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14 CHAPTER 6 TECHNIQUE SELECTION SYSTEM SELECTION STRATEGY AND APPROACH Netscape). The program, which contains rules to screen all 44 techniques and all attendant files, may be read from the Greenbank is a rule-based selection software program accompanying CD. (Once the CD is started, an icon at the developed for use by DOT or consulting engineers. The gov- lower right corner of every screen opens the support tool.) erning rules relate the strengths and weaknesses (hydraulic, A typical Greenbank consultation begins with the software geotechnical and environmental) of each technique to the rel- asking the user to provide information about the proposed evant site conditions and project constraints. The knowledge project. Specific aquatic habitat requirements can be taken base is contained in a matrix that allows the user to examine into account in a systematic manner. The user is asked to the rules and the basic rationale or reasoning behind each specify environmental resources or aquatic attributes of technique. Such an approach avoids the controversy associ- interest from a list of 11 possibilities: (1) benthic habitat, ated with selecting techniques based on a particular stream (2) decreased sedimentation, (3) enhanced bed stabiliza- classification system. This approach also allows the user to tion, (4) fish rearing habitat, (5) holding areas for adult fish, conduct sensitivity analyses when reach descriptors are (6) in-stream and overhead cover, (7) pool and riffle based on estimates. enhancement, (8) public acceptance potential, (9) riparian This software is roughly patterned on an earlier expert sys- habitat, (10) velocity refugia for fish, and (11) water qual- tem known by the acronym ENDOW (Environmental Design ity improvement. Based on this initial response, the system of Waterways) developed in the mid-1980s at the U.S. Army asks for more specific information about environmental Engineer Waterways Experiment Station (Shields and Aziz, issues. 1992). The new selection system, Greenbank, was developed The user is then asked to characterize the erosion problem using a Windows-based tool known as Exsys CORVID. The as (1) gullying, (2) erosion or scour by stream flow or wave CORVID system allowed the development of a tailor-made, wash, or (3) mass wasting (i.e., slope failure). If the user is interactive decision-making tool. uncertain about the nature of the erosion at his or her site, As noted previously, some 44 discrete techniques were iden- links are provided with text and photos to help the user iden- tified and adopted. Design criteria for some (Level I Tech- tify the dominant erosion process(es). In the case of erosion niques) are highly developed, with abundant studies in flumes, or scour by stream flow, the user can also input hydraulic field experiments, and mathematical analyses. Others (Level II criteria (i.e., design velocity and boundary shear stress), and Level III Techniques) are less well documented and are which allows Greenbank to compare these criteria with supported only by anecdotal or qualitative observation. The available published allowable values. A worksheet is pro- rule-based system allowed the application of the current knowl- vided to assist the user in computing estimates of velocity or edge for each technique. In the case of hydraulic criteria, for shear stress. example, there is a tabulation from the literature prepared by The user is also asked to classify the spatial extent of the Fischenich (2001b), as well as others. Geotechnical criteria are problem as local or general. If the erosion is general, the user well known in some cases, unknown in others, and sometimes must identify the parts of the stream channel cross section that appear to be eroding: top bank, middle bank, toe, or not applicable. Due to regional differences in ecosystems, channel bed. Through this dialog, the user is led to identify much of the environmental criteria must be general and generic. the dominant erosion mechanisms operative at the site in In any case, the Greenbank system is intended not to provide question. Up to 4 erosion mechanisms may be selected from detailed design criteria, but rather to offer a list of techniques a master list of 12 processes. The user is then asked to spec- that match (1) dominant erosion processes and (2) environ- ify the maximum acceptable unit cost (relative to a riprap mental resources of special concern at the site in question. blanket). Greenbank then assigns a score to each of the 44 tech- GREENBANK DECISION SUPPORT TOOL niques based on the technique's overall feasibility. This fea- sibility score takes into account suitability for a particular The Greenbank decision support tool can be run using a type of erosion problem, spatial location of the problem, web browser (i.e., recent versions of Internet Explorer or environmental attributes specified as important, and price the
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15 user is willing to pay. The top techniques are then output to nique guideline within the manual portion of the CD, so that the user, who may elect to change any or all of his or her pre- the user can learn more about the recommended technique. vious responses and obtain new recommendations. Each The logic flow, or selection methodology, used in Greenbank technique recommended is linked to the corresponding tech- is described in more detail in Appendix B.