Fundamentals of Fluvial Geomorphology and Channel Processes
stream and deliver it to another location. Diversions are often needed for water supply, irrigation,
hydropower, flood control, or environmental reasons. The system effects and complexities are similar to
those downstream of major dams. According to Lane's balance the sediment load in the receiving stream
will be increased due to extra, transport capacity of the increased discharge. In time, the erosion of bed
sediments decreases as the slope is reduced through bed degradation.
An increase in discharge due to a flow diversion can have a significant impact on the channel plan
form as well as the vertical stability. Schumm (1977) proposed a qualitative relation similar to Lane's that
included meander wavelength. His relation states that:
where Q is the discharge, b is the width, d is the depth, S is the slope, and L is the meander wavelength.
The above relation indicates that an increase in discharge may result in an increase in the meander
wavelength which would be accomplished through accelerated erosion of the streambanks. Therefore,
whenever diversions such as this are proposed the potential for increased meander activity must be
considered. If a stream is in the process of increasing meander wavelength, then stabilization of the bends
along the existing alignment is likely to be unsuccessful and is not recommended.
Basin Wide Factors. Sometimes the changes in the controlling variables can not be attributed to
a specific upstream or downstream factor, but rather are occurring on a basin-wide basis. This often results
from a major land use change or urbanization. These changes can significantly modify the incoming
discharge and sediment loads to a channel system. For example, urbanization can increase peak flows and
reduce sediment delivery, both of which would tend to cause channel degradation in the channel system.
A land use change from forest to row crop on the other hand might cause a significant increase in the
sediment loading resulting in aggradation of the channel system. Unfortunately, it is difficult, if not
impossible, to predict when basin wide changes such as these will occur. Therefore, the best the designer
can do in most cases is to simply try to design the bank protection measures to accommodate the most
likely future changes in the watershed. For instance, if there is a possibility of future urbanization in the
upper watershed, then additional launching stone may be needed to protect the bank from the destabilizing
impact of any future bed lowering.
188.8.131.52 Complexities and Multiple Factors
Lane's balance and other geomorphic analyses of initial morphological response to system
disturbance provide a simple qualitative method for predicting the channel response to an altered condition.
However, it does not take into account the magnitude of the change and the existing morphologic condition
of the stream. For instance, according to Lane's balance a channel cutoff should induce degradation. While
this is often the case, there are many examples where there may be no observable change in the channel
morphology following the construction of cutoffs. Brice (1981) documented the stability of streams at 103
sites in different regions of the United States where channels had been relocated. He found that following