FLOOD04049

• County: Denver
• Community: All
• Stream Name: All
• Basin: South Platte
• Title: Riverine Erosion Hazard Areas Mapping Feasibility Study Executive Summary
• Date: 9/1/1999
• Prepared For: State of Colorado
• Prepared By: FEMA
• Floodplain - Doc Type: Educational/Technical/Reference Information

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Erosional and depositional processes in alluvial channels are defined as follows:

Degradation:

Aggradation:

General Scour.'

Local Scour.

Deposition:

Lateral Migration.

Lowering of the channel bed on a substantial reach length
occurring over a relatively long period of time in response to
disturbances that affect general watershed conditions, such as
sediment supply, runoff volume, and artificial channel controls.

Raising of the channel bed as a resull of disturbances in
watershed conditions that produce the opposite effect to those
leading to degradation.

Lowering of the streambed in a general area as a consequence
of a short-duration event such as the passage of a flood.
Examples are the erosion zones near bridge abutments and
those in the vicinity of gravel pits.

Lowering of the bed due to localized phenomena such as vortex
formation around bridge piers.

Raising of the streambed due to a specific episode. An example
is the formation of a sand bar after a flood event Deposition is
used in this document as the counterpart to general scour.

Shifting of the streambank alignment due to a combination of the
above vertical eroSional and depositional processes. The most
common example is meander migration in the floodplain. Bank
retreat due to mass failure is another example,

Vertical variations in the streambed are additive in that the net change is the resull of long-
and short-term processes. For instance, a reach that is undergoing aggradation due to
increased sediment yield from the watershed can also experience general and local scour
as a consequence of flood events.

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Streams are constantly progressing towards a state of dynamic equilibrium involving water
and sediment. The geometry of the stream undergoes adjustments so that the sediment
transport capacity of the water is in balance with the sediment supply. Natural and artificial
factors can upset this state of equilibrium. Earthquakes, large floods, climatic changes,
urbanization, and construction of civil works in the waterway introduce changes in the
sediment supply and amount of runoff reaching the stream. For example, development in
the watershed typically increases the impervious area and hence the volume of runoff.
Similarty, c1ear-cutting of forests increases the sediment yield to the stream. Dams trap
sediment and have a regulating effect that increases low flows and reduces high flows.
Channelization projects reduce channel length and therefore increase slopes, Diversions
for irrigation or public water supply reduce the effective flows. Finally, an event such as a
large flood can dramatically reshape the floodplain and increase channel width.

Evaluation of Channel Changes

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Mathematical representation of fluvial fluid mechanics is difficult due to imperfect
knowledge of the complex physical phenomena involved, The many attempts to modeling
of fluvial processes have shortcomings largely due to the fact that sediment transport
equations commonly overpredict or underpredict sediment loads by orders of magnitude of
actual measured sediment transport rates,

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