Question

Channelization and levee construction may reduce local flood hazards, but they may worsen the flood hazards elsewhere along a stream. Explain. How can this lead to such drastic action as deliberate breaching of a levee during a flood event?

Answer 1

Flood risk management seeks to reduce the risk from flood events to the people who are located in floodprone areas

The magnitude of that risk is a function of the flood hazard, the characteristics of a particular location (its elevation, proximity to the river or coast, and susceptibility to fast-moving flows and surges, etc.), measures that have been taken to mitigate the potential impact of flooding, the vulnerability of people and property, and the consequences that result from a particular flood event

The initial risk is represented by the area’s characteristics without consideration of mitigation and risk transfer measures and the vulnerability of the population.

Each mitigation and risk transfer measure reduces the overall risk to some degree, but it is impossible to completely eliminate risk.

A flood risk management strategy identifies and implements measures that reduce the overall risk and what remains is the residual risk

Levees represent one method of reducing the impacts of flooding on a community or a region.

Levees keep the floodwaters away from the area behind the levee until the point at which the levee is overtopped or fails and the area behind the levees is inundated and the people and property are effected

The risk to those behind levees is a function of the characteristics of the levee (height, strength), their location, and the mitigation and risk transfer measures and vulnerability reduction actions that they have taken or have been taken on their behalf.

As has been previously discussed, every location within a floodplain, regardless of the presence or absence of a levee and whether or not the levee is accredited, is subject to some level of risk.

It is important for those located in the floodplain and those responsible for activity in the floodplain (public officials, investors, and those relying on activities in the floodplain, etc.) to ensure that those in the floodplain understand the nature of the risks they face and the steps that may be taken to reduce this risk.

Structural Mitigation Measures ::

Levees, Floodwalls, Seawalls, and Other Appurtenant Structures

These structures are designed to prevent floodwaters and storm surges from reaching areas that are at risk. Consequences of failure can be catastrophic because those behind the structure can be subject to rapid inundation and flooding conditions more severe than if the floodwaters had risen gradually.

Dams

Barriers that impound hydrologic flows, dams retain floodwaters before they reach areas at risk. For example, during high-precipitation periods, dams hold upstream floodwaters that are released gradually to minimize the likelihood of damage to downstream communities. However, during exceptionally large events, the storage capacity of a dam can be exceeded and uncontrolled flood flows are passed downstream. Under these circumstances, downstream levees may not be able to contain floodwaters and will fail. This condition occurred in 2011 during spring floods on the Missouri River (USACE, 2012b). Under exceptional circumstances, dams can fail and send significant quantities of water downstream, resulting in damage or destruction of levees and communities below the dams.

Floodways, Spillways, and Channels

Floodways, spillways, and channels are constructed to carry floodwaters around a community or region where the capacity of a river to pass a large volume of floodwaters past a critical location is limited. Under some circumstances, river channels can be modified to increase their flood carrying capacity.

Structural Mitigation Through Improved Levee Design or Modification

During a flood, levees are under continuous stress that threatens their integrity. The most serious challenges result from wave action against a levee face, the erosion of the land side of a levee as the levee is overtopped or subject to waves breaking over its top, and seepage under the levee that destroys the levee from within.