Chapter 17 Streams and Floods.pdf

Earth: Portrait of a Planet, 3 rd edition 2008, by Stephen Marshak © W. W. Norton & Company Chapter 17: Streams and Floods: The Geology of Running Water PowerPoint slides prepared by Ronald L. Parker Earlham College, Department of Geosciences Richmond, Indiana parkero@earlham.edu 1 Streams and Floods: The Geology of Running Water Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Streamflow ! Streams ? Water that flows down channels. ! Runoff ? Water in motion over the land surface. ! Stream runoff provides " Drinking water. " Transportation. " Waste disposal. " Recreation. " Irrigation. " Energy. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Streamflow ! Stream runoff is an important geologic agent. " Flowing water? #Erodes, transports, and deposits sediments. #Transfers mass from continents to ocean basins. " Earth: only planet in the solar system with flowing water. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak The Hydrologic Cycle ! Stream runoff is a component of the hydrologic cycle. ! Hydrologic cycle processes. " Evaporation. " Transpiration. " Precipitation. " Infiltration. " Runoff. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Forming Streams ! Streamflow begins as water is added to the surface. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Forming Streams ! Streamflow begins as moving sheetwash. ! Sheetwash erosion creates tiny rill channels. ! Rills coalesce, deepen, and downcut into channels. Earth: Portrait of a Planet, 3 rd edition 2008, by Stephen Marshak © W. W. Norton & Company Chapter 17: Streams and Floods: The Geology of Running Water PowerPoint slides prepared by Ronald L. Parker Earlham College, Department of Geosciences Richmond, Indiana parkero@earlham.edu 2 Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Forming Streams ! Rapid erosion lengthens the channel upslope. ! This process is called headward erosion. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Forming Streams ! Over time, nearby channels merge. ! Smaller tributaries join a larger trunk stream. ! The array of linked channels is a drainage network.is a drainage net k. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Drainage Networks ! Drainage networks often form geometric patterns. ! These patterns reflect underlying geology. ! Common drainage patterns. " Dendritic ? Branching, ?treelike? due to uniform material. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Drainage Networks ! Common drainage patterns. " Radial ? From a point uplift (mesa, volcano, etc.). Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Drainage Networks ! Common drainage patterns. " Rectangular ? Controlled by jointed rocks. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Drainage Networks ! Common drainage patterns. " Trellis ? Alternating resistant and weak rocks. Earth: Portrait of a Planet, 3 rd edition 2008, by Stephen Marshak © W. W. Norton & Company Chapter 17: Streams and Floods: The Geology of Running Water PowerPoint slides prepared by Ronald L. Parker Earlham College, Department of Geosciences Richmond, Indiana parkero@earlham.edu 3 Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Drainage Basins ! Land areas that drain into a specific trunk stream. ! Also known as catchments or watersheds. ! Divides are uplands that separate drainage basins. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Drainage Divides ! Watersheds exist in a variety of scales. ! Continental divides Contine tal di de separate flow to different oceans. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Permanent vs. Ephemeral ! Permanent streams " Water flows all year. " At or below the water table. " Humid or temperate. #Sufficient rainfall. #Lower evaporation. " Discharge varies seasonally. ! Ephemeral streams " Do not flow all year. " Above the water table. " Dry climates. #Low rainfall. #High evaporation. " Flow mostly during rare flash floods. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak ! The amount water flowing in a channel. " Volume of water passing a point per unit time. #Cubic feet per second (ft 3 /s). #Cubic meters per second (m 3 /s). ! D = cross-sectional area (A c ) x flow velocity (V). ! Varies seasonally due to precipitation and runoff. Discharge Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Discharge ! Velocity is not uniform throughout a channel. " Friction slows water along channel edges. #Greater in wider, shallower streams. #Lesser in narrower, deeper streams. " In straight channels, highest velocity is in the center. " Few natural channels are straight. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Discharge ! Velocity is not uniform in all areas of a channel. " In curved channels, max. velocity traces the outside curve. #The outside curve is preferentially scoured and deepened. Earth: Portrait of a Planet, 3 rd edition 2008, by Stephen Marshak © W. W. Norton & Company Chapter 17: Streams and Floods: The Geology of Running Water PowerPoint slides prepared by Ronald L. Parker Earlham College, Department of Geosciences Richmond, Indiana parkero@earlham.edu 4 Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Discharge ! Stream flow is characteristically turbulent. #Abundant mixing. #Swirling eddies. ! Turbulence caused by? " Flow obstructions. " Shear in water. ! Turbulent eddies scour the channel bed. