Part 4Part 4

NICK POINTS, BED NICK POINTS, BED EROSION, EROSION,

AGGRADATION AGGRADATION and LOCAL BASE and LOCAL BASE

LEVELLEVEL

• Resistant features can locally perturb the channel profile

• Waterfalls represent flow systems that are out-of-equilibrium and generally short lived

• These figures show the normal progression of planation

• Many channels in the western United States are Many channels in the western United States are actively down cutting their beds, as shown in these actively down cutting their beds, as shown in these examples from the San Francisco East Bay region.examples from the San Francisco East Bay region.

BANK EROSIONBANK EROSION

• Bank erosion is common within soft, unconsolidated flood plain sediments, whenever the channel migrates during high flow

• Nick points are generally indicative of out-of-equilibrium conditions, including hydraulic chokes caused by sediment influx or landslide dams

• Local base level can be perturbed by lakes or reservoirs, impacting the channel profile above and below the reservoir

• Diagram illustrating the four principal types of sediment commonly deposited in reservoirs: 1) bottomset beds; 2) forset beds; 3) topset beds; and 4) turbidity current deposits

Long term impacts on Long term impacts on channel profilechannel profile

• Sediment-laden runoff is decanted in reservoirs, where it accumulates upstream of and within the upper portion of the lake.

• The clear water that is released from reservoirs has greater capacity to erode the natural channel below the dam, as sketched here.

Measurements of check Measurements of check dams and their impacts on dams and their impacts on channel slope, as measured channel slope, as measured by Dr. Luna B. Leopold in by Dr. Luna B. Leopold in New Mexico between 1939 New Mexico between 1939 and 1974.and 1974.

Note the aggradation of Note the aggradation of channel grade, well channel grade, well upstream of the structuresupstream of the structures

Check DamsCheck Dams

• Check dams are commonly employed to reduce rill and gully erosion through checking of bed down cutting

• Empirical procedures exist to determine the spacings between drop structures

Ratio of check dam Ratio of check dam height to length of height to length of

affected reachaffected reach

• Check dam tests carried out by Luna Leopold between 1938-75 in New Mexico, summarized in the plot at lower left

• Note the importance of hydraulic grade, s