Environmental Engineering Reference
In-Depth Information
At least five basic types of botanical evidence of geomorphic events are useful, as shown in Fig. 2.55
(Sigafoos, 1964; Hupp, 1988): ķ corrasion scars; ĸ adventitious sprouts; Ĺ eccentric growth;
ĺ secondary roots, and Ļ buried sapling with adventitious roots showing ages and depth of sediment
deposition. Corrasion scars and sprouts from tilted parent stems yield accurate (usually within one year)
dates of bank failure. Increment cores or cross sections are taken through scars or at the base of tilt
sprouts to determine when the trees were impacted. Figure 2.56(a) shows sprouts from a tilted parent
stem of a tree by a small river in central China. The sprouts are about 6 months old, from which it is
estimated that a flood event occurred 6 months ago and caused the bank erosion and the tree tilted.
Figure 2.56(b) shows a dying tree in the Xiaojinchuan River in Sichuan Province, which explains that
there was a relative stable vegetative island in the river that had existed for several tens of years. A major
event occurred recently and the island was scoured away. It is estimated that the flood was an major
event with high sediment-removal capacity.
(a) (b)
Fig. 2.56 (a) Sprouts from tilted parent stem of a tree by a small river in central China. It is estimated that a flood
event occurred 6 months ago and caused the bank erosion and the tree tilted; (b) A dying tree in the Xiaojinchuan
River in Sichuan Province, which indicates that there was a relatively stable vegetated island in the river.
Eccentric growth often occurs when a stem is tilted off centre and is easily determined from cross
sections by noting where relatively concentric ring formation abruptly shifts to eccentric ring formation.
Eccentric ring patterns provide accurate dates of tilting, often to the season of occurrence. Estimates of
channel widening are made first by determining the ages of stem deformations associated with bank
failure, and then subsequently measuring the width of the slump block or the distance between affected
stems and the present top-bank edge (Fig. 2.57). Slump blocks of varying ages and entrained woody
plants provide a history of recent bank failure along a given reach.
Sediment carried by high water may be deposited around the bases of riparian trees and saplings
growing on various fluvial geomorphic surfaces. Buried trunks, branches, and adventitious roots permit
estimates of the sediment accretion rate. A rate of accretion may be estimated (Fig. 2.55) by digging the
ground adjacent to buried stems to the depth of their original root collar (germination point), coring the
stem for age determination, and subsequently dividing the depth of burial by stem age.
Over the last 25 years or so, there has been a substantial increase in the amount of plant ecological
and fluvial-geomorphic research on the role vegetation in geomorphic form and process and vice versa.
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