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Erosional Processes ! Erosional processes " The energy imparted to streamflow is derived from gravity. " Streams do work by converting potential to kinetic energy. ! Erosion is maximized during floods. " Large water volumes. " High water velocities. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Erosional Processes ! Streams scour, break, abrade, and dissolve material. " Scouring ? Running water picks up sediment and moves it. " The force of moving water can move material at channel bottom Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Erosional Processes " Abrasion ? Sediment grains in flow ?sandblast? rocks. #Bedrock exposed in channels is often polished smooth. #Gravel swirled by turbulent eddies drills holes in bedrock. Gravel led by turbulent edd es dri s holes in bedrock. $These bowl-shaped depressions are called potholes. " Dissolution ? Mineral matter dissolves in water. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Sediment Transport ! The material moved by streams is the sediment load. ! There are 3 types of load. " Dissolved load ? Ions from mineral weathering. " Suspended load ? Fine particles (silt and clay) in the flow. " Bed load ? Larger particles roll, slide, and bounce along. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Sediment Transport ! Competence ? The maximum size transported. ! Capacity ? The maximum load transported. ! Capacity and competence change with discharge. " High discharge ? Large cobbles and boulders may move. " Low discharge ? Large clasts are stranded. Earth: Portrait of a Planet, 3 rd edition 2008, by Stephen Marshak © W. W. Norton & Company Chapter 17: Streams and Floods: The Geology of Running Water PowerPoint slides prepared by Ronald L. Parker Earlham College, Department of Geosciences Richmond, Indiana parkero@earlham.edu 5 Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Sediment Deposition ! When flow velocity decreases? " Sediment grain sizes are sorted by water. #Gravels settle in channels. #Sands drop out in near channel environments. #Silts and clays drape floodplains away from channels. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Sediment Deposition ! Streams grade sediment with transport. " Coarsest particles typify steep gradients in headwaters. " Fine particles typify gentler gradients near the mouth. ! Fluvial sediments are called alluvium Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Longitudinal Changes ! The character of a stream changes with flow distance. ! In profile, the gradient describes a concave-up curve. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Longitudinal Changes ! The character of a stream changes with flow distance. " Near the headwater source of the stream? #Gradient is steep.Gradi nt steep. #Discharge is rapid but low. #Sediments are coarse. #Channels are rocky. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Longitudinal Changes ! The character of a stream changes with flow distance. " Toward the mouth? #Gradient flattens. #Higher discharges/ more volume. #Smaller grain sizes typical. #Channels describe broad meander belts. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Base Level ! The lowest point to which a stream can erode. " Ultimate base level is defined by the position of sea level. " A lake serves as a local (or temporary) base level. " Base level changes cause stream readjustments. #Raising base level results in an increase in deposition. re #Lowering base level accelerates erosion. Earth: Portrait of a Planet, 3 rd edition 2008, by Stephen Marshak © W. W. Norton & Company Chapter 17: Streams and Floods: The Geology of Running Water PowerPoint slides prepared by Ronald L. Parker Earlham College, Department of Geosciences Richmond, Indiana parkero@earlham.edu 6 Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Base Level " A ledge of resistant rock may define the local base level. " Erosive forces act to slowly remove the resistant layer. #This acts to restore the longitudinal profile. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Valleys and Canyons ! Land far above base level is subject to downcutting. " Valley ? Gently sloping trough sidewalls define a V-shape. " Canyon ? Steep trough sidewalls form cliffs. ! Determined by rate of erosion vs. strength of rocks. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Valleys and Canyons ! Stratigraphic variation often yields a stair-step profile. " Strong rocks yield vertical cliffs. " Weak rocks produce sloped walls. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Valleys and Canyons ! Valleys store sediment when base level is raised. ! Renewed incision creates stream terraces. " Terraces mark former floodplains. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Waterfalls ! Waterfall energy scours a plunge pool at the base. ! Basal erosion initiates collapse of overlying rocks. ! Waterfalls are temporary base levels. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Waterfalls ! Example: Niagara Falls. " Lake Erie drops 55 m flowing toward Lake Ontario. " Dolostone caprock is resistant; underlying shale erodes. " Niagara Falls continuously erodes south toward Lake Erie. " Erosion since deglaciation has formed Niagara Gorge. Earth: Portrait of a Planet, 3 rd edition 2008, by Stephen Marshak © W. W. Norton & Company Chapter 17: Streams and Floods: The Geology of Running Water PowerPoint slides prepared by Ronald L. Parker Earlham College, Department of Geosciences Richmond, Indiana parkero@earlham.edu 7 Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Waterfalls ! Niagara Falls. " Diversion of American Falls. " The rate of waterfall retreat is presently 0.5 m/yr. " Lake Erie will drain when the falls reach it. