Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
1 Significant resources are being applied by public and private road and rail organizations to design and construct restored streams in disturbed watersheds, as well as to provide for stable transportation crossings (bridges and culverts) of streams. Lacking in this effort is a scientifically supported method for defining the design hydrology for such efforts along with an understand- ing of how that design hydrology might change with land use changes. Current practice in hydrologic design of stable channels at stream crossings focuses on a single âdominantâ discharge that is assumed to be a reasonable surrogate for the entire range and temporal sequence of channel-forming flows. The channel-forming discharge is typically identified and âbankfullâ field indicators (a challenging task even in minimally disturbed chan- nels), recurrence interval analysis of peak flows (often extrapolated from gaged to ungaged sites), regional flood regression relationships, or a combination of these methods. Such methods can be problematic because they oversimplify the physical controls on channel form and response, and frequently result in channel designs that are unstable. A more robust alternative to stable channel design hydrology has converged on the idea that a sediment continuity or âsediment impact analysisâ should underpin the design of most alluvial channels (Copeland et al. 2005; Soar and Thorne 2001, 2011; Natural Resources Conservation Service (NRCS) 2007; Shields et al. 2003, 2008; Doyle et al. 2007), especially fine-grained or âlabileâ channels. One of the greatest impediments to adoption of these methods in practice has been the lack of (1) decision support and analysis tools for discerning which design situations require such an analysis and (2) practical tools generating key inputs and performing the analy- ses. This document addresses this gap by summarizing guidance and a set of decision support tools that are scientifically based and practical. The decision support tools presented herein are intended to be flexible and efficient in guiding users to an appropriate combination of design tools and depth of analysis for design hydrology in a given hydrologic and geomorphic setting. Additional data and effort are only required when necessary for the channel type and design situation of interest. Introduction C h a p t e r 1
