GEOL 1121 Earth Processes and Environments Wondwosen Seyoum Department of Geology University of Georgia e-mail: seyoum@uga.edu G/G Bldg., Rm. No. 122 Seyoum, 2015
Chapter 6 Streams and Flooding Seyoum, 2015
Streams: sculpturing the landscape Source: http://www.geologie.ens.fr/~meunier GEOL 1121: Earth Processes and Environments 3
Streams: as resources Source: http://www.watereducation.org/ GEOL 1121: Earth Processes and Environments 4
Streams hazard: Flooding Approx. takes the life of 140 and $6 billion in property damage annually in the US GEOL 1121: Earth Processes and Environments 5
The Hydrologic Cycle Hydrosphere: all water at or near the surface of the earth Components of the hydrologic cycle Evaporation Transpiration Condensation Precipitation Runoff Infiltration Percolation Figure 6 GEOL 1121: Earth Processes and Environments 6
Water Reservoirs Oceans 97.5 % Glacial Ice 1.81 % Ground Water 0.63 % Lakes and Streams 0.016 % Atmosphere 0.001% Soil Moisture 0.005 % Figure 6 GEOL 1121: Earth Processes and Environments 7
Streams and their Features Stream: a flowing water within a channel Drainage basin: a region from which a stream draws water The size of a stream is determined by basin area, climate, geology, vegetation Figure 6.3A GEOL 1121: Earth Processes and Environments 8
Streams and their Features Stream discharge: the volume of water flowing past a given point/cross section in a specified length of time Figure 6.4 GEOL 1121: Earth Processes and Environments 9
Sediment transport Transport: traction, saltation, suspended, and dissolved Load: the total quantity of material that a stream transports by all methods Capacity: a measure of the total load of material a stream can move Capacity > Load Figure 6.5 GEOL 1121: Earth Processes and Environments 10
Velocity, Gradient, and Base level Velocity: Gradient: Base level: Longitudinal profile: Figure 6.6 GEOL 1121: Earth Processes and Environments 11
Velocity, Sediment Sorting and Deposition Stream velocity impacts sediment sorting Slow moving water only carries fine-grained sediments Fast moving water carries the larger and denser grain sizes Sediments are commonly well sorted by size and density Depositional features of a stream Delta, a large, fan-shaped pile of sediment in still waters created by a stream Alluvial fan, a fan-shaped pile of sediment in a larger stream (slowly flowing) or a region between mountains and a plain formed by a small tributary stream GEOL 1121: Earth Processes and Environments 12
Velocity, Sediment Sorting and Deposition Alluvial fan Delta Photo: Jeremy T. Lancaster Figure 6.7 a, b, c GEOL 1121: Earth Processes and Environments 13
Channel and Floodplain Evolution Classic V-shaped valley: rapidly flowing and steep gradient, downcutting is rapid Figure 6.8 GEOL 1121: Earth Processes and Environments 14
Channel and Floodplain Evolution Meanders: streams don t flow in straight lines and erode old banks and create new banks, and thus bends form in the streams. Meanders are curves in a stream (or river) Point bar Cut bank Figure 6.9 GEOL 1121: Earth Processes and Environments 15
Channel and Floodplain Evolution Oxbows: meanders don t broaden or enlarge indefinitely. Streams may make a shortcut, or cut off a meander, abandoning the old, twisted channel for a direct downstream route Floodplain: a broad, fairly flat expanse of land covered with sediment around the stream channel. An area into which the stream spills over during floods Figure 6.11 GEOL 1121: Earth Processes and Environments 16
Channel and Floodplain Evolution Figure 6.12 C Lower Mississippi R. GEOL 1121: Earth Processes and Environments 17
Channel and Floodplain Evolution Braided stream: localized sediments developed in the channel with obstacles and the localized sediments divide the channel into a complex system of many channels. Figure 6.10 GEOL 1121: Earth Processes and Environments 18
Flooding Flooding: in time of higher discharge, the stream overflow the banks, input exceeds output Factors governing flood severity: Quantity of water The rate of water entering the system Excessive rainfall and snowmelt, severe storms Soil porosity and permeability (high porosity high infiltration and less flooding) Topography (the steeper the higher the velocity) Vegetation (block and reduce velocity, enhance infiltration) GEOL 1121: Earth Processes and Environments 19
