How Will Channel Velocity Change Along The Longitudinal Profile Of A Stream

EENS 111	Physical Geology
Tulane Academy	Prof. Stephen A. Nelson
Streams and Drainage Systems

Streams

A stream is a body of water that carries rock particles and dissolved ions and flows downwards slope forth a clearly defined path, called a channel . Thus, streams may vary in width from a few centimeters to several tens of kilometers. Streams are of import for several reasons:

Streams carry nigh of the h2o that goes from the land to the sea, and thus are an important office of the water bicycle.
Streams bear billions of tons of sediment to lower elevations, and thus are one of the principal transporting mediums in the production of sedimentary rocks.
Streams conduct dissolved ions, the products of chemical weathering, into the oceans and thus make the ocean salty.

Streams are a major part of the erosional process, working in conjunction with weathering and mass wasting. Much of the surface mural is controlled past stream erosion, evident to anyone looking out of an plane window.
Streams are a major source of water, waste matter disposal, and transportation for the world's human population. Most population centers are located next to streams.
When stream channels fill with water the backlog flows onto the the country as a inundation. Floods are a mutual natural disaster.

The objectives for this give-and-take are every bit follows:

How do drainage systems develop and what do they tell us about the geology of an area?
How do stream systems operate?
How do streams erode the landscape?
What kinds of depositional features result from streams?
How do drainage systems evolve?
What causes flooding and how tin we reduce the damage from floods?

Development of Streams - Steamflow begins when h2o is added to the surface from rainfall, melting snow,and groundwater. Drainage systems develop in such a way as to efficiently move water off the land. Streamflow begins as moving sheetwash which is a thin surface layer of h2o. The h2o moves downwardly the steepest slope and starts to erode the surface past creating small rill channels. Every bit the rills coalesce, deepen, and downcut into channels larger channels form. Rapid erosion lengthens the aqueduct upslope in a process called headward erosion . Over fourth dimension, nearby channels merge with smaller tributaries joining a larger trunk stream. (See effigy 17.3 in your text). The linked channels get what is known as a drainage network . With continued erosion of the channels, drainage networks change over fourth dimension.

Drainage Patterns - Drainages tend to develop along zones where rock blazon and structure are almost easily eroded. Thus various types of drainage patterns develop in a region and these drainage patterns reflect the construction of the stone.

Dendritic drainage patterns are most common. They develop on a country surface where the underlying rock is of uniform resistance to erosion.
Radial drainage patterns develop surrounding areas of high topography where elevation drops from a cardinal high expanse to surrounding low areas.

Rectangular drainage patterns develop where linear zones of weakness, such as joints or faults cause the streams to cut down along the weak areas in the rock.
Trellis drainage patterns develop where registrant rocks break upward the landscape (see figure 17.4 in your textbook).

Drainage Basins - Each stream in a drainage system drains a sure area, chosen a drainage basin (also called a catchment or a watershed). In a single drainage basin, all water falling in the basin drains into the aforementioned stream. A drainage divide separates each drainage basin from other drainage basins. Drainage basins tin can range in size from a few km², for small streams, to extremely large areas, such equally the Mississippi River drainage bowl which covers well-nigh 40% of the contiguous U.s.a. (run into figure 17.5c in your text).

Continental Divides - Continents can exist divided into large drainage basins that empty into different ocean basins. For example: N America can exist divided into several basins west of the Rocky Mountains that empty into the Pacific Ocean. Streams in the northern office of North America empty into the Chill Sea, and streams East of the Rocky Mountains empty into the Atlantic Ocean or Gulf of United mexican states. Lines separating these major drainage basins are termed Continental Divides. Such divides usually run along loftier mountain crests that formed recently enough that they have non been eroded. Thus major continental divides and the drainage patterns in the major basins reflect the recent geologic history of the continents.

Permanent Streams - Streams that menses all year are chosen permanent streams. Their surface is at or below the water tabular array. They occur in humid or temperate climates where there is sufficient rainfall and low evaporation rates. Water levels rise and fall with the seasons, depending on the discharge.

Ephemeral Streams - Streams that merely occasionally have water flowing are called imperceptible streams or dry washes. They are above the h2o table and occur in dry climates with low amounts of rainfall and high evaporation rates. They flow mostly during rare flash floods.

