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Planet earth streams and floods notes Planet earth streams and floods notes Document Transcript

  • Prof. C.ValentiPlanet Earth Streams and Floods Notes 1Stream SystemsA stream is defined as a body of running water that is confined in a channel. Itcarries rock particles and dissolved ions and flows down and moves downhill underthe influence of gravity. Doesn’t matter what size or shape all are called streams.Thus streams may vary in width from a few centimeters to several kilometers.Streams are important for several reasons • Streams carry most of the water that goes from the land to the sea, and thus are an important part of the water cycle. • Streams carry billions of tons of sediment to lower elevations, and thus are one of the main transporting mediums in the production of sedimentary rocks. • Streams carry dissolved ions, the products of chemical weathering, into the oceans and thus make the sea salty. • Streams are a major part of the erosional process, working in conjunction with weathering and mass wasting. Much of the surface landscape is controlled by stream erosion. • Streams are a major source of water and transportation for the worlds human population. Most population centers are located next to streams.Planet Earth Streams and Floods Notes
  • Prof. C.ValentiPlanet Earth Streams and Floods Notes 2Geometry and Dynamics of Stream ChannelsThe stream channel is the conduit for water being carried by the stream. Thestream can continually adjust its channel shape and path as the amount of waterpassing through the channel changes The volume of water passing any point on astream is called the discharge. Discharge is measured in units of volume/time(m3/sec).Longitudinal Profilea plot of elevation versus distance. Usuallyshows a steep gradient near the source of thestream and a gentle gradient as the streamapproaches its mouth.The Longitudinal profile of a stream is thetypical stream viewed from its side. Thestream begins in steep mountains and flows outacross a gentle plain into the sea.• The headwaters of a stream are in the upper part of the stream near its source in the mountains. Also referred to as upstream.• Lower reaches of a stream are referred to as downstream regions.• The mouth is the place where a stream enters the sea, a lake or a larger stream.• Base level is the theoretical limit to which the stream can erode. The base level is, in effect, the elevation of the streams mouth, where the stream enters an ocean, lake, or another stream. The ultimate base level is sea level.Water on a slope has potential energy and gravity pulls the water downstreamwhere the potential energy is converted to kinetic energy. The amount of astreams potential energy is proportional to the streams gradient, the vertical dropof a channel over a horizontal distance. As potential energy is converted to kineticenergy, the stream performs the work of erosion. When the stream reaches itslowest elevation, the stream has little kinetic energy, loses its capacity to do work,and the sediment load is dropped out of suspension.Planet Earth Streams and Floods Notes
  • Prof. C.ValentiPlanet Earth Streams and Floods Notes 3Longitudinal Profile (Aerial View = the stream from above)How water gets to the stream, the drainage basin.Water may enter a stream in a variety of ways. Ultimately the drainage basin fora particular stream is the portion of the ground surface that collects water anddrains it to that stream. It consists of a main channel and all of the tributaries that flow into it. Water enters streams via tributaries, which are smaller streams that feed (discharge) into a larger one. It is bounded by a divide (ridge), beyond which water is drained by another system. The drainage basin is surrounded by areas of high relief; topographic divide - a ridge or topographic separation between adjacent drainage basins.. Think of it as a cup of land surface receiving water. • For example, the Mississippi River is very large and therefore has a large drainage basin. It is divided by the Continental Divide, a ridge of high elevation, where everything that falls on one side flows west and the other side flows east to the Mississippi River.Planet Earth Streams and Floods Notes
  • Prof. C.ValentiPlanet Earth Streams and Floods Notes 4 Drainage basins are of variable size depending upon your point of reference. As the reference point moves downstream, the size of the drainage basin increases (because tributaries increase).  Water that directly falls on the land within the drainage basin can enter a stream by runoff, or overland flow; transports sediments into the stream.  Water can enter the stream by infiltration into groundwater; seeping into the ground and flowing downhill into the stream. Water seeps into the ground based on: • (1) intensity of rainfall (increases overland flow because cannot infiltrate very quickly • (2) prior wetted condition of the soil (soil is saturated, it will not absorb more water) • (3) soil texture (sands allow more infiltration) • (4) slope of land (steeper slopes favor overland flow, flat areas favor infiltration) • (5) vegetation (vegetation removes water from soil).  Water can enter a stream by direct precipitation.♦ The collecting system consists of a network of tributaries in the headwater region that collect and funnel water and sediment to the main stream. It commonly has a dendritic (treelike) drainage pattern, with numerous branches that extend upslope toward the divide. They develop on a land surface where the underlying rock is of uniform resistance to erosion. The surface of the ground slopes toward the network of tributaries, so the drainage system acts as a funneling mechanism for water.♦ The transporting system is the main trunk stream, which functions as a channel through which water and sediment flow from the collecting area toward the ocean. Although the major process is transportation, this system also collects additional water and sediment.♦ The dispersing system consists of a network of distributaries at the mouth of the stream, where sediment and water are dispersed into an ocean, lake, or dry basin. The major processes are the deposition of the sediment load and the river waters into the basin.Planet Earth Streams and Floods Notes
