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Environmental Geology Chapter 9 Rivers and Flooding • Flooding In

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Environmental Geology Chapter 9 Rivers and Flooding • Flooding in Pakistan 2010 -1600 killed/20000 affected • The hydrologic cycle is powered by the Sun – The cycle includes evaporation, precipitation, infiltration, runoff & transpiration ….. This chapter is about the “runoff” part of the cycle: the process that “sculpts” Earth’s surface • Streams = Small rivers. • River components: – Network of streams – Watershed or drainage basin – Base level and slope/gradient – Latitudinal profile – Longitudinal profile – Grading processes • Drainage Basin - The land area that contributes to the flow of a stream • Streams flow down to their base level = the low point at which the water stops moving and collects. Sea level = the ultimate base level. • Lakes and reservoirs = temporary or local base levels. • Lowering the base level steepens the overall slope, causing streams to increase in velocity and to erode. • Raising base level will lessen the overall slope, causing streams to slow down and deposit sediment. • River Sediment Transport Load = transported material – Bed load - Coarse particles moving along the bottom of river channel (less than 10 percent of the total load) – Suspended load: Accounts for about 90 percent of its total load and makes river look muddy - 2- – Dissolved load: Carried in chemical solutions, such as HCO3 , SO4 , Ca +, Na +, Mg+ • Stream Competence & Capacity – Competence = the maximum size transported by a river (largest particle it can move) – Capacity = the total amount a river capable of transporting (the maximum load of solid particles it can carry) – Note: If stream velocity doubles, competence increases by four times. • River Dynamic Characteristics • Continuity equation: – Discharge: The volume of water passing through a given location of a river per unit of time Q = W * D * V – W = Width of flow – D = Depth of flow – V = velocity – Q = Discharge in CUBIC Meters per second • A stream’s ability to erode and carry away sediment is controlled by its velocity, which is a function of gradient (steepness of slope), cross- sectional shape, channel size and roughness • PROFILES - Low gradient versus High gradient streams • V shape = High gradient (downcutting – erosional) = high velocity & competence • Low gradient – low velocity & competence (near base level) • Low-gradient streams (low slopes) typically erode only side-to-side in their floodplains, producing wide valleys with flat floors, most of which is the floodplain. • Low-gradient streams commonly meander back and forth across the floodplain. • Floods are common geological events and involve the stream leaving its channel and filling part or all of the floodplain, blanketing it with fresh sediment after the flood subsides. The new sediment is a beneficial result of flooding – related to farming. • When meanders get “cut off”, an oxbow lake results. • River Erosion Effectiveness is related to: • Stream velocity • Stream discharge • Stream load • Nature of the rocks • Regional topographic relief • Base level • Climatic conditions • River Erosion • Erosion types – Abrasion by sediments transported by river – Hydraulic action of moving water – Chemical corrosion • Erosion location - types – Downcutting – Lateral: Concentrating on the outer bends (cutbanks) – Headward = erosion in the upstream direction • Streams erode-downcut under certain conditions such as when: • sea level drop during an ice age • a stream is captured (via stream piracy – arrrrgh!) by another stream with a lower base level • there is gradual uplift and steepening of mountain slopes through plate tectonics • Deposition Condition: The flow of a river slows down as a result of: – Decrease of stream gradient – Decrease of velocity – Decrease of discharge – Change of channel shape – Change in the amount of stream load (e.g., land-use change, vegetation cover) – Change of geologic setting (e.g., rock types along the river banks) • Construction of a dam raises the base level (creates a temporary base level) resulting in a depositional condition. A wedge of sediment accumulates behind the dam that decreases the gradient of the stream. • River Dynamic Characteristics - Rock types along river channels – Harder rocks will produce boulders – Softer rocks will produce mud/sand – Boulders in channel slow down flow the most – Mud slows down flow the least • Smaller clasts produce less friction along streambed and river banks • River Sediment Deposition - Deposition features – Floodplain – Natural levee – Point bar – Island bar – Alluvial fan – Delta – Note: Stream sediment deposits in general are known as alluvium. • Flooding in Thailand 2010/2011 – One of the worst floods in memory – Heavy rains – Bangkok below sea level – Up to 