H. (2007) The Habitats Directive, coastal habitats and climate change - case studies from the south coast of the U.K. Norwich: The Tyndall Centre for Climate Change Research. (http://www.tyndall.ac.uk/sites/default/files/wp108.pdf)

Measure category

Flood and river bypasses

giacomo.cazzola — Thu, 08 Sep 2016 12:46:52 +0000

Flood and river bypasses giacomo.cazzola Thu, 09/08/2016 - 14:46

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)

Lowland rivers and estuaries are naturally often flanked by vast areas of floodplain that was periodically flooded. The extent of inundation varied between years and formed an integrated system together with the river for moving water from the continental interiors to the ocean. With settlements and farming activities in these floodplain areas, these areas were disconnected to the river system.

With the idea of flood bypasses, these portions of the historic floodplain are reconnected to the river and become inundated during major flood events. They act as relief valves in two ways: conveyance and storage. If this attempt is used in area were these bypasses are not based on historic floodplains, the term relief channels is used.

A common grey infrastructure solution to control riverine flooding is the building of levees and thus the disconnection of rivers from their floodplains. However, levees may create new problems (upstream/downstream flooding) and they are prone to failures. Levee setback is one green infrastructure (GI) solution to this; another is establishing flood bypasses. Flood bypasses are often portions of the historic floodplain that, during major flood events, are reconnected to the river and become inundated.

Flood bypasses act as flood relief valves in two ways: by providing conveyance and storage.

Conveyance. They increase the cross-sectional area available to move floodwaters safely through a particular stretch of river. This is analogous to opening up more lanes at a bridge toll crossing during rush hour to manage intense traffic. For example, the Yolo Bypass conveys approximately 80 per cent of the volume of major floods safely around the city of Sacramento. By increasing conveyance, strategically placed floodways can also reduce “backwater flooding,” which is caused by the“piling up” of floodwaters at and behind a bottleneck, such as where bluffs constrict the river. Similar to levee setbacks, the vegetation within a bypass can influence its hydraulic roughness and affect the ability to convey floodwaters. Thus, some bypasses are managed for vegetation with low roughness.
Storage. Flood bypasses can detain and store water, functioning similarly to a flood-control reservoir. While conveyance is analogous to adding lanes, a bypass providing storage can be viewed as a parking lot alongside a major freeway. During a particularly heavy period of traffic, a large number of cars exit the highway and park in the lot, staying there until traffic ebbs. The highway“downstream” of the parking lot will experience lower peak traffic because of the cars parked in the lot. The effect is known as “peak shaving”- reducing the height of the flood peak experienced at some downstream point. The Jianjiang Flood Detention Area along the Yangtze River is intended to function in this manner with floodgates that can be opened as the flood is rising. It has the capacity to hold five billion cubic meters of water, reducing the height of the peak against the levees that protect cities with millions of inhabitants.

Flood bypasses can provide a mix of conveyance and storage benefits that vary with the size of the feature, its location in the river system and the characteristics of the flood. Bypasses also vary in the frequency with which they are used. The Yolo Bypass is inundated relatively frequently - almost every year - while some of the floodways on the Mississippi have been used only a few times in 80 years.

Co-benefits

Because the floodways are only inundated during floods they can be used for a variety of economic activities, with landuse varying with the frequency of inundation. For example, the New Madrid Floodway has been used rarely (twice since the 1930s). It is almost entirely farmed and includes 200 homes whereas the Bonnet Carre Spillway has been used 10 times in that period. Due to the frequent inundation, the Bonnet Carrre spillway is uninhabited with land managed for fishing, hunting and recreation. Because in California the flood season (winter to early spring) and the growing season (spring to fall) have little overlap, much of the Yolo Bypass is in productive agriculture, despite the fact that it is flooded nearly every year. The agriculture in the bypass is in annual crops that are not jeopardized by up to months of inundation.

