Flood (coastal, fluvial)

A coastal flood is triggered by a high-water event resulting from storms, storm surges, or high tides. Coastal floods are exacerbated by sea level rise. A fluvial flood occurs when the water level in a river or lake rises and overflows to the surrounding land. In the Netherlands, fluvial floods are often caused by excessive rainfall. Flood events resulting from extreme rainfall are called pluvial floods.

A distinction can be made between protected and unprotected areas. Areas are considered protected when they are safeguarded by naturally high-lying topography or flood defences with a legal flood protection standard (see policies & regulations). This means that flood defences must meet specific probability requirements and are regularly inspected. A safety assessment every 12 years ensures that they continue to meet these standards. Areas that are not safeguarded by naturally high-lying topography or flood defences with a legal flood protection standard are considered unprotected.

Flooding of protected areas occurs when the protection fails (dike breach or overtopping). Flooding of unprotected areas happens when the water level rises higher than the adjacent land. The European Floods Directive (EFR), a European directive aimed at reducing flood risks in the member states, identifies four types (PDF) of coastal and fluvial floods: flooding of protected and unprotected areas in the main water system, and flooding of protected and unprotected areas in the regional water system.

Figure shows the different types of flooding in the Netherlands.
The image displays the different types of flooding in the Netherlands. (A) Flooding of unprotected areas along the main water system. (B) Flooding of protected areas along the main water system. (C) Flooding of protected areas along the regional water system. (D) Flooding of unprotected areas from the regional water system. (E) Pluvial flooding. (F) Groundwater flooding. Source: adapted from Klopstra & Kok and Ministry of Infrastructure and Water Management (PDF).

Main water system

Flooding of protected areas along the main water system due to the overtopping or failure of primary flood defences (B). In the Netherlands, primary flood defences include levees, dunes, storm surge barriers, locks, and dams that protect the land from major water bodies. A breach in these primary flood defences will result in flooding.

Flooding of unprotected areas (such as floodplains) along the main water system (A). This type of flooding occurs due to high discharges from the primary rivers of the Netherlands, storm surges on the North Sea and storms across Lake IJssel, or a combination of the events. It affects areas that are not protected by the primary flood defence systems.

Regional water system

Flooding of protected areas along the regional water system due to the overtopping or failure of regional flood defences (C). Similar to the breaches in primary flood defences, breaches can also occur in regional flood defence systems. These systems protect land from smaller bodies of water, such as canals and lakes.

Flooding of unprotected areas from the regional water system (D). Some regional brooks and rivers do not have flood protection. Overflows from these water bodies, caused by high water levels originating in the catchment area, are also recognised as fluvial floods.