Geomorphic Assessment of Channel Systems
cream and insect repellant. If snakes are likely to be encountered and the location is remote
from medical aid, then a snake bit kit should be carried. If any boat work is involved, life
jackets must be worn.
Given the rapid expansion in the area covered by mobile telephones globally, a mobile
phone constitutes an excellent safety item to be carried on a reconnaissance trip.
18.104.22.168 What to Look For in the Field
It is not possible to provide an all inclusive list of features that should be recorded in
the field that would cover all applications, but it is the intent of this section to list some of the
more common types of features that you may encounter in the field.
Bed Controls. Channel degradation is the result of an imbalance between sediment
transport capacity and supply. One of the most common causes of this imbalance is
channelization of the stream which increases the bed slope, causing an excess sediment
transport capacity in the channel. Channel degradation occurs as this oversteepened zone
migrates upstream, a process referred to as headcutting (see 22.214.171.124). A field indication of
the headcutting process occurs in the form of knickpoints and knickzones (Figures 2.17 and
2.18). A knickpoint occurs when the degrading channel encounters resistant bed material and
an abrupt overfall is formed. An oversteepened reach of channel representing the headward
migrating zone is referred to as a knickzone. Knickzones may consist of a fairly uniform
oversteepened reach, or may have a highly irregular profile with numerous small knickpoints.
They may extend over several hundred to several thousand feet of channel and, over time
often represent 10 to 20 feet of degradation. The shape of a knickpoint or knickzone, as well
as the rate with which it migrates up the channel is primarily a function of the composition of
the bed material. Therefore, when in the field, it is helpful to document the type of material
comprising the knickpoint or knickzone and to assess the amount of drop through this area.
It is also helpful during the field investigation to determine how long a
knickpoint/knickzone has been present at its current location. Local residents, supervisors,
or soil conservation officers familiar with the area can provide helpful information on the
history of the channel. If the knickpoint consists of very resistant material and has been
stationary for many years, then it may serve as a geologic grade control that can be relied
upon to provide long term grade control. However, geotechnical investigations of the vertical
and lateral extent of the material must be performed to ensure that the geologic control will
not be undermined or flanked.
Berms and Terraces. The formation of berms can indicate an attempt by the channel
to establish stability. Berms form after channel incision, widening and slope flattening have
progressed to the point where the sediment transport capacity is reduced. This impedes the
hydraulic removal of failed bank material at the toe of the bank and also allows sediment
deposition to begin (Figure 3.1). The stability of the berms increases after vegetation
(particularly woody species such as willow, birch and sycamore) is established. Studies of