There are two simple ways to improve drain water quality and reduce acid water flowing into rivers. You can open floodgates and let river water into the drain, or you can retain very acid water to prevent it from flowing into the river. This leaflet describes the main designs for floodgate opening devices and retention structures, and outlines their advantages and disadvantages.
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Various designs exist. They usually consist of an aperture within the existing floodgate with another smaller floodgate attached. A floating arm opens the gate and allows water exchange with each tide. The gate opens on the low tide and closes with the rising tide.
Water level control: Control is very good as the float arm can be adjusted to stop inflow at the desired water level.
Advantages: Automatic operation of the gate by the tide. Excellent water level control. The amount of exchange and maximum height of tidal influence on the inside water level can be adjusted. This design is flood secure and automatically closes as outside water level rises.
Disadvantages: There is a minor risk of being jammed open (as with normal gates). May require a new gate to be made in some cases.
Sluice gates consist of an aperture within existing floodgate with a sliding plate cover that can be opened to varying degrees. This opening can be vertical, horizontal or rotational in design.
Water level control: The aperture size can be adjusted to vary the amount of inflow to suit site conditions so water level control is good. The position of the aperture in the floodgate can also be varied and will affect water level control.
Advantages: The variable aperture size means sluice gates provide excellent water level control. The simple design is low cost and low maintenance.
Disadvantages: Requires manual operation and manual closure in floods.
Various designs exist including, a) winch and cable mechanism which allows the existing floodgate to be lifted open either horizontally or vertically and b) a worm drive mechanism that opens the gates vertically.
Water level control: Depends on the design. Horizontal winch gates have limited water level control and are either fully open or closed. Vertical lift gates have good water level control and can be set in any position, from fully closed to fully open.
Advantages: Winch gates can allow large, rapid inflows of river water and can be fully raised to assist outflow after flooding.
Disadvantages: These gates require intensive manual operation and manual closure in the event of flooding. Horizontal winch gates have a greater risk of causing overtopping when open. Large forces can be involved in winch and cable system. Vertical winch gates can experience closing difficulties due to friction.
Weirs and fixed sills consist of a partial block in the drain. A variety of designs exist and many materials can be used (i.e. sandbags, rock/fill, concrete, steel).
Advantages: Weirs and sills provide an ability to retain a guaranteed minimum water level. There is potential to vary the depth of water retention depending on the design (i.e. to retain only groundwater or to retain shallow surface water). These structures are generally low maintenance and low cost, depending on materials.
Disadvantages: It may be difficult to vary the minimum water level once installed, depending upon the design. They can be difficult to remove from drain, again depending on design and materials.
These are a sluice gate or vertical lift gate placed on the landward side of a culvert.
Advantages: Penstocks provide a good seal, good water level control and rarely fail. They are low maintenance if made from stainless steel.
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Disadvantages: They can be expensive. They are manually operated and can jam open during outflow periods if they are the vertical winch type design. A screw thread design prevents this.
This simple design consists of boards placed in slots in front of any culvert on the landward side.
Advantages: Water level depth can be adjusted to a desired level. Low cost.
Disadvantages: They are manually operated and the boards can be difficult to remove under a significant head of water. They are suitable only for low volume drains. They often have some leakage between the boards.
The type of floodgate opening device or retention structure you choose depends on your drainage system and management objectives. All stakeholders in the system need to be consulted before doing any works. Remember that retention structures will impede fish moving upstream, so these should only be used where there is significant benefit - such as reducing acid water flowing into the river.
It is important to match any works with the characteristics of the drainage system, so it is essential to conduct a thorough site assessment. This will help to determine what style of management the drainage system is best suited to. Various state and local government organisations have expertise in this kind of assessment and can be contacted for assistance.
For further information, advice and assistance on floodgate and drain management contact NSW DPI.
