Reduction

Water flow regulation

Flood and storm surge barrier

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

Based on information from the project website

General description

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

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

Governance aspects

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

Innovative aspects

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

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

Key lessons learnt

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

Relevant case studies and examples

Further Readings

CityLab: Venice's Vast New Flood Barrier Is Almost Here

Measure category

EXAMPLE: Seawall at Skara Brae, Scotland (UK)

nst — Tue, 24 Jan 2017 08:37:41 +0000

EXAMPLE: Seawall at Skara Brae, Scotland (UK) nst Tue, 01/24/2017 - 09:37

Skara Brae is one of Scotland’s most significant and famous UNESCO World Heritage Sites and it has been under constant threat of damage due to coastal erosion for decades. Fortunately, a seawall protects the base of this archaeological site from the erosive power of waves and storm events.

Based on information from Historic Environment Scotland and UNESCO World Heritage Centre.

General description

The 5000 year old settlement of Skara Brae is one of the four monuments that make up the Heart of Neolithic Orkney, a UNESCO World Heritage Site. It is also quite possibly one of Scotland’s most at-risk historic sites due to coastal erosion. Ironically, Skara Brae was only discovered as a result of coastal erosion from major storm events since the 1800’s.

The risk of coastal erosion to Skara Brae was addressed in the early days of its heritage management. The first seawall was constructed in the late 1920’s, and has been refortified several times since then. Like all seawalls, this 4-meter high wall serves as a protective barrier that is able to absorb the brunt of wave action and thereby shield vulnerable infrastructure, or in this case archaeological structures, from eroding.

Historic Environment Scotland, the public body responsible for managing Scotland’s heritage sites, has been over the years maintaining the integrity of the wall, with the support of other organizations. There has also been extensive monitoring of the entire bay area to determine the rate and location of erosion so that additional fortification can be made. Skara Brae is a cherished piece of Scotland’s history and therefore has much public support for protection from coastal erosion.

Innovative aspects

Considering the early adoption of a seawall protection measure for Skara Brae, Scottish authorities have been quite proactive in ensuring the long term reduction of coastal erosion at this heritage site. With the first wall erected in the late 1920’s, this measure was a relatively pioneering tactic for heritage conservation. Today, Historic Environment Scotland has also employed the latest technologies of 3D and 4D digital surveying to monitor the state of erosion along the coast and measure the ongoing effectiveness of the sea wall.

Key lessons learnt

When it comes to vulnerable heritages sites along eroding coastlines, time is of the essence. While powerful storms throughout history revealed the existence of Skara Brae to the world, these same storm events and constant wave action threaten the longevity of the site as a place for future generations to enjoy. The prompt action of Scottish authorities to construct a seawall to protect the archaeological site decades ago has since been proven to have been a wise decision. Knowing what we know about the possibility of more hidden archaeological sites in the area, it is important to continue the monitoring efforts to not only assess the stability of Skara Brae, but also the impacts of the seawall as an element of the natural environment. The Bay of Skaill is a dynamic and ever changing system, and it is possible that the seawall might increase erosion from intensified wave action on the unprotected sand dunes on either side of Skara Brea.

Relevant case studies and examples

Seawall or Revetment

Further Readings

BBC report on Skara Brae

Measure category

EXAMPLE: Dune rehabilitation in Praia de Faro (PT)

nst — Mon, 12 Dec 2016 11:00:36 +0000

EXAMPLE: Dune rehabilitation in Praia de Faro (PT) nst Mon, 12/12/2016 - 12:00

Dune strengthening, rehabilitation and restoration

Ecosystem based approach

A construction of an elevated wooden pathway alongshore and cross-shore of about 1500 m, and the construction of a dune fences were implemented in the coastal town of Praia de Faro (Portugal). The fences helped to trap sand in the dune areas leading to a growth of the dune system. The wooden path played also an important role in the dune recovery.

The Ria Formosa coastal lagoon is a RISC-KIT Case Study site and is located on the southern coast of Portugal. The coastal town of Praia de Faro is located partly on a natural dune area separating a lagoon from the open sea. Due to tourism and housing activities the dunes were in bad conditions, so two main actions were undertaken in 2000/2001: construction of an elevated wooden pathway alongshore and cross-shore of about 1500 m, and the construction of a dune fences. A row of fences was placed along the ocean side for about 1 km and additionally fences were built as a a reticulate for 670m (continuous rectangles or about 7x5.5 m). The fences helped to trap sand in the dune areas leading to a growth of the dune system. Over the past 15 years the dunes grew around 10 meters in width and about 1.3 meters in height, leading to an almost natural ecological state. The wooden path played an important role in the dune recovery, because it led to reduction of wild paths through the dunes. Wild path through the dunes caused a destabilization of the dune system. With the pathway, tourists, fishers, and inhabitants use the path to reach the shore or other areas. The fences were also an additional obstacle which minimized the usage of wild paths through the dunes.

