Types of Flooding

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In an earlier article Coastal Innovations discussed the flooding issues currently wreaking havoc on the Lake Ontario shoreline. In this article, Coastal Innovations is taking a step back and taking a broader view on the different types of flooding as well as how and why they occur.

Pluvial Floods

Flash floods and surface water flood events, also known as pluvial flooding, are caused by extreme rainfall that causes flooding not related to an overflowing body of water. In simpler terms, it’s not caused by the rise in water level in a nearby river or lake, but rather the accumulation of water due to torrential downpour or snow melt. One doesn’t even have to be anywhere near a body of water to be at risk and suffer the consequences of pluvial floods.

Surface Water Floods

Surface water floods happens when the urban drainage systems are unable to handle the sheer volume of water, and water flows onto the streets and nearby structures. The process happens in a gradual manner as the water accumulates faster than the process of draining it. Unlike extreme flood events like storm surge, surface water flooding is usually shallow and rarely more than three feet. In terms of the overall risk profile, surface water flooding doesn’t carry any significant threat to life. However, it may lead to significant destruction of buildings and other infrastructure resulting in financial loss to both homeowners and communities.

As for surface flood mitigation, the primary counter measure is increasing capacity of sewer and drainage systems. The primary challenge with such initiatives is the need for large investments which may not be financially viable for the areas in question. Another long term mitigation strategy on a city planning level is making sure the topography is adapted to divert water away from buildings and structures, but this often easier said than done.

Flash Floods

Flash floods occur when torrential rain falls on elevated terrain, which causes the water to flow to lower ground at a high velocity. Another cause of flash floods is rapid snow falls, or abrupt release of water from levees placed upstream or dams. Unlike river flooding which happens gradually, flash floods can happen within minutes of the triggering event. On previous occasions we’ve discussed the dangers of water flowing at a high velocity in the context of storm surges, but the same principle applies here. Water at a height of just 6 inches is sufficient to knock a person off his/her feet, and flash flooding at a height of 2-feet can move vehicles. For this reason, flash flooding carries a much higher risk of injuries and deaths than its more gradual counterpart, leading to approximately 200 deaths in the US each year.

Here’s a great video showing flash flooding in Utah.

Fluvial floods

Fluvial floods refer to when rising water levels in rivers overflows into surrounding shores, banks and land areas, which occurs for a multitude of reasons, the most common being excessive rain or melting of snow. Usually this type of flooding will affect smaller rivers downstream which can cause damage to dams and dikes which can cause additional flooding of nearby areas. Fluvial floods have the potential to inflict tremendous damage on surrounding areas, depending on a range of factors. The three main components used to determine the severity of floods are duration of the flood event itself, the intensity measured by volume, and finally the level of soil water saturation from precipitation preceding the river flooding.

Scientists measure the risk of river flooding based on a range of inputs, including historic precipitation data, forecasted precipitation, current water levels as well as the topography and soil saturation. On flat terrain, flooding tends to rise more gradually and remain relatively shallow. Fluvial flooding on flat terrain may persist for days.

Hilly terrain offers a different set of challenges. Similar to flash floods, river floods in hilly terrain can occur within minutes causing widespread damage due to the flow of debris. On a positive note, flooding in hilly or mountainous areas drain very quickly and usually doesn’t linger for long.

Coastal Flooding

Coastal flooding happens when land that is normally low lying and dry gets flooded by seawater. Coastal flooding can be caused by storm surges, rise in sea levels, and tsunami events. Although we’ve covered storm surges extensively in previous articles, storm surge happens when water is pushed inland by storms at the speed of which the storm system itself is moving. Storm surge may become particularly destructive if coupled with high tide.

Coastal flooding due to rise in sea levels is a relatively slow process. The Intergovernmental Panel on Climate change estimates a global mean rise in sea levels to be between 9 and 80 centimeters from year 1990 to 2100. This will pose a huge risk to low lying land and infrastructure as sea levels will gradually exceed the elevation of land in certain coastal areas. Major sea-level changes are predicted over the next 100 years.

A final cause of coastal flooding is tsunami events. A tsunami occurs when a massive amount of ocean water becomes rapidly displaced. This can happen due to earthquakes, volcanic eruptions under water, underwater landslides. Detecting tsunamis in deep waters is challenging due to extraordinary speed (500+ mph) and sometimes low wave heights (wave heights can be as low as 3-feet). As a tsunami approaches shallow waters, the wave slows down and its height increases. A notable example includes the 2004 Indian Ocean tsunami which claimed over 230,000 lives and was one of the deadliest natural disasters ever recorded. The largest tsunami ever recorded occurred in Alaska in 1958 when an earthquake loosened 40-million cubic yards of rock high above the bay causing a massive wave that removed all trees and vegetation from elevations up to 1720 feet.

Although it’s not possible to remain protected against all types of flooding, a better understanding of the type of flood risk in any particular area allows individuals and local governments to more easily tailor flood protection solutions to the types of flooding they may experience. This helps limit the risk for injuries or death, as well as potential damage to buildings and infrastructure. Stay tuned to Coastal Innovations for more information on flooding, erosion, and innovative solutions.