Chapter 9Chapter 9

Hydrologic Hazards at the Hydrologic Hazards at the Earth’s SurfaceEarth’s Surface

The Hydrologic CycleThe Hydrologic Cycle

Oceans and

Salt Lakes 97.41%

Lakes and rivers, 0.0071%

Ground Water 0.592%

Ice and Snow 1.984%

(Soils, wetlands, and biota, 0.0059%)

Atmospheric water, 0.001%

100 liters (26 gallons)

2.59 liters (0.7 gallon)0.003 liter

(1/2 teaspoon)

Readily available

freshwater 0.003%

Freshwater 2.59%

Total water 100%

Reservoirs

A closer look at river A closer look at river systemssystems

Systems vary with respect to:Systems vary with respect to:

1. 1. sizesize – width, depth and length – width, depth and length

2. 2. pathpath – straight, curvy… – straight, curvy…

3. 3. shapeshape of channel – smooth, rough of channel – smooth, rough

4. 4. velocityvelocity of flow – faster/slower of flow – faster/slower

5.5. volumevolume of flow – of flow – DISCHARGEDISCHARGE (m (m33/sec)/sec)

6. 6. steepnesssteepness (gradient) – longitudinal (gradient) – longitudinal profileprofile

See any “interdependence?”See any “interdependence?”

Drainage BasinsDrainage Basins

The fundamental The fundamental geographic unit or geographic unit or tract of land that tract of land that contributes water contributes water to a stream or to a stream or stream systemstream system

Drainage basins Drainage basins are separated by are separated by dividesdivides

DischargeDischarge

The amount of water flowing in a The amount of water flowing in a stream channelstream channel

Factors combine to produce discharge:Factors combine to produce discharge:– Runoff/drainage areaRunoff/drainage area– Subsurface flowSubsurface flow– Rainfall/snowfallRainfall/snowfall– UrbanizationUrbanization– VegetationVegetation

A river’s total load is known as its A river’s total load is known as its CAPACITY. it is directly related to the CAPACITY. it is directly related to the

river’s DISCHARGE. river’s DISCHARGE.

The largest particle a river can carry is The largest particle a river can carry is the river’s COMPETENCE which is the river’s COMPETENCE which is

directly related to the river’s directly related to the river’s VELOCITYVELOCITY

Type of load shown?

Bed load: contains largest and heaviest sediment. moved by high energy water. is bouncing and scraping along bottom.Suspended load: moved by water, but is suspended in the channel.Dissolved load: composed of ions, in solution

ErosionErosion

Erosive power is a Erosive power is a function of flow function of flow velocity - the velocity - the greater the greater the velocity, the velocity, the greater the erosiongreater the erosion

DischargeDischarge Channel shapeChannel shape GradientGradient

Base LevelBase Level

The lowest level to The lowest level to which a stream or which a stream or stream system can stream system can erodeerode

Sea levelSea level Temporary base Temporary base

levels, such as levels, such as lakes, dams, and lakes, dams, and waterfallswaterfalls

Graded StreamGraded Stream

Stream that has Stream that has reached a balance reached a balance of erosion, of erosion, transportation transportation capacity, and the capacity, and the amount of material amount of material supplied to the supplied to the riverriver

Common stream channel patternsCommon stream channel patterns

Braided Meandering

Braided: typically have a significant sediment load. Have lots of energy, competence and capacity. Steep gradient, much wider than they are deep. Have many bar deposits

Meandering: can carry a lot of sediment, but it is typically smaller (low competence, high capacity). Gradient is gentle, have deep and shallow areas. Notice point bars.

AlluviumAlluvium

Sediment Sediment deposited by a deposited by a stream, either stream, either inside or outside inside or outside the channelthe channel

Alluvial FanAlluvial Fan

Buildup of alluvial Buildup of alluvial sediment at the sediment at the foot of a mountain foot of a mountain stream in an arid stream in an arid or semiarid regionor semiarid region

Note sharp change Note sharp change in gradient!in gradient!

DeltaDelta

Deltas are formed Deltas are formed where a sediment-where a sediment-laden stream flows laden stream flows into standing waterinto standing water

Fig. 9-13a, p. 256

Fig. 9-13b, p. 256

Fig. 9-14, p. 256

Meanders, Oxbow Lakes, and Meanders, Oxbow Lakes, and CutoffsCutoffs

Flowing water will Flowing water will assume a series of assume a series of S-shaped curves S-shaped curves known as known as meandersmeanders

The river may cut The river may cut off the neck of a off the neck of a tight meander loop tight meander loop and form an oxbow and form an oxbow lakelake

FloodplainFloodplain

Low area adjacent Low area adjacent to a stream that is to a stream that is subject to periodic subject to periodic flooding and flooding and sedimentation sedimentation

The area covered The area covered by water during by water during flood stageflood stage

FloodsFloods

Highland floods come Highland floods come on suddenly and move on suddenly and move rapidly through rapidly through narrow valleysnarrow valleys

Lowland floods Lowland floods inundate broad inundate broad adjacent floodplains adjacent floodplains and may take weeks and may take weeks to complete the flood to complete the flood cyclecycle

Fig. 9-15, p. 257

HydrographHydrograph

A graph that plots A graph that plots measured water measured water level (stage) or level (stage) or discharge over a discharge over a period of timeperiod of time

Fig. 9-19, p. 259

Recurrence IntervalRecurrence Interval

The average length The average length of time (T) between of time (T) between flood events of a flood events of a given magnitudegiven magnitude

T = (n+1)/M, where T = (n+1)/M, where N is the number of N is the number of years of record and years of record and M is the rank of the M is the rank of the flood magnitude flood magnitude

Flood ProbabilityFlood Probability

The chance that a The chance that a flood of a particular flood of a particular magnitude will magnitude will occur in a given occur in a given year based on year based on historical flood historical flood data for a data for a particular locationparticular location

Flood MitigationFlood Mitigation

DamsDams Retaining basinsRetaining basins Artificial leveesArtificial levees Elevating Elevating

structuresstructures Flood Insurance?Flood Insurance?

Flood MitigationFlood Mitigation

Do we really Do we really need to stop need to stop them?them?

What are the What are the benefits of benefits of floods?floods?

Urban DevelopmentUrban Development

Urbanization Urbanization causes floods to causes floods to peak sooner during peak sooner during a storm, results in a storm, results in greater peak runoff greater peak runoff and total runoff, and total runoff, and increases the and increases the probability of probability of floodingflooding