Topography Resulting from Stream Deposition
4.11 NEOTECTONICS AND EARTHQUAKES
During the cycle of erosion the land surface is affected not only by the erosional forces but also by the internal forces, which cause displacement of earth’s surface due to movement of earth’s plates and resulting stress building. This displacement is usually slow and can be gradual uplift, subsidence or lateral displacement. Neotectonics refers to these gradual and presently active aseismic crustal deformations. If this happens in the vicinity of an alluvial stream, uplift or subsidence can cause degradation or aggradation respectively thereby altering the gradient upstream, at the axis of movement and in the downstream reach.
The minimum rate of uplift estimated by Zeuner (see Schumm 1977) for the Alps and the Himalayas are a millimetre per year. In California the average mountain building rate in modern times is 0.80 mm/year. The present rate of isostatic uplift in North America is 0.50 mm/year. The subsidence in the surrounding area caused by the storage of water and sediment in Lake Mead, U.S.A. was 1.3 mm/
year. According to Schumm et al. (1987) many streams such as the Mississippi and Rio Grande in U.S.A. and Amazon, Niger, Tigris, Euphrates, Rhine and Indus are affected by such structural
Fig. 4.20 Observed bed topography of south Esk bend 10 M
5 6
7
4 3 3 2 1
instability. In Northern Iraq (i.e., Ancient Mesopotamia) Diyala River that is the tributary of the Tigris has incised into its alluvial deposit due to uplift during the past 1000-1200 years. As a result the inundation canal system developed in the earlier times has had to be abandoned. Upwarping of the Brahmaputra basin is found to be partly responsible for flood problems in Bangladesh. Similarly, tectonic uplift is likely to be at least partly responsible for the shifting of the river Kosi through 110 km to the west in the past 200 years. Such uplift and downwarping may look innocuous during a short period but can cause aggradation, degradation or change in plan form in different stretches of the stream. This aspect has been studied by Ouchi (1985) in the laboratory and his results are summarised in the Table 4.6.
Reach A: from 2.0 to 3.5 m where no significant uplift or subsidence occurred. Reach B: from 3.5 to 4.65 m, the upstream half of the uplifted or subsided zone. Reach C: from 4.65 to 5.75 m downstream of uplifted or subsided zone.
Reach D: from 5.75 to 7.0 m where no significant uplift or subsidence occurred.
The lateral movement along the fault may cause a lateral shift in the stream crossing the fault. Such a shift has been observed in the case of Narmada River in India. It has also been reported that prior to Uttarkashi earthquake of 20th October 1991 of magnitude 7.1, horizontal and vertical movements were noticed in Garhwal, Himalayas during 1972-1978. Horizontal movements were about 30 to 150 mm while vertical movements ranged from 10 to 90 mm.
Earthquakes in Zone 1 can cause large-scale land slides and mass movement and produce enormous amount of sediment which eventually reaches the stream and can cause aggradation, change in plan form, shifting of tributaries and flooding in Zones 1 and 2. This is what happened in the Brahmaputra after 15th August 1950 earthquake of 8.6 magnitude, see Gee (1951). The effects that were observed immediately after that earthquake and in subsequent years were
Table 4.5 Stream variables during different times (Schumm 1977)
Variable Geologic Time graded Steady
Time I N.R. N.R.
Initial relief I N.R. N.R.
Geology (Lithology and Structure) I I I
Palaeo climate I I I
Palaeo hydrology D I I
Relief or volume of system above base level D I I
Valley dimensions (width depth, slope) D I I
Climate (mean temperature, precipitation, seasonality) X I I
Hydrology (mean discharge of water and sediment) X I I
Channel morphology X D I
Observed Qw, Qs X X D
Hydraulics of flow X X D
I = Independent D = Dependent
NR = Not relevant X = Indeterminate
Table 4.6 Effect of uplift and subsidence on channel morphology (Ouchi 1985) zone of uplift or subsidence
i) Some tributaries got blocked by temporary dams created by the debris falling in them from land slides;
ii) Subsequent bursting of these dams caused large floods;
iii) A large quantity of sediment was brought down in the Brahmaputra causing aggradation of the order of two to three metres over several kilometers; and
iv) Some tributaries shifted their course.
According to Walters (1975) channel widening and meander cut-offs in the Mississippi river in the early 19th century were due to New Madrid earthquakes of 1811 and 1812.
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