inland floods
non-coastal regions from excess rain or melt, stream floods most common, urban floods when insufficient drainage, and catastrophic floods when dams/levees fail
streams and flowing water
1. parts of stream
2. longitudinal profile
3. flowing water
- headwater: stream begins flow, usually higher up, mouth: where stream empties into a level body of water, gradient: slope in downstream direction
- elevation v. axis, distance from mouth on h. axis
- moving water carriers sediment as dissolved (ions), suspended (clay/mud), or bed load (larger materials like pebbles and rocks)
flood topography
1. floodplains
2. base level
3. natural levees
4. delta
- area on either side of stream which is submerged during flood, good for agriculture, usually where human settlements are
- elevation where stream surface does not drop, lakes and tributaries are local base levels, sea is ultimate base level
- sediment ridges on banks of stream, corm as water slows and spills into floodplain
- low lands where sediment collects at mouth of stream, form as water empties into standing body of water, liable to flooding
Mississippi River floodplain
infrastructure changes shape to relatively straight river, evidence of channel position changes from abandoned meanders and oxbow lakes where water abandoned, delta naturally change but most changes due to infrastructure
permanent v. ephemeral streams
- permanent fed by sufficient ground water, stream bed submerged in water table and flows all year, temperate and tropical regions with high rainfall
- ephemeral fed by insufficient ground water, stream beds often exposed, streams dry up during year, water table below stream bed, semiarid and arid regions
drainage networks of North America
continental divide (high land areas) separates east Atlantic and west Pacific drainage networks flowing into different oceans, water in great basin evaporates and does not drain into sea
stream discharge
discharge is volume passing through CSA, measured using gauging station to record stage (elevation below streambed) and avg velocity for m^3/sec, give information on flooding and diluting pollution
flood stage
when water rises above a stream bank, sig areas outside channels are submerged, large elevation increase in flood stage when stream channel lies in narrow valley, small elevation increase in flood stage when stream channel lies within low-lying floodplain
hydrographs of floods
discharge increases as flood dev, decreases as flood recedes, resembling bell curve, flood crest when water reaches highest stage, lag time is time between event triggering flooding and flood crest, warning time to prepare
slow onset floods
1. overview
2. Yangtze river flood 1931
- water rises over days/weeks and recedes over weeks/months in a region, affects streams downstream, wide curve hydrographs, very deadly
- worst flood in history, 16m above flood stage, millions of people died and displaced
Mississippi River floods
1. 1993
2. annual peak discharge
3. 2011
- heavy rains over headwaters for weeks, spilling into floodplains, stopping transport lines, 75 towns inundated, mass loss of fresh water due to contamination and destruction of agricultural lands, displaced, property dmg, and economic dmg
- get data for trend, usually around 100 years but still have unpredictability
- late season snowfall melted under heavy rains, worst flooding since 1927, levees risked collapse so used strategic breaching of small towns and farms to avoid cities and evacuated people accordingly
flash floods
intense rain for hours in small area, fast moving, short lived, upstream, fast water often catch people and motorists by surprise, helicopters may be required to save people, turbulent water causes rapid erosion, dynamic pressure of sediment laden water destroys structures, sediment makes it hard to tell depth, heavy debris batter and buries areas downstream
urban flooding
caused by impermeable surfaces that prevent water infiltration, short event, increasing discharge over shorter lag time or inadequate drainage, flood control channels may speed up water removal, danger with toxic runoff causing environmental dmg and higher flood crest
levee failure
- overtopping: water rises enough to flow over levee
- underselling/mining: water moving beneath levee, can form springs and sand volcanoes, increasing risk of mass movement with water permeating levee
atmospheric rivers
narrow bands of moist air flowing inland from Pacific ocean along south side of mid lat cyclone; BC, California, Oregon, and Washington at risk, way pesist for days increasing flood risk from rains
flooding Abbotsford BC
in floodplain of Fraser River, drained for rich agricultural land, atmospheric river rain caused high flooding, eroded highways, logistics issues with supplies in and out, state of emergency declared but cell phone alert sys not used bc fear of panic, have resources for human evacuation, farm animals unable to evacuate causing unsafe water due to dead animals, long clean up effort
Pakistan Monsoonal floods, 2010
flash flooding, 20% of Pakistan submerged, many people and livestock dead, 20 mill people displaced, road and power lines destroyed, secondary disaster with food and water scarcity and malaria and cholera endemic due to contamination for years
monsoon
seasonal reversal in wind direction, causing shift in precipitation either dry or rainy season, when ITCZ drifts N during N summer and Indian Ocean moisture spawns heavy rainfall in southeast Asia
