P C S 2 | FLOOD CONTROL AND DRAINAGE ENGINEERING

MODULE 1: FLOOD AND ITS CAUSES

A flood is an overflow of water that submerges land that is usually dry. Floods are an area of
study in the discipline of hydrology. They are the most common and widespread natural severe
weather event. Floods can look very different because flooding covers anything from a few inches of
water to several feet. They can also come on quickly or build gradually.

According to our friends at the National Severe Storms Laboratory, there are five types of floods. They
include:

RIVER FLOOD

A river flood occurs when water levels rise over the top of riverbanks. This flooding can happen
in all river and stream channels. This includes everything from small streams to the world’s largest
rivers.

River flooding typically happens for four reasons. They are:

 Excessive rain from tropical storm systems making landfall

River floods can happen suddenly or slowly. Sudden river flooding events occur more often on
smaller rivers, rivers with steep valleys, rivers that flow for much of their length over impermeable
terrain, and normally dry channels.

On the other hand, low-rising river floods typically occur in large rivers with large catchment
areas. In case you did not know this already, a catchment area is any area of land where precipitation
collects and runs off into a common outlet.

COASTAL FLOOD

A coastal flood is the inundation of normally dry land areas along the coast with seawater.

Coastal flooding is typically a result of a combination of sea tidal surges, high winds, and barometric
pressure. These conditions typically come from storms at sea like tropical cyclones, tsunamis, and/or
higher-than-average tides.

STORM SURGE

Storm surge is an abnormal rise in water level in coastal areas over and above the regular
astronomical tide.

Storm surge is always a result meteorological storms that cause higher than normal tides on
the coast. There are three things in a storm that create storm surge: wind, waves, and low atmospheric
pressure.

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Storm surge is an extremely dangerous form of flooding. It can flood large coastal areas at the
same time. It can also cause flooding very quickly. Extreme flooding occurs when storm surge happens
at the same time as high tide. This can cause storm tides to reach over 2o feet. Our meteorologists
always stress that storm surge is the most dangerous aspect of any tropical system. It poses the most
threat to both life and property. In the past, we have seen truly devastating storm surge impacts.

INLAND FLOODING

Some organizations refer to inland flooding as urban flooding. A flash flood is also a type of
inland flood. An inland flood is flooding that occurs inland or not in a coastal area. Therefore, coastal
flooding and storm surge are not inland floods.

Rainfall is usually to blame for inland floods. Rain causes inland flooding in two ways. It can
happen with steady rainfall over several days or it can happen because of a short and intense period of
rainfall. Snowmelt also causes inland floods, although rainfall is a more common cause. Another
reason of inland flooding is when debris, ice, or dams block waterways.

Inland floods are often worse in urban areas because there is little to nowhere for the water to
go. The following urban features can create urban flooding or make inland floods worse:

 Paved roads and streets

FLASH FLOOD

The most well-known and deadly type of flood is a flash flood. A flash flood is flooding that
begins within 6 hours, and often within 3 hours, of heavy rainfall (or other cause).

Flash floods can happen for several reasons. Most flash floods happen after extremely intense
rainfall from severe thunderstorms over a short period of time (normally 6 hours or less). There are
two key elements to determine is flash flooding is likely:

Rainfall rate

Rainfall duration

Flash floods also happen when damns break, when levees fail, or when an ice jam releases a
large amount of water.

Flash floods are extremely powerful. They have enough force to roll boulders, tear trees from
the ground, destroy buildings and bridges, and scour out new channels. This type of flood is
characterized by raging torrents that rip through riverbeds, urban streets, or canyons, wiping out
everything in their paths. With heights reaching 30 feet, flash floods can completely cover
communities.

Another reason why flash flooding is so dangerous is that it can happen with little to no
warning. This is especially true when dams or levees break.

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FLOOD CAUSES & EFFECTS

1. Heavy Rains

The simplest explanation for flooding is heavy rains. No matter where you live, you are surrounded by
infrastructure and systems designed to move rainwater into appropriate basins and reservoirs. In most
cases, the infrastructure does its job, and you never have to think about where the rain goes when it
runs off.

When it rains heavily, however, those systems are overwhelmed, and that water doesn’t drain nearly
as quickly as it needs to. In short, the drainage systems back up, and the water rises — sometimes into
homes. This typically happens only in cases of sustained heavy rains over a long period.

2. Overflowing Rivers

You do not necessarily need to have heavy rains to experience flooding in your area. For example, if
you live along a river and areas upstream from you experience heavy rains, it could lead to a serious
overflow where you live. Most larger rivers include a series of dams to help manage large amounts of
rainfall, and most river systems are managed by government authorities.

Sometimes, however, those authorities have to make tough decisions about how to operate dams. They
often can manage the water and prevent flooding altogether — but not always.

3. Broken Dams

Much of America’s infrastructure was built in the 20th century, so it is getting old. When heavy rains
come, and water levels rise, aging dams can fail and unleash torrents of water on unsuspecting
households.

This is part of what happened after Hurricane Katrina hit New Orleans in 2005. Levees failed and
made the flooding far worse than it would have been otherwise. While we have come to depend on
20th-century architecture, and much of it does its job well, there is always a possibility that a structure
will fail.

4. Urban Drainage Basins

Many of our cities are made of mostly concrete and other impermeable material. When you have an
urban drainage basin that is made of concrete, there is no ground for water to sink into. So, when those
drainage basins fill up, it is going to mean flooding for low-lying areas.

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This is mostly the case in large urban areas — think Houston and Los Angeles. When heavy rains strike,
the basins used to drain them cannot always handle the load.

5. Storm Surges and Tsunamis

Rain is not always the culprit when it comes to flooding. Storm surges related to hurricanes and other
storms can lead to significant flooding, as can tsunamis that are sometimes caused by underwater
earthquakes.

Given modern technology, we often know about storm surges and tsunamis before they arrive, but this
is not always the case. For example, in 2004, an earthquake off the coast of Indonesia created a
tsunami that gave little warning before coming ashore.

6. Channels with Steep Sides

Flooding often occurs when there is fast runoff into lakes, rivers and other reservoirs. This is often the
case with rivers and other channels that feature steep sides. It is a similar issue to having a lack of
vegetation, which is explained in more detail below.

7. A Lack of Vegetation

Vegetation can help slow runoff and prevent flooding. When there is a lack of vegetation, however,
there is little to stop water from running off. This can be a bit of a conundrum after a drought.

While area residents likely welcome the rain, the lack of vegetation after the drought can cause flash
flooding. This does not always happen given that basins and reservoirs are close to empty, but it can
occur in cases of extreme rains following long periods of drought.

8. Melting Snow and Ice

A winter of heavy snow and other precipitation can lead to a spring of flooding. After all, that snow
and ice have to go somewhere when they melt. Most mountainous areas experience relatively
consistent snowfall totals from year to year, but an unusually heavy winter of precipitation can spell
bad news for low-lying areas around the mountains when spring hits.

The good news is that sustained winter precipitation provides a long lead time to prepare for potential
flooding. That is a silver lining at the very least.

These are just a few examples of common causes of floods, but there does not need to be an
incredible weather event for you to experience flooding at your home. You can experience devastating
flooding simply due to a clogged or broken pipe as well as other plumbing issues. No matter how or
why you experience a flood, you need a qualified water restoration professional with quality tools to
help you get your home back into working order again.

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