5.2 Overflow#
Overflow of a dike occurs when the still water level rises above the crest, resulting in a continuous and unhindered flow of water over the crest and inner slope of the dike into the hinterland. The overflow failure mechanism occurs when water flow erodes the inner slope, which can eventually lead to a dike breach.
The most important parameter for an overflow calculation is the freeboard of a dike, \(R_c\), defined as the difference between the crest elevation of a dike and the still water level. Thus, the freeboard is defined as positive when the still water level is below the crest and negative when the still water level is above the crest. According to the \citet{EUROTOP2018}, the amount of water flowing over a dike due to overflow can be determined by equation (26):
In equation (26), \(q_{overflow}\) is the amount of water flowing over the crest of the dike in [m\textsuperscript{3}/s/m]. The EurOtop Manual notes that the coefficient 0.54 is a constant value calibrated on experiments, which may vary depending on the crest geometry.
In most assessments where overflow is of importance, waves are often present and overtopping should also taken into account by summing both effects to find the total discharge over the crest of the dike. However, for a large negative freeboard, it is clear that the discharge due to overflow is dominant. Therefore, EurOtop suggests neglecting the effects of overtopping when \(R_{c} < -0.3\:H_{m0}\). As many of the dikes in the Netherlands were designed and constructed with extra freeboard to cover uncertainty in hydraulic loads, overflow is generally not a dominant failure mechanism in the assessment and improvement of existing dikes. In fact, in the current Dutch safety assessment (i.e., the standardized testing procedure) any amount of overflow is considered to be a failure of the flood defence. However, a dike design can deviate from this limitation if justified with a custom site-specific analysis.