Over the time span of 15 years, the measure showed an improvement of the dune systems where the pathway and the fences were built. Before the measure was implemented there were overwashes in the area, but these did not occur in the last years. With no extreme storm events or human intervention, the dunes should stay rehabilitated for the next decades.

Political & social feasibility:

The constructed wooden path was well received by inhabitants and tourists. For recreational purposes the measure had a positive impact, because it is now easier and faster to get to different spots ate the oceanic shore and the back barrier beach, where the area is used for bathing, surfing, or kite-surfing. To get to these areas, the pathway is used.

Cost of implementation & maintenance

The initial costs of the measures were around 1250000€. 4/5 of the costs were paid with EU funds, 1/5 was paid with national funding. There is no money assigned for maintenance costs. If maintenance is needed, is overtaken by local fisherman or house owner (basically by replacing broken parts of the wooden path or the fence). For a more detailed cost effectiveness analysis see here.

Ecological feasibility

With the measure the ecological status of the dune system is near to natural values with natural (autochthon) species dominating the ecosystem. Allowing the natural evolution of the dune ecosystem, this approach can be classified as ecological feasible. Problems could arise if the new system attract a lot more tourists, that could put additional pressure on the ecosystem (e.g. too many bird watchers, too many visitors at the beach, etc.).

Key lessons learnt

The rehabilitation of dunes with the help of two strategies worked very well in this case study area. The damages of dunes due to wild paths are limited by the guidance of visitors and inhabitants along the wooden paths. This alleviated impairments of the dunes. Additionally the set-up of fences helped to trap sand and accelerate the process of dune rehabilitation.

Relevant case studies and examples

Further Readings

RISC-KIT Case Study

Cost-Effectiveness analysis of a wooden path over the dune (Praia de Faro)

Measure category

EXAMPLE: Dynamic dune management, Terschelling (NL)

nst — Thu, 03 Nov 2016 10:14:51 +0000

EXAMPLE: Dynamic dune management, Terschelling (NL) nst Thu, 11/03/2016 - 11:14

Dune strengthening, rehabilitation and restoration

Ecosystem based approach

This case study is about enhancing the natural dynamics of the dunes and the sand transport on the Wadden Sea island of Terschelling, Netherland

Based on the WWF study "Klimaanpassung an weichen Küsten - Fallbeispiele aus Europa und den USA für das schleswig-holsteinische Wattenmeer"

Terschelling is one of the West Frisian Wadden Sea islands and feature one of the largest dune areas in the Netherlands. The dunes have been dynamically evolved by deflation, wandering dunes and periodic flooding leading to valuable ecosystems with the characteristic of rejuvenating dunes. These natural processes have been disrupted in the past by planting or measures with the aim of fixing the coastline. With the introduction of drainage systems at Terschelling the groundwater level has been lowered and wetlands in the dune system are declining. Simultaneously, by limiting the sediment transport from the beach to the dunes and the plant covering of the dunes, the dunes are aging. Overall, natural dynamics at the dunes were hindered and because of the limiting of the sediment transport, the adaptive capacity to sea level rise (and resulting higher floods) are also limited.

Therefore, a solution was sought to enable a more natural and dynamic development of the dune system at Terschelling.

Technical feasibility

In 1994, on a 5 km stretch of the northern coast line of Terschelling, the outer dunes were mechanically opened to start a rejuvenation process of the dune system. With the help of reed fences along these cuts, wind regimes were channeled so over the years a considerable amount of sand was blown from the beach through the dunes into the inland.

Additionally an artificial wetland was created (called ‘Eldorado’) by the removal of pine forest and other vegetation. This has rejuvenated the dune and to keep the vegetation limited, grazing is undertaken. This additional project was carried out by the dutch public organization Staatsbosbeheer (http://www.staatsbosbeheer.nl/english) as part of a LIFE project called ‘Saving the Dutch dunes’(LIFE05 NAT/NL/000124). It is important to note that the approach of the dynamic dune management has no negative impact on the safety of the people of Terschelling during storm floods. The natural sand transport from the beach has the effect that the dune system is ‘growing’ with the sea level rise.

