chapter 3 fundamentals of fluvial geomorphology and stream ...

and slope of the reach are not significantly altered. However, itshould be kept in mind that the stream may respond to stabilizationof one bend through accelerated morphological activity in adjacentfree bends. Hence, care must be taken to ensure thatmanagement of local instability at one location does not transfer orconcentrate this natural process elsewhere in a way that isdetrimental to the dynamic stability of the system.The causes of local instability are not limited to the stream.This type of instability can also be triggered by activities insurrounding riparian and floodplain areas. For example, a reach ofstream may display local stream widening due to trampling andover-grazing by cattle, while upstream and downstream reachesthat are not directly affected and are to maintain dynamicallystable. In this situation, a local management solution, based onrestricting access by fencing, construction of suitably reinforcedaccess ramps at water points, and reinstatement of the regimewidth are all that are needed to alleviate a site-specific problem.Site-specific instability problems may respond satisfactorily todesign alternatives developed using reference reach techniques.In practice, however, it is not always easy to establishwhether a local instability problem results from and is amenable toa local solution, or is symptomatic of more serious, system-scaleimpacts and adjustments. Even if the engineer suspects that localinstability results from adjustments of the fluvial system to streaminstability, human activities, or watershed land-use changes, theymay lack the authority or resources to address off-site and nonpointcauses. Under these circumstances, the engineer may haveto modify the adopted solution by constructing a local structurewith the capability for it to continue functioning successfully evenwhen system-driven stream adjustments have significantly alteredlocal conditions. For example, local bank stabilization may berequired at the outside of a migrating bend on a river that ispredicted to degrade in the future due to system instabilitydownstream. Ideally, the system-scale problem (degradation)should be addressed directly using one or more grade controlstructures, but this may be institutionally or financially unfeasible.Recognition that the problem is not entirely local is nonetheless stillvaluable, as it allows the engineer to determine the degree ofadditional toe scour protection necessary to ensure that the bankprotection measures can withstand the additional bed loweringassociated with degradation during the design life of the project.3.7.4 System InstabilityAdjustments involved in system instability typically involveaggradation (increasing bed elevation), degradation (decreasingbed elevation), or planform metamorphosis (abrupt alterationfrom one planform pattern to another). The response of an alluvial48 Fundamentals of Fluvial Geomorphology and Stream Processes