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

The Hydrologic Cycle

(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, 0.0071% (Soils, wetlands, and biota, 0.0059%) 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%

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 1995. This does not include fresh water supplied for agricultural use or water that is self-supplied.

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?”

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

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

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?

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

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

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

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.

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

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

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

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

Figure 9.13: Erosion and deposition patterns on a meandering stream. (b) Rillito Creek rampaged through Tucson, Arizona, during the flood of 1983. 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

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

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

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

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

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

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

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

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

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

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

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

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