|
HYDROLOGICAL
CYCLE
|
|
The
hydrological cycle is a basic process of material cycling that is
crucial to all life on earth. Precipitation waters crops for us to
eat, replenishes streams, lakes, and wetland habitats, supported the
growth of forests, and recharges the groundwater supplies that provide
the water we drink. Evaporation withdrawals water from earth and
stores it in the atmosphere as clouds, until weather conditions
stimulate a rain shower.
|
|
|
|
The drainage system includes the geographic area surrounding the stream
system that captures precipitation, filters and stores water, and
determines water release into stream systems. The stream system is the
visible, aboveground portion of this larger drainage system.
The connectivity of
the stream system refers to the physical connection between tributaries
and the river, between surface water and groundwater, and between wetlands
and water. Connectivity is the primary reason for doing aquatic
assessments at the watershed level. Because water moves downstream,
any activity that affects the water quality, quantity, or rate of movement
at one location can affect locations downstream. For this reason, everyone
living or working within a watershed needs to cooperate to ensure good
watershed conditions.
|
|
|
|
watershed
definition | water cycle
| floodplains | streams
| groundwater | runoff
|
|
|
|
|
Floodplains
are nature's method of disaster damage control. Not all ecosystems can
tolerate inundation, or the damage and destruction left by raging
floods. Floodplains are special parts of the valley where rising
waters can flood. Specially evolved plant communities occupy these
areas. Grass-like sedges and rushes capture rushing sediments and help
stabilize the stream banks. Trees like red maples and box elders like
to grow with their roots in water, and can re-sprout from these roots
if their trunks get broken by rushing debris during a flood. Other
trees like sycamores are strong and solid and can withstand many
floods.
|
|
|
|
Rivers
and streams overflow predictably into the floodplain. This fact makes
building in the floodplain a very dangerous option. Even building on a
natural terrace slightly above the floodplain could spell disaster in
the next 100 years.
|
|
|
|
The
illustration above shows the probable extent of flooding over long
time intervals on a hypothetical floodplain similar to the Chartiers
Creek floodplain. The 2-3 year flood inundates the existing
floodplain (Terrace 1). It is only this flooding which the Fulton
Flood Control Project was designed to eliminate. The 100 year
flood inundates both the existing floodplain and a higher one (Terrace
2), which formed when the river stood at a higher level. The 500
year flood event inundates an even higher terrace and all lower
terraces and floodplains.
After
many flooding disasters, the government has realized the danger and
expense of building on floodplains. Some regulations restrict the
construction of new buildings within certain limits of the
floodplain.
.
|
|
|
|
In
the illustration above, the regulatory floodway is kept open to carry
flood water. No building or fill is permitted. Use in the
regulatory floodway fringe is permitted if protected by fill,
flood-proofed or otherwise protected. The regulatory flood limit
is based on technical study and is the outer limit of the floodway
fringe. The standard project flood (SPF) limit is the brown area
subject to possible flooding by large floods. This SPF area was
the floodplain inundated by Hurricane Ivan.
However, in the past,
planners did not recognize the power of water and the danger of the
floodplain. Today, neighborhoods like Carnegie, Heidelberg and
Bridgeville lie in the floodplain. The water may not be visible, but
the land still remembers its floodplain identity.
|
|
|
|
watershed
definition | water cycle
| floodplains | streams
| groundwater | runoff
|
|
|
|
|
Water is one of the
most powerful forces known and has been harnessed by humans for
thousands of years. Even today, the combined strength of water and
gravity power mills and run hydroelectric power generating stations.
All ecosystems on earth are shaped and influenced by water, or a lack
of water. The eastern United States is a living, life-sized tribute to
the action of water and rivers. Fast moving water can move
very large particles, as this diagram shows. As water slows down, it
starts to deposit particles. This power of flowing water was evident
in the wake of hurricane Ivan.
|
|
Hjulstrom
diagram of threshold stream velocities for erosion, transportation and
deposition of varying particle sizes. Note that a higher water
velocity is required to erode clay and silt than to move sand.
|
|
|
As a river flows, it
deposits sediments at intervals. These deposition spots change the
velocity of the river and encourage the deposition of more sediments.
