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Rivers and the Environment

by: Kerrian Johnson

Rivers and the Environment EESC117001

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Kerrian Johnson

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Class notes.
Rivers and the Environment
Noah Synder
Rivers and the Environment, environmental science
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Date Created: 01/17/16
Rivers and the Environment: Course Layout 9/4/13 I. Functions of Water 1. Stream-flow starts as runoff in uplands o With water flows sediment o Rivers move pollutants through the environment o Rivers are vital as habitats and as migration pathways 2. Rivers erode bedrock… slowly lowering mountains, creating new sediment o i.e. Grand Canyon o The amount of sediment in the river determines the morphology of the river  Rivers downstream of glaciers often have more grazed morphology. o Deposited sediment creates new land (i.e. Mississippi River Delta) II. River Management 1. Example: Hoover Dam, Colorado River supplies water to Nevada/Arizona region (Las Vegas). III. Salmon and Rivers 2. Salmon Hatcheries 3. Aquaculture 4. Salmon Habitat IV. What rivers do you know? 1. Rivers near you. 9/6/13 Hydrologic Cycle >>The circulation of Earth’s water supply between various reservoirs and states. Reservoirs: (lakes/impounded water behind dams are usually called reservoirs)  Oceans – 96%  Ice – 3%  Groundwater - ~1%  Lakes and rivers (surface water) – (0.009%)  Atmosphere  Biosphere (humans, animals, plants) 1 States:  Solid  Liquid  Gas How much? 1.46x10 km water in oceans 6 2 9.63x10 km land area VOLUME 1.46x10 km 3 1.46 x103 =152 km AREA 9.63x10 km 2 9.63 Water Cycle: I. Evaporation/Transpiration/Sublimation: 1. Evaporation: water to gas 2. Transpiration: water in plants to gas 3. Sublimation: solid to gas 4. Evaporation rate is proportional to temperature T... Evap rate … Water vapor concentration  II. Precipitation 1. PV=nRT (Ideal Gas Law) 2. PV is proportional to T 3. P is proportional to T/V 4. As air rises in the atmosphere, pressure goes down. Temp also goes down. Air rises… P  … T  5. Lifting of air masses to higher, cooler elevations. 6. Orographic Lifting: 2 7. Frontal Wedging: Cloud development because of frontal lifting of warm moist air. 8. Low-pressure systems over the eastern U.S.  winds blow counter-clockwise. III. Runoff (overland flow) IV. Infiltration 9/8/13 Global Weather: Weather: the atmospheric conditions at a given time and place. Climate: weather averaged over time. Both are forecasted computer models. Earth’s Climate System: 1. Driven by the sun 2. Incoming solar energy (Visible=short wavelength). 3. Heat radiating from Earth must equal solar input (Infared=long wavelength). 4. Atmospheric gas composition: a. N 2 78% b. O 2 21% c. Ar d. CO 2 e. Others (aka. Greenhouse gasses): i. H2O ii. O3 iii. CH4 5. Greenhouse Effect: a. If you increase the amount of greenhouse gasses in the atmosphere, the Earth will get warmer because they are insulating. 3 b. The greenhouse gasses work to trap heat within the atmosphere, which is a good and natural effect. o c. Without the greenhouse effect, the Earth would be -19 C (temp of moon, which doesn’t have the greenhouse effect). d. The excess amount of greenhouse gasses  Problematic climate change. e. The Carbon Cycle: i. The ocean is the largest reservoir of carbon. ii. We’re changing where carbon is stored in the environment… Geologic reservoirs  Atmosphere/Ocean iii. Excess carbon in ocean leads to ocean acidification. iv. Atmospheric carbon conc. is increased by ~3.3Gtons per year v. The Keeling Curve: Atmospheric CO con2entrations has been increasing dramatically since ~1960. vi. In Mauna Loa, Hawaii, the CO co2c increases from the low point in October to the high point in March/April b/c in the winter, plants die and photosynthesis increases and in the spring, cellular respiration increases. f. Ice-Albedo Feedback i. Albedo: reflective ability; How much a surface reflects radiation back toward space. ii. High-albedo  Very reflective iii. Low-albedo (ocean)  very absorptive iv. Ice is very reflective... there is a warming trend (positive feedback loop). v. As the climate becomes hotter, there will be more evaporation, more water vapor in the air, and thus, more clouds.  These clouds will be reflective, and might prevent rapid global warming. vi. Some other scientists disagree, and do not think the clouds’ albedo will be that important in preventing global warming. g. Ice Cores: these cores of ice trap bubbles that provide a record of past atmospheric composition. (Ice cores have recorded the past 800,000 years of atmospheric compositions). i. Shows that there have been rises and falls in the CO 2 conc. h. Climate Computer Models: we can try to forecast out how the high levels of CO i2 going to affect our future. i. Due to the warmer Earth, we’re going to be in for a more rigorous water cycle. 