Geometry and Dynamics of Stream Channels

Discharge

The stream channel is the conduit for water being carried by the stream. The stream can continually accommodate its channel shape and path as the amount of water passing through the channel changes. The volume of water passing any point on a stream is called the belch . Belch is measured in units of volume/time (m³/sec or ft³/sec).

Q = A 10 V

Discharge (m³/sec) = Cross-sectional Area [width 10 average depth] (m²) x Average Velocity (one thousand/sec).

As the corporeality of water in a stream increases, the stream must adjust its velocity and cross sectional surface area in order to class a rest. Discharge increases every bit more water is added through rainfall, tributary streams, or from groundwater seeping into the stream . As discharge increases, more often than not width, depth, and velocity of the stream as well increase.

Velocity

A stream'south velocity depends on position in the stream channel, irregularities in the stream channel acquired by resistant stone, and stream gradient. Friction slows water along channel edges. Friction is greater in wider, shallower streams and less in narrower, deeper streams.

In directly channels, highest velocity is in the eye. In curved channels,The maximum velocity traces the outside bend where the channel is preferentially scoured and deepened. On the inside of the curve were the velocity is lower, deposition of sediment occurs. The deepest part of the channel is called the thalweg, which meanders with the curve the of the stream. Period around curves follows a spiral path.

Stream menstruation can be either laminar, in which all h2o molecules travel along similar parallel paths, or turbulent, in which individual particles take irregular paths. Stream period is characteristically turbulent. This is chaotic and erratic, with abundant mixing, swirling eddies, and sometimes high velocity. Turbulence is acquired by menses obstructions and shear in the water. Turbulent eddies scour the channel bed, and tin can keep sediment in interruption longer than laminar menstruum and thus aids in erosion of the stream bottom.

Cross Exclusive Shape

Cross-sectional shape varies with position in the stream, and discharge. The deepest part of channel occurs where the stream velocity is the highest. Both width and depth increment downstream because discharge increases downstream. As discharge increases the cross exclusive shape volition change, with the stream becoming deeper and wider.

Erosion by Streams

Streams erode considering they accept the power to pick up rock fragments and transport them to a new location. The size of the fragments that can exist transported depends on the velocity of the stream and whether the flow is laminar or turbulent. Turbulent flow can go on fragments in suspension longer than laminar flow.

Streams can besides erode by undercutting their banks resulting in mass-wasting processes like slumps or slides. When the undercut cloth falls into the stream, the fragments can be transported away by the stream.

Streams tin can cut deeper into their channels if the region is uplifted or if there is a local alter in base level. As they cutting deeper into their channels the stream removes the material that once made up the channel bottom and sides.

Although slow, as rocks move along the stream bottom and collide with one another, chafe of the rocks occurs, making smaller fragments that can then be transported past the stream.

Finally, because some rocks and minerals are easily dissolved in water, dissolution also occurs, resulting in dissolved ions being transported by the stream.

Sediment Ship and Deposition

The rock particles and dissolved ions carried by the stream are the called the stream's load. Stream load is divided into three categories.

Suspended Load - particles that are carried forth with the h2o in the main office of the streams. The size of these particles depends on their density and the velocity of the stream. College velocity currents in the stream can carry larger and denser particles.

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. Annotation that sediment tin move between bed load and suspended load as the velocity of the stream changes.

Dissolved Load - ions that have been introduced into the h2o by chemical weathering of rocks. This load is invisible because the ions are dissolved in the water. The dissolved load consists mainly of HCO₃ ^- ² (bicarbonate ions), Ca⁺ⁱⁱ, And so₄ ^-2, Cl^-, Na⁺ⁱⁱ, Mg⁺ⁱⁱ, and Grand⁺. These ions are eventually carried to the oceans and give the oceans their salty graphic symbol. Streams that have a deep underground source generally have higher dissolved load than those whose source is on the Earth's surface.

The maximum size of particles that can exist carried as suspended load by the stream is called stream competence . The maximum load carried by the stream is called stream capacity . Both competence and capacity increase with increasing discharge. At loftier belch bedrock and cobble size material can move with the stream and are therefore transported. At low discharge the larger fragments go stranded and only the smaller, sand, silt, and dirt sized fragments motion.