Bypasses can provide significant environmental benefits. Approximately one third of the Yolo Bypass is in wildlife refuges, including managed wetlands. Much of the land within the three Mississippi River floodways that are in Louisiana are in natural vegetation and support abundant fish and wildlife. Even floodways that are in agricultural land use can provide environmental benefits, particularly during periods of inundation. When bypasses are frequently inundated for long periods of time, the floodwaters are able to percolate into the soil and recharge the groundwater. This recharge can serve as a valuable “groundwater bank” during a drought (Jercich 1997).

Costs

Costs of establishing a flood bypass at any location consists of the investment needed in construction works (including weir or gate to direct water into bypass, and levees to delineate the floodway), as well as any costs associated with easements or title for land to ensure access to the floodplains.

The costs of the above-mentioned variables are highly location-dependent, as appropriate management practices, land costs and construction costs come into play. The maintenance costs-benefits are also dependent on the characteristics of the floods – e.g. some floodplains, such as the Yolo Bypass, are inundated frequently, while others only once in several decades, or even less (Opperman 2014). In general, projects of such magnitude can involve very high investment, though the costs can often be counterbalanced by avoided grey infrastructure investments (Opperman et al. 2011), reduced flood damage (particularly in economically active urban areas) and the wide range of co-benefits that are brought about to people and wildlife.

Relevant case studies and examples

EXAMPLE: Relief channels, Wroclaw floodway system (PL)

Further Readings

Adapted from Jeffrey J. Opperman. (2014). “A Flood of Benefits: Using Green Inf…

Literature sources

Jercich, S. A. (1997). California’s 1995 Water Bank Program: Purchasing water supply options. Journal of Water Resources Planning and Management-Asce, vol. 123, pp. 59-65.

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.

Opperman, J.J. (2014). A Flood of Benefits: Using Green Infrastructure to Reduce Flood Risks. The Nature Conservancy, Arlington, Virginia. http://nature.ly/floodofbenefits

Scale

Measure category

Mitigation

Flood storage systems

giacomo.cazzola — Thu, 08 Sep 2016 10:24:09 +0000

Flood storage systems giacomo.cazzola Thu, 09/08/2016 - 12:24

Flash floods

Natural flood, runoff, catchment management

EXAMPLE: Reconnecting lakes to the Yangtze River (CHN)

Ecosystem based approach

If fluvial systems don't have sufficient room for natural detention of floodwater in the floodplain, the development and management of flood storage within and adjacent to the natural floodplain is recommended and described in more detail in this measure. It addresses aspects like the process of selecting where to locate the flood storage, deciding how much storage is needed, how to measure the storage capacity, selecting appropriate flow control structures, analysing how the works will perform and making sure that the flood storage scheme is safe in extreme floods.

Based on "J C Ackers, J M Bartlett (2009): 10 Flood storage works. 28p. In: UK Environmental Agency (2009): Fluvial Design Guide (Contains public sector information licensed under the Open Government Licence v3.0.)"

There are two main reasons for providing temporary detention of floodwater:

to compensate for the effects of catchment urbanisation;
to reduce flows passed downriver and mitigate downstream flooding.

Although these may be separate drivers for a flood storage scheme, they are in essence identical. The flood storage works are designed to reduce the peak flood flow passed downstream, spreading the overall volume passed downstream over a longer period. Alternative methods of providing this flood protection would be to:

enlarge the river channel;
raise the riverbanks;
construct floodbanks set back from the river;
provide specific protection around flood-prone buildings or groups of buildings.

Providing flood storage is thus one of a portfolio of options for managing and controlling the risk of flooding. In some cases it can provide sufficient flood protection on its own; in other cases it may be chosen in conjunction with other measures.

The advantage of flood storage is that the flood alleviation benefit generally extends further down-stream, whereas the other methods benefit only the local area, and may increase the flood risk downstream.

