Page 38 - Fundamentals of Geomorphology

WHAT IS GEOMORPHOLOGY? 21

chaotic nature of much landscape change (e.g. Phillips provisionally until further field work was carried out,
1999; Scheidegger 1994). Jonathan D. Phillips’s (1999, that events occurring once or twice a year perform most
139–46) investigation into the nature of Earth surface geomorphicwork(WolmanandMiller1960).Somelater
systems, which includes geomorphic systems, is par- work has highlighted the geomorphic significance of rare
ticularly revealing and will be discussed in the final events. Large-scale anomalies in atmospheric circulation
chapter. systems very occasionally produce short-lived super-
floods that have long-term effects on landscapes (Baker
Magnitude and frequency 1977, 1983; Partridge and Baker 1987). Another study
revealed that low-frequency, high-magnitude events
Interesting debates centre on the variations in process greatly affect stream channels (Gupta 1983).
ratesthroughtime.The‘tame’endofthisdebateconcerns The ‘wilder’ end engages hot arguments over gradual-
arguments over magnitude and frequency (Box 1.4), the ism and catastrophism (Huggett 1989, 1997a, 2006).
pertinent question here being which events perform The crux of the gradualist–catastrophist debate is the
the most geomorphic work: small and infrequent events, seemingly innocuous question: have the present rates of
medium and moderately frequent events, or big but rare geomorphic processes remained much the same through-
events? The first work on this issue concluded, albeit out Earth surface history? Gradualists claim that process

Box 1.4

MAGNITUDE AND FREQUENCY

As a rule of thumb, bigger floods, stronger winds, where T is the recurrence interval, n is the number
higher waves, and so forth occur less often than their of years of record, and m is the magnitude of the
smaller,weaker,andlowercounterparts.Indeed,graphs flood (with m = 1 at the highest recorded discharge).
showing the relationship between the frequency and Each flood is then plotted against its recurrence inter-
magnitude of many geomorphic processes are right- val on Gumbel graph paper and the points connected
skewed, which means that a lot of low-magnitude to form a frequency curve. If a flood of a particu-
events occur in comparison with the smaller number lar magnitude has a recurrence interval of 10 years,
of high-magnitude events, and a very few very high- it would mean that there is a 1-in-10 (10 per cent)
magnitude events. The frequency with which an event chance that a flood of this magnitude (2,435 cumecs
of a specific magnitude occurs is expressed as the return in the Wabash River example shown in Figure 1.11)
period or recurrence interval. The recurrence inter- will occur in any year. It also means that, on average,
val is calculated as the average length of time between one such flood will occur every 10 years. The magni-
events of a given magnitude. Take the case of river tudes of 5-year, 10-year, 25-year, and 50-year floods
floods. Observations may produce a dataset comprising are helpful for engineering work, flood control, and
the maximum discharge for each year over a period of flood alleviation. The 2.33-year flood (Q 2.33 )isthe
years.To compute the flood–frequency relationships, mean annual flood (1,473 cumecs in the example),
the peak discharges are listed according to magnitude, the 2.0-year flood (Q 2.0 ) is the median annual flood
with the highest discharge first.The recurrence interval (not shown), and the 1.58-year flood (Q 1.58 )isthe
is then calculated using the equation most probable flood (1,133 cumecs in the example).
n + 1
T =
m