Channel Geometry

only indicate stability, but also provide insight into the dominant discharge for the stream.

Terraces are another feature that provide information on channel morphology and

the history of channel instability. When a channel degrades, it creates a relatively high

erosional escarpment which was previously the top bank (Figure 3.2). This is called a terrace

or inactive floodplain. Terraces are usually higher than the active floodplain (berms) and may

only be overtopped by extreme flood events. In many instances, there may be several different

terraces, each the result of separate episodes of degradation. For this reason, a detailed

survey of the terraces may yield valuable information about the erosional history of the

channel.

Channel Geometry. An alluvial stream will size itself in accordance with the

magnitude and frequency of the discharges imposed upon the system. This is accomplished

through the rearrangement of the bed and bank materials within the channel. Therefore, it is

important during the field investigation to observe the dimensions and geometry of the stream,

particularly the width and depth. In some instances this may be the only survey information

you will be able to obtain, while in others, it may be used as supplementary data for the

existing field surveys. The width and depth of the stream should be measured at low water

and top bank conditions. If berms or terraces are present, then the width and depth associated

with these features should also be measured. As a general rule of thumb, measurements

should be made about every 15 to 20 channel widths along the channel. If the stream shows

little variation for long distances, then measurements may be made less frequently.

Conversely, if the channel geometry is varying widely along the channel, then more frequent

measurements may be necessary.

Bank Stability. Heights and angles of the channel banks should be field-determined

to assist in a bank stability assessment. These data can be determined from surveyed cross

sections, but field verification is recommended since surveyed cross sections may not be

representative of the entire reach. Field measurements include estimation of bank height with

a survey rod or cloth tape, and of bank angle with an inclinometer.

During the field investigation it is also important to observe the bank stratigraphy,

mode of bank failures (slab, rotational failures, etc.), and indicators of potential instability

such as tension cracks in the upper bank. Proper identification of the bank stratigraphy and

its role in channel stability is best determined by an investigator with a background in geology

or sedimentology. The classification of the general composition of the observed layers and

the percent of the total bank composed by each layer should be recorded.