If the primary driver for flood storage is to compensate for the effects of urbanisation (see Box 10.1), the objective is normally to detain the additional and faster runoff that results from an increase in the impermeable area. This is then released downstream at a slower rate, designed to mimic the natural runoff from the non-urbanised catchment, avoiding any increase in flood depths and frequencies being propagated downstream.

According to Design of flood storage reservoirs (Hall et al, 1993), the provision of flood storage: ‘has much in its favour. The capacity of the reservoir both attenuates the incoming flood peak to a flow that can be accepted within banks by the downstream channel and delays the timing of the flood so that its volume is discharged over a longer time interval’.

Hall et al (1993) go on to point out that, where several flood storage reservoirs are deployed in a river basin, their overall effect has to be considered carefully. For example, a flood storage reservoir on a minor tributary could delay the peak of the tributary flood so that it coincides with the peak flood coming down the main stream, thereby making matters worse. It is normally obvious if there is a risk of this occurring, but the application of appropriate hydrological and flood modelling approaches can demonstrate if this is indeed a problem. Such studies need to cover a range of rainstorm and flood scenarios – including the effect of rainstorms moving across the catchment – to ensure that the provision of flood storage at a site is a robust solution that does not have detrimental effects at other locations downstream.

Adaptability

Flood storage works sometimes lend themselves to adaptability to changed circumstances such as:

increased upstream catchment runoff;
a change in the vulnerability of downstream communities to flooding.

Increases in catchment runoff may be due to progressive urbanisation or a change in climate, while the downstream vulnerability might change with increased urban development or the implementation of a flood alleviation scheme.

Adaptability of the flood storage works could include such features as:

the ability to raise the impounding embankment to increase the flood storage capacity;
adjusting the settings of gates or orifices that control downstream releases;
if applicable, changing the setting of weirs and gates that admit flows to the reservoir.

At the time when flood storage works are proposed and designed, it is vital to consider:

whether it may be desirable to adapt the design of the scheme in the future;
whether any such flexibility should be incorporated into the design.

Types of flood storage

Flood storage works can usually be described as one of the following:

online – in which the water is temporarily stored within the river channel and its floodplain;
offline – in which the water is diverted from the river channel, stored in a separate area (which may be part of the floodplain) and subsequently released back to the river or to another watercourse.

In general, online storage works are normally located in the upper catchment (where the catchment area is modest) while offline storage works are more common on larger rivers with wide floodplains. Some complex flood storage schemes include a combination of online and offline components, de-signed to act in conjunction.

Floodplains that are modified to augment their natural flood storage and attenuation characteristics are often described as washlands – a term that can be used in the context of either online or offline flood storage.

To learn more about different type of flood storage, click here.

Relevant case studies and examples

Literature sources

Hall, M J, Hockin D L and Ellis, J B (1993). Design of flood storage reservoirs, B014. CIRIA and Butterworth-Heinemann.

Scale

Measure category

Mitigation

Exposed element relocation and removal

giacomo.cazzola — Tue, 06 Sep 2016 08:59:51 +0000

Exposed element relocation and removal giacomo.cazzola Tue, 09/06/2016 - 10:59

Flash floods

Removal or relocation

Based on kindly provided information on the Flood Management Tools Series by the Associated Programme on Flood Management

Non-structural measure

Moving a building out of the existing flood hazard area is the safest solution among several retrofit-ting methods; however it is also usually the most expensive method (FEMA, 2009). When a community acquires a flood-prone home from the owner, relocation is often applied, as well as demolition of the building. The relocation is not only limited to buildings, it can also be applied to other exposed coastal infrastructure.

Relocation includes the following process: lifting up a building from its foundation, placing it on a trailer, transporting it to a new safe area, and setting it onto a new foundation. As with the elevation of a building, a relocated building must be structurally sound enough to withstand all the stresses during the relocation process. Similar techniques as used for the elevation of buildings are used for lifting and setting a building structure. The moving process requires trailer wheel sets to be placed beneath steel beams supporting the building. The size and weight of a building affects the relocation process and the necessary equipment. A single- story, wooden framed building with a rectangular shape is easier to be relocated than a multi- story, solid masonry one.

