Fundamentals of Engineering Design
4. significant morphological features;
5. nature of bank materials and evidence of instabilities;
6. vegetation; and
188.8.131.52 Field Identified Features
The typical types of field-identified features include:
a. Knickpoints/Knickzones. As discussed in Chapter 3, channel degradation is the result of an
imbalance in the sediment transport capacity and supply. A field indication of degradation occurs in the
form of knickpoints or knickzones. These are often referred to as headcuts. However, there is
considerable confusion in the terminology. According to Schumm et al. (1984) a headcut is defined as a
headward migrating zone on incision. A knickpoint is a location on a thalweg profile of an abrupt change
of elevation and slope. A steeper reach of channel representing the headward migrating zone is referred
to as a knickzone.
b. Berms. The formation of berms can indicate an attempt by a channel to establish stability.
Berms from after a channel has degraded and channel widening and slope flattening have progressed to the
point where the sediment transport capacity is reduced. This in turn reduces the hydraulic removal of failed
bank material at the toe of the bank and also allows sediment deposition to occur at the toe of the bank.
The stability of berms is improved after vegetation (particularly woody species such as willow, river birch,
and sycamore) is established. Berms may be associated with the incision channel's development of a new
c. Terraces. A terrace is another feature that provides information on channel behavior. Terraces
are erosional features resulting from bed lowering, while berms are depositional features which form as the
channel regains stability following bed lowering. When a channel degrades, it leaves an erosional
escarpment which was previously the top bank. This is called a terrace or inactive floodplain. Therefore,
terraces are indicators of past degradation in a channel. The tops of terraces are usually much higher than
the active floodplain and may only be overtopped by extreme flood events.
d. Sediment Sources/Samples. Major sediment sources to the channel are recorded during the
field investigation. These sources include the bed and bank of the channel, tributaries, gullies, drainage
ditches from roads and highways, and watershed (upland) erosion. In the degrading channels, the major
sources of sediment are the deteriorating channel banks and beds. In this case, the sediment is introduced
into the system over a sometimes lengthy reach of channel. In contrast, tributaries that are undergoing
similar instabilities may be points of concentrated sediment input. During the field investigation, note the
amount of sediment deposited at the downstream of the confluence of tributaries.