Flood control
methods are used to reduce or prevent the detrimental effects of flood waters.
[1]
Flood relief methods are used to reduce the effects of flood waters or high water levels.
Flooding can be caused by a mix of both natural processes, such as extreme weather upstream,
and human changes to waterbodies and runoff. Though building hard infrastructure to prevent
flooding, such as flood walls, can be effective at managing flooding, increased best
practice within landscape engineering is to rely more on soft infrastructure and natural systems,
such as marshes and flood plains, for handling the increase in water. For flooding on
coasts, coastal management practices have to not only handle changes water flow, but also
natural processes like tides.
Flood control and relief is a particularly important part of climate change adaptation and climate
resilience, both sea level rise and changes in the weather (climate change causes more intense
and quicker rainfall), means that flooding of human infrastructure is particularly important the
world over.[2]
Causes of flooding[edit]
Relationship between impervious surfaces and surface runoff
Floods are caused by many factors or a combination of any of these generally prolonged heavy
rainfall (locally concentrated or throughout a catchment area), highly accelerated snowmelt,
severe winds over water, unusual high tides, tsunamis, or failure of dams, levees, retention
ponds, or other structures that retained the water. Flooding can be exacerbated by increased
amounts of impervious surface or by other natural hazards such as wildfires, which reduce the
supply of vegetation that can absorb rainfall.
Periodic floods occur on many rivers, forming a surrounding region known as the flood plain.
Video of a tree and branch catching centre on the banks of the River Ely, Cardiff, Wales. The area is
monitored by CCTV and trees removed before they block the river downstream.
During times of rain, some of the water is retained in ponds or soil, some is absorbed by grass
and vegetation, some evaporates, and the rest travels over the land as surface runoff. Floods
occur when ponds, lakes, riverbeds, soil, and vegetation cannot absorb all the water. This has
been exacerbated by human activities such as draining wetlands that naturally store large
amounts of water and building paved surfaces that do not absorb any water.[3] Water then runs off
the land in quantities that cannot be carried within stream channels or retained in natural ponds,
lakes, and man-made reservoirs. About 30 percent of all precipitation becomes runoff[1] and that
amount might be increased by water from melting snow. River flooding is often caused by heavy
rain, sometimes increased by melting snow. A flood that rises rapidly, with little or no warning, is
called a flash flood. Flash floods usually result from intense rainfall over a relatively small area, or
if the area was already saturated from previous precipitation.
Severe winds over water[edit]
Even when rainfall is relatively light, the shorelines of lakes and bays can be flooded by severe
winds—such as during hurricanes—that blow water into the shore areas.
Unusually high tides[edit]
Coastal areas are sometimes flooded by unusually high tides, such as spring tides, especially
when compounded by high winds and storm surges. This was the cause of the North Sea flood
of 1953 which flooded large swathes of the Netherlands and the East coast of England and
which remains the greatest recorded natural disaster in England.
Effects of floods[edit]
Flooding has many impacts. It damages property and endangers the lives of humans and other
species. Rapid water runoff causes soil erosion and concomitant sediment deposition elsewhere
(such as further downstream or down a coast). The spawning grounds for fish and other wildlife
habitats can become polluted or completely destroyed. Some prolonged high floods can delay
traffic in areas which lack elevated roadways. Floods can interfere with drainage and economical
use of lands, such as interfering with farming. Structural damage can occur in bridge abutments,
bank lines, sewer lines, and other structures within floodways. Waterway navigation
and hydroelectric power are often impaired. Financial losses due to floods are typically millions of
dollars each year, with the worst floods in recent U.S. history having cost billions of dollars.
Benefits of flooding[edit]
There are many disruptive effects of flooding on human settlements and economic activities.
However, flooding can bring benefits, such as making the soil more fertile and providing it with
more nutrients. Periodic flooding was essential to the well-being of ancient communities along
the Tigris-Euphrates Rivers, the Nile River, the Indus River, the Ganges and the Yellow River,
among others. The viability for hydrologically based renewable sources of energy is higher in
flood-prone regions.
Methods of flood management[edit]
Some methods of flood control have been practiced since ancient times.[1] These methods
include planting vegetation to retain extra water, terracing hillsides to slow flow downhill, and the
construction of floodways (man-made channels to divert floodwater).[1] Other techniques include
the construction of levees, lakes, dams, reservoirs,[1] retention ponds to hold extra water during
times of flooding.
Coastal management[edit]
This section is an excerpt from Coastal management.[edit]
Oosterscheldekering sea wall, the Netherlands.
Coastal management is defence against flooding and erosion, and techniques that stop erosion
to claim lands.[4] Protection against rising sea levels in the 21st century is crucial, as sea level
rise accelerates due to climate change. Changes in sea level damage beaches and coastal
systems are expected to rise at an increasing rate, causing coastal sediments to be disturbed by
tidal energy.