When menstruum velocity decreases the competence is reduced and sediment drops out. Sediment grain sizes are sorted by the water. Sands are removed from gravels; muds from both. Gravels settle in channels. Sands drop out in near channel environments. Silts and clays drape floodplains away from channels.

Changes Downstream

As one moves along a stream in the downstream direction:

Discharge increases, as noted above, because water is added to the stream from tributary streams and groundwater.
Equally discharge increases, the width, depth, and average velocity of the stream increment.
The gradient of the stream, however, will decrease.

It may seem to exist counter to your observations that velocity increases in the downstream direction, since when 1 observes a mountain stream near the headwaters where the gradient is high, it appears to accept a college velocity than a stream flowing along a gentle gradient. But, the water in the mountain stream is likely flowing in a turbulent way, due to the large boulders and cobbles which brand upwards the streambed. If the flow is turbulent, then it takes longer for the water to travel the aforementioned linear distance, and thus the average velocity is lower.

As well as ane moves in the downstream direction,

The size of particles that make upward the bed load of the stream tends to decrease. Even though the velocity of the stream increases downstream, the bed load particle size decreases mainly because the larger particles are left in the bed load at higher elevations and chafe of particles tends to reduce their size.
The composition of the particles in the bed load tends to change along the stream equally dissimilar bedrock is eroded and added to the stream's load.

Long Profile

A plot of elevation versus distance. Commonly shows a steep slope or gradient, well-nigh the source of the stream and a gentle gradient equally the stream approaches its rima oris. The long contour is concave up, as shown by the graph below.

Base of operations Level

Base level is defined as the limiting level beneath which a stream cannot erode its aqueduct. For streams that empty into the oceans, base level is sea level. Local base of operations levels tin occur where the stream meets a resistant trunk of rock, where a natural or artificial dam impedes further aqueduct erosion, or where the stream empties into a lake.

When a natural or artificial dam impedes stream flow, the stream adjusts to the new base level past adjusting its long profile. In the example here, the long contour to a higher place and below the dam are adjusted. Erosion takes identify downstream from the dam (peculiarly if it is a natural dam and h2o tin can flow over the acme). Just upstream from the dam the velocity of the stream is lowered so that degradation of sediment occurs causing the gradient to become lower. The dam substantially go the new base of operations level for the part of the stream upstream from the dam.

In full general, if base level is lowered, the stream cuts downward into its channel and erosion is accelerated. If base of operations level is raised, the stream deposits sediment and readjusts its profile to the new base level.

Valleys and Canyons

Land far above base of operations level is subject to downcutting by the stream. Rapid downcutting creates an eroded trough which tin become either a valley or canyon. A valley has gently sloping sidewalls that bear witness a V-shape in cantankerous-section. A Canyon has steep sidewalls that class cliffs. Whether or valley or coulee is formed depends on the rater of erosion and strength of the rocks. In general, slow downcutting and weak, easily erodable rocks results in valleys and rapid downcutting in stronger rocks results in canyons.

Because geologic processes stack strong and weak rocks, such stratigraphic variation frequently yields a stair step contour of the coulee walls, as seen in the Yard Canyon. Strong rocks yield vertical cliffs, whereas weak rocks produce more gently sloped coulee walls.

Active downcutting flushes sediment out of channels. Only after the sediment is flushed our can farther downcutting occur. Valleys store sediment when base level is raised.

Rapids

Rapids are turbulent h2o with a rough surface. Rapids occur where the stream gradient of a sudden increases, where the stream flows over large clasts in the bed of the stream, or where there is an abrupt narrowing of the channel. Sudden change in gradient may occur where an active fault crosses the stream channel. Large clasts may exist transported into the stream by a tributary stream resulting in rapids where the ii streams join. Abrupt narrowing of the stream may occur if the stream encounters potent rock that is not easily subject to erosion.