  • Prof. C.ValentiPlanet Earth Streams and Floods Notes 5 If the gradient of the stream suddenly changes by emptying into a flat-floored basin, an ocean basin, or a lake, the velocity of the stream will suddenly decrease resulting in deposition of sediment that can no longer be transported. This can result in deposition of such features as alluvial fans and deltas. Deltas. Most streams ultimately flow into the sea or into large lakes. A stream flowing into a body of standing water builds a delta, a body of sediment deposited at the mouth of a river when the stream’s velocity decreases. The surface is marked by distributaries, small shifting channels that carry water away from the main river channel and distribute it over the surface of the delta. Delta’s resemble the Greek letter delta and are triangluar in shape. Alluvial Fans. Some streams, in dry climates, do not reach the sea or any other body of water. They build alluvial fans instead of deltas. An alluvial fan is a large, fan or cone shaped pile of sediment that usually forms where a stream’s velocity decreases as it emerges from a narrow mountain canyon onto a flat plain. An alluvial fan builds up gradually as streams deposit sediments from flash flooding conditions at the top of the mountain. The sudden loss of velocity when a stream flows from steep mountains onto a plain causes the sediment to deposit on top of the fan in layers, due to the decrease in gradient and the widening of the channel.Planet Earth Streams and Floods Notes
  • Prof. C.ValentiPlanet Earth Streams and Floods Notes 6Cross Sectional Shape - varies with position in the stream and discharge. Thedeepest parts of a channel occur where the stream velocity is the highest. Bothwidth and depth increase downstream because discharge increases downstream. Asdischarge increases the cross sectional shape will change, with the streambecoming deeper and wider.Upstream – V shaped channels – high velocity water cuts into stream bed.Downstream – U shaped channels – high volumes of water, therefore wide anddeep.Stream cross section shows the stream channel which normally contains all waterflowing in the stream.• Banks/valley walls are the sides of the channel.• Stream bed is the bottom of the channel.• Stage is the level of water in the stream.• Levees are slightly elevated areas that parallel the stream (formed by deposition of sediment during flooding). During a flood the waters of the stream may rise and spill over the banks onto the flat flood plain on adjacent sides of the stream beyond the levees.• Flood Plain. A flood plain is a broad strip of land built up by sedimentation on either side of a stream channel. During floods, flood plains become inundated with water (and suspended load). As water spreads over a floodplain, velocity decreases causing most of the sediment to be deposited near the stream channel. Over and over, this process builds up natural levees, low ridges of flood deposited sediment that form on either side of a stream channel and thin away from the channel. Sediment near and within the channel is the coarsest, and fines as you move away from the stream.Sudden decreases in velocity can result in deposition by streams. Within a streamwe have seen that the velocity varies with position, and, if sediment gets moved toPlanet Earth Streams and Floods Notes
  • Prof. C.ValentiPlanet Earth Streams and Floods Notes 7the lower velocity part of the stream the sediment will come out of suspension andbe deposited. Other sudden changes in velocity that affect the whole stream canalso occur. For example if the discharge is suddenly increased, as it might beduring a flood, the stream will overtop its banks and flow onto the floodplain wherethe velocity will then suddenly decrease. This results in deposition of such featuresas levees and floodplains.Load - The rock particles and dissolved ions carried by the stream are the calledthe streams load. Stream load is divided into three parts:Suspended Load - particles that are carried along with the water in the main partof the stream. The size of these particles depends on their density and thevelocity of the stream. Higher velocity currents in the stream can carry larger anddenser particles. The suspended load is what gives most streams their muddylooking appearance and brown or red color.Bed Load - coarser and denser particles that remain onthe bed of the stream most of the time but move by aprocess of saltation (jumping) as a result of collisionsbetween particles and turbulent eddies. Note thatsediment can move between bed load and suspended loadas the velocity of the stream changes.Dissolved Load - ions that have been introduced into the water by chemicalweathering of rocks. This load is invisible because the ions are dissolved in thewater. The dissolved load consists mainly of HCO-3 (bicarbonate ions), Ca+2, SO4-2,Cl-, Na+, Mg+2, and K+. These ions are eventually carried to the oceans and give theoceans their salty character.• Capacity is the maximum amount of material a stream can transport.• Competence is the maximum grain size a stream can carry, dependent on velocity alone.Planet Earth Streams and Floods Notes