Given that relocation requires a moving route between the old and new sites, this adds additional consideration because of the route restrictions, such as width of roads, load limits on bridges, and clearance of facilities along the route. If a building is too large to fit on any moving route, it may be cut into sections, moved separately, and reassembled at the new site. Taking public roads and changing utility lines requires the necessary permits from local governments or utility companies. The relocated building also needs to meet all zoning ordinances and building codes in the new site.

Because relocation is a costly but effective method to prevent recurrence of flood damage, it is often used for preserving historical buildings and monuments. The City of Grand Forks, North Dakota, USA was severely hit by the Red River flood in April 1997 (FEMA, 2001). The Boomtown Building, one of the city’s oldest structures and a property of the National Register of Historic Places, was also a casualty of the flood. In order to make way for a new dike, the building had to move to another location with the financial support of the city.

Another famous example of relocation of an historical monument is shown by the Abu Simbel tem-ples in Egypt. Following the rise of the Nile waters as a result of the construction of the Aswan High Dam, a multinational team of archaeologists, engineers and skilled heavy equipment operators working together under the UNESCO banner, began in 1964 the salvage of the Abu Simbel temples. Between 1964 and 1968, the entire site was carefully cut into large blocks (up to 30 tons, averaging 20 tons), dismantled, lifted and reassembled in a new location 65 meters higher and 200 meters back from the river, for a total cost of some USD 40 million at the time (De Carvalho, 1966).

Key lessons learnt

Success factors:

In areas with low population densities, the costs of retreat (including compensation and infrastructure costs) could be significantly less than other grey or green measures to protect assets where they are.
The retreat of settlements and infrastructure can be combined with the recreation of natural features, such as vegetation buffers, wetlands, dunes, that can provide landscape and biodiversity benefits as well as protection against erosion, debris flows and floods.
Retreat policies are likely to be more successful and receive stronger public support if they are designed in a long-term perspective.

Relevant case studies and examples

EXAMPLE: Managed Retreat at Surfer’s Point, California (USA)

EXAMPLE: Relocation in Criel sur Mer, Normandy (FR)

EXAMPLE: Relocation of Clavell Tower, Dorset (UK)

Further Readings

ClimateAdapt on relocation

UN Migration Network: Changing climate, moving people: Framing migration, displ…

Literature sources

De Carvalho, G., 1966: The sun rises as Pharaoh Planned, LIFE Magazine, Vol. 61, No. 23, 2 December 1966.

FEMA (Federal Emergency Management Agency), 2001a: Journeys - North Dakota’s Trail Towards Disaster Resistance. www.fema.gov/about/regions/regionviii/journeys.shtm

FEMA (Federal Emergency Management Agency), 2009: Homeowner’s Guide to Retrofitting - Six Ways to Protect Your Home From Flooding. FEMA P.312, Second Edition. www.fema.gov/library/viewRecord.do?id=1420

Scale

Individual - private

Measure category

Coastal and river setbacks

giacomo.cazzola — Wed, 31 Aug 2016 12:38:11 +0000

Coastal and river setbacks giacomo.cazzola Wed, 08/31/2016 - 14:38

Flash floods

Avoidance

EXAMPLE: Coastal setbacks on the island of Kauai (USA)

Non-structural measure

Coastal setbacks are an demarcated area where all or certain types of development are prohibited. Coastal setbacks can be measured either as a minimum distance from the shoreline for new buildings or infrastructure facilities, or may state a minimum elevation above sea level for development. Setbacks determined by distance from the shore are used to combat coastal erosion, while setbacks determined by evaluation are used to control flooding.

Based on kindly provided information by the ClimateTechWiki and the TNA Guidebook on 'Technologies for Climate Change Adaptation' by Matthew M. Linham & Robert J. Nicholls

A set back can be determined at a fixed distance or it can be ‘floating’ and adapt t the dynamic of an area’s topography and shoreline movement. Setbacks can also cover parts of a shoreline or area, or an entire administrative zone. It is important that setbacks are strategically placed in relation to historic erosion or water level rates, rather than by arbitrary placement.