Political & social feasibility

Initially, there was skepticism among the inhabitants of the island towards this measure. Especially lowering the outer dunes so that the ocean was visible from the inner dunes worried the inhabitants. It was also criticized that grassland that was turned to arable land in former times, should again be covered by sand. These issues could partly be explained by insufficient communication between all stakeholders.

For tourism, a main economic sector of Terschelling, this a beneficial approach, since 90% if visitors stated in a local survey being fond of the ‘living dunes’.

Ecological feasibility

Without further intervention new dunes have formed through sand drift. Especially for Natura 2000 habitats such as primary and white dunes this can be seen as very positive. Goats will help to keep the dune areas open, so the valuable habitats will be preserved.

Key lessons learnt

Instead of stabilizing the dunes consistently by planting vegetation and other measures, aim was to allow dynamic coastal processes with a stable safety standard. An important part of this new management approach was the improvement of the natural sand transport from the beach to the dunes.

The example of Terschelling showed that a more natural sand transport from the beach over the dunes to the hinterland was actually possible. Habitats such as primary and White dune re-emerged or developed closer to nature, additional sand deposits accumulated in the dunes and serve as long-term adaptation to sea level rise. However, this more local effect will not solve the ‘sand-deficit’ on outer coasts, therefore additional sand deposits (for example beach nourishment) may be required. Overall, this example shows that a restoration of natural processes can contribute to a long term security of the island and their inhabitants.

Relevant case studies and examples

Further Readings

WWF Study on Climate adaptation (in German)

Video link about the LIFE project

Literature sources

De Jong, B.; Keijsers, J. G. S.; Riksen, M.; Krol, J.; Slim, P. A. (2014): Soft Engineering vs. a Dynamic Approach in Coastal Dune Management: A Case Study on the North Sea Barrier Island of Ameland, The Netherlands. In: Journal of Coastal Research, Vol. 30, No. 4, S. 670–684.

Ecomare (o. J. (b)): Wanderdünen auf Terschelling. Webseite. http://www.ecomare.nl/de/lexicon/mensch-und-umwelt/naturschutz/naturentwicklung/wanderduenen-auf-terschelling/ (02.09.2014).

Löffler, M.; van der Spek, A.J.F.; van Gelder-Maas, C. (2013): Options for dynamic coastal management. A guide for managers. Deltares. dtvirt35.deltares.nl/products/30539 (01.09.2014).

Petersen, J.; Janssen, G.; Lammerts, E. J.; Menn, I.; Mulder, S. (2005): Chapter 9: Beaches and Dunes. In: Essink, K.; Dettmann, C.; Farke, H.; Laursen, K.: Lüerßen, G.; Marencic, H.; Wiersinga, W. (Hrsg.). Wadden Sea Quality Status Report 2004. Wadden Sea Ecosystem No. 19. Trilateral Monitoring and Assessment Group, Common Wadden Sea Secretariat, Wilhelmshaven, Germany.

Salman, A.; van der Neut, R. (o. J.): Kustgids. Region: Terschelling, Nature and Landscape. Webseite. http://www.kustgids.nl/terschelling-en/index.html (02.09.2014).

Staatsbosbeheer (2012): Life Dunes. Report on six years of dune restoration in the Netherlands. Staatsbosbeheer Regio (http://www.staatsbosbeheer.nl/duinherstel)

Nord, Groningen. http://www.staatsbosbeheer.nl/English/LIFE%20Nature/LIFE%20Dune%20restoration.aspx (21.01.2015).

Measure category

Cliff stabilization

giacomo.cazzola — Tue, 13 Sep 2016 10:20:44 +0000

Cliff stabilization giacomo.cazzola Tue, 09/13/2016 - 12:20

Combined approach (grey + green)

Cliff stabilization is a coastal management erosion control technique. Generally speaking, the cliffs are stabilised through anchoring (the use of terracing, planting, wiring or concrete supports to hold cliffs in place), smothing the slope, or dewatering (drainage of excess rainwater to reduce water-logging).