  • Prof. C.ValentiPlanet Earth Streams and Floods Notes 8Velocity - A streams velocity depends on position in the stream channel,irregularities in the stream channel caused by resistant rock, and stream gradient.The average velocity is the time it takes a given particle of water to traversea given distance. 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. Average linear velocity is generally greater in laminar flow than in turbulent flow.It may seem to be counter to your observations that velocity increases in thedownstream direction, since when one observes a mountain stream near theheadwaters where the gradient is high, it appears to have a higher velocity than astream flowing along a gentle gradient. But, the water in the mountain stream islikely flowing in a turbulent manner, due to the large boulders and cobbles whichmake up the streambed. If the flow is turbulent, then it takes longer for the waterto travel the same linear distance, and thus the average velocity is lower.Discharge - The discharge of a stream is the amount of water passing any point ina given time. Q=AxVDischarge (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 velocityand cross sectional area in order to form a balance. Discharge increases as morewater is added through rainfall, tributary streams, or from groundwater seepinginto the stream. As discharge increases, generally width, depth, and velocityof the stream also increase.• In general, if an increase of water enters a stream, the stream must adjust itself to handle this increase in water. (Vice Versa)• Increase width or depth to increase area, or increase velocity. Typically when velocity increases, streams erode at base and sides therefore area increases. So, increase in velocity increases the erosional capabilities of the stream and the channels widen. Conversely, decrease in velocity, favors deposition, which decreases area, increases friction.Planet Earth Streams and Floods Notes
  • Prof. C.ValentiPlanet Earth Streams and Floods Notes 9Changes Downstream - As one moves along a stream in the downstream direction:Discharge increases, as noted above, because water is added to the stream fromtributary streams and groundwater.As discharge increases, the width, depth, and average velocity of the streamincrease.The gradient of the stream, however, will decrease.Planet Earth Streams and Floods Notes
  • Prof. C.ValentiPlanet Earth Streams and Floods Notes 10Channel PatternsStraight Channels - Straightstream channels are rare. Wherethey do occur, the channel isusually controlled by a linear zoneof weakness in the underlying rock,like a fault or joint system. Even instraight channel segments waterflows in a sinuous fashion, with thedeepest part of the channelchanging from near one bank tonear the other. Velocity is highestin the zone overlying the deepestpart of the stream. In theseareas, sediment is transported readily resulting in pools. Where the velocity of thestream is low, sediment is deposited to form bars. The bank closest to the zone ofhighest velocity is usually eroded and results in a cutbank.Meandering Channels - Because of thevelocity structure of a stream, andespecially in streams flowing over lowgradients with easily eroded banks,straight channels will eventually erodeinto meandering channels. Erosion willtake place on the outer parts of themeander bends where the velocity ofthe stream is highest. Sedimentdeposition will occur along the innermeander bends where the velocity islow. Such deposition of sediment results in exposed bars, called point bars.Because meandering streams are continually eroding on the outer meander bendsand depositing sediment along the inner meander bends, meandering streamchannels tend to migrate back and forth across their flood plain.If erosion on the outside meander bends continues to take place, eventually ameander bend can become cut off from the rest of the stream. When this occurs,the cutoff meander bend, because it is still a depression, will collect water andform a type of lake called an oxbow lake.Planet Earth Streams and Floods Notes
  • Prof. C.ValentiPlanet Earth Streams and Floods Notes 11Braided Channels –In streams having highly variable discharge andeasily eroded banks, sediment gets deposited toform bars and islands that are exposed duringperiods of low discharge. In such a stream thewater flows in a braided pattern around theislands and bars, dividing and reuniting as it flowsdownstream. Such a channel is termed a braidedchannel.During periods of high discharge, the entirestream channel may contain water with the islandscovered to become submerged bars. During suchhigh discharge, some of the islands could erode,but the sediment would be re-deposited as thedischarge decreases, forming new islands orsubmerged bars. Islands may become resistant toerosion if they become inhabited by vegetation.Planet Earth Streams and Floods Notes