4 ii. High latitudes will get wetter, low latitudes will get dryer. iii. The magnitude and frequency of floods have been increasing. 9/11/13 Watersheds and Watershed Geology:  Getting flow to the stream.  Watershed = drainage basin = catchment:  Watershed: Area of land bounded by divides that contribute flow to a river. o Divide: a ridge where water flows two different ways on each side. o Tributary stream: brings the water from the divide down towards the… o Trunk stream/river: brings the water from the tributary stream towards… o Outlet: Where all the water in a watershed drains. o Channels: lower, trough-shaped parts of a water shed where water flow is concentrated.  Types:  Rills (sm)  Gullies (sm)  Ravines (sm)  Brooks (sm)  Creeks (lg)  Rivers (lg)  Often dry = ephemeral  Usually flowing = perennial o Hillslopes: the un-channeled part of a watershed.  Watersheds are closed systems, so one can keep track of pollutants, etc.  BC’s in HUC 01090001 Charles River, Ipswitch River, etc. o Charles River Watershed 308 mi /797 km 2  Watershed Geologic Basics: o Rocks and minerals- minerals are basic building blocks of rocks  Rock (granite) is made up of quartz, feldspar, etc (minerals) o Rocks:  Igneous: melting of rocks in deep crust, and upper mantle 5  Sedimentary: weathering and erosion of rocks exposed at surface  Metamorphic: rocks under high pressure. o Weathering is the process that alters rocks.  Chemical weathering: decomp of rocks by chemical reactions on mineral surfaces.  Chemical weathering makes soil.  Physical weathering: breaks up rocks; mechanical breakdown (disintegration). 9/13/13 Hillslope Water Budget  Rates-change over time o Ex. Precipitation rate (rainfall intensity) o Depth of water Time 6 inches x 2.54 cm x 10 mm x 1 day = 6.4 mm/hr day 1 in 1 cm 24 hr Processes of water transfer between reservoirs: o Precipitation: atmosphere  watershed o Evaporation: watershed  atmosphere  Also lump in interception, or rainfall directly on vegetation o Runoff or overland flow: land surface  channels (and streams) o Infiltration: land surface  groundwater  Deeper infiltration occurs along cracks  Write a balanced equation for a hillslope water budget o Precipitation rate (P) o Evapotranspiration rate (ET) o Runoff rate (R) o Infiltration rate (IN) o P = ET + R + IN o R = P – ET – IN o During a rainstorm, evaporation is not that great o If P > (IN + ET), then runoff occurs o What controls infiltration rate?  Temperature  Soil characteristics  Porous, land use, grain size, soil moisture  Slope of the surface  Vegetation  Bare soil, pavement, impervious surface  Rill  Gully  Ravine  Small stream 6 9/16/13 Hillslope Hydrology: Flow pathways to channels >>Runoff (fast) + Infiltration (slow) 1. Horton Overland Flow (HOF) a. “Infiltration Capacity Curve” b. Rainfall rate > infiltration rate c. Rapid, intense rainfall over dry land… Water runs down surface. d. For 4 billion years of Earth’s history, there were no land plants. e. For most of Earth’s history, Horton Overland Flow was probably the main cause of runoff. 2. Subsurface Storm Flow (SSF) a. Not getting runoff, as all rain infiltrates into the soil. b. There is a perched water table on the soil/bedrock interface; water flows below surface on this interface. c. This occurs commonly in vegetated landscapes (most rain performs this type of flow). 3. Saturation Overland Flow (SOF) a. Rain is for the most part infiltrating, and it is feeding flow of water in the saturated zone (subsurface). b. Once the “sponge” becomes too saturated, the flow must become overland flow. c. Return flow/temporary springs = right after a storm, water seeps out of the ground because the sponge is full. Hurricane Irene:  Late August 2011  Inland flooding event along the eastern U.S. coast.  49 people died, $~10.1 billion damage Motivation:  Why do we study hillslope hydrology? o Determines whether hillslope water reaches the channel quickly and at the same time after a precipitation event, or not. o Therefore helps explain why some streams flood after a given rainfall event and others do not. The processes of hillslope hydrology also explain why rivers flow even when there is no rain. 7  Hyetograph: measures rainfall intensity 9/18/13 Groundwater:  Infiltration: downward flow of water from the surface into the soil or rock.  Water flow in the subsurface: o Downward (vertical) in the unsaturated zone (interflow). o Flow parallel with the water table surface in the saturated zone.  