Mitigation of flood events is a long-term problem and
solution. the immediate problem is that
cities are areas where people live and work and the transportation systems that
move them from these location to the other. When you add entertainment to the mix it gets
more complicated. We prefer personal
transportation vehicles to public ones, and this means that the roadways and
the parking spaces for these vehicles are large flat spaces where water cannot
penetrate and so must flow overland. These
flat areas are sloped to cause the water to move to drainage areas. This is fine for low volume rain events when
it rains only a couple of millimetres per hour but when the rain comes in
faster it overwhelms these systems.
Metric is perfect for these calculations. A hectare is 100m x 100m and that is a large
space, but about the size of a parking lot.
A hectare has exactly 10 000 square meters in it. when it rains on that hectare it is easy to
calculate how much water has fallen on it as there is 1000mm in a meter, every
millimeter of rain fall equals 10 cubic meters of water. So, when it rains 10mm in an hour that means
there is 100 cubic meters of water that has to be moved off it every hour. When 100mm of rain falls that is 1000 cubic
meters of water that has to be moved in an hour. If the drainage system can only more 600 cubic
meters an hour, that means that 400 cubic meters has to be moved to adjacent hectares
or else sit in place, and water like to move.
A mall will have many hectares of parking lot, roads have
more drainage places but are also prone to the drains are in a line and can be
overwhelmed from all the water pouring through them. Homes also count as impermeable space, the
water flows from them into the street or on to grass but many homes are
surrounded by impermeable asphalt and concrete.
There are green spaces around some homes. People like their lawns. Sometimes there is a
large tree there too, maybe some shrubs.
But cities are mostly impermeable.
There are many types of ground. And people think of ground as being porous
for water and it can be. Rock is the worst,
and it promotes a lot of over land flow because the water does not penetrate it. Gravel allows good water penetration as it
has lots of space between the gravel.
Sand is just as good. Clay is the
more like rock when dry and when wet slows water penetration. Bare ground is good until the air holes are
filled with water and only if the air holes can fill with water. Better than bare ground is plants. Plants have roots that penetrate ground and
established wild plants have added organic mater to the ground and soil.
Grass is the worst, modern lawns the worst of the
worst. Lawns have shallow root
penetration a couple of centimeters at best and that is as far as the overland
water can penetrate easily, if lawn cuttings are left on the lawn they will
create Organic matter in a few months and that will hold water like a sponge. Better than grass are shrubs, the roots of shrubs
will try to penetrate deep into the soil, about a meter into the soil. This means that water flowing overland into
some shrubs will soak into the ground about a meter and until this level fills
up the overland water will penetrate the ground faster. Trees are the next best. Tree roots will often penetrate the ground
more deeply. As the tree grows taller
the roots grow deeper to support the tree above ground and to find deeper sources
of water. The overland water encounters
a tree and follows the roots deep into the ground and to disperse at those
lower levels in the deeper soils. Trees drop
a lot of leaves in the city these leaves are taken away and disposed of, but in
nature they form an organic matter shield that slows overland flow to allow it
to penetrate the ground through the roots and acts like a sponge to fill with
water. The absolute best is a natural
park. The mix of shrub and trees with a
blanket of multi-year leaf drift holds the overland flow back and helps it
penetrate the ground for groundwater storage.
The organic matter in the soil (at all levels) acts as a sponge holding
the water in place.
Healthy natural greenspaces inside urban environments and
green spaces outside cities also help, the green belts ensure less overland
flow from regions outside area of the city from reaching the city. Back to the math. A square kilometer is about the size of a neighbourhood,
there are a hundred hectares in a square kilometer. A big city is about 10km x 10 km and so has
100 square kilometers or about 10 000 hectares.
If it rains 10mm on the city, the city has to deal with 100 cubic meters
of water per hectare, or about 1 000 000 cubic meters of water. If the water can penetrate the ground into
the soil then the city does not need to do anything. If it rains on the city 100mm of rain and the
city can only take care of half of the rainwater, then the rest will flow
overland. Half of that is 5 million
cubic meters. If the city can increase the
ground penetration or delay the overland flow of the water less water will
cause problems for the city. More trees,
more natural green spaces, more organic matter in the ground, on the ground,
less lawns more complex front yards.
An Ounce of Prevention
We hear that global warming is the cause of the
flooding. How? There is a relationship
with the temperature of the air and the amount of water it can hold. The relationship is exponentially
related. Cold air holds less water and
warm air holds more. As soon as the air
becomes warm it will start to hold more water until the temperature hits a
point where all the air can be water vapour.
That is a long way off and there will be other problems if the air gets
that warm. But the amount of water that
the air will hold when it is one degree warmer in the summer is a lot more and
rain events that drop 30 mm in an hour will become events that drop 50mm in an
hour and infrastructure that is designed to carry away water in 100-year events
will experience those event more often and events that are expected every thousand
years or never, every few years instead.
What is a 100-year event? if we are expecting to get 100mm of rain in
the month of July, and every 10 years we expect to get 150mm of rain or only
50mm of rain every once every 10 years, based on statistics. We might expect that a drought in July where
we get no rain might happen every 50 years and every 50 years we get 175mm of
rain, both occurrences would be 50-year events.
A hundred-year event would change according to the regional statistics,
but it might only a bit more than the 50-year event, so say 180mm of rain. If the air were warmer, its carrying capacity
would increase a lot. If the carrying
capacity rose 5% for every local temperature rise, and the temperature was 5
degrees warmer than usual, the amount of rain that would fall would be 20%-25%
higher than average and 150mm would increase by 30mm-37.5mm and make 100-year events happen every 10
years.
Carrying capacity works two ways though, it also works to
dry out the land faster too. This means
that when the air warms the amount of water it can hold increases, so while 20
degree wind over a northern forest can hold 20g of water per cubic meter of
air, 30 degree wind will hold 30g of water meaning that the air will dry out
the land 50% more than cooler wind, dry forests are forests that burn
more. Global warming and Climate Change go
hand in hand. Additionally, CO2 is
considered a weak greenhouse gas and water vapour a stronger one. The burning forests up the in the air and the
warmer air adds more water vapour to the atmosphere so the world will get
warmer faster.
What can we do? Reduce
carbon emissions. Stop adding to the
problem. Increase carbon capture, this will
take carbon out of the air, and it will reduce the temperature around the globe
a little bit, but the lowering of the temperature will decrease the carrying
capacity of the air and reduce the water vapour in the air and further reduce
the temperature of the planet.
Short term mitigation, more natural green spaces inside
cities to decrease overland flow and to reduce the flash part of a flood, slow
the overland water down. Increase diversification
of urban residential greenspaces (lawns) more permeable hard spaces. More plants around parking spaces has other
positive benefits like calming effects and cooler cities. Permeable parking lots with more green spaces
would divert water to ground water and not to storm water. Public Transit would decrease parking needs
allowing for greater greenspaces and less overland water in addition to less
greenhouse gasses. Less lawns means less
motorized yard maintenance with is less greenhouse gasses used, better water
retention and cooler houses. Less grassy
lawns would also increase water penetration and lessen overland flow allowing
more penetration of the ground and flooding in the homes.
Some household suffer from more flooding than other
locations. They sit closer to floodplains,
or they sit in places where rivers and streams historically existed before
people developed them into houses. Those
natural water corridors should be revived to help channel water away from houses
and along natural pathways. Reviving a
stream will keep the water out of the storm drain system and direct it slowly
to more natural pathways. Natural streams
have denser vegetation that helps slow and recharge water tables.
