Overtopped

Overtopped describes a condition in which water flows over the crest of a water-retaining structure, such as a levee, dike, dam, or coastal wall, during flood, storm surge, or high tides. It indicates that the crest height and associated freeboard are insufficient to completely contain the water level under the prevailing hydraulic conditions.

Causes of overtopping include extreme inflows from rainfall or river discharge, storm surge and high tides in coastal areas, and waves acting on seawalls. The extent of overtopping depends on factors such as crest height relative to the water level, the geometry of the structure, the duration of the high-water event, and downstream flow conditions. Weather, conditions of the soil, and maintenance status can also influence the likelihood and rate of overtopping.

The consequences of overtopping can range from localized erosion of the crest and slopes to internal erosion, piping, or scour at the toe, which may undermine the structure and lead to a breach. Overtopping can injure the protective capacity of a levee or seawall, increase sediment transport, and raise the risk of downstream flooding.

Engineering practice treats overtopping through design, monitoring, and mitigation. Designers aim to provide sufficient freeboard and structural resilience to limit overtopping rates. Mitigation measures include increasing crest height, enhancing toe protection with armour or riprap, improving drainage and seepage controls, and implementing maintenance regimes to preserve structural integrity. Overtopping is typically analyzed with hydraulic models to inform risk assessments and emergency planning.