Waterfalls

Waterfalls are temporary base levels caused past strong erosion resistant rocks. Upon reaching the strong rock, the stream then cascades or costless falls down the steep slope to form a waterfalls. Because the rate of flow increases on this rapid modify in slope, erosion occurs at the base of the waterfall where a plunge pool forms. This tin initiate rapid erosion at the base, resulting in undercutting of the cliff that acquired the waterfall. When undercutting occurs, the cliff becomes subject area to rockfalls or slides. This results in the waterfall retreating upstream and the stream eventually eroding through the cliff to remove the waterfall.

Niagara Falls in upstate New York is a skilful example. Lake Erie drops 55 grand flowing toward Lake Ontario. A dolostone caprock is resistant and the underlying shale erodes. Blocks of unsupported dolostone collapse and fall.
Niagara Falls continuously erodes southward toward Lake Erie. In temporary diversion of the water that flows over the American Falls section revealed huge blocks of rock. The rate of s retreat of Niagara Falls is presently 0.5 m/yr. Eventually the falls will reach Lake Erie, and when that happens Lake Erie will drain.

Channel Patterns

Straight Channels - Straight stream channels are rare. Where they do occur, the channel is usually controlled by a linear zone of weakness in the underlying rock, similar a fault or articulation system.

Even in straight aqueduct segments h2o flows in a sinuous mode, with the deepest function of the channel changing from nigh one bank to about the other. Velocity is highest in the zone overlying the deepest part of the stream. In these areas, sediment is transported readily resulting in pools . Where the velocity of the stream is depression, sediment is deposited to form bars .

The bank closest to the zone of highest velocity is usually eroded and results in a cutbank .

Meandering Channels - Because of the velocity structure of a stream, and specially in streams flowing over low gradients with easily eroded banks, directly channels will eventually erode into meandering channels . Erosion will take identify on the outer parts of the meander bends where the velocity of the stream is highest. Sediment deposition will occur along the inner meander bends where the velocity is low. Such deposition of sediment results in exposed bars, called indicate bars . Considering meandering streams are continually eroding on the outer meander bends and depositing sediment along the inner meander bends, meandering stream channels tend to migrate back and along across their flood plain.

If erosion on the outside meander bends continues to take place, eventually a meander curve tin can become cut off from the residuum of the stream. When this occurs, the cutoff meander bend, because it is even so a depression, will collect water and form a type of lake called an oxbow lake .

Braided Channels - In streams having highly variable belch and easily eroded banks, sediment gets deposited to form confined and islands that are exposed during periods of low discharge. In such a stream the water flows in a braided pattern around the islands and bars, dividing and reuniting as information technology flows downstream. Such a channel is termed a braided aqueduct . During periods of high discharge, the unabridged stream channel may contain water and the islands are covered to get submerged bars. During such loftier discharge, some of the islands could erode, but the sediment would be re-deposited every bit the discharge decreases, forming new islands or submerged bars. Islands may become resistant to erosion if they become inhabited by vegetation

Stream Deposits

Sudden changes in velocity can result in deposition by streams. Within a stream we have seen that the velocity varies with position, and, if sediment gets moved to the lower velocity part of the stream the sediment will come out of suspension and be deposited. Other sudden changes in velocity that affect the whole stream tin can besides occur. For example if the belch is suddenly increased, as information technology might be during a flood, the stream will overtop its banks and flow onto the floodplain where the velocity will so suddenly subtract. This results in deposition of such features as levees and floodplains. If the gradient of the stream of a sudden changes by emptying into a flat-floored bowl, an body of water basin, or a lake, the velocity of the stream will all of a sudden decrease resulting in deposition of sediment that can no longer be transported. This can result in deposition of such features equally alluvial fans and deltas.

Floodplains and Levees - As a stream overtops its banks during a flood, the velocity of the flood will commencement be loftier, but will of a sudden subtract as the water flows out over the gentle gradient of the floodplain. Because of the sudden decrease in velocity, the coarser grained suspended sediment will be deposited along the riverbank, somewhen building up a natural levee. Natural levees provide some protection from flooding because with each overflowing the levee is built higher and therefore discharge must be college for the next flood to occur. (Annotation that the levees we see along the Mississippi River hither in New Orleans are non natural levees, only man made levees, built to protect the floodplain from floods. Still, the natural levees do form the loftier ground as evidenced past the flooding that occurred as a event of levee breaches during Hurricane Katrina).