Flooding Figure 6.12 1993 flooding Missouri River near Glasgow GEOL 1121: Earth Processes and Environments 20
Flood Characteristics Velocity, water level (Stage), and discharge of a stream increase during a flood Stage - the elevation of the water at any point Flood stage when stream level exceeds the bank height Crest the higher level or stage reached Upstream flood occurs in a small, localized, upper part of a basin Downstream flood occurs in a larger, lower part of a drainage basin Flash flood type of upstream flood characterized by a rapid rise of stream stage, occurs in urban areas, deserts GEOL 1121: Earth Processes and Environments 21
Stream Hydrograph Hydrograph: a plot of stream Stage or discharge (Y axis) over time (X axis) Useful to monitor stream behavior Figure 6.16 GEOL 1121: Earth Processes and Environments 22
Flood-Frequency Curves Flood-frequency curve: a curve showing discharge as a function of recurrence interval for a particular stream. Recurrence interval: how frequently a flood of that severity occurs. Can infer probability of given event Useful in assessing flood hazard Figure 6.17 GEOL 1121: Earth Processes and Environments 23
Flood-Frequency Curves Flood-frequency curve: a curve showing discharge as a function of recurrence interval for a particular stream. Recurrence interval: how frequently a flood of that severity occurs. Can infer probability of given event Useful in assessing flood hazard Figure 6.17 GEOL 1121: Earth Processes and Environments 24
Human Impacts and Flood-Frequency Curves The way a stream respond to a given rain 100 years ago is quite different from today Flood frequency curves changes with time. Decreased by ½ after dam construction Figure 6.18 Green River, WA GEOL 1121: Earth Processes and Environments 25
Human Impacts and Flood-Frequency Curves The way a stream respond to a given rain 100 years ago is quite different from today Flood frequency curves changes with time. Increased after urbanization Figure 6.18 Mercer Cr GEOL 1121: Earth Processes and Environments 26
Development in Floodplains Why would anyone live in a floodplain? Ignorance of the extent of flood hazard Flat, cheaper and easier for building construction Fertile land Access for transportation Scenic feature The more people settle and build in flood plains, the more the damage is Floodplain development increase the likelihood or severity of flooding GEOL 1121: Earth Processes and Environments 27
Development in Floodplains Effects of development on flood plain: Asphalt and concrete - reduce infiltration, increase runoff Buildings - replace water volume, raises stream height Filling in floodplain land - reduces volume Storm drains/sewers - rapid delivery of storm water to streams Vegetation loss - farm lands and urban areas remove natural vegetation and expose the soil, increase erosion and fills up stream with sediments GEOL 1121: Earth Processes and Environments 28
Development in Floodplains Figure 6.20 GEOL 1121: Earth Processes and Environments 29
Development in Floodplains Figure 6.19 GEOL 1121: Earth Processes and Environments 30
Strategies for Reducing Flood Hazards 1. Restrictive Zoning: similar to strategies applicable to reducing damage from seismic and other geologic hazards 2. Retention Pond: trap some of the surface water runoff Figure 6.22 GEOL 1121: Earth Processes and Environments 31
Strategies for Reducing Flood Hazards 3. Diversion Channel: comes into play as stream stage rises, and redirects some of the water flow into other safe places (away from built-up areas) Confluence of Upper Mississippi and Ohio R. at Cairo, IL GEOL 1121: Earth Processes and Environments 32
Strategies for Reducing Flood Hazards 4. Channelization, various modifications of the stream channel itself to increase the velocity of water flow, the volume of the channel, or both The Channel can be deepened or widened Meander cutoff (reroute) GEOL 1121: Earth Processes and Environments 33
Strategies for Reducing Flood Hazards 4. Levees, raised banks along a stream channel Confining the stream may increase velocity downstream Figure 6.23 GEOL 1121: Earth Processes and Environments 34
Strategies for Reducing Flood Hazards If breached, unexpectedly inundated or leave water behind levee Figure 6.24 GEOL 1121: Earth Processes and Environments 35
Strategies for Reducing Flood Hazards 4. Flood Control Dams and Reservoirs Figure 6.25 GEOL 1121: Earth Processes and Environments 36