The longitudinal profile of fans may be concave. Two types of concavity are recognized. The
first is due to intermittent uplift of the mountains which gradually steepens the fan head. The
other case is due to trenching and the building out of a low flatter reach of recent alluvium at
the toe of the fan. Normally the coarsest material is found at the fan apex, although fan-head
trenching might result in a slight increase in sediment size with the fan radius.
Fluvial (wet) fans can become very large, which contrasts with dry fans. The almost random
distribution of erosion and deposition patterns on the arid fan is often replaced by a
progressively shifting channel. Lateral migration of streams on fluvial fans can be anticipated
by the concavity of the contours (i.e., topographic lows or swales). New orientation of a river
channel is also an equally possible shifting process.
The potential for avulsion, deposition, and channel blockage and channel incision are
important for highway design. To minimize these impacts on highways, a reconnaissance of
the fan and its drainage should be undertaken so that potential changes can be identified
and countermeasures taken. The ideal result of any study of alluvial fans is a geomorphic
map delineating active and inactive portions of the fan and the identification of problem sites
within the active portions of the fan. For example, local aggradation in a channel can lead to
avulsion because avulsion is likely to occur in places where deposition has raised the floor of
the channel to a level that is nearly as high as the surrounding fan surface. This condition
can be identified in the field by observation or by the surveying of cross-fan profiles
(Schumm and Lagasse 1998).
Experimental studies show that growth at the fan-head is intermittent, being interrupted by
periods of incision, sediment reworking and downfan distribution of sediment. The greatest
variation in sediment yield is related to fan-head trenching and aggradation. Geomorphic
thresholds controlling fan growth are sketched in Figure 5.1 (see also Section 5.2.5).
Threshold concepts must be considered when evaluating fan-related hazards to highways.
For example, identification of relatively recent debris flow deposits, which suggests very high
sediment delivery from the drainage basin may, in fact, be an indication of future stability.
That is, stored sediment has been flushed from the drainage basin, and it may be a very
long time before sufficient sediment accumulates again to produce debris flows even under
extreme rainfall. This situation has been documented along the Wasatch Mountain front
north of Salt Lake City (Keaton 1995; Lowe 1993), where drainage basins that produced
debris flows in 1983 do not contain sufficient stored sediment to produce debris flows at
present. Therefore, not only the fan itself, but its drainage basin requires investigation.
5.2.4 Nickpoint Migration and Headcutting
Abrupt changes in the longitudinal profile of the stream are shown in Figure 5.2. This break in
the profile induces a perturbation moving upstream, especially during floods. Above and below
the profile break the river may be stable. As the perturbation migrates past a point, a dramatic
change in channel morphology and stability occurs. These perturbations are of two types: the
first is a sharp break in profile which forms an in-channel scarp called a headcut (Figure 5.2a),
and the second, called a nickpoint, has a gradual change in elevation over a greater length of
channel, but still represents an oversteepened reach with respect to the overall channel slope
(Figure 5.2b).
5.3
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