Based on Mangor, Karsten (2013): Cliff stabilisation. Available from http://www.coastalwiki.org/wiki/Cliff_stabilisation [accessed on 23-01-2017]

Background

Coastal cliffs can be unstable due to the combined effect of several factors, such as:

Erosion of the foot of the cliff caused by wave action and storm surge
Sliding or weathering of the slope due to geo-technical instability. The erosion of the foot of the cliff normally initiates geotechnical instability, but the sliding/collapse can be of different nature depending on the geo-technical conditions of the slope. There are basically three different situations:

If the material is non-cohesive material, the weathering of the cliff ill normally occur simultaneously with the erosion of the foot as a talus formation, which is the collection of fallen material forming a slope at the foot of the cliff.
If the material is a mixture of clay, silt, sand and boulders, such as in the case of moraine till, the slope can be very steep for a period due to the cohesive forces, but the slope will eventually collapse. Smaller or bigger fractions of the cliff will fall in connection with groundwater pressure, frost impact or general weathering, or by sliding. Sliding will especially occur in connection with groundwater pressure.
If the material consists of plastic clay or silty clay, the collapse of the cliff will be in the form of slides, which can go far behind the top of the cliff.

Weathering of the cliff by wind transport of sand. This will be most pronounced if the cliff material is sand; however, also exposed cliffs consisting of other types of material can be eroded by sand blown over the cliff from the beach.

Method

The basic cause of cliff instability is normally the marine erosion of the foot of the cliff, mitigation of this is covered under the protection method: Revetment. Installing the revetment will exclude further erosion of the foot, but at that stage the slope of the cliff may very well be so steep that weathering and sliding may still occur. This can be counteracted by the following means:

Artificial smoothing of the slope, if there is enough space at the foot as well as at top of the cliff for this. This will counteract future uncontrolled weathering and sliding.
Smoothing of the slope by filling with granular material at the foot of the cliff. This requires that there is sufficient space at the foot of the cliff for the filling.
Establish a vegetation cover on the cliff. This can best be done by following the above-mentioned smoothing of the slope. Good vegetation protects against weathering and groundwater seepage, and thereby to some extent against sliding
Drainage of groundwater. This can be used if the cliff suffers from sliding due to high groundwater pressure and poor drainage conditions. Horizontal and vertical drains can be used as well as the regulation of the surface runoff.

Cliff slopes are often “protected” by dumping assorted rubbish, such as branches etc., over the cliff. It is a bad “solution” because it does not stop the risk of sliding. On the contrary, it spoils the vegetation and thereby increases the risk of sliding.

Functional characteristic

Cliff stabilisation presupposes that the foot of the cliff has been stabilised. Stabilisation counteracts the natural behaviour of cliffs to slide and weather. Such an active cliff is part of the dynamic coastal landscape and should therefore in principle be maintained as an integrated part of this landscape.

Applicability

Cliff stabilisation can be applied at all moderately exposed to exposed coasts; however, in order to preserve the dynamic coastal landscape cliff stabilisation should only be used sparingly. Preserving the active cliff at densely populated coasts is normally not feasible due to the limited space. Consequently, cliff stabilisation is normally only used when there is sufficient space in the backland to allow some smoothing.

Further Readings

Developing a management strategy for coastal cliff erosion hazards in South Aus…

Literature sources

Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.

Measure category

Shingle beach restoration

giacomo.cazzola — Tue, 13 Sep 2016 09:18:09 +0000

Shingle beach restoration giacomo.cazzola Tue, 09/13/2016 - 11:18

Combined approach (grey + green)

Shingle beaches are mobile structures developed in high-energy environments that are very efficient at absorbing and dissipating wave energy. Restoration of shingle beaches on the foreshore can create a more desirable morphological profile that is better able to dissipate wave energy and attenuate storm surge.

Based on kindly provided information by SEPA's Natural Flood Management Handbook (p. 58f.)

Shingle beaches are composed of pebbles or gravel as opposed to consisting only of sand. They can be naturally occurring or manmade and are extremely effective for absorbing and dissipating wave energy and storm surges. Gravel and pebble size varies significantly from 2mm to 200mm. Shingle beaches are typically steep because as the waves flow through and over the coarse and porous surface of the beach, the effect of backwash erosion is reduced creating a steeply sloping beach.

Restoring a shingle ridge

Shingle ridges require upkeep and restoration, particularly after a storm or when the ridge becomes particularly high and narrow. Restoring and repairing a shingle beach is complicated as there are risks relating to ensuring the integrity of vegetation and invertebrates. For example, care should be taken that re-profiling does not reduce the availability of sediment downdrift, potentially increasing flood or erosion risk there.

A wide shingle beach profile will have greater absorption capacity. Historically shingle beaches were made more vertical, putting the shingle ridge in contact with greater wave energy in place a of a naturally expansive shoreline. A steep shingle ridge on the one hand decreases the possibility of overtopping but is at risk of a catastrophic breach in the case of a significant storm.

Restoring vegetated shingle

While composed mostly of rock or pebble, shingle beaches also have a sensitivity to neighboring vegetation. The amount and type of vegetation is largely determined by the stability of the beach and by composition of the finer sediment which can be either sand, silt, clay, or organic matter.

