Chapter 6 Environmental Geology

Front 1

Flood

Back 1

Rising and overflowing of a body of water, especially onto normally dry land.

Front 2

Factors that affect flooding

Back 2

Climate (Too much rain/snow), slope (gradient in mountainous regions), infiltration versus runoff (soil type and moisture content), vegetation, moon/tide, geographic location

Front 3

Upstream Floods

Back 3

Floods that affect only small, localized areas are sometimes called upstream floods. i.e. flash floods, rapid snow melt, dam failure, etc.

Front 4

Flood Characteristics

Back 4

They are measured from stage or discharge (bank height exceeded), Crest, when maximum stage reached.

Front 5

Downstream Floods

Back 5

Floods that affect large stream systems and large drainage basins. i.e. prolonged rain events

Front 6

Downstream Flood Characteristics

Back 6

Prolonged, extensive area, crest may occur long after flood causing event, dissipate relatively slowly.

Front 7

Stream Hydrographs

Back 7

Temporal record of stream stage/discharge. Floods recorded as peaks in hydrograph. Height and width of peaks diagnostic of type of flood event and distance from flood event.

Front 8

Flood Frequency Curves

Back 8

Discharge as a function of recurrence interval (R). A measure of how frequently a flood of certain severity occurs on average for that stream.

Probability = 1/R (smaller means more likely)

Problems include frequent events are well constrained, major flood events poorly constrained, hard to correctly define recurrence interval of rare floods.

Front 9

Estimating recurrence interval from yearly maximum stage data.

Back 9

R = (N + 1) N= Number of years in data.

______ M= Rank in terms of discharge/stage.

M

Front 10

Reasons to live in floodplains

Back 10

Urbanization- concrete and asphalt are impermeable, increase surface runoff.

Buildings consume volume on floodplain.

Storm water/sewage systems rapid discharge to stream.

Front 11

Floodplains

Back 11

A nearly flat plain bordering a stream or river that is naturally subject to flooding.

Front 12

Restrictive zoning/ floodproofing

Back 12

Appropriate land-use allocation, easier new development, land requirements in cities are intense.

Front 13

Channelization

Back 13

Widening/deepening/straightening rivers. Increase discharge capacity to reduce upstream flooding.

Front 14

Problems of channelization

Back 14

Stream has natural tendency to meander. Expensive, may increase chances of downstream flooding, ecological impacts.

Front 15

Leevees

Back 15

Artificial elevation of channel banks to contain higher discharge/stage.

Front 16

Problems of Leevees

Back 16

False sense of security, failure can lead to serious flooding, sedimentation that would naturally occur happens in channel, downstream flooding can happen too.

Front 17

Dams

Back 17

Artificial retention/storage of stream discharge, regulated release.

Front 18

Problems of Dams

Back 18

Channel modification- downstream erosion/upstream deposition, ecosystem damage- flooding, effect on fish, expensive maintenance (dredging), false sense of security.