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atTa (~ arlrle eS1gI1 ail O11 lOellglrlE~eI'111g by Bryan Ripp, P.E., P.G., Intuition & Logic Increasingly, streams and rivers are effectively managed using a technique called Natural Channel Design, which often incorporates soil. bioengineering. This design approach represents a convergence of the fluvial geomor- phology and civil engineering knowledge accumulated over the past half century. At times working independently, investigators and designers such as the legendary Luna Leopold and E.W. Lane published the seminal works that still underlie all modem river analysis. Fundamental con- cepts such as stream forming flow, channel evolution and the role of energy dissipation in channel stability exert powerful influences on both analysis and design. Engineering v_-- ---_ requirements _ - :~~, y __ Plant _ ~.l Plant characteristics - - ~' selection Site -~_ a ------ conditions `= _-- -___ increases soil tensile strength. Recently, the National Sedimentation Laboratory reported that the evapotranspira- tion effect on forested soils increases strength by 10-fold during wet, average and dry years. However, this increase did sharply diminish during extremely wet years. In Natural Channel Design, project objectives such as flood or erosion protection are achieved by working with rather than controlling the forces shaping the stream. This re- quires an understanding of stream processes and the ability to design treatments that honor these processes. Natural Channel Design requires careful in-stream investigation to uncover the root causes of stream instability. This approach contrasts sharply with the more symptomatic treatment characterized by bank treatments such the wide- spread use of rip rap, gabion baskets, Reno mattresses, and concrete lining. In many cases, treating the underlying causes of a problem such as incision or channel downcutting, takes enough stress off the system that other repairs can be achieved using methods much less intrusive .than conventional means. It is in this context, the robust qualities of properly designed soil bioengineering result. This approach also sharply contrasts to the prevalent experimental or trial and error applications of soil bioengi- neering, which compels many to question whether engineering is involved at all. Perhaps the most visible feature of Natural Channel Design is the use of native riparian vegetation. These vegetative stabilization methods are often referred to as soil bioengi- neering. In sharp contrast to conventional methods that clear vegetation from streambanks, modern management techniques take advantage of the stabilization qualities of trees and shrubs. Riparian vegetation stabilizes streams hydrologically, mechanically and hydraulically. Hydrologically, trees can withdraw immense quantities of water from the soil through evapotranspiration. The resulting suction of soil moisture The mechanical stabilization through reinforcement devel- ops through the load transfer from soil to root fibers under shear stress/strain conditions. Contrary to popular belief, the mechanisms of soil- root interaction and the magnitude of root reinforcement are not loosely based on empirical data. Donald Gray and Andrew Leiser, Gray and Harukazu Ohashi, Fu-Chun Wu and others have rigorously investigat- ed these mechanisms. Through root reinforcement, soil shear strength is approximately doubled in well-vegetated soils. Finally, riparian vegetation protects streambanks more indirectly through hydraulic interactions. The hydraulic roughness afforded by flexible vegetation shifts the high velocity flows away from the streambank. By selectively shifting the high-energy, faster velocities toward the center of the channel, the shear stress on the streambank is reduced thereby reducing scour at the toe of the slope. = ~ t ~ ~ ±- r.~ R,,. ~, t } t ~. r t E ,~~. ~~; ~ ,~s ~~ ~~ The selective and advantageous use of hard points is a less obvious but even more important feature of Natural Channel Design. This allows the designer to balance energy, water flow and sediment flow by adjusting channel planform and profile. The simplest application of hard points is energy 14