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Alluvial Fans ! Alluvial fans build at the base of a mountain front. ! Sediments create a conical, fan-shaped structure. " Coarsest material found near the stream source. " Sediments fine and thin away from canyon stream. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Braided Streams ! Form where channels are choked by sediment. ! Flow is forced around sediment obstructions. " Diverging - converging flow creates sand and gravel bars. " Bars are unstable, rapidly forming and eroded away. ! Flow occupies multiple channels across a valley. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Meandering Streams ! Channels form looping curves (meanders). " Along the lower portion of the profile/low gradient. " Where streams travel over a broad floodplain. " When substrates are soft and easily eroded. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Meandering Streams ! Maximum velocity swings back and forth across flow. " Fast water erodes one stream bank. " The opposite bank collects sediment. " Cut bank and point bar Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Meandering Streams ! Erosion accentuates the cut bank. " High-velocity flow scours the outside of the meander bend. " Collapsed cut bank material is transported away. ans ! Deposition builds the point bar. " Sediment accumulates inside the meander bend. " Continued addition expands the point bar laterally. Earth: Portrait of a Planet, 3 rd edition 2008, by Stephen Marshak © W. W. Norton & Company Chapter 17: Streams and Floods: The Geology of Running Water PowerPoint slides prepared by Ronald L. Parker Earlham College, Department of Geosciences Richmond, Indiana parkero@earlham.edu 8 Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Meandering Streams ! Sinuosity increases until a meander is cut off. " Cut banks converge and a meander neck thins. " During flooding, high-velocity flow cuts through the neck. " The meander cut-off forms an oxbow lake. " The oxbow fills with sediment, leaving an arcuate scar. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Meandering Streams ! Occupy only a small part of the floodplain. ! During floods, the floodplain may be immersed. ! Natural levees form ridges parallel to the channel. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Deltas ! Deltas form when a stream enters standing water. " Current slows and loses competence; sediments drop out. ! Stream divides into a fan of small distributaries. ! Shape due to the interplay of flow, waves, and tides. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Deltas ! The Mississippi is a river-dominated bird?s foot delta. ! Distinct lobes indicate past depositional centers. ! The river periodically switches course. " River breaks through a natural levee upstream. " Establishes a shorter, steeper path to the Gulf of Mexico. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Deltas ! Abandoned delta lobes are sediment-starved. ! Sediments deposited before avulsion slowly subside. ! Eventually, abandoned delta lobes are submerged. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Deltas ! Subsidence is a problem for cities built on deltas. " Lack of regular flooding leads to sediment starvation. " Subsidence below sea level magnifies flood risks. #New Orleans is an example. Earth: Portrait of a Planet, 3 rd edition 2008, by Stephen Marshak © W. W. Norton & Company Chapter 17: Streams and Floods: The Geology of Running Water PowerPoint slides prepared by Ronald L. Parker Earlham College, Department of Geosciences Richmond, Indiana parkero@earlham.edu 9 Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Drainage Evolution ! Landscapes evolve over time. ! Streamflow is the cause of most landscape changes. ! Example: " Uplift sets a new base level. " Stream cuts into former surface. " Valleys widen; hills erode. " Landscape denuded to base level. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Drainage Evolution ! Stream piracy. " One stream captures flow from another. " Results from headward erosion. " A stream with more vigorous erosion (steeper gradient), intercepts another stream. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Drainage Evolution ! Drainage reversal. " Tectonic uplift can alter the course of a major river. " In the early Mesozoic, South America drained westward. " Western uplift raised the Andes, diverting flow to east. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Drainage Evolution ! Superposed streams ? Cross deformed terrain ignoring structure. " Streams initially develop in younger, flat strata. " The stream then chainsaws into underlying rocks. " Stream maintains its initial geometry. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Drainage Evolution ! Antecedent drainages. " Tectonic uplift may raise ground beneath established streams. " If erosion keeps pace with uplift, the stream will cut through the uplift. #Called antecedent drainage. " If the rate of uplift exceeds erosion, the stream is diverted by the range. