Selection of Site-specific Stabilization Techniques
5.3.1 COST OF ALTERNATIVE TECHNIQUES
Because costs vary widely with location and time, discussion here is limited to general
concepts, which are universal and timeless.
Suitable methods can be identified using the matrix approach presented in the next
section. A preliminary cost estimate can then be used to eliminate cost-prohibitive methods,
followed by more precise estimates to be used in final selection.
The final estimate can take into consideration incidental items such as rights-of-way,
engineering and design, supervision and inspection of construction, operation and
maintenance, and contingencies. Institutional policy may specify that these items simply be
estimated as a percentage of construction cost, or a more precise estimate may be
appropriate. In the selection phase, it matters only if there are substantial differences in these
factors among the methods being considered, which is seldom the case. Some possible
Significant differences in cost of rights-of-way may occur if one method could
be constructed with floating plant, but another method would require
extensive rights-of-way on the bank in a developed area.
Significant differences in the cost of engineering and design may exist if
methods are being considered for which standard specifications exist, or for
which design assistance is available from the manufacturer. These would
require less engineering and design effort than methods for which original
specifications must be developed. Also, the data required for analysis and
design, may vary between methods. For example, precise riprap design
requires hydrologic and hydraulic analysis, and precise geotechnical design
requires costly field and analytical work. Protection techniques which involve
pile-driving may require borings to determine sub-surface conditions.
Methods requiring a long period of time to construct, such as labor-intensive
methods, or methods requiring intensive quality control, such as underwater
placement of stone, would have a higher supervision and inspection cost than
techniques that are quickly constructed with minimal supervision.
In practice, the cost of operation and maintenance for well-designed work is
usually low, and quantitative comparison of various methods is difficult unless
a method is being considered which requires unusually intensive monitoring,
maintenance, and reinforcement. A sophisticated analysis would examine
"life-cycle" costs, the procedure for which will usually be specified by
institutional policy. Some of the environmental features discussed in 5.2 often
require more long-term attention than "hard" structures. Vegetative measures
and land use management often require monitoring and maintenance in order