GLG 110

Part 2

Chapter 5

Rivers and Flooding

5.1 River Processes

            People live on floodplains because of quality of soils, water supply, ease of waste disposal, and ease of commerce

            Floodplain: flat surface adjacent to river channel, temporarily flooded by river

Streams and Rivers:

            Part of hydrologic cycle

            Runoff is discharged through rivers and streams

            Drainage basin (watershed): region (surface area) drained by a particular river (Fig. 5.1)

            Stream gradient (slope): vertical drop per unit of stream length

            Base level: theoretical lowest level to which a river may erode, commonly the ocean, sometimes a lake or dam

            Stream power: product of volume per unit time of water flowing by a point (discharge), the water surface slope (about same as channel slope), and unit weight of water (a constant)

Sediments in Rivers

            Bed load: moves along the bottom of a river by rolling, bouncing, or skipping

            Suspended load: carried above the bed by turbulence of flowing water

            Dissolved load: carried in chemical solution

River Velocity, Erosion, and Sediment Deposition

            Rivers are transporters of sediment and sculpture landscapes

            Discharge: volume of water passing a point in a given time

            Alluvial fan: fan-shaped deposits at mountain base (Fig. 5.2)

            Delta: triangular land mass extending into the ocean or lake (Fig. 5.3)

            River competency: largest particle (diameter) a river may transport

            River capacity: total load a river carries per time unit

Effect of Land-Use Changes

            Rivers try to maintain hydraulic equilibrium

            Equilibrium between: erosion, deposition, and transport (Fig. 5.6)

            Any change in stream will upset this equilibrium (e.g. land-use change from forestry to agriculture)

Channel Patterns and Floodplain Formation

            Two main patterns exist:

                        - braided stream (Fig. 5.7): sediment rich, with islands and several small channels (e.g. Salt River)

                        - meandering stream (Fig. 5.8): sinuous channel, with bends (meanders), wide floodplains

pools (deep areas in river) and riffles (shallow areas in river) (Fig. 5.9, 5.11)

point bars: form on inside of river bend, erosion on outside of river bend

5.2 Flooding

            Flooding: is natural process of overbank flow

            Flood stage: elevation of the water surface has reached a high-water condition likely to cause damage (human definition)

            Flooding will continue as long as we build and live on flood-prone areas

            Upstream and Downstream Floods (Fig. 5.12)

            Upstream flood: in upper part of drainage system, caused by intense rainfall

            Downstream flood: (e.g. 1993 Mississippi) cover wide area, due to severe storms that saturate the soil and increased runoff

5.3 Development and Flooding

            Human use of land usually increases flooding potential due to increase in impervious cover (e.g. roofs, pavement, cement)

            Storm sewers are important because they allow runoff to reach channels quickly

            Usually, small to intermediate drainage basins are mostly affected

            Flashy discharge: rapid rise and fall of water (e.g. here in Phoenix)

            Extend of land-use change greatly influences runoff and flooding potential

5.4 The Nature and Extent of Flood Hazards

            Lives lost to river flooding in US: about 100 annually

            Property damage annually: about $4 billion

            Factors that control damage caused by floods include:

                        - land use of floodplain (residential vs. agricultural)

                        - magnitude of flooding (depth and velocity of the water and frequency of flooding)

                        - rate of rise and duration of flooding

                        - season (e.g. plant cover)

                        - sediment load deposited (mud and debris)

                        - effectiveness of forecasting, warning, and emergency systems

            Effects can be primary (caused by the flood; e.g. loss of life, damage to structures, loss of soil and vegetation) or secondary (disruptions and malfunction of services; e.g. short-term pollution, hunger and disease, displacement of people, fires)

5.5 The Response to Flood Hazards

            Historically, response has been prevention of flooding, e.g. construction of dams, modification of streams by levee building, rebuilding entire stream so it drains more “efficiently”

            Floodplain regulation: adjustment measure, used more recently

            Prevention: Physical Barriers (Fig. 5.18)

e.g. levees along banks of river, flood walls, reservoirs to store water, on-site storm-water retention basins, channel improvements (channelization), channel diversions

            Adjustment: Floodplain Regulation

                        “design with nature”: physical barriers and floodplain regulation

                        Flood-hazard mapping: provides information about land-use of floodplain  (includes vegetation patterns, soil studies, long-term water discharge measurements, etc.)

Channelization Controversy

Channelization is straightening, deepening, widening, clearing, or lining a stream channel

                        Adverse Effects of Channelization:

                                    - bad for fish and wetland wildlife

                                    - aesthetic degradation of stream

                        Benefits of Channelization and Channel Restoration

                                    - in urban areas: decrease in flooding

                                    - improved navigation

            Perception of Flooding

                        On the institutional level: perception is adequate

                        On individual level: perception is not clear

5.6 Mississippi River Flooding, 1973 and 1993

Review questions are located on page 97.