FLOOD01647

• County: Statewide
• Title: Optimal Sizing of Urban Flood Control Systems
• Date: 3/1/1974
• Prepared For: US
• Prepared By: COE
• Floodplain - Doc Type: Educational/Technical/Reference Information

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of the alternatives and development of implementation strategies. An objective
within the successive refinement of alternatives is usually to determine the
system, which can include physical works and other non structural measures.
that will in the aggregate perform their function most economically. The most
economically efficient size for a system exists when the difference between
the total annual benefits and the total annual cost is maximized, which is termed
the scale of maximum net benefits. In studies with a few components,e.g"
two or less, the usual approach is to nominate a few selected component sizes.
determine their performance, and graphically estimate the particular component
scales that would accomplish the economic objective, For more than two
components, graphical analysis is virtually impossible.
The next step in formulation is usually to "select" a performance standard,
giving appropriate weight to social and environmental objectives. The performance
standard is usually expressed as the "degree of protection," which is the
exceedence interval of the hydrologic event that can be controlled so that flood
damages do not result. A 50-yr degree of protection would be provided by
a system that reduced the stages at potential damage areas for a 50-yr exceedence
interval flood to stages below damaging levels,
Another sizing problem exists upon having selected a performance standard:
To determine the size of each system component that will accomplish the target
degree of p,rotection most efficiently and economically. The usual approach
is to size the facilities so that they accomplish the target performance standard
at the least overall annual cost. A better approach would be to size the facilities
to satisfy the target performance standard while, to the extent possible. maximizing
system net benefits. This concept recognizes that different components, such
as reservoirs and levees, perform differently for events that exceed the magnitude
of the performance target event.
The determination of the size of each component in a system that will maximize
net benefits or accomplish the performance standard is by no means trivial
when more than two major components can take on a range of sizes. For
complex urban flood management systems, the analysis can be extremely tedious
and consume a very large portion of the efforts and energies of those performing
the studies, if they are done at all,
The issue of timing or sequencing of implementation of system components
once the desirable components have been sized has been examined by James
(8). Because of land-use projection uncertainties and questions pertaining to
policies related to implicit consideration of future economic growth, the technique
presented herein does not directly deal with the issue, Instead, as subsequently
pointed out, it is suggested that the sensitivity of the solution to timing, particularly
as represented by future development if timing is believed of significance, be
determined by varying the assumed discounted damage relationships.

OPTIMIZATION TECHNIQUE

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The strategy for developing the technique consisted of first devising a computer
simulation model for simulating the hydrologic and economic performance of
flood-control systems, then structuring an automatic search procedure that would
exercise the simulation model by successively adjusting the scales of each
component of the system until the solution is found,