Vegetated shingle habitats are relatively rare as the nutrient bed associated with shingle beaches is often not nutrient rich and are prone to invasive species and weeds. There are however several approaches to restoring vegetated shingle beaches (Forbes et al 2015):

allowing natural regeneration;
using the natural seed bank;
sowing seeds; and
planting container-grown plants.

Costs

The costs of shingle beaches is dependent on several factors. Firstly, if it requires recharge on a short time scale, there will be costs of maintenance. The scale or size of the shingle beach also influences the cost of implementation.

It should also be noted that shingle beaches are effective in storm barriers however they are generally considered disadvantageous for tourism and beach activities.

Download Factsheet

SEPA-NFMH-58-59.pdf

Literature sources

Heather Forbes, Kathryn Ball and Fiona McLay (2015): Natural Flood Management Handbook. Published by Scottish Environment Protection Agency. (https://www.sepa.org.uk/media/163560/sepa-natural-flood-management-handbook1.pdf)

Measure category

Drainage system management

giacomo.cazzola — Tue, 06 Sep 2016 14:39:06 +0000

Drainage system management giacomo.cazzola Tue, 09/06/2016 - 16:39

Urban floods

Surface Water Management

Limited intervention

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.

Urban drainage systems need to be able to deal with both wastewater and stormwater whilst minimizing problems to human life and the environment, including flooding. Urbanization has a significant effect on the impact of drainage flows on the environment: for example, where rain falls on impermeable artificial surfaces and is drained by a system of pipes, it passes much more rapidly to the receiving water body than it would have done when the catchment was in a natural state. This causes a more rapid build-up of flows and higher peaks, increasing the risk of flooding (and pollution) in the receiving water. Many urban drainage systems simply move a local flooding problem to another location and may increase the problem. In many developed counties there is a move away from piped systems, towards more natural systems for draining stormwater.

Where the drainage system of an urban area is piped, by a ‘sewer system’, there are two approaches in use: ‘combined’ or ‘separate’.

The older parts of many cities (New York being an example) are drained using the combined system, whereby wastewater and stormwater are mixed and are carried together. The system takes the combined flow to the point of discharge into the natural water system, commonly via a wastewater treatment plant that discharges treated effluent. During heavy rainfall events, the stormwater flow will greatly dominate the wastewater flow in terms of volume, but it is hardly ever viable to provide sufficient capacity throughout the system for stormwater resulting from heavy rainfall, as the system would operate at a small fraction of its capacity during dry weather. Instead, structures are included in the system to permit overflow to a nearby watercourse. During significant rainfall events a significant volume of the flow is likely to overflow, rather than to continue to the wastewater treatment plant. As the overflowed water is generally a dilute mixture of wastewater and stormwater, these structures are designed hydraulically to prevent larger, visually offensive solids from being discharged to the river. However, the inescapable fact is that combined sewer overflows inevitably cause some pollution (Butler and Davies 2011).

In the urban areas served by a combined system, capacity is similarly exceeded by extreme stormwater flows. Under these circumstances, the ‘surcharging’ of the system may cause flooding of the urban surface and, as the flood water will include wastewater, there are associated pollution and health implications.

In a separate system, wastewater and stormwater are drained by separate pipes, often constructed in parallel. Wastewater is carried to the wastewater treatment plant, whereas stormwater is usually discharged direct to the nearest watercourse. The problem of combined sewer overflows is thereby avoided, but there are still challenges: stormwater discharge is usually untreated, and this may cause pollution. Stormwater may enter the wastewater pipe either through mistaken or unauthorized connections; there may also be infiltration of groundwater at pipe imperfections. Because of the relative proportions of wastewater and stormwater during heavy rainfall, these additional inputs may significantly reduce the capacity of the pipe for the wastewater it was designed to carry.

In urban areas without conventional piped sewer systems, disposal of excreta and wastewater is likely to be localized, though in some cases simplified (shallow and small diameter) pipes are used. Stormwater is most likely to be carried by open drains, typically unlined channels along the side of the street. Better constructed channels may be lined with stone or concrete, and may be integrated into the urban landscape. Open drains are far cheaper to construct than stormwater sewers, and although they can easily become blocked by debris or refuse from the surface, such blockages are more easily monitored and removed than in piped systems.

Maintenance is vital, not only to remove obvious obstructions, but also cleaning out deposited sediment, and then disposing of the material so that it does not go back into the drain. In heavy rain, the capacity of an open urban drainage channel may quickly be exceeded; in a well planned system, overflow should be to a specified ‘major system’ such as a road which can act as a drainage channel.

