Fundamentals of Fluvial Geomorphology and Channel Processes
The primary point of this delightful sarcasm is that we should not fall into the trap of attempting to plan a
project based on "...wholesale returns of conjecture out of such a trifling investment of fact." Empirical
relationships can be very useful. We cannot be certain that New Orleans and St. Louis will have a common
Board of Aldermen on September 13, 2604; however we must be certain that the data from which the
relationship was developed is valid for the project location, for the scale of the project, and that the
relationship makes physical sense in application to the project.
3.1.5
CHANNEL CLASSIFICATION
Several primary methods of river classification are presented in the following paragraphs, and these
methods can be related to fundamental variables and processes controlling rivers. One important
classification is either alluvial or non-alluvial. An alluvial channel is free to adjust dimensions such as size,
shape, pattern, and slope in response to change and flow through the channel. The bed and banks of an
alluvial river are composed of material transported by the river under present flow conditions. Obviously,
a non-alluvial river is not free to adjust. An example of a non-alluvial river is a bedrock controlled
channel. In other conditions, such as in high mountain streams flowing in very coarse glacially deposited
materials or significantly controlled by fallen timber would suggest a non-alluvial system.
Alluvial channels may also be classified as either perennial, intermittent, or ephemeral. A perennial
stream is one which has flow at all times. An intermittent stream has the potential for continued flow,
but at times the entire flow is absorbed by the bed material. This may be seasonal in nature. An ephemeral
stream only has flow following a rainfall event. When carrying flow, intermittent and ephemeral streams
both have characteristics very similar to perennial streams.
Another classification methodology by Schumm (1977) includes consideration of the type of
sediment load being transported by the stream, the percentage of silt and clay in the channel bed and banks,
and the stability of the channel. Sediment load refers to the type or size of material being transported by
a stream. The total load can be divided into the bed sediment load and the wash load. The bed sediment
load is composed of particles of a size found in appreciable quantities in the bed of the stream. The wash
load is composed of those finer particles that are found in small quantities in the shifting portions of the bed.
Frequently, the sediment load is divided into the bedload, those particles moving on or near the bed, and
the suspended load, those particles moving in the water column. The size of particles moving as
suspended load may include a portion of the bed sediment load, depending on the energy available for
transport (Vanoni, 1977). For example, the suspended load frequently reported by U.S. Geological
Survey publications usually includes a portion of the bed sediment load and all of the wash load. Sediment
discharge is the rate at which the sediment load is being supplied or transported through a reach.
For purposes of this classification system, a stable channel complies with Mackin's definition of a
graded stream. An unstable stream may be either degrading (eroding) or aggrading (depositing). In the
context of the definition of a graded stream being in balance between sediment supplied and sediment
transported, an aggrading stream has excess sediment supply and a degrading stream has a deficit of
sediment supply.
37
Previous Page