Porosity and permeability: relates to the rate/ability of water flow. Groundwater aquifers: o A geologic formation capable of storing and transmitting enough water to supply wells.  Rephrase: Body of rock or sediment that can move enough water through it that it can supply at least one well. o Necessary Properties of an Aquifer:  Below the water table (Water table= top of the saturated zone, below the Earth’s surface).  SurfaceUnsaturatedwater tableSaturated  Porosity: % void space in a rock or sediment formation.  Permeability: ability of a rock or sediment formation to transmit a fluid.  HIGH  Porosity/permeability  LOW  Sediment Gravel Sand Silt Clay  Bedrock Sandstone Fractured rock UnF rock  Recharge from precipitation o Unconfined aquifer: groundwater surface that mimics land topography o Perched aquifer: has places that allow water flow (permeable), but also places that does not allow water to flow. o Confined aquifer: becomes trapped and can become pressurized  Artesian well: water flows freely to land surface because it is pressurized. o If you have a well and you pump water at a higher rate than it is being recharged, you can create a “cone of depression”. o Gaining stream: typical of wet climates, water table intersects water surface, aquifer contributes flow to stream 8 o Losing stream: typical of dry climates, water table below water surface, stream recharges aquifer. 9/20/13 Stream Discharge Volume of water = Q (discharge) time  Hyetograph: a graph of rainfall intensity against time. o Depth/time  Hydrograph: a graph of discharge against time.  Discharge: volumetric flow rate through a given stream cross section. (Q) o How much water is moving through a stream cross section at a given time (found instantaneously).  Rational Runoff Method: used to estimate discharge during storms. o Postulate: during storms, for small watersheds o Q=CIA  Q=discharge  C=rational runoff coefficient=fraction of rainfall that is runoff  % impervious surface (roads and roofs) HOF  % saturated ground SOF  I=rainfall intensity  A=watershed drainage area  Measuring Q: velocity measurements in intervals across a channel. o Measurements at different flow depths (stages, gauge heights) allow for construction of a rating curve relationship between stage and discharge.  The deeper the water, the more discharge you have.  The shallower the water, the less discharge there is.  As depth increases, so does velocity (and thus, discharge).  Rating curve: empirical relationship between flow depth and discharge (excel—add fit line). o Discharge: x-axis o Flow stage/depth: y-axis  Application: Flood Insurance Rate Maps  National Flood Insurance Program  not all private insurance companies will give flood insurance (depends on whether or not you live in a dangerous flooding region on flood map).  Importance of water/discharge: 9 o Drinking water o Recreation o Flood magnitude o Forecasting Watershed Exercise: #5: List of gauging stations  (Search my watershed?) find out the important ones. Ones with largest discharges will be the ones with the largest rivers. 9/23/13 Motivation:  Manage and operate dams, reservoirs, diversions  Protect water quality  Forecast floods and droughts  Create flood mitigation strategies o Flood insurance rate maps o Cost-benefit analysis of flood control structures  Understand the role of infrequent events in forming the landscape we see  Understand whether/how humans affect flood frequency Flood statistics: Terms   Flood: high flow events when streams overflow onto their valley floor (floodplain). Magnitude: a flood of a given discharge (Q). Frequency: how often a given flood magnitude occurs. o Low Q: occurs often (most days) o High Q: occurs rarely (i.e. once every 100 years). Recurrence interval (RI): average number of years between flood discharge of a given size o Weibull equation: RI=(n+1)/rank  n= number of years of discharge records for a given gauging station. o Exceedence probability (P): the probability that an event of a given magnitude will occur in a given year (P=1/RI).  “100-year flood”: RI=100yr, P=1/100=1% Effect of Urbanization: Example of two watersheds near Seattle:  different amounts of urban development  same climate, geology and soils 9/24/13 Sediment Transport is Important: 10  Builds habitat  Erodes landscape  Erodes structures  Reduces reservoir capacity Sediment Transport by Rivers:  Function of: o Available sediment (sediment supply and size) o Amount of water flow o Rate of water flow o Turbulence of water flow  Feedback with morphology o Sediment load affects channel shape and habitat Flow  Air (cyclone)  Rock (Gneiss)  Ice in the form of glaciers o Glaciers are laminar (straight flow)  Water o Water is turbulent (swirling) Flow Facts:  Sir Osborne Reynolds—studied turbulence  Reynolds number (Re)  NO UNITS! o Re = velocity x depth x water density viscosity = inertial forces viscous forces = flow a resistance  Physics: laminar vs. turbulent flow o Which kind of flow?  