Flood Types

Flooding in Trinidad and Tobago has become this country’s most frequent natural disaster. Trinidad and Tobago is highly vulnerable to several types of flooding.

Flooding at Pluck Road, Woodland on November 16th 2018 due to the South Oropuche River overtopping its banks.

Flooding happens when the inflow of water into an area is faster than the outflow. In Trinidad and Tobago, the following types of floods may occur.

Although floods may be categorized differently, one category may merge into another. The 3 main types of flooding that affect Trinidad and Tobago are:

Pluvial Flooding
—— Street/Urban Flooding
—— Pluvial/Rural Flooding
Fluvial Flooding
—— Flash Flooding
—— Riverine Flooding
Coastal Flooding
Groundwater Flooding – An uncommon flood type in Trinidad and Tobago, but has been observed as recently as the October 2018 floods.

Pluvial or Surface Flooding

Undoubtedly, street flooding, also known as urban or pluvial flooding, is the most common type of flooding in Trinidad and Tobago. Pluvial flooding is the umbrella term that covers all surface flooding, meaning heavy rainfall created a flooding event independent of an overflowing water body such as a drain, stream or river.

Street/Urban Flooding

Street Flooding along the Eastern Main Road, St. Augustine on November 6th 2018 due to 30 minutes of heavy rainfall and blocked drainage.

Urban/Street flooding, a type of pluvial flooding, is more common in urbanized areas of Trinidad and Tobago, where heavy rainfall overwhelms the drainage system and water flows into streets and occasionally into structures. This is frequently seen along the East-West Corridor, along the nation’s highways where inadequate drainage exists and most notoriously, Port of Spain. In Tobago, it favors Bon Accord and Scarborough.

Urban and street flooding can be mitigated but it requires corporation from both government and individual citizens. Proper drainage needs to be present for water to be able to runoff in relatively urbanized areas. However, even in areas where sufficient drainage exists, littering and illegal dumping is a pervasive problem across the islands. Litter block drains during rainfall and these blockages are sometimes directly responsible for the street/urban flooding in uncharacteristic locations.

Traditional Pluvial Flooding

The other type of pluvial flooding occurs in rural areas or hillsides, where rainfall exceeds the rate at which soils can absorb the rain, causing water to pool (on a flat surface) or runoff (where topography exists). Hillsides with recent forest fires are notorious sources of pluvial floods, as are suburban communities on hillsides.

Unfortunately, traditional pluvial floods are much harder to avoid, particularly living in a tropical area where heavy downpours are the norm. However, by reducing the amount of clearing that occurs on soil, and allowing vegetation to exist, soils will be able to retain significantly more water and prevent extreme runoff or ponding to occur.

During the dry season, the slash and burn method of clearing agricultural land is popular in Trinidad and Tobago. This method, year after year, triggers a number of bush/wildfires across the country, reducing the soil’s absorption. By reducing the slashing and burning method to clear land, we have a better chance to combat pluvial flooding.

Fluvial (River) Flooding

Fluvial flooding occurs when excessive rainfall, either over a short or long period of time, causes a watercourse to exceed its capacity. This type of flooding in Trinidad and Tobago is usually the most damaging, as it encompasses both flash and riverine flooding.

Flash Flooding

Flash Flooding along Conrad Street, Santa Cruz on September 28th 2018.

Flash flooding, a type of fluvial flooding, is the next frequent type of flooding experienced in Trinidad and Tobago. Flash flooding is characterized by an intense, high-velocity torrent of water that occurs in an existing river channel with little to no notice. Flash floods are very dangerous and destructive not only because of the force of the water, but also the hurtling debris that is often swept up in the flow.

Flash floods are difficult to predict in Trinidad and Tobago and usually occur within a period of minutes to hours (usually less than 2 – 3 hours in T&T). Because of the sudden onset and the high traveling speed of the water, flash floods can be very dangerous. The water can transport large objects like rocks, trees, and cars. Never drive through a flash flood, even if it doesn’t seem to be very deep: the car may be swept away by the sheer speed of the water.

Riverine Flooding

As the Caroni River overtopped its banks in a spectacular and unprecedented fashion, strong currents of water moved across both sides of the Uriah Butler Highway during the historic floods of October 17th-21st 2018

Riverine flooding, another type of fluvial flooding, occurs when water from a primary watercourse, or river, overtops its banks. Rainfall over an extended period and an extended area can cause major rivers to overflow their banks. The water can cover enormous areas. Downstream areas may be affected, even when they didn’t receive much rain themselves.

The severity of riverine flooding is determined by the amount of precipitation in an area, how long it takes for precipitation to accumulate, the previous saturation of local soils, and the terrain surrounding the river system.