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Drainage Evolution ! Some antecedent streams have incised meanders.incised meand " Meanders initially develop on a gentle gradient. " Uplift raises the landscape (dropping the base level). " The meanders erode down into the uplifted landscape. Earth: Portrait of a Planet, 3 rd edition 2008, by Stephen Marshak © W. W. Norton & Company Chapter 17: Streams and Floods: The Geology of Running Water PowerPoint slides prepared by Ronald L. Parker Earlham College, Department of Geosciences Richmond, Indiana parkero@earlham.edu 10 Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Floods ! During a flood " Flow exceeds the storage volume of a stream channel. " Velocity (thus, competence and capacity) increase. " Initially, moving water and debris scour floodplains. " Water slows, dropping sediment. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Floods ! Floods result from a number of circumstances. " Torrential rains. " Soil saturation decreases inflitration " Rapid winter snow melt. " Failure of natural or artificial dams. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Floods ! Seasonal floods recur on an annual basis. " Monsoons ? Tropical rains of the Indian subcontinent. #Generate long periods of rain and severe flooding. #Many people live in floodplain and delta plain settings. #In 1990, a monsoon killed 100,000 people in Bangladesh. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Floods ! Flash floods ? Water rises rapidly with little warning. " Stem from unusually intense rainfall or dam failures. " May strike with little warning and may be very destructive. #Johnstown, Pennsylvania, 1889., Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Floods ! Case history: Mississippi and Missouri Rivers, 1993. " Spring 1993: the jet stream moved over midwestern U.S. #Trapped moist, humid air from the Gulf of Mexico. Sustained rain " July 1993: Flood waters #Covered 40,000 mi 2 . #Lasted 79 days. #50 people died. #55,000 homes destroyed. " $12 billion in damage. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Raging Waters ! Case history: Big Thompson Canyon, Estes Park, COmpson Canyon, Estes Par , CO " July 31, 1976: Rising moist air drenched the Rockies. enc #Heavy rain (7.5? in an hour). #Rock and soil added to the flow. #Houses, bridges, and roads vanished; a 275-ton rock moved. #144 people died. Earth: Portrait of a Planet, 3 rd edition 2008, by Stephen Marshak © W. W. Norton & Company Chapter 17: Streams and Floods: The Geology of Running Water PowerPoint slides prepared by Ronald L. Parker Earlham College, Department of Geosciences Richmond, Indiana parkero@earlham.edu 11 Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Floods ! Case history: Ice-Age floods. " 11 Ka, ice dams failed, releasing Glacial Lake Missoula. " Water scoured the landscape of eastern Washington. " Created the channeled scablands. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Living with Floods ! Flood control is expensive and, sometimes, futile. " Dams on tributaries hold water back from trunk streams. " Artificial levees and flood walls increase channel volume. #Levees transmit intensified flood problems downstream. #Extreme conditions - Levees are overtopped or undermined. $1993, Mississippi River. $2005, New Orleans. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Living with Floods ! Case study: New Orleans in the aftermath of Katrina. " Numerous levees and floodwalls failed in the days after. " Possible failure mechanisms include: #Overtopping water eroding soil on the landward side. #Soil instability beneath levees. #Water pressure. " More than 1,300 died. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Living with Floods ! People living in floodplains face hard choices.hard choices. " Move or expect eventual catastrophic loss. ! Land use changes may mitigate flood damage. " Establish floodways ? Places designed to transmit floods. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Living with Floods ! Flood risks are borne by homeowners, insurance companies, lenders, and government agencies. ! Hydrologic data are used to produce flood risk maps. ! Maps allow regulatory agencies to manage risks. ! Building in flood-prone settings is tightly regulated. d. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Living with Floods ! Flood risks are calculated as probabilities. " Discharges are plotted against recurrence intervals. " On semi-log paper, this plots as a straight line. " The probability (% chance of occurrence) of a given discharge can be determined by graph inspection. Earth: Portrait of a Planet, 3 rd edition 2008, by Stephen Marshak © W. W. Norton & Company Chapter 17: Streams and Floods: The Geology of Running Water PowerPoint slides prepared by Ronald L. Parker Earlham College, Department of Geosciences Richmond, Indiana parkero@earlham.edu 12 Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Urbanization ! Cities cover large areas with impermeable surfaces. ! Storm water runoff from cities is distinctive. " Shorter lag time between rainfall and flood flow. " Larger discharges for shorter durations. Chapter 17: Streams and Floods: The Geology of Running WaterEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak A Vanishing Resource? ! Rivers have directed human settlement patterns. " Drinking water, food, transport, energy, recreation, and waste disposal. ! Settlement has directed flow. " Urbanization. " Agriculture. " Pollution. " Dams. wolflor Microsoft PowerPoint - Ch17_Lecture_Earth3.ppt