Where there is no adequate system for disposal of wastewater, there is a high likelihood that open drains will be contaminated by foul sewage. This could come from contributions from areas without sewers, or from discharge from simplified sewerage which does not lead to an adequate treatment facility. Open drains may also be misused for the disposal of domestic solid waste. Where the quality of stormwater carried in open drains is an issue for these reasons, there may be limited opportunities for using semi-natural systems of urban drainage that rely on the storage or infiltration of stormwater because of public health issues.

Literature sources

Butler D. and Davies J.W. 2011 Urban Drainage, 3rd edition. UK: Spon Press.

Scale

Individual - private

Local

Measure category

Mitigation

Seawall or Revetment

giacomo.cazzola — Tue, 06 Sep 2016 14:01:40 +0000

Seawall or Revetment giacomo.cazzola Tue, 09/06/2016 - 16:01

Based on the information available on the EEA ClimateAdapt Platform

A seawall or a revetment is a structure made of concrete, masonry or sheet piles, built parallel to the shore at the transition between the beach and the mainland or dune, to protect the inland area against wave action and prevent coastal erosion. Seawalls are usually massive structures designed to resist storm surges.

The height of a seawall will at least cover the difference between the beach level and the mainland, though commonly seawalls are built higher to protect the land against wave overtopping. Seawalls are also used to stabilize eroding cliffs and protect coastal roads and settlements. The crest of the wall often extends into a stone covered part that may be used for a road, promenade or parking area. A seawall creates a distinct separation between the beach and the mainland. Seawalls are often found in the case of narrow or steep beaches, where a typical breakwater is either too large or not economical.

Stakeholder participation

If an EIA is undertaken, the EU Directive provides for the right to access information and to participate in the environmental decision-making procedures to the public concerned by the project. If a project could create a significant impact on a Natura 2000 sites, the ‘appropriate assessment’ of the infrastructure project may include a public participation process, though this is not mandatory. Similarly, the Floods Directive, the Water Framework Directives and the Maritime Spatial Planning Directive establish public participation processes that may be relevant for these projects.

Several categories of stakeholders can be affected by strengthening seawalls. A seawall can negatively affect the landscape and the use of the beach, or can be used to artificially widen the beach to create recreational space. Tourists and tourism industry as well as other recreational users will therefore be affected. However, the protection against flooding offered by the seawall can benefit settlements and economic activities.

Success factors

A seawall provides a high degree of protection against coastal flooding and erosion. It fixes the boundary between the sea and land which can be beneficial if important infrastructure or buildings are located on the shoreline.

Seawalls have a lower space requirement than other coastal defences such as dikes. Seawalls can be heightened to face sea level rise, which requires simultaneously a widening of the foundation.

The high level of security provided by a seawall can favour the development of the hinterland. The crest of the seawall often extends into a stone covered part providing other functions, e.g. road, promenade or parking places.

Limiting factors

Artificial structures such as seawalls can have adverse effects on the coastal environment. Seawalls often interfere with natural processes such as habitat migration, causing the reduction of intertidal habitats. However, these effects depend very much on the main wave and sediment transport direction and the design of the seawall. The choice of coastal defences will be made according to site-specific conditions and primary and secondary goals (such as wave protection, road stabilization, space conservation and mooring capabilities). Where sufficient space is available and no conflict with other primary or secondary goals exists, green measures (such as beach nourishments and dune restoration) are often preferred.

Seawalls often do not stop erosion in front of the structure but prevent erosion of the dunes and hinterland. Vertical seawalls often reflect wave energy instead of dissipating it, which makes the shoreline more subject to erosion. Many seawalls have therefore been more recently conceived to integrate slopes.

When seawalls are regularly overtopped, or when this occurs in major storms, the water can remove soil or sand behind the wall and weaken it. Overtopping water saturates the soil and increases pressures from the landward side, which can cause structural collapse. Sea rise level and potential overtopping must be taken into account in the construction of the seawall. In general, continued erosion can undermine the foot of the structure and threatened its stability.

Seawalls can have negative impact on the landscape and can reduce the attractiveness of the landscape. However, seawalls have also been used, more or less successfully to artificially widen a beach at steep shores to create recreational space.

Costs and Benefits

Construction costs are high but these structures usually require low maintenance. Construction costs vary according to the shape of seawall structures: the volume of the seawall depends on the required crest level, the foundation level, the wave loading, and acceptable overtopping rates. In the Netherlands, it has been estimated that a seawall would cost 300-500€ per m3 of concrete (Deltares, 2014).