In flatter areas, floodwater tends to rise more slowly and be more shallow, and it often remains for days. This is usually common in areas surrounding the Caroni River and South Oropuche River.

With large rivers, such as the Caroni River and South Oropuche River, the process is relatively slow. While rain may fall directly into the rivers, much of the rise of major rivers comes from its tributaries. Smaller streams and rivers usually are the first to overtop their banks, as the water flows towards the larger rivers. When there is excessive rain over an extended period, the rise in the major rivers usually takes 24-72 hours, as water from smaller rivers converges into the main watercourse. It takes time for all the rainwater to reach the river, but once it is in the river it has to flow downstream to the sea.

While the water level slowly rises, officials can decide to evacuate people before the river overflows. The area that is flooded can be huge, as seen from the October 2018 floods across Central Trinidad.

On a governmental level, much can be done to mitigate riverine flooding. De-silting of major watercourses, clearing primary and secondary watercourses of shrubbery that hinder water flow and increased leveeing along rivers in flood-prone areas, to name a few methods. However, due to improper infrastructural planning across the country, with a number of settlements built in the flood plains of rivers, riverine flooding is inevitable.

On an individual level, we see yet again that illegal dumping is a major issue. Proper disposal of waste, whether that be trash, old furniture, and appliances, to tree branches and shrubbery is paramount to ensuring watercourses remain as free-flowing as possible when a heavy rainfall event occur. For areas that are located in flood-prone areas, sandbagging and preparation are key by securing your personal property and evacuating before conditions deteriorate.

Coastal flooding due to storm surge in Miami due to Hurricane Florence in September 2017.

A coastal flood, as the name suggests, occurs in areas that lie on the coast of a sea, ocean, or other large bodies of open water. It is typically the result of extreme tidal conditions caused by severe weather.

In Trinidad and Tobago, we usually see coastal flooding during a long period swell event, where large battering waves affect the coastlines, or during exceptionally high tide events known as Spring Tide and the rare King Tide events.

Coastal flooding also occurs in association with tropical cyclones and is dubbed storm surge. Storm surge is generated with high winds associated with a tropical cyclone push water onshore. Water overwhelms low-lying land near the coast and can cause devastating loss of life and property.

Coastal flooding can be categorized into three levels:

  • Minor: A slight amount of beach erosion will occur but no major damage is expected.
  • Moderate: A fair amount of beach erosion will occur as well as damage to some homes and businesses.
  • Major: Serious threat to life and property. Large-scale beach erosion will occur, numerous roads will be flooded, and many structures will be damaged. Citizens should review safety precautions and prepare to evacuate if necessary.

The severity of a coastal flood is determined by several factors, including the strength, size, speed, and direction of the storm. The onshore and offshore topography also play an important role.

Coastal flooding is regularly seen along the South Trunk Road, along Mosquito Creek during high tide events, as well as along the Manzanilla Mayaro Road in Trinidad. Long period swells, causing battering waves do cause coastal flooding and beach erosion across many of Trinidad and Tobago’s coastlines.

Coastal erosion causing massive landslides along Trinidad’s southwestern coastline, at Cedros in February 2018.

Unfortunately, as the planet continues to warm and ice caps continue to melt, a rising sea level may cause devastating impacts to coastal communities in extreme events, and become a nuisance during regular high tide.

Beyond creating sufficient man-made coastal defenses, such as higher sea walls, raising the road levels and creating artificial sand or rock dunes, there isn’t much we can do to prevent coastal flooding. Coastal mangroves do protect inland areas, but in a similar situation to riverine flooding, much of the coastal settlements already exist where there is not much protection from mangroves. As seas continue to warm and expand, as well as rise due to melting ice caps, this problem is likely to become more frequent and severe.

Groundwater Flooding

Groundwater Flooding in Ireland. Credit: Geological Society Ireland

As opposed to flash floods, groundwater flooding takes time to occur. As rain falls over an extended period, the ground becomes saturated with water until it cannot absorb any more. When this happens, the water rises above the ground’s surface and causes flooding. You can read more about groundwater flooding here from Kent City Council in the UK.

This is a fairly common problem in parts of Western Europe and the United Kingdom, but mostly unheard of in Trinidad and Tobago until recently. During the October 2018 floods, residents living near the Gandia River, along Chin Chin road, reported that water rose from the ground, instead of flowing into their homes from nearby drains which were at threshold levels.

In this case, unfortunately, there isn’t much a homeowner can do to protect their property from this type of flooding. Mitigation and water management measures, such as contracting additional drainage on your property to aid runoff, as well as investing in pumps to remove water from your home are typically recommended.

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