(Also known as a flood levee, main levee, embankment, stop bank, dyke, dike, summer dike, confinement dike, ring dike, bund)
What is an earthen levee: A earthen levee is an embankment built along a river with the primary purpose of providing flood protection to adjacent land or human settlement from inundation (Duivendijk 1999). It is predominantly an embankment consisting of consolidated earth properly keyed into the underlying soil, with an impermeable packed clay, reinforced concrete or sheet pile core and flat batters either side (Lees 2010). A levee operates in most cases by confining and increasing the discharge capacity of the river. This is achieved as the raised embankments make the channel deeper giving it an ability to hold more water before it overtops and floods adjacent land.
An earthen levee generally improves community access and recreational use. Why? The levee embankment can increase public space that may be suitable for walking, cycle paths, improved recreational fishing access and sometimes roads.
An earthen levee usually causes equality issues and impacts individual members of the community. Why? As levees rely on their mass to resist floodwater pressure, to provide structural stability, resist erosion and allow the sides (batters) to be mown, batters have to be designed with a 4 to 1 or 5 to 1 batter slope in order to permit the operation of mowing machinery and prevent slump and failure. As a result, a levee at its base is usually 6 to 8 times its height and hence, requires a significant land area for construction. This land is usually both river frontage and located on private land. As a result this causes the situation whereby a property owner rather than having a river view and a level backyard now (with the construction of a 3 metre high levee) has a large mound of earth covered in grass, no backyard and no river views.
Levees in general provide an additional level of flood protection to the community and allow communities to function during long-duration floods provided they are constructed and maintained to the best design practices. However, it has been evident that levees provide a false sense of security to the people living behind them, as residents continually choose not to evacuate their houses believing a levee would protect them. Why? If people stay rather than evacuate and the levee overtops or fails, floodwater rapidly inundates the township placing those residents and sometimes emergency management personnel at significant risk to life. Please note: that 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 could overtop or fail.
An earthen 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 that the population is aware of the risk of overtopping, is informed about emergency management plans in the event of overtopping and does not lapse into the common belief that a levee provides protection against all floods (NFRAG, 2012).
A typical levee has significant negative environmental and ecological impacts. Why? A major levee is usually designed to 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 pushing the flood peak downstream, increased erosion and scour, habitat destruction, and interrupting natural animal migration patterns.
A new approach is set back levees. These levees are sufficiently set back from the river’s edge, allowing the river to maintain its natural low flow meandering channels and preserving to a degree the critically important riparian vegetation while also providing property protection during flood events due to increased flood storage.
A typical levee causes negative water quality impacts. Why? As noted above a levee is designed to increase conveyance (move water downstream quicker) which in turn diminishes the capacity of the river to regulate its flows, which causes scour and erosion, increases turbidity and reduces the ability for the floodplain to recharge which collectively decreases water quality (Freitag, Bolton, Westerlund, & Clark, 2009).
Levees typically due to the length required have a major initial costs 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 (NSW Government 2005). Why? Levees are relatively simple to construct, have low cost materials and are easy to maintain. In NSW the construction costs for an earthen levee are typically around $1,100 per linear metre for a 2 metre high levee and $1,300 per linear metre for a 3 metre high levee. However, the cost and availability of: borrow 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 levee has 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, 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) Mowing and general maintenance of the levee and associated drainage systems. On average this equates to an annual maintenance cost of $30 per linear metre/ per annum (derived from the Hunter Flood Mitigation Scheme, NSW).
Levees significantly reduce flood related damages in large flood prone areas. Why? As detailed above Levees can cost a lot of money however, they can substantially reduce damage costs particularly for more frequent flood events. Note: When a levee does overtop or fail it can cause significant economic damages to the township.
A levee can 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 of levees.
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
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.