Estimates made by Scottish Natural Heritage in 2000 show that in the UK, costs at the time varied from £200,000 to £500,000 (250,000 – 625,000€)/100m length for seawalls and impermeable revetments. The English Environment Agency (2007), gives an average construction cost for seawalls of US$ 2.65 million (at 2009 price levels; about 1.85€ million) including direct construction costs, direct overheads, costs of associated construction works, minor associated work, temporary works, compensation events and delay costs. No indication on the length of the seawall was provided. These and other sources note that costs for seawalls vary according to the type of construction, dimensions, availability and proximity of construction materials, anticipated rates of future erosion and wave loadings, facilities such as walkways and steps or slipways.

Legal Aspects

The construction of coastal works to mitigate erosion and hard sea defences ‘capable of altering the coast’, such as seawalls, fall into Annex II of the EIA Directive (codified as Directive 2011/92/EU): Member States decide whether projects in Annex II should undergo an EIA procedure, either on a case-by-case basis or in terms of thresholds and criteria. However, this requirement does not affect the maintenance and reconstruction of these works. Any infrastructure project likely to have a significant impact on a Natura 2000 site must be subjected to an ‘appropriate assessment of its implications for the site’ to determine whether the project will adversely affect the integrity of the site.

The Water Framework Directive calls for the good status of Europe’s water bodies, including coastal waters. Coastal defences could alter the hydromorphological characteristics of coastal waters – for example in terms of water flow, sediment composition and movement– and thus to a deterioration of ecological status. Any projects that do so would need to meet criteria set out in Art. 4(7) of the Directive. The EU Floods Directive (2007/60/EC) provides a legal framework for flood actions and defence. The construction and restoration of dikes could be part of measures under flood risk management plan. The 2014 Maritime Spatial Planning Directive requires consideration of the interactions between land and sea, along with maritime activities and adaptation to climate change. Seawalls could affect these land/sea interactions.

Implementation Time

Depending on the complexity of the structure 5 to 20m per day can be realised during construction phase. Preparation before construction might double the implementation time.

Life Time

Typically, 30-50 years life expectancy before major repair.

Revetment

There are several ways how to implement a revetment. In their guide to managing coastal erosion in beach/dune systems, the Scottish Natural Heritage has provided information on different kind of revetments. The following measures are more described in detail on sub-pages of this guide.

Relevant case studies and examples

EXAMPLE: Seawall at Skara Brae, Scotland (UK)

Further Readings

CIRA (2007): The Rock Manual.

SNH: IMPERMEABLE REVETMENTS AND SEAWALLS

UNEP: TNA Handbook - Technologies for Climate Change Adaptation

climatetechwiki on seawalls

Measure category

Riverine or slow rise floods

Dry proofing - sealing and shielding

giacomo.cazzola — Tue, 06 Sep 2016 08:23:16 +0000

Dry proofing - sealing and shielding giacomo.cazzola Tue, 09/06/2016 - 10:23

Flash floods

Estuarine floods

Urban floods

Deal with the effects

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

Dryproofing makes a building watertight and substantially impermeable to floodwaters (FEMA, 1993). Compared to wetproofing, dryproofing requires a more reinforced building structure to withstand floodwater pressures and impact forces caused by debris. Other important factors to be considered in dryproofing are watertight closures for doors and windows, prevention of floodwater seepage through walls, and check valves to prevent reverse flows from sewage.

Before selecting dryproofing as a viable floodplain management tool, numerous factors must be considered, such as flood warning time, purpose of building usages, mode of building entry and exit, flood depths, floodwater velocities, floating debris impact, flood frequency, etc. The flood proof function must work sufficiently for design flood level and additional freeboard is recommended because flood depth estimation includes a certain error and may be influenced by future development in the basin.

Sufficient warning time, which is calculated by the rate of floodwater rise and the existing flood warning system, is necessary for evacuation from a flood prone building, for installation of removable flood shields or gates, and for operation of sump pumps and check valves. If the warning time is limited, for example the structure is located in a flash flood area, flood proofed buildings should not be considered as the necessary operations to make it flood proof will require too much time. FEMA suggests flood velocity of 5 ft/s (1.5 m/s) and flood depth of 3 feet (90 cm) as thresholds for adopting dryproofing. If the flood exceeds these limits, the cost of dryproofing may become too significant and the dryproofing method is therefore not feasible. Any areas susceptible to severe debris flow, such as mountainous regions or areas facing ice flow in winter, are not suitable for flood proofed buildings in a cost.effective manner.

