(Also known as a Flood Wall, Flood Levee, Main Levee, Dyke, Dike)
What is a concrete levee: A concrete levee is a wall built along a river with the primary purpose of providing flood protection to adjacent land or human settlement from inundation. They are typically gravity floodwalls constructed of solid concrete and use weight for stability or cantilever floodwalls consisting of a wall and footing constructed of cast-in-place concrete that relies partly on the weight of the floodwater and soil for stability (FEMA, 2009). A levee operates in most cases by confining and increasing the discharge capacity of the river. This is achieved as the raised walls make the channel deeper giving it an ability to move and hold more water before it overtops and floods adjacent land.
A concrete levee is generally neutral in improving community access and recreational use. Why? A well designed concrete levee may increase public space that may be suitable for recreation such as sporting fields, parks, walking and cycle paths etc. However, a Concrete levee may also create physical barriers between the township and watercourse partially if it is tall.
A concrete levee may cause equality issues and impact individual members of the community. Why? A concrete levee may reduce access, views of the water and require the acquisition or easement of private property depending on its size and location. For example, a property owner rather than having a river view and direct access to the water now has a large 3 metre high concrete wall.
Levees have the potential to slightly improve safety for the community and allow communities to function during long-duration floods up to their design height provided they are constructed and maintained correctly and internal drainage is not an issue. Recent events however, have shown that levees may decrease safety for the community as they provide a false sense of security, as residents continually choose not to evacuate their houses believing the levee will protect them. Why? If people stay rather than evacuate and the levee overtops or fails, floodwater can rapidly inundate the township placing those residents and emergency management personnel at significant risk to life. Please note: A levee is designed to protect property and not people, and should be treated that way during emergency management responses as every flood is different and the levee may overtop or fail. In addition if rain falls inside the levee, the community may still experience flooding as the water may not be able to drain out without the use of water pumps.
A concrete levee can improve community awareness and understanding of the local flood risk however, as noted above it can also lead to a false sense of security leading to significant risk to life. Why? Its presence can be an everyday reminder of the potential flood water levels and the role it plays in the community to reduce damages during times of flood. It is essential that ongoing community education is conducted to ensure the community is aware that levees do not provide protection against all floods and there is significant risk when they do overtop or fail (NFRAG, 2012). Some Council’s actually use the space on flood walls to create public art and murals about local flood impacts, with the aim to increase community flood awareness.
A traditional concrete levee can have significant negative environmental and ecological impacts. Why? A concrete levee is usually designed to reduce inundation, straighten the river channel and reduce roughness, which in effect quickens the flow of water through the stream channel. This increased conveyance of water has the negative environmental and ecological impacts of increasing the flood peak downstream, increased erosion and scour, habitat destruction, and interrupting natural animal migration patterns.
An alternative approach are set back levees. These levees are sufficiently set back from the river’s edge, allowing the river channel to maintain its natural meander stream functions and preserves to a degree the critically important riparian vegetation. This has multi objectives as it can provide property protection during flood events due to increased flood storage as well as recognise the importance of the natural stream functions.
A concrete levee has the potential to cause negative water quality impacts. Why? As noted above a levee is designed to increase conveyance (move water downstream quicker) which in turn increases velocities and flow in the river channel which can lead to increased scour and erosion causing bank failure and turbidity issues in addition to reducing the ability for the floodplain to recharge.
Levees typically due to the length required have a major initial cost but are frequently utilised in flood mitigation, as they are an economically attractive measure as they protect a lot of pre-existing development in large flood prone areas up to their design height (NSW Government 2005). Why? Concrete levees although generally more expensive than earthen levees are still relatively simple to construct, have moderate cost materials and are easy to maintain. In NSW, the construction costs for a concrete levee would typically be around $1,300 per linear metre for a 2 metre high levee and $1,600 per linear metre for a 3 metre high levee. However, the cost and availability of: materials; machinery; labour/ project management; design and feasibility studies, easements and/or the acquisition of land; resolving internal drainage issues and legislative costs can skew this typical cost per linear metre significantly.
A concrete levee generally has minor to moderate ongoing maintenance costs. Why? As levees remain unused for long periods of time and are required to perform to a predetermined level at short notice, it is vital that ongoing maintenance is undertaken. Maintenance includes: 1) Inspecting for rabbit burrows, trees, scour of banks, cracking, concrete cancer, the build up of debris or weed growth, slump or failure; 2) Repairing any faults that which would affect the capacity, and consequently the function of the levee; 3) General maintenance of the levee and associated drainage systems; and 4) testing the operation of flood gates. Maintenance of concrete levees are generally are less than earthen levees.
Levees can reduce flood related average annual damages in large flood prone areas (NSW Government 2005). Why? As detailed above an earthen levee can cost a lot of money however, it can reduce damage costs particularly for more frequent flood events below their design height. Note: When a levee does overtop or fail it can cause significant and potentially greater economic damages to the township then if no levee was present in the first place.
A levee has the potential to cause adverse flood impacts to other areas. Why? As mentioned previously a levee is designed to increase water flow through the stream channel. This amplified conveyance therefore moves flood water downstream quicker and with more energy, causing possible damage to downstream assets. Also mentioned previously when a levee is overtopped, floodwaters can rapidly inundate the township with increased velocities causing structural damage. As a result feasibility and detailed design studies for a full range of flood events (from regular to extremely rare floods) are required to assess the upstream and downstream impacts levees. Typically offset works are required to reduce these impacts such as flood storage areas.
Duivendijk, J. V. (1999). Assessment of Flood Management Options: Prepared for Thematic Review IV.4. Acceesed on 15/9/2012 at: http://www.dams.org
FEMA (2009). Homeowner’s Guide to Retrofitting: Six Ways to Protect Your Home From Flooding. Federal Emergency Management Agency. Washington DC.
Lees, S. (2010). CHAPTER 6: MANAGING EXISTING FLOOD RISKS- OPTIONS & THEIR ASSESSMENT. Floodplain Management in NSW. UTS: Sydney, Australia.
AEMI (2013). Managing the floodplain: a guide to best practice In flood risk management In Australia. Australian Government: Canberra, Australia.