Estuarine floods

Groynes are cross-shore structures designed to reduce longshore transport on open beaches or to deflect nearshore currents within an estuary. On an open beach they are normally built as a series to influence a long section of shoreline that has been nourished or is managed by recycling. In an estuary they may be single structures.

A breakwater is a coastal structure (usually a rock and rubble mound structure) projecting into the sea that shelters vessels from waves and currents, prevents siltation of a navigation channel, protects a shore area or prevents thermal mixing (e.g. cooling water intakes). A breakwater typically comprises various stone layers and is typically armoured with large armour stone or concrete armour units (an exception are e.g. vertical (caisson) breakwaters). A breakwater can be built at the shoreline or offshore (detached or reef breakwater). This measure is not directly addressed to protect the coast in flood events, but can indirectly stabilize the coast by preventing erosion.

The county of Sogn og Fjordane frequently experiences avalanches and landslides, storm surges and flooding. A demonstration project explored the potential for an effective, reliable and cost-efficient early warning system that has a multi-hazard approach and makes use of location and population-based communication technologies, such as mobile phones, as well as social media such as Facebook and Twitter. The system was tested with a sample warning followed by a survey and data analysis to judge its efficacy.

In 1996 the Dutch Department of Transport, Public Works and Water developed a program called ‘Roads to the Future,’ and a component of this project was the testing of a pilot floating road. The testing of the pilot took place in 2003 and aimed to create a 70 meter stretch of road in the town of Hedel, the Netherlands to mitigate against rising ground water levels. The floating road was designed to maintain access and flexibility in traffic and movement and prevent the isolation of a village otherwise cut off by flooding.

Located on England’s North Norfolk coast, the Titchwell Marsh is a key piece of the North Norfolk Coast Special Protection Area (SPA) and Special Area of Conservation (SAC). This coastal wetland ecosystem includes freshwater and brackish habitats and is currently protected from the erosive power of waves by seawalls which are becoming increasingly weakened.

The Titchwell Marsh Coastal Change Project aims to protect vital freshwater habitats from both coastal erosion and sea level rise through managed realignment and seawall reinforcement, and mitigate and compensate for the loss of important brackish habitats.

Coastal and marine environments are usually characterized by beautiful landscapes and rich ecosystems of great importance, offering elements such as rich biodiversity. They also attract human activities such as tourism and industrial uses. However, the co-existence of human activities and natural resources often creates conflicts of use in the coastal zone.

Management policies are an important means of implementing planning in order to minimise, prevent or resolve use conflicts. The development of a coastal and marine spatial planning system presents an opportunity for the implementation of an overall strategy of conservation, sustainability and management to maximise future economic profit.

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.

Flood forecasting is an essential tool for providing people still exposed to risk with advance notice of flooding, in an effort to save life and property.

To minimize the loss of lives and reduce other flood impacts, an area should be evacuated when the depth of standing water due to flooding is already or is expected to become high. Such floods are defined as those which are expected to cause buildings, including residential houses, to be washed away or seriously damaged by the flooding.

The purpose of early warning systems (EWS) is simple. They exist to give advance notice of an impending flood, allowing emergency plans to be put into action. EWS, when used appropriately, can save lives and reduce other adverse impacts.

It is vital to recognize that even after the implementation of non-structural flood mitigation measures residual flood risk will remain. It is of paramount importance to make plans to deal with flood events and their aftermath. This involves multiple activities which can be included as part of a flood emergency plan. In this section there is an overview of the elements central to emergency planning.

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

Flood risk awareness is the cornerstone of non-structural flood risk management. All actions to minimize the impact of flooding hinge upon stakeholders becoming aware these are both necessary and desirable. Ignorance of flood risk encourages occupation of the floodplain, in the first instance, and can allow appropriate building design practices to fall into disuse. In the event of a flood, the lack of awareness of risk can result in a failure to heed warnings to evacuate, thereby endangering lives.

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.

Saltmarsh and mudflats are usually located together with mudflats in front of the saltmarsh. Saltmarsh vegetation and saltmarsh creeks help manage floods by dissipating wave and tidal energy.  They are valuable barriers to the risks of flood, as they dissapte wave and tidal energy. Saltmarshes used in combination with other measures can have beneficial outcomes to managing climate change impacts. Even a small width of fronting saltmarsh can significantly reduce the height of sea walls required to achieve the same level of protection and thus also reduce initial construction costs. Having saltmarsh fronting will also significantly reduce maintenance costs due to the reduced exposure to wave and tidal energy.

A breakwater is a coastal structure (usually a rock and rubble mound structure) projecting into the sea that shelters vessels from waves and currents, prevents siltation of a navigation channel, protects a shore area or prevents thermal mixing (e.g. cooling water intakes). A breakwater typically comprises various stone layers and is typically armoured with large armour stone or concrete armour units (an exception are e.g. vertical (caisson) breakwaters). A breakwater can be built at the shoreline or offshore (detached or reef breakwater). This measure is not directly addressed to protect the coast in flood events, but can indirectly stabilize the coast by preventing erosion.

A sea dike is a manmade structure designed to protect low-lying areas from flooding from the sea or ocean. They typically are designed with several components including a sand core, a watertight outer protective layer, toe protection and a drainage channel. Sea dikes are intended to withstand and resist water and wave action. They are widely used in countries with low lying geographies such as Vietnam, Bangladesh, Thailand, the Netherlands and parts of the United States.

A temporary flood barrier is one that is only installed when the need arises (that is, when high flood levels are forecast). A demountable flood defence is a particular form of temporary defence that requires built-in parts and therefore can only be deployed in one specific location. The removable stoplog defence is a particular form of demountable defence applicable only for small openings in a permanent defence. The more commonly adopted gate option for closing off a gap in a floodwall is neither temporary nor demountable, as it is part of the permanent defence and is left in place all the time (albeit normally in an open position).

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

Dredging is the general term used for the excavation of material below water level either as a maintenance activity or as part of channel enlargement works. The main purpose of dredging is either to maintain the navigation depth or the flood capacity, or sometimes both.

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.

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

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

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.

'Elevation of buildings' and ' Land raising' are two separated measures with the aim to elevate exposed elements.

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.

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.

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.

Wetproofing (or wet floodproofing) is different from dryproofing in that it allows flood water to enter a structure, though both floodproofing methods have the same purpose, that of preventing damage to the structure and its contents and creating no additional threats to public safety (FEMA, 1993).

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

Artificial reefs are shore parallel rock mound structures set part way down the beach face. They may be long single structures or form a series of reefs extending for some distance alongshore. They are submerged for at least part of the tidal cycle, and are therefore less intrusive on the coastal landscape, have less impact on upper beach longshore processes and add a new intertidal habitat to sandy foreshores.

Groynes are cross-shore structures designed to reduce longshore transport on open beaches or to deflect nearshore currents within an estuary. On an open beach they are normally built as a series to influence a long section of shoreline that has been nourished or is managed by recycling. In an estuary they may be single structures.