Streams and Floods

Well, these aren't really class notes, but you will probably find them at least as helpful.

  1. What are the discharge, velocity, capacity and competence of a stream and how are they related to one another?

The discharge is the volume of water that moves past a point in the stream in a unit of time (a flux). The discharge is the velocity of the stream multiplied by its cross-sectional area. The capacity of the stream is the total amount of load that the stream can move, while the competence measure the largest particles that the stream can transport. The capacity is controlled largely by discharge while the competence is controlled largely by the velocity

  1. Where the Comal River leaves Landa Park in New Braunfels, it has a rectangular cross section due to brick walls along its banks. The river is 15 feet wide and 6 feet deep. If the water is flowing at 4 feet per second, what is the discharge of this river?

The discharge is the volume of water that moves past a point in the stream in a unit of time (a flux). The discharge is the velocity of the stream multiplied by its cross-sectional area. The area of this stream is 15x6 square feet, or 90 square feet. The discharge is thus: 4x90 cfs or 360 cfs. This is slightly above the average discharge of Comal Springs of around 300 cfs.

  1. Define the following terms
    1. alluvium

Alluvium is any sediment deposited directly by running surface water.

    1. base level

Pipken and Trent, page 276... the lowest level to which a stream can erode, determined by the body of water into which the stream empties. Ultimate base level is sea level, but many streams empty into other bodies of water such as lakes, reservoirs and other streams.

    1. drainage basin

Pipken and Trent, page 286

    1. drainage divide

Pipken and Trent, page 286

    1. entrenched meanders

Entrenched meanders are large meander loops with a youthful V shaped valley profile. Entrenched meanders are caused when an old-aged stream is forced to begin downcutting by uplift of surrounding land, or a drop in base level. This is called rejuvenation. The Grand Canyon, and the other spectacular canyons of the Colorado Plateau are characterized by entrenched meanders.

    1. alluvial fan

Pipken and Trent, page 277 and 280

  1. braided stream

A braided stream is characterized by many intertwining channels separated by gravel or sand bars. Braided streams occur when a stream has more sediment supply than it can carry with its normal discharge. Braided streams are common in deserts where stream often only have significant discharge after a storm event. Braided streams are also found in association with glaciers, where the glacier supplies large amounts of sediment.

 

  1. Describe the types of loads carried by a stream.

Dissolved load consists of ions dissolved in the water. Suspended load consists of the material that is held in suspension by the water energy. This is the material that falls in the transport region of the Hjulstrom diagram. Bed load consists of material that is dragged along the stream bed, particularly at flood stage.

  1. Describe the concept of a graded stream and how this controls the longitudinal profile of a stream.

A  graded stream carries out no net erosion or deposition, but merely transports its load. An ideal graded stream would has a smooth, concave upward longitudinal profile from head to mouth. It would have a gradually lower gradient as it moves from head to mouth. No real streams are graded from head to mouth. The idea of a graded stream should be though of as a goal that a stream tries to achieve. The actions of a stream, either eroding or depositing material, can be viewed as the response of the stream trying to achieve the goal of being graded.

  1. Based on the following Hjulström diagram:
    1. What is a stream flowing 10 cm/sec doing to 10 mm gravel sediment and 0.1 mm sand?

It is depositing the gravel, and transporting the sand.

    1. How fast would the stream have to flow in a flood to begin to erode 0.1 mm sand from its bed? To erode 0.01 mm clays?

It would have to flow at about 40 cm/sec to erode 0.1 mm sand, and about 100 cm/sec to erode 0.01 mm clay.

  1. Describe how stream cutting and mass wasting work together to form a youthful V shaped stream valley.

In a youthful stream, the stream is far from graded, has not achieved its ideal longitudinal profile, and is therefore predominantly cutting downward. As the steam cuts downward, the slopes of the valley are oversteepened. Mass wasting reduces the slopes as material moves downhill under the influence of gravity and is carried away by the stream. So, although the stream must ultimately transport away all of the mass wasted material, it does not directly erode all of this material from the valley walls. Thus the V shape develops.

  1. Describe, and sketch, the evolution of a stream valley. At each stage in the evolution, indicate the major features of the valley and the primary action of the stream.

In a youthful stream, the stream is far from graded, has not achieved its ideal longitudinal profile, has a steep gradient and is therefore predominantly cutting downward. Youthful streams have V shaped valleys in which the stream channel occupies most of the bottom of the valley. There is little or no floodplain and the stream is reasonably straight with only small bends and meanders. Often there are still waterfalls and rapids as the stream has not yet cut down to its equilibrium longitudinal profile.

Mature streams have reached an equilibrium longitudinal profile and have a lower gradient. They have thus stopped most downward cutting and are mostly engaged in lateral cutting. Mature streams are actively cutting (widening) their flood plain.  Active (steep) cut banks and point bars are present as the stream cuts a flood plain. The meanders occupy most of the flood plain.

An old-age stream has a very low gradient, is usually close to base level and is mostly engaged in recutting its own flood plain sediments. The flood plain is wide, usually much wider than the active meanders, with gentle slopes. Numerous oxbow lakes are present as remnants of former meanders. Natural levees and Yazoo tributaries abound!

  1. Carefully describe, using a series of sketches, the formation of an oxbow lake.

Pipken and Trent, page 287, figure 9.9

  1. What is a stream hydrograph? What is bank-full discharge?

A stream hydrograph is a plot of discharge of a stream as a function of time. Bank-full discharge is the amount of discharge needed to fill the stream channel to its bank. This level of water is called the flood stage of the stream. Any higher discharge and the stream is in flood stage and overflows into its flood plain.

Pipken and Trent, page 287-288,  figure 9.13, 9.14, 9.17 and 9.18

  1. Use a stream hydrograph show the difference between upland and lowland floods. Use a hydrograph to explain the effects of urbanization on flooding events.

Pipken and Trent, page 290,  figure 9.14

Pipken and Trent, page 300-301,  figure 9.26

  1. What is channelization? How does it improve flooding control? How can it increase flooding potential?

Pipken and Trent, page 294-296,  Artificial levees

  1. Explain several ways that vegetation acts to mitigate floods.

Vegetation physically slows down the flow of water, both sheet flow across the ground, and flow in channels. Vegetation also absorbs water, much of which is returned directly to the atmosphere via transpiration and is thus removed from runoff.

  1. You have collected flood data for 20 years. The 7 largest floods covered a bridge in your neighborhood. Use the Weibull equation to determine the recurrence interval of any flood that covers the bridge.

The Weibull eqation is:

T =  (N+1)/M

Where T is the recurrence interval, N is the number of years of recorded data, and M is the rank of the flood event. So we are interested in the recurrence interval for a flood of rank 7 in 20 years of data. So:

T = (20 +1)/7 = 3 years

  1. Discuss some of the major problems with the National Flood Insurance Act of 1968. What was chosen as the line of demarcation for flood insurance purposes?
  2. Why have North Dakota, Iowa and Minnesota communities experience several "100 year" floods in the last few decades?

Pipken and Trent, page 297-301