After heavy rainfall, water discharges in small streams can increase many times above their average. The period between the rainfall event and the arrival of a flood is usually short, with limited time available for flood warnings.
In contrast to small streams floods in large rivers are strongly influenced by long-lasting or frequent precipitation events over a larger area, as well as snowmelt in temperate or polar climatic zones. The water levels usually rise more slowly with more time available for flood warnings. The causes of floods and subsequent inundations can be manifold and potentially occur in combination and reinforce each other.
Further causes of inundations are storm surges and overflowing of flood protection structures. Flood protection structures include dikes, protection walls, retention basins or mobile systems. Generally, they do not completely eliminate the risk of flooding for the people protected by them, this especially during extreme weather conditions.
High-water levels can potentially lead to the overtopping of flood protection barriers and subsequent inundation of the backcountry, while prolonged periods of high-water levels can lead to the saturation of dikes causing instabilities and potentially dike failures.
Stormwater runoff and in extreme cases flash floods from heavy rain are weather related phenomena that will occur more frequently in the future. This type of flood can occur anywhere and affect even those located in safe distances from a water body. Flood risks are present anywhere where water accumulates or discharges over the ground surface. The topography and terrain features, either natural or a man-made, determine the flow paths of the water. Houses located in such flow paths are under risk of flooding, regardless of whether it is located on a steep or gentle slope, local depression, or built-up areas in a flat terrain.
Furthermore, houses must professionally be designed to route stormwater from heavy rains away to prevent flooding of the foundation or damages to the structure. Correct installed roofing, gutter designs and drainages are therefore as crucial as regular maintenance activities.
Every public sewer system has a limited capacity to hold water. During heavy rainfall, those capacities can be exceeded and lead to flooding of your home. Generally, sewer systems work by force of gravity, where water flows in direction of the natural slope of the pipe.
A blockage of the pipe and especially surcharges during heavy rain events can trigger a reversed water flow in the pipe and subsequently a sewer backflow. During a sewer backflow the sewage water level can rise beyond the ground level. In such case sewage water can either spill over the street and into your home or enter your house directly through pipes connected to your house. Commonly sewage water spills out of toilets, sinks or floor drains in the basement, which not only leads to damages but also creates severe health hazards.
All this is not uncommon!
Luckily, an overflow of sewage water can easily be prevented by installation of a backflow prevention valve added to your house plumbing system.
What many may not know: Water can seep into your property from the underground due to a rising water table. This type of flooding occurs when the natural drainage system of the underground cannot drain access water from rainfall quick enough leading to a rising water table. Although this happens more slowly, it can lead to permanent and extensive damage due to the length of time the house structure and materials will be exposed to the water until groundwater levels will go back to normal.
This type of flooding might occur when your house is located in a local depression or at a slope. Further than that groundwater flooding can also be associated with a river flood, particular in proximity of larger rivers. Due to a flood in the river, the natural groundwater flow towards the river will be reversed pressing the water into the aquifer adjacent to the river with the consequences of a rising water table. Even if your house is protected from a river flood due to its location behind a flood protection barrier or dike it will not be protected from a rising water table related to groundwater flooding.
If you are interested in moving to a new house, make sure you get well informed about past floods and known damages at your new location by local authorities, the community or neighbours.