Environmental Engineering Reference
In-Depth Information
holes ultimately migrate back upstream through the bridge opening, posing a threat to the stability of the
bridge and threatening public safety.
Trees and tree branches delivered to incised channels by bank failures often become trapped on the
upstream sides of bridge piers and abutments. Accumulation results in trapping of additional debris and
the redirection of flows. Secondary flows result in scour around piers and abutments, undercutting of
bank toes, and slab failures. Large woody debris and its effects have been cited as a cause for bridge
failures in incised channels (Robbins and Simon, 1983; Melville and Dongol, 1992; Wallerstein and
Thorne, 1996; Hupp, 1997). On the other hand, large woody debris may mitigate the channel incision by
creating high resistance and trapping sediment. Figure 3.2 shows large woody debris deflecting the flow
in a tributary stream of the Yalong River in Sichuan, China, which results in sedimentation mitigation of
channel incision.
Fig. 3.2
Large woody debris mitigates incision in a tributary stream of the Yalong River in Sichuan
In field investigations the diagnostic feature of incised rivers is the ā€œVā€-shape channel. If the lower
banks have higher slope than upper banks the river experienced accelerating incision in the past decades
and is of particular danger of bank failures or landslides. The banks of incised rivers are mostly at the
critical slope, which means that a slight increase in bank slope may cause bank failures or even landslides.
On incised rivers the bank slope may be higher than 30 e . In some cases it may be as high as 45 e . Lithologic
characters are the most important factor for the bank slopes. Limestone, granite and sandstone support
high slopes but shale, antigorite and phyllite can only sustain low slopes. Poor vegetation is associated
with reduced slope. Figure 3.3(a) shows the incised Liwu River in Taiwan, China with ā€œVā€ shape valley.
The bank slope is so steep and unstable that rainstorms often cause bank failures and landslides. Because
bank failures bring a lot of sediment into the river the sediment concentration in the river is quite high.
Figure 3.3(b) shows a bank failure due to incision on a river in Sichuan.
3.1.2 Causes of Incision
The causes of channel incision are numerous and they can be grouped into six categories (Table 3.2) that
at least partly reflect the different time and space scales at which formative processes operate (Darby and
Simon, 1999). Geologic and geomorphic causes may require many years to develop a response, whereas
climatic and hydrologic variability, animal grazing, and human activities can have a more immediate
impact. For example, a wetter climate (C2) will increase discharge (D1), and increased rainfall intensity
(C3) will increase peak discharge (D2). Also, there is feedback from human and animal activities to
hydrologic controls. For example, human activities, 1, 3, 4, 5, 13, 14, and 15 will modify water discharge
and sediment loads.
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