Terraces - Terraces are exposed former floodplain deposits that result when the stream begins down cutting into its alluvion plain (this is usually caused by regional uplift or by lowering the regional base of operations level, such as a drop in sea level).

Alluvial Fans - When a steep mount stream enters a flat valley, there is a sudden decrease in gradient and velocity. Sediment transported in the stream volition suddenly become deposited along the valley walls in an alluvial fan. As the velocity of the mount stream slows it becomes high-strung with sediment and breaks up into numerous distributary channels.
Deltas - When a stream enters a standing body of water such every bit a lake or ocean, again there is a sudden subtract in velocity and the stream deposits its sediment in a eolith chosen a delta. Deltas build outward from the coastline, but volition only survive if the sea currents are non strong plenty to remove the sediment.

As the velocity of a stream decreases on inbound the delta, the stream becomes choked with sediment and conditions become favorable to those of a braided stream channel, but instead of braiding, the stream breaks into many smaller streams called distributary streams.

Over the terminal 1,000 years, most of the country that makes up southern Louisiana has been built by the Mississippi River depositing sediment to class delta lobes. These delta lobes have shifted back and along through time as the River's class inverse in response to changes in sea level and the River trying to maintain the shortest and steepest path to the Gulf of United mexican states (run into figure 17.25a)

Drainage Evolution

Landscapes on Earth'southward surface evolve over fourth dimension with the chief crusade of modify being streamflow and the resulting erosion and deposition. For example:
Uplift sets a new base level which causes streams to cut deeper, resulting in widening of valleys and erosion of hills. If these erosional processes were to continue, the mural would be eroded to base level.

Stream Piracy

Stream piracy is where one stream erodes headward to capture the drainage of another stream. The stream with more vigorous erosion (steeper gradient), intercepts another stream and water from the captured stream no flows into the pirating stream (run across figure 17.26 in your text).

Drainage Reversal

Drainage reversals can occur as a consequence of tectonic processes. For example, in the early Mesozoic when Africa and South America were role of the same continent, South America drained due west. Eventually Africa separated from South America to form the Atlantic Sea on the eastern side of S America. On the west coast, subduction began and the resulting compression caused the uplift of the Andes mountains. Equally the uplift occurred, the drainage had to reverse to menstruum to the east into the Atlantic Ocean (run across figure 17.27 in your text).

Superposed and Ancestor Streams

In looking at the landscape, it is often evident that streams sometimes cut through plain-featured terrain seemingly ignoring the geologic structures and hardness of the rock. If a stream initially develops on younger flat strata made of soft textile and so cuts downward into the underlying plain-featured strata while maintaining the course developed in the younger strata, information technology is referred to as a superposed stream , because the stream design was superposed on the underlying rocks. In such cases much of the original soft strata is removed. (see effigy 17.29 in your text).

If tectonic uplift raises the basis beneath established streams and if erosion keeps pace with uplift, the stream volition cut downward and maintain its original course. In such a case, the stream is called an antecedent stream, because the stream was present before the uplift occurred . (See figure 17.30 in your textbook).

Some antecedent streams have incised meanders. The meanders initially develop on a gentle slope then uplift raises the landscape (dropping the base level) and the meanders cut downward into the uplifted mural (see effigy 17.28 in your text for an instance).

Floods

Floods occur when the discharge of the stream becomes also high to exist accommodated in the normal stream aqueduct. When the discharge becomes too high, the stream widens its aqueduct by overtopping its banks and flooding the low-lying areas surrounding the stream. The areas that become flooded are called floodplains .

Floodwaters are devastating to people and property. During a flood belch exceeds the storage volume of the stream channel. Velocity (thus, competence and capacity) increment and water leaves the channel and flows onto adjacent land. H2o slows away from the thalweg, dropping sediment.

Causes of Flooding

Heavy rains dump large volumes of water on the landscape increasing the amount of water flowing into the stream.
If the soil has become saturated every bit a effect of rain and then that there is no room in the soil for water to infiltrate, the water instead will come across stream channels and increment the discharge.
In the wintertime, if a sudden increase in temperature rapidly melts snow causing an influx of h2o into the drainage organisation.
When a natural or bogus dam breaks or levee breaks, releasing water into a channel with a sudden increase in discharge or releases water from the channel onto the surrounding floodplain.