Variables that matter: velocity, depth, viscosity  Viscosity (μ): resistance to flow  Honey: high viscosity  Water: low viscosity  Air: very low viscosity o Laminar flow  Low Re  Glaciers o Turbulent flow  High Re  Rivers  Modes of sediment transport: o Suspended load: particles supported by turbulent eddies, rarely stop (deposit). 11  Most sediment flow is suspended flow is settling velocity (vs) is less than turbulent velocities in the water column.  Stokes law:  Settling velocity depends on the relative density of the particle and the fluid it’s flowing through  Most important term is particle diameter SQUARED.  Course particles (boulders) have large diameters, and thus have large settling velocities (will likely be carried as bedload along the bottom of the riverbed). o Bedload: saltation (impacts) 9/27/13  Entrainment: initial setting in motion of a particle  Transport: particles in motion  Deposition: particles settling to the bed  Cohesive sediment: clay  Competence: the maximum particle size that can be entrained by a given flow  Capacity: the maximum quantity of sediment that can be transported by a given flow  Sediment load: total sediment transport rate of a river (bedload + suspended load) o Mass/time (tons/year) or volume/time (m /year)  Sediment yield: watershed sedimen2 local/drainage area o Mass/area/time (tons/km /yr) o Range: 2-2000 tons/km /yr 2 Bedload transport:  Sets the competence of a river  coarsest material in transport is always bedload  Threshold process  most of the time no bed is moving, need big events to move sediment (floods)  Set by the force of the water moving over the bed (shear stress). o Force/unit area = stress o Shear stress is proportional to velocity, depth, channel slope  Saltation: particles hop, impact the bed, dislodge other particles o Gravel-bed rivers  Bedforms: ripples, dunes, etc. o Form with size and shape in relation to flow strength o Sand-bed rivers 12  Armoring: sediment that must be eroded in place. 9/30/13 Fluvial Geomorphology: Motivation: 1. Why are streams shaped the way they are? 2. Many river channels are self-formed, reflecting a balance among discharge, sediment load, and river shape. Change one variable, and you change the rest. a. Stream geomorphology responds to climate change, land use, engineered structures, etc. Self-formed rivers:  “Alluvial rivers”  alluvium: sediment deposited by flowing water.  Mobile bed and banks Imposed-form rivers:  “Bedrock rivers”  Immobile bed and banks Terms:  Uplands (mountains around a valley)  Valley  Floodplain (area at the bottom of a valley) How does a channel become self-formed?  Bankfull discharge o Geomorphic observation: for alluvial rivers, the discharge when flow spills over the banks onto the floodplain is also the “effective” discharge or flow that transports the most sediment. o Hydrologic observation: Q typbcally has recurrence interval of 1.5-2 years (50-67% chance of happening). o Means alluvial rivers can adjust as factors change. o Probability: 1/recurrence interval Wednesday Oct. 2, 2013: NEED! 10/4/13 Fluvial Geomorphology III: River Responses and Imposed­Form Channels (bedrock  rivers)  How do rivers vary downstream? o Increase in discharge because tributaries join the main stem channel o Less exposed bedrock  o Finer sediment o Decrease in channel slope  Rivers have  o Branching networks 13  Like in the bodyveins and capillaries  Efficient ways to transport stuff through anything  Tributaries add discharge and continues flow to main stem o Capacity increases o At same time sediment that bounced on bed is broken up into small pieces   Smaller particles go down stream  Bigger particles get stuck along the way o As amount of water in channel increases, amount of sediment increase o ***Check PowerPoint on this one***  Graded Stream: stream with smooth, concave longitudinal profile o Indication that a stream has achieved a balance among erosion, transport  and deposition  Lane’s Balance  What happens? o If we  have a  river and decrease  stream  power  (decrease slope or discharge) or increase resistance to transport (increase  sediment size or supply volume)  Pushing down the left­side of lane’s balance which means that  more and bigger sediment will result in deposition  Why does this happen? o Increase in sediment supply  From mining, clear­cutting, or forest fire  Drought  Sediment Supply greater than stream capacity (recall what gold mining in  California did  Alluvial Fans: fan­shaped landforms developed where rivers