It is a rainy season and overflowing of river water is a natural tendency. With this,
they also deposit a huge sediment beyond the bank of the river.
What does this river water carry? What does it give rise to? What kind of a wall
does it create which leave behind the deposits ?
Answers
A flood results when a stream runs out of its confines and submerges surrounding areas.
In less developed countries, humans are particularly sensitive to flood casualties because of high population density, absence of zoning regulations, lack of flood control, and lack of emergency response infrastructure and early warning systems.
In industrialized countries the loss of life is usually lower because of flood control structures, zoning regulations that prevent the habitation of seriously vulnerable lands, and emergency preparedness. Still, property damage and disruption of life takes a great toll, and despite flood control structures and land use planning, floods still do occur.
Causes of Flooding
From a geological perspective, floods are a natural consequence of stream flow in a continually changing environment. Floods have been occurring throughout Earth history, and are expected so long as the water cycle continues to run. Streams receive most of their water input from precipitation, and the amount of precipitation falling in any given drainage basin varies from day to day, year to year, and century to century.
The Role of Precipitation
Weather patterns determine the amount and location of rain and snowfall. Unfortunately the amount and time over which precipitation occurs is not constant for any given area. Overall, the water cycle is a balanced system. Water flowing into one part of the cycle (like streams) is balanced by water flowing back to the ocean. But sometimes the amount flowing in to one area is greater than the capacity of the system to hold it within natural confines. The result is a flood. Combinations of factors along with exceptional precipitation can also lead to flooding. For example, heavy snow melts, water saturated ground, unusually high tides, and drainage modifications when combined with heavy rain can lead to flooding.
Dam & Levee Failures
Stream Systems
A stream is a body of water that carries rock particles and dissolved ions and flows down slope along a clearly defined path, called a channel. Thus streams may vary in width from a few centimeters to several kilometers. Streams are important for several reasons
Velocity - A stream's velocity depends on position in the stream channel, irregularities in the stream channel caused by resistant rock, and stream gradient.
Friction slows water along channel edges. Friction is greater in wider, shallower streams and less in narrower, deeper streams. In straight channels, highest velocity is in the center.
In curved channels - maximum velocity traces the outside curve where the channel is preferentially scoured and deepened. On the inside of the curve were the velocity is lower, deposition of sediment occurs.
Stream flow can be either laminar, in which all water molecules travel along similar parallel paths, or turbulent, in which individual particles take irregular paths. Turbulent flow can keep sediment in suspension longer than laminar flow and aids in erosion of the stream bottom.
Q = A x V
Discharge (m3/sec) = Cross-sectional Area (width x average depth) (m2) x Average Velocity (m/sec)
As the amount of water in a stream increases, the stream must adjust its velocity and cross sectional area in order to form a balance. Discharge increases as more water is added through rainfall, tributary streams, or from groundwater seeping into the stream. As discharge increases, generally width, depth, and velocity of the stream also increase. Increasing the depth and width of the stream may cause the stream to overflow is channel resulting in a flood.
Load - The rock particles and dissolved ions carried by the stream are the called the stream's load. Stream load is divided into three parts:
Suspended Load - particles that are carried along with the water in the main part of the stream. The size of these particles depends on their density and the velocity of the stream. Higher velocity currents in the stream can carry larger and denser particles. The suspended load is what gives most streams their muddy looking appearance and brown or red color. When the velocity of the stream is decreased, that particles in the suspended load that can no longer be suspended are deposited.
Bed Load - coarser and denser particles that remain on the bed of the stream most of the time but move by a process of saltation (jumping) as a result of collisions between particles and turbulent eddies. Note that sediment can move between bed load and suspended load as the velocity of the stream changes.