Flood Stage

The term stage refers to the height of a river (or whatsoever other trunk of water) above a locally defined tiptop. This locally defined height is a reference level, often referred to as datum. For example, for the lower function of the Mississippi River, reference level or datum, is sea level (0 feet). Currently the Mississippi River is at a phase of near 12.5 feet, that is 12.v feet above ocean level. Other river systems have a reference level that is not body of water level. Most rivers in the United States have gaging stations where measurements are continually made of the river's stage and belch. These are plotted on a graph called a hydrograph , which shows the stage or belch of the river, as measured at the gaging station, versus time.
When the discharge of a river increases, the channel may become completely total. Whatsoever discharge above this level volition issue in the river overflowing its banks and causing a overflowing. The stage at which the river will overflow its banks is called bankfull stage or flood stage. For instance, the graph below is a hydrograph of the Mississippi River at St. Louis, Missouri during the time menstruum of the 1993 flood. Discharge is plotted on the Y-axis, and dates are plotted on the ten-axis. Note that stages corresponding to diverse discharges are shown on the left-hand y-axis, and that the spacing between equal units of stage are non equal along the y-axis.

missrivflooddis.gif (18467 bytes)

Notation that for the 1993 Mississippi River Flood, the river reached overflowing stage of 30 feet in a higher place datum on about June 26 and peaked (or crested) at only under 50 feet to a higher place datum on August 1. The sudden drops seen in discharge around July 15 and July 20 corresponded to breaks in the levee system upstream from St. Louis that caused water to flow onto the floodplain upstream, thus reducing both the stage and discharge measured at St. Louis.
To illustrate, for the Mississippi River flood at St. Louis, idealized cross sections of the River are shown below for points a, b, and c in the diagram higher up.

Lag Time

The time difference between when heavy atmospheric precipitation occurs and when peak belch occurs in the streams draining an area is chosen lag time.

Lag fourth dimension depends on such factors as the amount of time over which the rain falls and the amount of water that can infiltrate into the soil.

If the corporeality of rain is high over a short time menstruation, lag time is curt. If the corporeality of rain is loftier over a longer time period, lag time is longer. Lack of infiltration and interception reduce lag time

Wink floods occur when the charge per unit of infiltration is low and heavy rains occur over a brusk period of fourth dimension. Because they come with niggling warning, wink floods are the well-nigh dangerous to homo lives. Such floods stem from unusually intense rainfall or dam failures, strike with little alarm,an they are oft deadly. (Meet the example of the Big Thompson Canyon flash flood in your text, p. 644).

Any time the surface materials of the Earth are covered with impermeable materials like concrete, asphalt, or buildings, the infiltration of h2o into the soil is prevented. Urbanization tends to reduce infiltration, and thus water must collect in tempest sewers and somewhen in the main drainage systems. Thus, all-encompassing urbanization too decreases the lag time and increases the elevation discharge fifty-fifty further. Urbanization can therefore lead to a college incidence of wink floods.

urbanization.gif (11218 bytes)

Flooding Risk

Discharge data collected over a long flow of time on streams can be used to calculate alluvion probability. The data are plotted on a graph of Peak Discharge for each year versus recurrence interval. Annotation that the logarithm of the recurrence interval is used. As an case, such a graph is shown for the Cerise River of the N at Fargo, North Dakota below.

From such a graph one tin determine the stage or belch for different recurrence intervals. The 10 year overflowing is defined as the discharge that would have a 10% probability of occurring every year. Similarly, the 100 year flood is the belch that has a 1% chance of occurring every year. Note that the 100 year flood does not necessarily occur only once every 100 years. For case, the graph for the Red River of the North, above, shows that two 250 twelvemonth floods occurred in an viii year period.

F lood Hazard Mapping

Food hazard mapping is used to decide the areas susceptible to flooding when discharge of a stream exceeds the bank-total stage. Using historical data on river stages and belch of previous floods, forth with topographic data, maps can be constructed to prove areas expected to be covered with floodwaters for various discharges or stages.

