Consideration of risk in hydraulic design of bank protection using Reno matresses and riprap

This report discusses channel stabilization design by erosion protection measures considering risk analysis. Although many different erosion protection measures have been used, this report only considers rip-rap and wire enclosed rock (in mattresses) which are two of the most widely used methods of erosion protection. The purpose of erosion protection and channel stabilization is, of course, to maintain a channel and its banks and/or bed in a relatively fixed location to protect property, structures or other development from erosion related damage. Because rivers are quite dynamic and development continues along rivers and other types of channels, the need for erosion protection and bank stabilization will continue. Chapter II discusses briefly the dynamic nature of rivers and the need for erosion protection and channel stabilization. Various types of engineering analysis are required in developing an erosion protection design. These include hydrology, hydraulics, geomorphic, erosion and sediment transport. Where ship or barge traffic is of concern, ship-generated waves and other related impacts should also be accounted for. Chapter III discusses the data and analysis required for designing erosion protection and channel stabilization projects. This is by no means a comprehensive discussion of these topics but is meant to serve as an introduction and guide to the subject. The reader is encouraged to refer to the references given on these topics for further information. Chapter IV discusses common failure modes of erosion protection projects. There are a number of distinct and independent causes of failure. Each one alone can be sufficient to induce failure or they may, in some cases, act in concert to induce failure. A fairly comprehensive list of potential modes of failure is included ranging from the obvious to the more obscure and often overlooked. Flowing water causes forces or stresses to act on the bed and banks of a river. These are the forces that cause erosion and attempt to damage any erosion protection. They basically include shear, lift and drag forces acting on each individual particle on the bed and banks. The beginning portion of Chapter V analyzes the hydrodynamic forces due to flowing water acting on erosion protection works. Then based on the most important or frequent modes of failure and an evaluation of the forces acting on rip-rap or wire enclosed rock, methods to evaluate the probability of failure for each of the selected modes were developed. The remqinder of Chapter V covers this topic. The methods presented can be used either directly in a design application or as a tool in evaluating an existing design. Chapter VI outlines these methodologies in step by step format and then gives an example to follow for each procedure. Because the individual modes of failure can be treated independently, the total probability of failure is obtained by summing the individual probabilities. If the probability of failure thus obtained is too high for the given event, redesign is required. The next chapter (Chapter VII) is called "Risk Analysis." This type of an analysis actually is an economic optimization procedure but it has been previously called risk analysis in the literature. Risk analysis consists basically of designing a structure or whatever in order to minimize overall costs. These overall costs include both the initial construction costs as well as long term maintenance and repair costs. It is based on the logic that one may design something that can withstand the forces of a 1,000 year flood but the initial cost and therefore the total cost would be unreasonably high. One may also design a project to withstand the forces of a two year event which would have a very low initial cost but again the overall cost would be very high because of the frequency of maintenance and/or repair and replacement costs. The purpose of risk analysis is to find the minimum overall cost by varying the hydrologic return period of a project and associated designs and their costs in order to minimize the overall project cost. The design is selected that then gives this minimum cost or at least is in the range where costs are near the minimum. Chapter VIII contains the summary and conclusions. References and an appendix of terminology follows the final chapter