Coastal zones occupy less than 15% of the Earth's land area, while they host more than 40% of
the world population. Nearly 1.2 billion people live within 100 km of shoreline and 100 m of sea
level, with an average density 3 times higher than the global average for population.[5] With three-
quarters of the world population expected to reside in the coastal zone by 2025, human activities
originating from this small land area will impose heavy pressure on coasts. Coastal zones
contain rich resources to produce goods and services and are home to most commercial and
industrial activities.
Dams[edit]
Flood Discharging at Xin'an River Dam during 2020 China floods
Many dams and their associated reservoirs are designed completely or partially to aid in flood
protection and control. Many large dams have flood-control reservations in which the level of a
reservoir must be kept below a certain elevation before the onset of the rainy/summer melt
season to allow a certain amount of space in which floodwaters can fill. Other beneficial uses of
dam created reservoirs include hydroelectric power generation, water conservation, and
recreation. Reservoir and dam construction and design is based upon standards, typically set out
by the government. In the United States, dam and reservoir design is regulated by the US Army
Corps of Engineers (USACE). Design of a dam and reservoir follows guidelines set by the
USACE and covers topics such as design flow rates in consideration to meteorological,
topographic, streamflow, and soil data for the watershed above the structure.[6]
The term dry dam refers to a dam that serves purely for flood control without any conservation
storage (e.g. Mount Morris Dam, Seven Oaks Dam).
Diversion canals[edit]
This section is an excerpt from Flood control channel.[edit]
Tujunga Wash is an example of a concrete flood control channel.
Flood control channels are large and empty basins which let water flow in and out (except
during flooding) or dry channels that run below the street levels of some larger cities, so that if
and when a flood occurs, the water will run into these channels, and eventually drain into
a river or other body of water. Flood channels are sometimes built on the former courses of
waterways as a way to reduce flooding.
Floodplains and groundwater replenishment[edit]
Excess water can be used for groundwater replenishment by diversion onto land that can absorb
the water. This technique can reduce the impact of later droughts by using the ground as a
natural reservoir. It is being used in California, where orchards and vineyards can be flooded
without damaging crops,[7] or in other places wilderness areas have been re-engineered to act as
floodplains.[8]
River defenses[edit]
In many countries, rivers are prone to floods and are often carefully managed. Defenses such as
levees, bunds, reservoirs, and weirs are used to prevent rivers from bursting their banks. A weir,
also known as a lowhead dam, is most often used to create millponds, but on the Humber
River in Toronto, a weir was built near Raymore Drive to prevent a recurrence of the flood
damage caused by Hurricane Hazel in October 1954.
The Leeds flood alleviation scheme uses movable weirs which are lowered during periods of high
water to reduce the chances of flooding upstream. Two such weirs, the first in the UK, were
installed on the River Aire in October 2017 at Crown Point, Leeds city centre and Knostrop. The
Knostrop weir was operated during the 2019 England floods. They are designed to reduce
potential flood levels by up to one metre.[9]
Coastal defenses[edit]
Coastal flooding has been addressed with coastal defenses, such as sea walls, beach
nourishment, and barrier islands.
Tide gates are used in conjunction with dykes and culverts. They can be placed at the mouth of
streams or small rivers, where an estuary begins or where tributary streams, or drainage ditches
connect to sloughs. Tide gates close during incoming tides to prevent tidal waters from moving
upland, and open during outgoing tides to allow waters to drain out via the culvert and into the
estuary side of the dike. The opening and closing of the gates is driven by a difference in water
level on either side of the gate.
Living Breakwaters Case Study
The Living Breakwaters initiative is the result of an Obama-era competition for innovative designs
to prevent further flooding in coastal communities during harsh weather conditions.
In October 2012, Hurricane Sandy hit the east coast of the United States causing more than $65
billion in damages and economic loss. One of the areas that got hit hardest was Staten Island's
South Shore where the beach community of Tottenville saw sixteen-foot waves that destroyed
homes and killed two residents (Dejean).
In response to the devastation, the Department of Housing and Urban Development created the
Hurricane Sandy Rebuilding Task Force to develop "implementable solutions to the region's most
complex needs (Hurricane Sandy Design Competition)." Shortly after its formation, the task force
introduced Rebuild by Design, a competition that promised a total of $920 million from the
Community Block Disaster Recovery program for the winners to implement their plans
(Tottenville Shoreline Protection Project). Ten teams submitted designs and six of them were
eventually awarded funding for their respective projects.
One winning design was the Living Breakwaters initiative proposed by landscape architect Kate
Orff. Her firm, SCAPE, envisioned what Orff describes as a "living piece of infrastructure"
including a barrier that protrudes from the water and houses an oyster reef (Dejean).
SCAPE's green infrastructure solution will be implemented in three stages beginning in
Tottenville, subsequently expanding to the surrounding areas, and to be completed in 2021
(Melcher). The plan aims to protect the South Shore of Staten Island from future storm damage,
employing oysters to purify the water and restore the coastline. They claim that the breakwaters
will protect the coastline from the intense storm conditions caused by climate change and the
wave protection will reduce erosion which has greatly contributed to habitat loss in the area
(Dejean).
