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Lecture 9

Lecture 9.docx

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Barbara Murck

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Lecture 9 River Flooding Hydrometeorological Hazards River Flooding • Fluvial Hydrology & Flooding – Processes in drainage basins & river systems – Causes & characteristics of flooding – Measuring & predicting floods • Human Intervention – Channelization & urbanization – Impacts on flood magnitude, frequency & prediction • Institutional Frameworks – FEMA in the U.S. – PSEPC in Canada Fluvial Hydrology • Drainage basins – Stream: body of water that flows downslope in a well-defined channel – Drainage basin: total area that contributes water to a stream – Divide: topographic “high” that separates adjacent drainage basins – Watershed: drainage basin + vegetation + inhabitants and their activities • Integrated watershed management Flooding: Causes • Temporary overflow of a river onto adjacent land not normally covered by water (floodplain). – Flow in the channel (discharge) exceeds the capacity of the river to handle the flow. – Imbalance: • Amount of precipitation • Rate of infiltration, throughflow, overland flow • Channel capacity • Floodplain: lands adjacent to the channel, covered by water during flooding events; flat-lying, fine sediments deposited by floodwaters over time. Fluvial Hydrology • Overland flow and throughflow are affected by: – Amount & intensity of precipitation – Changes in surface storage – Rate of infiltration, in turn influenced by: • Soil permeability • Vegetation & interception • Climatic factors (frozen ground) • Human use factors (impermeable surfaces, storm drainage, etc.) • Discharge: quantity of water flowing past a point on the streambank in a given time interval – Measured in units of volume per time – Channel cross-sectional area (width x depth) – Average velocity of water • Gradient (vertical drop between two points along the channel), smoothness, straightness of channel • Sediment load nd – Discharge (2 definition): area where groundwater flows out to join surface water • Q = V x CSA – Where: – Q = discharge (m s )-1 – V = velocity ( m s ) 2 – CSA = cross-sectional area (m ) • Deeper larger discharge • Wider larger discharge • Faster larger discharge • Straighter larger discharge Flooding: Characteristics • Stage: level of water surface over a datum • Bankfull (or flood) stage: channel is full – Bankfull discharge: discharge in the channel at flood stage • Peak (or crest): highest stage reached during an event
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