Citation: Johnston S,Floodgate and drain management on coastal floodplains leaflet 6: Floodgates and retention structures - choosing the right design. (NSW Agriculture: Wollongbar)
Due to the complexity of the engineering solutions, the significant costs of construction and maintenance, and the possible expected environmental impacts, proposals for storm surge barriers require wide and prolonged stakeholder and public participation. Moreover, these structures generally require an Environmental Impact Assessment procedure that, according to the EU EIA Directive, must ensure the right to access information and to participate in the environmental decision making. Similarly, the EU Floods Directive, and the EU Water Framework Directive establish public participation processes that may refer to these projects as well.
The construction phase requires considerable consultancy with engineers, local communities, NGOs, local authorities and representatives of policy sectors that can be affected by the measure (e.g. fisheries, maritime transport, tourism, etc.). A strong political support and wide public consensus, together with a long-term vision, is needed to ensure success in the implementation of such complex measures.
Storm surge gates and flood barriers provide a high degree of protection of low lying coastal areas by providing a physical barrier against flooding. In particular, they are used to protect highly vulnerable and precious coastal urban and infrastructure areas. Existing gates and barriers (Netherlands, UK, Venice, St. Petersburg) have provided effectiveness against storm surges. The use of mobile barriers, instead of fixed structures, allows waterways to remain open during normal conditions. They allow to limit the (environmental, social, economic) impacts related to a permanent closure. Success examples of mobile barriers in the world are shared through I-Storms, the international network for storm surge barriers. It aims at facilitating knowledge exchange and collaboration of experiences of barrier planners and operators facing similar challenges.
One key limiting factor of the storm surge gates is the high capital and maintenance costs since significant investment is required to build these structures and to continually maintain them. The environmental impact of such measures is another key issue to be considered. The construction of mobile barriers can cause large modifications of natural environments and related environmental impacts must be properly assessed and minimised in the design phase. If too frequently operated, mobile gates and flood barriers can limit water exchange in estuarine and lagoon habitats.
Another important issue is the extent to which these barriers will remain viable in the face of future climate change and sea-level rise. In the case of London, the Thames Barrier is expected to continue to protect the city to its current standard up until . The Thames Estuary Plan was designed to be adaptable to different rates of sea level rise and changes affecting the estuary. The plan identifies different options for improving or replacing the Thames Barrier. Full review and update of the Plan is scheduled every 10 years.
Other limiting factors are related to the capacity of the forecasting systems to early predict in a reliable way the flooding event, thus allowing to activate the procedures of gates closing on time. The time needed to close the barriers can vary according to both specific technical aspects and to complex management issues of the whole area. It can imply the interruption of navigation, port services and other activities. Continuous investment in research and technological innovation is essential to improve the reliability and precision of forecasting systems and their use under operational conditions.
Finally, the technical failure of the system (e.g. a barrier not closing properly) can be perceived as a large risk by the public. Acceptance of the work by the public and stakeholders can be fostered by an overall transparency in the decision-making process. , Proper stakeholder engagement, public consultation and informative workshops are proven means for transparent process settings.
Storm surge gates and flood barriers provide a high degree of protection for urban settlements and infrastructure against seaward storm surges and related flooding. Compared to fixed gates, this type of infrastructure provides a more flexible solution. It allows waterways to be open in normal conditions for the natural water exchange and the movement of aquatic species as well as for human activities such as shipping and fisheries.
Large capital and maintenance costs are required to design, build, and maintain storm surge gates and flood barriers. Investment in monitoring hydrological parameters, flood forecasting and warning systems must also be ensured, to improve robustness and precision of information needed for the activation of the system in a prompt manner.
The construction of the Thames Barrier cost GBP 535 million in (about GBP 1.7 billion or EUR 2.5 billion in ), according to the UK Environment Agency. Operational costs are about GBP 8 million a year (about EUR 9.5 million in prices). Building the Mose system (including four mobile barriers at the Venice lagoon inlets) cost EUR 5.49 billion, according to official estimates. The estimate also includes two additional activities, i.e; the requalification of the facilities of the Venice Arsenal for the maintenance and operation of the MOSE system, and the requalification works needed to improve the integration of the mobile barriers within the lagoon environment.
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