Chapter 6 Techniques This chapter provides information on specific techniques used in integrated streambank protection. Table of Contents Flow-Redirection Techniques........................................................................................................................... 6-3 Groins ....................................................................................................................................................... 6-3 Buried Groins....................................................................................................................................... 6-15 Barbs ......................................................................................................................................................... 6-23 Engineered Log Jams........................................................................................................................ 6-31 Drop Structures ................................................................................................................................. 6-39 Porous Weirs ........................................................................................................................................ 6-47 Structural Techniques ........................................................................................................................................... 6-55 Anchor Points ...................................................................................................................................... 6-55 Roughness Trees ................................................................................................................................. 6-61 Riprap ....................................................................................................................................................... 6-67 Log Toes .................................................................................................................................................. 6-79 Roughened-Rock Toes .................................................................................................................... 6-89 Log Cribwalls........................................................................................................................................ 6-99 Manufactured Retention Systems ............................................................................................ 6-107 Biotechnical Techniques ...................................................................................................................................... 6-119 Woody Plantings ................................................................................................................................ 6-119 Herbaceous Cover ........................................................................................................................... 6-133 Soil Reinforcement............................................................................................................................ 6-139 Coir Logs ................................................................................................................................................ 6-149 Bank Reshaping ................................................................................................................................... 6-157 Internal Bank-Drainage Techniques .............................................................................................................. 6-163 Subsurface Drainage Systems ..................................................................................................... 6-163 Avulsion-Prevention Techniques .................................................................................................................... 6-171 Floodplain Roughness ..................................................................................................................... 6-171 Floodplain Grade Control ............................................................................................................ 6-177 Floodplain Flow Spreaders ........................................................................................................... 6-183 Other Techniques................................................................................................................................................... 6-189 Channel Modifications..................................................................................................................... 6-189 Riparian-Buffer Management....................................................................................................... 6-201 Spawning-Habitat Restoration ................................................................................................... 6-207 Off-Channel Spawning and Rearing Habitat ...................................................................... 6-217 Chapter 6 6-1 6-2 Chapter 6 Groins GROINS Flow-Redirection Techniques DESCRIPTION Groins, also called spur dikes, are large roughness elements that project into the channel from the bank and extend above the high-flow, water-surface elevation. They are usually constructed in a series and act together hydraulically to provide continuous bankline roughness. Though commonly constructed of rock, groins can be built with large woody debris or pilings that collect debris. Figure 6-1 (at the end of this technique discussion) shows various applications of groins through- out Washington State. The main functions of groins are to redirect flow away from a streambank and to reduce flow velocities near the bank, which, in turn, encourages sediment deposition. As more sediment is deposited behind the groins, banks are further protected. Groins tend to induce scour near their tips, and scour holes are likely to form in those locations. Depending upon factors such as the angle of attack of flood flows and depositional patterns, eddies may form between groins, which may lead to scour along the bases of groins or adjacent streambanks. In general, however, deposition can be expected between groins that are properly designed and installed in an appropriate location. Barbs and groins are often mistaken for one another because they look similar, and both function to redirect flow. The primary difference between groins and barbs is that groins are higher-profile structures that tend to deepen the thalweg and narrow the stream, while barbs have less of an effect on the cross-sectional shape of the stream. APPLICATION Groins are used to realign a channel or redirect flow away from a streambank to protect it from erosional forces. They are also used to increase channel roughness at locations that lack roughness elements. Groins are best applied as bank protection in long, uniform bends where the upstream flow approach remains relatively constant over time. Frequently, groins are applied to reduce flow velocities and shear stress along eroding banks. In certain cases, groins can be used to narrow the channel in low-gradient, aggrading reaches causing flow velocities and sediment transport rates to increase. Prior to applying groins as a bank-protection technique, it is important to understand the existing physical characteristics and geomorphic processes present in a potential project reach (see Chapter 2, Site Assessment and Chapter 3, Reach Assessment for guidance). Groins work best in wide-radius bends where they can even out the hydraulic effect along the bank. In tight- radius bends or other constricted reaches, groins may not be very effective, and their application can further exacerbate existing erosion problems or move them upstream. Care in sizing and spacing the groins is crucial to avoid creating a constriction. Use of groins within a channel migration zone is also not recommended because it interrupts the natural riverine channel- Chapter 6 6-3 migration process and may cause future erosion problems upstream and downstream. Refer to the screening matrices in Chapter 5, Identify and Select Solutions for more guidance on the applicability of groins based on the mechanism of failure and causes of streambank erosion. Groins are often installed as a combination of habitat enhancement and bank protection. However, recent work has called into question the use of rock structures for habitat enhance- ment and, therefore, as mitigation. Density at rock groins were less than those found at adjacent, untreated banks.1 Variations Groins can be set back from the active channel as an eventual line of bank protection. This type of groin is referred to as a buried groin. Buried groins are discussed as a separate technique in this chapter. Groins can be constructed to be permeable or impermeable. An impermeable groin (e.g., solid- rock groin) allows minimal flow-through, whereas a permeable groin (e.g., log groin) allows flow to pass through it easily. A permeable groin acts as sieve and tends to collected a greater amount of woody material than an impermeable groin. As material is collected at the perme- able groin, it eventually functions more like an impermeable groin. Impermeable groins tend to be more effective at redirecting the flow than permeable groins with no accumulated debris. Tight-Radius Scour Holes As mentioned earlier, groins are not particularly effective in tight-radius bends. Indeed, they can do more harm than good. A tight-radius bend with a deep scour pool acts as an energy sink, significantly dissipating stream energy. Partially filling the bend and pool with groins and/or shortening