The building structure must be able to resist four types of flood.related forces: (1) hydrostatic flood force that freestanding water exerts on a submerged object; (2) buoyancy force that a building receives from surrounding floodwaters; (3) hydrodynamic force that vertical surfaces receive from moving floodwaters; and (4) debris impact force to withstand the flood.borne debris strikes on the side of building. FEMA provides an estimation formula for each force (Appendix 2). For more detailed standards of dryproofing structure design, FEMA has a comprehensive guidance and case study report “Engineering Principles and Practices for Retrofitting Flood.Prone Residential Structures” (FEMA, 2012).

Flood Shields for Openings

Doors, windows and air vents of buildings are potential flow paths where flood water runs into properties (DCLG, 2007). Raising the threshold of doors as high as possible without disturbing accessibility is a primary prevention measure. Sealed polyvinyl chloride (PVC) framed doors are a more preferable option than wooden doors and the doors should be properly fitted to their frames. Windows are also vulnerable to flood water and preventive measures of fitting and sealing similar to those for doors should be taken. The windows should adequately resist the pressure of flood water and prevent damage that could be caused by debris flows. Regarding ventilation vents, special designs of air vents that prevent water from entering into the premises are available on the market.

Temporary flood protection system

A temporary and removable flood protection system is provided in locations where permanent flood defences would not be suitable because they are not technically, economically or environmentally feasible (DEFRA, 2011). The temporary system includes a pre-installed system that requires operation; the system may be installed in a pre.constructed foundation, or it may also be a system where the whole of it is movable and needs to be installed. DEFRA defines the first two systems as “demountable systems” and the third one as a “temporary system” . These systems are further classified by their different structures, such as earth filled containers, air and water filled tubes, and panel type flood barriers.

In contrast to a permanent flood protection system, a temporary system brings an additional risk of operational failure. Taking this fact into consideration, a permanent system should be given priority if it is feasible and locally acceptable. In the event of a temporary system being adopted, it should be ensured that the movable parts of the system are at a minimum and that the reliability of all the operational processes including mobilization, installation and closure are at a maximum. If the temporary system requires significant preparation time, it is suitable for location at the downstream of a large river basin.

A temporary flood protection system can allow a dual function by ensuring effective flood control performance without obstructing the ordinary use of the building, for example access through a floodwall, or parking lot turning into a flood protection site. A temporary system also adds additional safety to a permanent system, which is often the case in critical disaster situations. There are several factors affecting the risk of operational failure, such as sufficient lead.in time, reliability of flood forecasting and warning, system maintenance, and training of operators. Because the flood warning system usually triggers the operational process of the temporary system, technical and human operational reliability is a pre-requisite for the temporary system. Regular training and emergency exercises together with flood operation manuals increase the reliability of the total system. Different temporary systems need different levels of installation skills and preparation time. Site.specific conditions, such as the location of the stockyard of the system parts, transportation means, and available resources of personnel and equipment, also affect the selection of an appropriate temporary system. Detailed advantages and disadvantages of different temporary systems and commercially available products are explained in “Temporary and Demountable Flood Defences” (DEFRA, 2011).

Further Readings

Climate Tech Wiki on Flood proofing

Literature sources

FEMA (Federal Emergency Management Agency), 1993: Non-Residential Floodproofing - Requirements and Certification for Buildings Located in Special Flood Hazard Areas in accordance with the National Flood Insurance Program. FIA.TB.3. www.fema.gov/library/viewRecord.do?id=1716

FEMA (Federal Emergency Management Agency), 2012: Engineering Principles and Practices for Retrofitting Flood-Prone Residential Structures (Third Edition). https://www.fema.gov/media-library-data/20130726-1506-20490-2593/fema259_complete_rev.pdf

DCLG (Department for Communities and Local Government), 2007: Improving the Flood Performance of New Buildings – Flood Resilient Construction. https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/7730/flood_performance.pdf

DEFRA (Department for Environment, Food and Rural Affairs ), 2011: Temporary and Demountable Flood Protection Guide. https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/290837/scho0711buak-e-e.pdf

Scale

Individual - private

Measure category

Mitigation

Wet proofing - Sealable buildings

giacomo.cazzola — Wed, 31 Aug 2016 12:38:10 +0000

Wet proofing - Sealable buildings giacomo.cazzola Wed, 08/31/2016 - 14:38

Riverine or slow rise floods

Flash floods

Estuarine floods

Urban floods

Deal with the effects