The river soon takes on a winding, meandering shape as continued
erosion and deposition occur.
Erosion
and deposition patterns on a meandering stream are shown at right.
Erosion of the cutbank and deposition of a point bar on the slip-off
slope. Arrow length is proportional to stream velocity.
|
|
|
These forces of erosion and
deposition happen continually. Sometimes the effect of these processes
is obvious overnight after a torrential rain. Other times, the subtle
movement of the streamcourse may not be visible at all in the course
of a human lifetime. However, rivers always move. Human attempts to
confine and alter the flow of river are usually not successful in the
long run. It is wiser to stay out of the floodplain and build in
upland locations.
|
|
|
|
watershed
definition | water cycle
| floodplains | streams
| groundwater | runoff
|
|
|
|
|
In
most areas of the Eastern United States, groundwater occupies the pore
spaces between soil particles at some depth below the surface. These
deposits can be fairly small, or enormous underground reservoirs, or
aquifers. Near streams and wetlands, the watertable is usually very
close to the surface. Streams and the watertable have a cooperative
relationship. After a rain when the rivers are full, they recharge the
underground water supply. When the streams run dry in the summer, the
groundwater feeds the stream to support the aquatic environment that
lives there.
|
|
When groundwater is extracted for residential,
commercial, and industrial uses the watertable can be altered. This
means the water resides further from the surface than it once did. If
more and more water is taken out, eventually the groundwater supply
may be exhausted. Under natural conditions, precipitation would
percolate through the soil and continually recharge the groundwater.
However, in cities, the precipitation is usually piped away as runoff
and released in a different area, or so quickly that it cannot seep
back into the ground.
|
 |
|
|
|
watershed
definition | water cycle
| floodplains | streams
| groundwater | runoff
|
|
|
|
|
Under
almost all situations, rainfall results in some amount of runoff. Any
water that cannot immediately seep into the ground flows downslope as
runoff. Ground permeability affects runoff significantly. Hard packed
clay soils, such as those prevalent in the Chartiers watershed, absorb
very little water while a loose sand might absorb almost all the
precipitation that falls onto it. The amount of runoff is
related to the amount of rain a region experiences.
However,
urban and rural areas experience the effects of runoff very
differently. The presence of vegetative cover slows the journey of
raindrops from sky to soil and reduces the amount of runoff.
Impermeable surfaces, such as concrete, absorb almost no water at all.
The management of storm runoff is a significant issue in cities,
especially when considering the destructive power of raging water.
|
|
The
Rational Equation is used to calculate amounts of storm water runoff.
The runoff coefficient is calculated based on the permeability of the
ground surface condition.
|
Q=CIA
Q = peak runoff rate
C = runoff coefficient
I = rainfall intensity
A = drainage area (acres)2 |
|
|
Note
that the C value for unimproved areas (forests, native meadows) is
very low, almost all the water is absorbed. The C value for downtown
areas containing a lot of asphalt, concrete, and roof surfaces is very
close to 1.0, which means almost all of the water runs off these
surfaces. Impermeable urban areas can create huge volumes of
stormwater runoff.
Cities
have a large proportion of paved areas and few natural areas with
trees and shrubs. Because so much of the city surface is impervious to
water, most of the precipitation that falls flows away as runoff.
Urban storm runoff is usually directed through storm sewers,
eventually emptying into nearby rivers. Under pre-urban conditions,
much of this volume of water would have absorbed into the ground.
Riverbeds often cannot accommodate this increased volume of water and
massive flooding results downstream from urban areas.
The
graphs below illustrate that the risks of severe flooding and flood
frequency increase with the percentage of area impervious to water as
a result of paving and urbanization.
|
 |
 |
|
The
urban machine does not always fit into the delicate balance of nature.
Urban areas and rural areas outside of the city are often in conflict.
The use and mismanagement of water illustrates this conflict.
Untreated sewage dumped into Chartiers Creek and its tributaries
contaminates the water of nearby neighbors. Urbanization also can
change the flow of streams and cause flooding in towns downstream. |
|
|
|
watershed
definition | water cycle
| floodplains | streams
| groundwater | runoff
|
|
|
