1. Hydrologic Hazards at the Earth’s Surface
Chapter 9
Hydrologic Hazards at the Earth’s Surface

2. The Hydrologic Cycle

3. (Soils, wetlands, and biota, 0.0059%)
Oceans
and
Salt Lakes
97.41%
Reservoirs
Ice and Snow
1.984%
Ground
Water
0.592%
Lakes and rivers, %
(Soils, wetlands, and biota, %)
Atmospheric water, 0.001%
100 liters (26 gallons)
Figure 8.1: The planet’s water budget. Only a tiny fraction by volume of the world’s water supply is fresh water available for human use.
2.59 liters (0.7 gallon)
0.003 liter
(1/2 teaspoon)
Readily
available
freshwater
0.003%
Freshwater
2.59%
Total water 100%

4. Figure 8. 3: The sources and users of the U. S
Figure 8.3: The sources and users of the U.S. public water supply in This does not include fresh water supplied for agricultural use or water that is self-supplied.

5. A closer look at river systems
Systems vary with respect to:
1. size – width, depth and length
2. path – straight, curvy…
3. shape of channel – smooth, rough
4. velocity of flow – faster/slower
5. volume of flow – DISCHARGE (m3/sec)
6. steepness (gradient) – longitudinal profile
See any “interdependence?”

6. Drainage Basins
The fundamental geographic unit or tract of land that contributes water to a stream or stream system
Drainage basins are separated by divides

7. Discharge The amount of water flowing in a stream channel
Factors combine to produce discharge:
Runoff/drainage area
Subsurface flow
Rainfall/snowfall
Urbanization
Vegetation

8. A river’s total load is known as its CAPACITY
A river’s total load is known as its CAPACITY. it is directly related to the river’s DISCHARGE.
The largest particle a river can carry is the river’s COMPETENCE which is directly related to the river’s VELOCITY
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
Type of load shown?

9. Erosion
Erosive power is a function of flow velocity - the greater the velocity, the greater the erosion
Discharge
Channel shape
Gradient

10. Base Level
The lowest level to which a stream or stream system can erode
Sea level
Temporary base levels, such as lakes, dams, and waterfalls

11. Graded Stream
Stream that has reached a balance of erosion, transportation capacity, and the amount of material supplied to the river

12. Common 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.

13. Alluvium
Sediment deposited by a stream, either inside or outside the channel

14. Alluvial Fan
Buildup of alluvial sediment at the foot of a mountain stream in an arid or semiarid region
Note sharp change in gradient!

15. Delta
Deltas are formed where a sediment-laden stream flows into standing water

16. Figure 9.13: Erosion and deposition patterns on a meandering stream.
(a) Erosion of the cutbank and deposition of a point bar on the gently sloping side of the meander. Longer arrows indicate faster flow. Shorter arrows indicate slower flow.
Fig. 9-13a, p. 256

17. Figure 9.13: Erosion and deposition patterns on a meandering stream.
(b) Rillito Creek rampaged through Tucson, Arizona, during the flood of Damage occurred when the cutbank of the meander migrated west (left) into vacant property immediately upstream from the townhouses (center left). This allowed erosion to occur behind a concrete bank that was protecting the townhouse property. Note the prominent point bar that developed (bottom center) as the meander migrated westward.
Fig. 9-13b, p. 256

18. Figure 9.14: (a–d) Over time, cutbanks erode, sometimes bringing meanders closer together (a and b). When the two meanders finally join (c), a cutoff forms, shortening the river channel but leaving an oxbow lake where the river used to flow (d).
Fig. 9-14, p. 256

19. Meanders, Oxbow Lakes, and Cutoffs
Flowing water will assume a series of S-shaped curves known as meanders
The river may cut off the neck of a tight meander loop and form an oxbow lake

20. Floodplain
Low area adjacent to a stream that is subject to periodic flooding and sedimentation
The area covered by water during flood stage

21. Floods
Highland floods come on suddenly and move rapidly through narrow valleys
Lowland floods inundate broad adjacent floodplains and may take weeks to complete the flood cycle

22. Figure 9.15: Relationship between natural levees and floodplain to a hypothetical stream. The coarsest sediment is deposited in the stream channel, where the stream’s velocity is greatest, and finer silt and clay are deposited on levees and the floodplain as velocity diminishes during overflows.
Fig. 9-15, p. 257

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

24. Figure 9.19: A stage hydrograph superimposed on a schematic stream valley illustrates the impact of increasing stage height on the flooded area.
Fig. 9-19, p. 259

25. Recurrence Interval
The average length of time (T) between flood events of a given magnitude
T = (n+1)/M, where N is the number of years of record and M is the rank of the flood magnitude

26. Flood Probability
The chance that a flood of a particular magnitude will occur in a given year based on historical flood data for a particular location

27. Flood Mitigation Dams Retaining basins Artificial levees
Elevating structures
Flood Insurance?

28. Flood Mitigation Do we really need to stop them?
What are the benefits of floods?

29. Urban Development
Urbanization causes floods to peak sooner during a storm, results in greater peak runoff and total runoff, and increases the probability of flooding