3m water – Devastation to homes and industry – Deadly – Expensive – Expensive • Point Bar Deposition • Stream Velocities and Channels in Meander – the highest velocity is on the cutbank side of the river in a meander. Deposition occurs on the point bar, on the bank opposite the cutbank. Meanders migrate laterally by erosion on the cut banks and by deposition on point bars. Channels often contain a series of regularly spaced pools and riffles • Braided vs. Meandering Channels – Low gradient streams - Braided streams result from an abundance of coarse bed load sediment. The channels tend to be wide and shallow compared with meandering rivers • Alluvial fan deposits are produced by velocity decrease at the edge of the mountains or when the stream enters a lake or sea • Drainage patterns (as seen from above) include dendritic - most common (uniformly eroded rock types) & radial, rectangular, and trellis. produced by specific geologic/landform conditions. • Dynamics of River Erosion and Deposition • Ever-changing processes: Time and rate of erosion and deposition • Reasons for the changes—Complex, but related to Ø Changes in river channel (width, depth and slope) Ø Composition of channel bed and banks Ø Vegetation cover Ø Variations of weather and climate pattern Ø Human activities, particularly land-use changes • Effects of Land-Use Changes • Changes in infiltration rate: Change of the amount of water flowing into a river • Soil erosion: Change in the amount of sediments in a river • Amount of water and sediments in river: Changes in the velocity of water flow • Changes in river’s velocity: Leading the change in river dynamics • Effects of Land-Use Changes • Forest to farmland Ø Increases soil erosion, stream deposition Ø Increases gradient and velocity Ø Increases river-channel erosion • Urban build-up Ø Increases impervious cover Ø Increases certain flood frequency Ø Reduces the lag time of flood • Flooding • Flooding: Overbank flow condition, discharge greater than channel’s holding capacity • Stage: The height of the water level in a river at a given location at a given time • Hydrograph: Graphic representation of a river’s discharge over time • Lag time: The amount of time between the occurrence of peak rainfall and the onset of flooding • Flooding Data Measurement • Frequency and Magnitude of Flood • Recurrence interval R = (N + 1)/M N as the number of years of record, M is the rank of individual flow within the recorded years • The probability of a given magnitude flood P = 1/R • Statistical probability versus reality Probability: One 25-year flood, on average, once every 25 years Reality: Two 25-year floods within the same year • Types of Flooding • By stream location Ø Upstream flood: Shorter duration, smaller area Ø Downstream flood: Longer duration, greater magnitude, larger area • By duration Ø Flash flood: High volume of flooding water in very short duration, characteristic short lag time • By time/magnitude Ø 100-year, 50-year, 25-year, 10-year floods • Factors Affecting Flood Damage • Regional land-use changes, such as urban development, deforestation, soil erosion, etc. • Land use on the floodplain • Frequency and magnitude of flooding • Lag time and duration of flooding • Sediment load • Effectiveness of forecasting, warning, and emergency management • Urbanization and Flooding • Impact on frequency and magnitude Ø Increase in both frequency and magnitude, especially in small drainage basins • Impact on a river’s discharge Ø Increase in runoff, without an increase in precipitation • Significant reduction in lag time or flashy discharge • Flood hazard increases as percentage of impervious area increases in an urban setting • Impact Effects of Flooding • Primary effects Ø Injury and loss of life, damage and destruction of property, erosion and deposition of sediments • Secondary effects Ø Water pollution Ø Fire Ø Diseases Ø Displacement of people Ø Interruption of social and economic activities • Adjustments to Flood Hazards • The structural approach Ø Physical/Engineering barriers: Levee augmentation (see next slide) Ø Channelization Ø River-channel restoration Ø Reservoir construction – to store excess water for later release Ø Storm sewers • Flood insurance: Shared responsibility and accountability • Flood-proofing: Raised foundation, floodwalls, waterproof doors and windows, pumps - can create false sense of security of living in floodplain • Levee Augmentation - Levees are NOT unbreakable & people should not think they are. After levee construction – land behind levees farmed & wetlands confined between levees • Channel Restoration Ø Viable alternative to deepening and straightening of channels Ø Less aesthetically and ecologically damaging to river system Ø Allows for variety of environments (pools vs. riffles, shaded vs. non-, high vs. low velocity flow,
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