Geology Reference
In-Depth Information
chaotic nature of much landscape change (e.g. Phillips
1999; Scheidegger 1994). Jonathan D. Phillips's (1999,
139-46) investigation into the nature of Earth surface
systems, which includes geomorphic systems, is par-
ticularly revealing and will be discussed in the final
chapter.
provisionally until further field work was carried out,
that events occurring once or twice a year perform most
geomorphic work (Wolman and Miller 1960). Some later
work has highlighted the geomorphic significance of rare
events. Large-scale anomalies in atmospheric circulation
systems very occasionally produce short-lived super-
floods that have long-term effects on landscapes (Baker
1977, 1983; Partridge and Baker 1987). Another study
revealed
Magnitude and frequency
that
low-frequency,
high-magnitude
events
Interesting debates centre on the variations in process
rates through time. The 'tame' end of this debate concerns
arguments over
magnitude
and
frequency
(Box 1.4), the
pertinent question here being which events perform
the most geomorphic work: small and infrequent events,
medium and moderately frequent events, or big but rare
events? The first work on this issue concluded, albeit
greatly affect stream channels (Gupta 1983).
The 'wilder' end engages hot arguments over
gradual-
ism
and
catastrophism
(Huggett 1989, 1997a, 2006).
The crux of the gradualist-catastrophist debate is the
seemingly innocuous question: have the present rates of
geomorphic processes remained much the same through-
out Earth surface history? Gradualists claim that process
Box 1.4
MAGNITUDE AND FREQUENCY
As a rule of thumb, bigger floods, stronger winds,
higher waves, and so forth occur less often than their
smaller, weaker, and lower counterparts. Indeed, graphs
showing the relationship between the frequency and
magnitude of many geomorphic processes are right-
skewed, which means that a lot of low-magnitude
events occur in comparison with the smaller number
of high-magnitude events, and a very few very high-
magnitude events. The frequency with which an event
of a specific magnitude occurs is expressed as the
return
period
or
recurrence interval
. The recurrence inter-
val is calculated as the average length of time between
events of a given magnitude. Take the case of river
floods. Observations may produce a dataset comprising
the maximum discharge for each year over a period of
years. To compute the
flood-frequency relationships
,
the peak discharges are listed according to magnitude,
with the highest discharge first. The recurrence interval
is then calculated using the equation
where
T
is the recurrence interval,
n
is the number
of years of record, and
m
is the magnitude of the
flood (with
m
1 at the highest recorded discharge).
Each flood is then plotted against its recurrence inter-
val on Gumbel graph paper and the points connected
to form a frequency curve. If a flood of a particu-
lar magnitude has a recurrence interval of 10 years,
it would mean that there is a 1-in-10 (10 per cent)
chance that a flood of this magnitude (2,435 cumecs
in the Wabash River example shown in Figure 1.11)
will occur in any year. It also means that, on average,
one such flood will occur every 10 years. The magni-
tudes of 5-year, 10-year, 25-year, and 50-year floods
are helpful for engineering work, flood control, and
flood alleviation. The 2.33-year flood (
Q
2.33
)isthe
mean annual flood (1,473 cumecs in the example),
the 2.0-year flood (
Q
2.0
) is the median annual flood
(not shown), and the 1.58-year flood (
Q
1.58
)isthe
most probable flood (1,133 cumecs in the example).
=
n
+
1
=
T
m