Advantages

Setbacks are considered low cost and less intrusive than solid barriers such as sea walls and dikes. Setbacks maintain natural vegetation and shorelines and allow for the natural dynamics and rhythms of the shoreline to exist and are considered an environmentally sustainable measure for this reason (NOAA 2010). Setbacks can also have spillover benefits by making the shoreline accessible and providing open public recreational space.

Disadvantages of the technology

One of the disadvantages of setbacks is that they are vulnerable to a changing sea line and specifically sea level rise. For this reason, setbacks must be re-evaluated over time to ensure that the buffer zone provides continued protection in light of a changing environment.

Another important point, is that setbacks are not necessarily capable of protecting against strong storm surges and associated flooding and therefore a certain level of risk remains when implementing them as a protection measure.

The nature of setback zones is also subject to controversy because it implies that an area cannot be built on and development of the coast in certain areas cannot take place. This can result in conflicts between users or in the case of a reassessment can even mean that existing structures are newly within the no-build zone. Often structures are allowed to remain, or are compensated for having to move.

Financial requirements and costs

Again, the costs of implementing a coastal setback approach will be variable, depending on local conditions. A number of costs will be incurred when implementing setback in any situation. They are discussed below.

Firstly, a decision must be taken as to how far to set back. Costs involved in taking this decision include the collection and analysis of historic erosion rates or water levels, the cost of modelling likely shoreline evolution, and the associated cost of buying in modelling services and expert consultation. The cost at this stage will vary depending on the method used to determine setback distance. Less technical solutions are likely to be cheaper.

Secondly, the setback policy must be communicated to relevant bodies in order that the policy is taken into account in the planning process. Costs involved at this stage may also involve the additional costs of incorporating coastal setback into local planning policies.

Finally, enforcement is essential. The cost of enforcement may however be low as it is possible to enforce setback via pre-existing local planning bodies.

Additional costs may be incurred if private landowners are required to be compensated for loss of development potential and also when the setback distance undergoes periodic review.

Implementation of a setback policy is likely to have the lowest costs when implemented proactively, before significant, inappropriate development occurs. In this way it should be possible to minimise compensatory payments to private landowners.

Barriers to implementation

One of the challenges associated with implementing setbacks is public opposition. Land ownership and coastal development are often contentious issues, particularly if setbacks require restrictions that affect individual landowners. In such cases, landowners can be compensated for costs of lost development, however, this also increases the costs incurred from implementing a setback.

Many coastal areas also experience pressure to develop the coast for touristic purposes. In many instances, coastal regulation that has been put in place to protect the coastline is overridden in the face of development (Sanò et al., 2010).

Opportunities for implementation

Implementing setbacks is an issue closely related to land use and building regulations and is therefore dealt with by the same policies that regulate building standards. Environmental policy, therefore, has an opportunity to be streamlined into these policy areas.

Setbacks are complimentary measures, meaning that they benefit when implemented in combination with other measures such as sand dune reconstruction or wetland restoration. Implementing a setback in unison with dune reconstruction or wetland restoration helps to ensure that these areas are left alone to properly develop and also improve the capacity to act as a buffer to coastal flooding and erosion.

Relevant case studies and examples

Literature sources

Main source: Matthew M. Linham & Robert J. Nicholls (2010): TNA Guidebook on 'Technologies for Climate Change Adaptation' UNEP , 166p.

NOAA (National Ocean and Atmospheric Administration) (2010) Construction Setbacks. Charleston, SC: NOAA.

Sanò, M., Marchand, M. and Medina, R. (2010) Coastal setbacks for the Mediterranean: a challenge for ICZM. Journal of Coastal Conservation, 14, 33-39.

Scale

Measure category