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Lecture 10

Lecture 10 - Rivers and Floods.docx

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Earth Sciences
Sergei Basik

Earth Sci 2GG3 – L10 Lecture 10 – Rivers and Floods  Floods cause of most # of fatalities in US  Great Mississippi River Flood 1993  100 miles overwhelmed with rain  10 million In damages  Hurricane Katrina  Breech of levees  Hard years of warning but no action  led to all the damage  Floods can become the largest natural disaster Flooding Factors  Intensity and Duration of Precipitation  Flash foods  Sediment type and availability  Not enough sediment (New Orleans)  Land Use  Most ppl live on coast lines ( New Orleans)  Engineering Modifications  Levees – things we do to control floods makes them worse at times  Floods disasters in Canada  Southern Ontario – 6 flood disasters  Manitoba – Red River Flooding (1950 and 1997)  Maritimes – storm surges, hurricanes, NorWesters  As # of floods increases  costs also increase (more people) Precipitation  Flooding from Precipitation  High intensity & short duration  Large amount of precipitation falling in a very short period; Flash Flood  Lower Intensity & Long Duration  Large amount of precipitation falls steadily over a longer time period  2 foot of water  enough to move a car  Flash flood most common cause of death in Grand Canyon Watershed (Drainage Basin)  Drainage Basin (Watershed)  The upstream area from which surface water flows towards the channel  All rivers drain in here (ex. Lake Ontario)  Drainage Divide  Topographic line or boundary separating watersheds (ex. Mountain Range) Discharge  m /s3  measured volume of water flowing past a cross section of a river in a given amount of time  Q = vA Earth Sci 2GG3 – L10  Q – discharge (m /s) v – velocity (m/s) A – width x depth (in m)  Stream gauge used to tell velocity and discharge of a river Stream Gradient & Floodplains  Gradient: slope of the river channel; typically decreases downstream  Steeper gradient at top of mountain  Floodplain: relatively flat lowland that borders a river, usually dry but subject to flooding  good for growing crops  Base Level: an elevation that a stream cannot erode past, controlled by level of the body of water which the stream discharges into (ex. Lake Ontario,Gulf of Mexico) Alluvial Fan  A fan-shaped deposit of sand and gravel at the mouth of a mountain canyon, where the stream gradient flattens at main valley floor Delta  Accumulation of sediment deposited by a river at its entrance into a basin  Ex. Nile River Delta  Good fertile soil here  highly populated (Ex. Bangladesh) Bankfull Channel (Width, Depth & Capacity)  Bankfull: water level in a river equal to the height of the banks  Flooding occurs when water rises above bankfull levels  Rivers do not reach bankfull stage every year  Determining channel bankfull width and depth can approximate the bankfull discharge and the capacity of the channel Stream Equilibrium  Graded River: a stream in equilibrium with its environment  Channel slope (grade) is adjusted to accommodate the amount of water and the amount sediment and grain sizes provided to it  More water  channel deeper  Less water  channel narrower  Hypothetical setting  Dynamic Equilibrium  The condition of a system in which the inflow and outflow of material is in balance Sediment Size  Suspended Load: finer particles, such as clay, silt, and fine sand, carried in suspension  Bedload: heavier sediment in a stream that is moved along the stream bed rather than in suspension  Roll by saltation  Increase velocity – inc
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