spill into valleys,  lose competence and capacity, and deposit sediment on valley floors  Deltas: fan­shaped landforms developed where rivers spill into standing bodies of  water, lose competence and capacity and deposit sediment 14 o Triangular shape ∆  When you build a dam you flatten the longitudinal flow of the river  Increase stream power (increase slope or discharge) or decrease resistance to  transport (reduce sediment size or supply volume) o Erosion o Example causes:   Decrease in sediment supply  Change to wetter or stormier climate  Increased slope due to tectonics over a long time period o Terraces are floodplains, abandoned due to changes in flood frequency &  magnitude, sediment flux, erosion rate  Imposed­form channels (bedrock channels) o Immobile bed and banks o Large, rare events set morphology  Huge floods, landslides, volcanic eruptions o Bedload transport rate is set by availability of transportable sediment     Limited by sediment supply, not stream power o Bedrock is often exposed and can be eroded directly by the river     Morphology is dictated by factors external to the channel (as opposed to  self­form channels)  Where do you find them o Channels that are actively eroding and/or have experienced a recent  perturbation  Mountains (Areas that are active in erosion)  Recently de­glaciated landslides  Eruptions  Landslides  Bedrock erosion by rivers in an important geologic process WATCH LAST 10 minutes online 10/7/13 What flows downstream?  1. Water 2. Sediment particles (bedload, suspended load) 3. Organic matter (suspended and dissolved) 4. Dissolved major ions (calcium, sodium, magnesium, potassium, bicarbonate, sulfate, chloride) a. Products of chemical weathering of rocks and minerals 5. Dissolved nutrients: N, P 6. Dissolved gasses: N , CO2, O 2more2soluble in cold water) 7. Dissolved trace metals 8. Other pollutants, chemicals Salinity: 15  Concentration of dissolved salts in water (mostly NaCl, Mg, and Ca sulfates and bicarbonates)  Ions: charged atoms or molecules Total Dissolved Solids (TDS):  Concentration of all dissolved ions and suspended matter <2 um in diameter.  Commonly measures in fresh water systems.  Units: milligrams of ions/liter of water (mg/I) = parts per million (ppm)  milligrams of ions mg = parts per million (ppm) liters of water L  1 mg 0.001g 1000 = 1 g density of 1 liter 1000 cm 3 1000 10 cm 3 water (1 3 g/cm )  1 g 1 cm 3 1 g of ions = 1 part of ions 6 3 10 cm  Water classification: o Fresh water: <1500 mg/l o Brackish water 1500-5000 mg/l o Saline water: /5000 mg/l  Example values: o Rain: ~5 mg/l o Streams: ~100 mg/l o Oceans ~35,000 mg/l  Sources of rain water TDS - + 2+ o Ocean spray  +l , 2+ , Mg o Soil dust  K , Ca o Pollution  Chemical weathering yields ions in solution.  Hydrologic cycle: o Evaporation concentrates solutes in surface water o Climate sets water quantity and weathering rates o Groundwater flow carries chemical weathering products to streams Outline: 1. Types and common sources of river pollution a. Point source (i.e. acid rain) b. Non-point source 2. Regulatory efforts a. Clean air act 16 b. Clean water act  Silent Spring by Rachel Carson  documented detrimental effects of pesticides on the environment particularly on birds. o Key moment in starting the environmental movement. Pollution:  Point source: o Wastewater discharge from a pipe or channel. o Environmental Protection Agency definitions: sewage treatment plants, industrial operations, active mines, oil fields, furl and chemical storage tanks. o Added: “wet weather point sources”: combined sewer overflows, construction sites. o Ex.  coal-fired power plant, sewage treatment plant outflow.  Combined sewer system: o Dry weather: sewage goes to treatment plant o Wet weather: combined sewage and storm water can overflow and drain into a river. o Cambridge has a combined sewage system. o No longer built, but still serve 40 million people in the U.S.  Construction sites: o Sediment fences keep sediment/pollutants contained.  Non-point source pollutants: o Pollutants picked up by water as it travels as runoff or groundwater  i.e. pollutants that enter streams via dispersed watershed flow processes. o Agricultural fields (irrigated) and residential lawns  Fertilizers, pesticides, herbicides o Urban runoff (that doesn’t enter the sewer system)  Oil, chemicals, bacteria (sewage leaks) o Roads  Salt, oil, sediment o Abandoned mines  Acid mine drainage o Pastures  Livestock wastes Acid Rain:  Acids  yield H ions in solution (pH<7) -  Bases  yield OH ions in solution (pH>7)  Particularly destructive on limestone. Clean Air Act 1970  Amendments in 1990 addressed acid rain. Clean Water Act 1972 17  To protect chemical, physical and biological integrity of U.S. waters (streams, rivers, lakes, bays, ocean).  