FloodMap.GIF (21479 bytes)

Inundation Control

Response to overflowing hazards can be attempted in two main means: An engineering science approach, to control flooding, and a regulatory arroyo designed to decrease vulnerability to flooding.

Applied science Approaches

Aqueduct modifications - By creating new channels for a stream, the cross-sectional area can be enlarged, thus create a state of affairs where a higher stage is necessary before flooding. Channelization also increases water velocity, and thus reduces drainage fourth dimension.
Dams - Dams tin be used to hold water back then that discharge downstream tin be regulated at a desired rate. Human being synthetic dams have spillways that tin be opened to reduce the level of h2o in the reservoir backside the dam. Thus, the h2o level can exist lowered prior to a heavy rain, and more water tin can be trapped in the reservoir and released later on at a controlled belch.
Retentivity ponds - Retentiveness ponds serve a similar purpose to dams. Water can be trapped in a retention swimming and so released at a controlled discharge to forestall flooding downstream.
Levees, Dikes, and Floodwalls - These are structures built along side the channel to increase the stage at which the stream floods.
Floodways - Floodways are areas that tin can be built to provide an outlet to a stream and allow it overflowing into an area that has been designated as a floodway. Floodways are areas where no construction is immune, and where the land is used for agronomical or recreational purposes when at that place is no threat of a flood, but which provide an outlet for inundation waters during periods of high discharge. The Bonnet Carrie Spillway due west of New Orleans is such a floodway. During depression stages of the Mississippi River the land between the River and Lake Pontchartrain is used for recreational purposes - hunting, fishing, and clay wheel riding for example. During loftier stages of the River when there is a potential for the River to rise to flood stage in New Orleans, the spillway is opened so that h2o drains into Lake Pontchartrain. This lowers the level of h2o in the Mississippi and reduces the possibility of a levee break or water overtopping the levee.

Regulatory Approaches

With a better agreement of the behavior of streams, the probability of flooding, and areas likely to be flooded during high discharge, humans can undertake measures to reduce vulnerability to flooding. Amongst the regulatory measures are:

Floodplain zoning - Laws tin can be passed that restrict construction and habitation of floodplains. Instead floodplains can exist zoned for agronomical use, recreation, or other uses wherein lives and belongings are not endangered when (note that I did not utilize the discussion if) inundation waters re-occupy the floodplain.
Floodplain edifice codes - Structures that are allowed within the floodplain could be restricted those that can withstand the high velocity of flood waters and are loftier enough off the ground to reduce risk of contact with water.
Floodplain buyout programs - In areas that have been recently flooded, it may be more cost effective for the government, which ordinarily pays for flood harm either through subsidized flood insurance or direct disaster relief, to buy the rights to the land rather than pay the cost of reconstruction and then have to pay again the next fourth dimension the river floods.
Mortgage limitations - Lending institutions could turn down to give loans to buy or construct dwellings or businesses in flood decumbent areas.

Case of a Flood

During Hurricane Katrina in 2005, much of New Orleans flooded, mainly equally a result of levee and floodwall failures that occurred on human made drainage and navigation canals. In lecture, this issue will be discussed in some detail. For details on the geological aspects of the flood events see the following web page and its included links - www.tulane.edu/~sanelson/Katrina.

Examples of questions on this material that could be asked on an exam

Define the following: (a) ephemeral stream, (b) stream gradient, (c) stream discharge, (d) suspended load, (e) bed load (f) dissolved load (yard) drainage bowl, (h) drainage divide
What happens to a stream'southward discharge as one moves down stream? Explain why this occurs.
List and give a brief description of the various types of drainage internet works..
What weather condition are necessary for stream to be meandering stream and a braided stream?
How do streams erode?
Define the following: (a) alluvial fan, (b) delta, (c) floodplain, (d) betoken bar, (e) stream piracy, (f) floodstage, (g) hydrograph, (h) flash overflowing, (i) stream terrace.
What are the main causes of floods?
What is the probability that the 100 year flood volition occur in whatever given yr?
How does homo development affect alluvion hazards?
What engineering approaches are available to reduce the risk of flooding?
Besides engineering solutions, what other steps can be taken to reduce vulnerability to flooding?