Editing Levee (section)

== Failures and breaches ==
{{Main|Levee breach}}

Both natural and man-made levees can fail in a number of ways. Factors that cause levee failure include overtopping, erosion, structural failures, and levee saturation. The most frequent (and dangerous) is a [[levee breach]]. Here, a part of the levee actually breaks or is eroded away, leaving a large opening for water to flood land otherwise protected by the levee. A breach can be a sudden or gradual failure, caused either by surface erosion or by subsurface weakness in the levee. A breach can leave a fan-shaped deposit of sediment radiating away from the breach, described as a [[crevasse splay]]. In natural levees, once a breach has occurred, the gap in the levee will remain until it is again filled in by levee building processes. This increases the chances of future breaches occurring in the same location. Breaches can be the location of [[meander cutoff]]s if the river flow direction is permanently diverted through the gap.

Sometimes levees are said to fail when water overtops the crest of the levee. This will cause flooding on the floodplains, but because it does not damage the levee, it has fewer consequences for future flooding.

Among various failure mechanisms that cause levee breaches, [[soil erosion]] is found to be one of the most important factors. Predicting soil erosion and scour generation when overtopping happens is important in order to design stable levee and [[floodwall]]s. There have been numerous studies to investigate the erodibility of soils. Briaud et al. (2008)<ref>Briaud, J., Chen, H., Govindasamy, A., Storesund, R. (2008). Levee erosion by overtopping in New Orleans during the Katrina Hurricane. Journal of Geotechnical and Geoenvironmental Engineering. 134 (5): 618–632.</ref> used Erosion Function Apparatus (EFA) test to measure the erodibility of the soils and afterwards by using Chen 3D software, numerical simulations were performed on the levee to find out the velocity vectors in the overtopping water and the generated scour when the overtopping water impinges the levee. By analyzing the results from EFA test, an erosion chart to categorize erodibility of the soils was developed. Hughes and Nadal in 2009<ref>Hughes, S.A., Nadal, N.C. (2009). Laboratory study of combined wave overtopping and storm surge overflow of a levee. Coastal Engineering.56: 244–259</ref> studied the effect of combination of wave overtopping and storm surge overflow on the erosion and scour generation in levees. The study included hydraulic parameters and flow characteristics such as flow thickness, wave intervals, surge level above levee crown in analyzing scour development. According to the laboratory tests, empirical correlations related to average overtopping discharge were derived to analyze the resistance of levee against erosion. These equations could only fit to the situation, similar to the experimental tests, while they can give a reasonable estimation if applied to other conditions.

Osouli et al. (2014) and Karimpour et al. (2015) conducted lab scale physical modeling of levees to evaluate score characterization of different levees due to floodwall overtopping.<ref>{{Cite book |doi = 10.1061/9780784479087.117|chapter = Scour Characteristics of Saturated Levees Due to Floodwall Overtopping|title = IFCEE 2015|year = 2015|last1 = Karimpour|first1 = Mazdak|last2 = Heinzl|first2 = Kyle|last3 = Stendback|first3 = Emaline|last4 = Galle|first4 = Kevin|last5 = Zamiran|first5 = Siavash|last6 = Osouli|first6 = Abdolreza|pages = 1298–1307|isbn = 9780784479087}}</ref><ref>{{cite web|url=https://www.researchgate.net/publication/275033990|title=Levee Erosion and Scour Potential Due to Floodwall Overtopping (PDF Download Available)|website=ResearchGate}}</ref>

Another approach applied to prevent levee failures is [[electrical resistivity tomography]] (ERT). This non-destructive geophysical method can detect in advance critical saturation areas in embankments. ERT can thus be used in monitoring of seepage phenomena in earth structures and act as an early warning system, e.g., in critical parts of levees or embankments.<ref>{{Cite journal|last1=Arosio|first1=Diego|last2=Munda|first2=Stefano|last3=Tresoldi|first3=Greta|last4=Papini|first4=Monica|last5=Longoni|first5=Laura|last6=Zanzi|first6=Luigi|date=2017-10-13|title=A customized resistivity system for monitoring saturation and seepage in earthen levees: installation and validation|journal=Open Geosciences|volume=9|issue=1|pages=457–467|doi=10.1515/geo-2017-0035|issn=2391-5447|bibcode=2017OGeo....9...35A|doi-access=free|hdl=11380/1151894|hdl-access=free}}</ref>