Waters should be “swimmable and fishable”.  Mostly addresses point sources. 10/11/13 Cultural Eutrophication:  Eutrophication: comes from the Greek word for “well-fed” o Nutrients enter a water body (fertilizer runoff or sewage) o Algae (mostly phytoplankton) grow (“algal bloom”) o Plants die, sink to the bottom o Bacteria consume the plants and dissolved oxygen through respiration o Water becomes hypoxic (depleted oxygen levels) or anoxic (extreme form of hypoxia), fish die  Water-related diseases: o Two main types, by route of infection:  “No plumbing” diseases (“fecal-oral diseases”)  Cholera  Fecal coliform diarrhea and gastroenteritis (stomach flu)  Typhoid fever  Guinea worm  Hepatitis A  Polio  Giardia  Vector borne  Malaria  River blindness (onchocerciasis)  Schistosomiasis o Most common in developing and tropical countries o All have water management and wastewater management issues at their core.  Sewage treatment: o Primary treatment: Physical separation of solids, greases, etc.  Removes 30-40% of pollutants.  These are dumped or landfilled elsewhere. o Secondary treatment: reduce organic matter  90% of organic matter is removed after this  Produces sludge o Discharge or tertiary treatment:  Discharge to surface water or groundwater …OR… 18  Send to tertiary treatment prior to discharge  Main purpose: to reduce N and P with filters, chemicals, or soil (with microbes).  May also reduce heavy metals, human-made chemicals, or microbes.  Still rare but use is increasing. Midterm: 1. Multiple choice, fill-in-the-blanks, matching 2. Short answers (words, simple sketches) 3. Photo identification 4. Multi-part answers a. Sketches b. Short text c. Focus on geomorphology and hydrology Different flow pathways o Know the different rates of rainfall becoming discharged in a river (relative to each other)  Look at hillslope hydrology from 9/16/13  Stokes Law o Settling velocity depends on the relative density of the particle and the fluid it’s flowing through  Most important term is particle diameter SQUARED. o Course particles (boulders) have large diameters, and thus have large settling velocities (will likely be carried as bedload along the bottom of the riverbed).  How do rivers vary downstream? o Increase in discharge o Less exposed bedrock o Finer sediment o Decrease in channel slope  Small, steep channels in mountains  change in river morphology  self-formed, meandering rivers in the lowlands… leads out to the delta.  Imposed form channels: o Formed via abrasion  creates sand particles o Plucking of bedrock blocks  boulders  Relation between bankfull discharge (Q ) bfd channel forming o More sediments moves at a higher Q o Q is more frequently small than larger 19 o River adjusts its width, depth etc., so that the transport is at an intermediate value (around the bankfull discharge) b/c more is transported at this value. o Most effective discharge does closely coincide with bankfull discharge. o Recurrence interval of about every 2 years. o Most transport is at an intermediate Q (Q ). bf o Long term sediment transport = bankfull discharge** (over time the most sediment is transported at a discharge value around the bankfull discharge).  Bed Sheer stress: o Force is mass x acceleration o Stress is force per unit area o Stress exerted on riverbed by water flowing over it. o Velocity of the river goes to 0 right at the bed (molecular level) o Right above the water bed, the velocity increases a little bit, sheering the river bed.  Hydrographs and hyetogrpahs o Hydrograph is discharge over time  Usually measured at a river cross-section.  Can be measures continuously, so we usually graph it as a smooth curve. o Hyetograph is graph of rainfall intensity over time o They are the two fundamental graphs of hydrology. Hjulstrom diagram: o Shows the relationship of sediment diameter to velocity. o Depicts threshold velocities for the transport of sediment (suspended load transport, versus bedload transport, versus deposition). o It’s easier to move a particle ones it’s in suspension (once a particle is moving, it’s easier to keep it moving). o Clay sticks together, so it takes very fast flows to erode clay off the bed. However, once it’s moving, it is not very hard to keep it moving.  Weibull Equation: o RI=(n+1)/rank  n= number of years of discharge records for a given gauging station. o Purpose is to make a plot that can project relationship between recurrence interval and annual maximum discharge. o Can predict the higher discharge/less frequent floods (often destructive).  Know: Orographic lifting, fronts, etc. and Greenhouse layer! 20 21


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