  • Prof. C.ValentiPlanet Earth Streams and Floods Notes 12Stream FloodingHaving covered the basics of stream systems we now turn our attention to thedetails of flooding associated with rivers and streams.Floods occur when the discharge of the stream becomes too high to beaccommodated in the normal stream channel. When the discharge becomes toohigh, the stream widens its channel by overtopping its banks and flooding the low-lying areas surrounding the stream.Discharge = volume of water passing a given point in a given amount of time.Contributing factors include • Heavy rains • Rapid snowmelt (if melting rate exceeds the gourn’s capcity to absorb the water • If ground is frozen and impermeable and increases runoff rate • If slope is steep increasing stream velocity and increases runoff rate • Dam failure • Human manipulation of the landscape (deforestation, paving)Flooding is a normal, natural event that occurs along streams over periods varyingfrom several times a year to every few years. Many of the worlds major cities arebuilt along streams due to their convenience in transporting things for business,energy supply etc. Factors that Affect FloodingWhen rain falls on the surface of the Earth, some of the water is evaporated andreturns to the atmosphere, some of it infiltrates the soil and moves downward intothe groundwater system, and some is intercepted by depressions and vegetation.What remains on the surface of the Earth and eventually flows into streams iscalled runoff. In general, then:Runoff = Precipitation - Infiltration - Interception – EvaporationEvaporation tends to be the least of these quantities, particularly over shortperiods of time, and thus precipitation, infiltration, and interception are themost important variables that determine runoff and eventual discharge intostreams.Planet Earth Streams and Floods Notes
  • Prof. C.ValentiPlanet Earth Streams and Floods Notes 13If rainfall is heavier than normal in a particular area and infiltration, interception,and evaporation are low then runoff can be high and the likelihood of flooding willincrease.The relation between the rainfall and thestream discharge is often shown in graphicalform as a hydrograph. A hydrograph showsthe relationship between stream discharge(which is a function of runoff) andprecipitation input. When rainfall occurs insome portion of the watershed, the runoffcollects and is concentrated in streams andtributaries contributing to the streamsdischarge.The lag time is the delay in response of the stream between precipitation and peakdischarge. Lag time depends on such factors as the amount of time over which therain falls and the amount of infiltration and interception that takes place along thepath to a stream.  If the amount of rain is high over a short time period, lag time is short.  If the amount of rain is high over a longer time period, lag time is longer.  Lack of infiltration and interception reduce lag time.The lag time is usually influenced by the type of land coverage around the rechargearea (where the rain water soaks into the ground).Reduction of infiltration - Any time the surface materials of the Earth arecovered with impermeable materials like concrete, asphalt, or buildings, theinfiltration of water into the soil is prevented. Urbanization tends to reduceinfiltration, and thus water must collect in storm sewers and eventually in the maindrainage systems. Thus, extensive urbanization also decreases the lag time andincreases the peak discharge even further. Urbanization can therefore lead to ahigher incidence of flash floods.• If the recharge area is natural, there will be greater infiltration and interception, runoff will be reduced, and therefore lag time will be greater and peak discharge low. Much of the water will be absorbed and transpired by trees and will be held up by groundwater to travel to the stream as base flow.Planet Earth Streams and Floods Notes
  • Prof. C.ValentiPlanet Earth Streams and Floods Notes 14 If the recharge area is covered by concrete material which does not allow water to penetrate, infiltration and interception will be low, and the precipitation will simply travel along the surface as runoff directly into streams. The lag time will be short and the peak discharge will be high as all the water that hits the ground runs directly into the stream channel. Natural: large lag time, broad and wide discharge peak, high residence time in soil. Peak discharge is lower. Urbanization: higher discharge with a shorter lag time between rainfall and maximum discharge. Urbanization with sewers: even higher discharge, lag time is shortest. water that hits the surface runs directly into the stream channel.Planet Earth Streams and Floods Notes
  • Prof. C.ValentiPlanet Earth Streams and Floods Notes 15Planet Earth Streams and Floods Notes
  • Prof. C.ValentiPlanet Earth Streams and Floods Notes 16Upstream (Flash) Floods• Caused by intense but brief periods of rainfall in a localized area.• Occur at Headwater regions• Steep gradient resulting in high runoff (little infiltration)• Channels are V shaped in cross-section with no floodplains or levees. Causes water to rise rapidly and flood areas surrounding the stream.• Increase in discharge increases water velocity resulting in an increase in erosion (Q=A*V).• Intense flooding but temporally brief. Flood waters rise quickly and recede quickly.• Flash floods (short lag time, maybe only a few hours), no prediction, high loss of life.• Downstream not affected due to large, wide channels.Downstream (Regional) Floods• Caused by large amounts of rain fall over an extended period of time over a large portion of the drainage basin.• Occur downstream closer to mouth of stream.• Shallow gradient resulting in high infiltration. Waters rise as soil becomes saturated.• Channels are wide, broad U shaped channels with floodplains so water can spread over a larger area. Floodplains protect areas surrounding the stream by catching excess water the stream cant contain.• Flooding is slow to rise, slow to fall.• Predictable and no loss of life but great property damage.• Floods affect the larger streams as well as tributary streams.Planet Earth Streams and Floods Notes