FLOOD07468

• County: Statewide
• Community: State of Colorado
• Stream Name: All
• Basin: Statewide
• Title: The Design of Encroachments on Floodplains Using Risk Analysis
• Date: 10/1/1980
• Prepared For: USDOT
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

The second step in the alternative selection process is to determine the
components of the various design strategies. These could include bridges,
culverts, embankment, protective measures and so on, and each alternative
may involve one or a combination of these components. There will also be
control variables associated with each design strategy. These may include
bridge length, embankment height, cul vert size, degree of longitudinal
encroachment and countermeasure parameters.

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Consider, for example, a crossing of a wide flood plain, 2900 feet with a
low water channel 440 feet wide. The alternative design strategies include
a main channel structure and may include auxiliary (relief) structure(s),
either culverts or smaller bridges. The control variables may be bridge
length, embankment height and possibly culvert size. Also, some or all of
the alternatives may require scour protection, thus requiring that additional
control variables be considered.

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In this illustration, the main channel opening could vary from a structure
less than 440 feet in length to one 2900 feet long, the embankment height may
vary from no embankment to the height controlled by the adjacent grade lines,
in any desired increments. Auxiliary opening and scour protection schemes
may involve one or more bridge lengths and/or culvert sizes and one or more
protection systems for all or only some of the alternatives.

The Control variables are designated as follows: main channel bridge length

as BLl --~ BLm' embankment height as EHl --- EHn' auxiliary opening

schemes as XOl --- XOo' and scour protention schemes as SPl -- SPp.

The possible set of design alternatives becomes a four dimensional matrix best

described: by index notation, A, , k 1 where i varies from 1 to n, j varies
1 tJ" ,
from 1 to m, k varies from 1 to 0 and 1 varies from 1 to p. If we assume n=4,
m=4, 0=4 and p=4 there will be 256 design alternatives in the analysis. However,
if auxiliary opening schemes are varied in combination with bridge lengths, the
analysis then looks,like a two variable analysis as illustrated in table 4.1,
and is manageable. Scour protection would be inherent in the design of the
alternatives, but sCour protection costs would still vary with the amount of
flow constriction, the extent of over-embankment flow and the extent of
auxil i ary openi ngs.

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From the above example, it is apparent that the alternative selection process,
especially where more than two control variables are involved, can rapidly
assume unmanageable proportions. Seldom will the selection process be as open
ended as in the previous example, however. In most cases, the selection of
alternative design strategies will be governed by a set of control criteria
based on previous experience, preliminary analysis or other constraints such as
those discussed above.

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