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°ce Creek ~~ letes Strea bank .reject by Derek Knapp The Rice Creek Watershed District (RCWD) recently completed a streambank stabilization project on Rice Creek in Fridley that utilized a streambank erosion control and stabili- zation technique that is relatively new to the Minneapolis/St. Paul metro area. The purpose of this article is to describe the stabilization method as well as provide project highlights, design advantages, and project costs. Ten partially-submerged, Class IV rip- rap bendway weirs were constructed to protect approximately 700 feet of se- verely eroded Rice Creek streambank at two separate creek bends. The upper portions of the 20'-30' high outer banks were regraded to provide more stable slopes, and Prairie Restoration, Inc. was contracted to provide intensive outer bank revegetation measures at both sites. The bendway weir application consists of partially submerged, streamflow-de- flecting rip-rap vanes angled upstream to protect and preserve the outer. bank and encourage thalweg migration to- wards the inside streambank. In a typi- cal unimproved bend (without protec- tion), surface water currents tend to move from the inside of the bend toward the outside, concentrating flow and increas- ing velocities along the outer bank. When properly designed and constructed, the bendway weirs redirect streamflow nearly perpendicular to the axis of the weir. Because the weirs are angled to point upstream, the flow is redirected away from the outer bank towards the inner bank. The redirected weir flow then "merges" with main channel flows, which begin to expand to use the entire channel capacity and begin moving back towards the outer bank. When spaced correctly, the next downstream bendway weir in- tercepts this erosive flow just prior to contacting the outer bank. Some bendway weir advantages include: creation of high-velocity, erosive flows flow can be redirected at predictable angles; good efficiency under high-flow, high-erosion conditions; fisheries habitat can be improved through incorporation of a "J-hook" at the streamward end of the weir (creating a candy cane shape); weirs can be retrofitted following project completion to improve per- formance; flow energy is slightly reduced; sedimentation usually occurs imme- diatelyupstreamand downstream of the weir, helping to further protect the outer bank; and weir construction costs are fa- vorable when compared to tradi- tional rip-rap armoring. Potential bendway weir pitfalls include: if the weirs are too large with relation to channel capacity; minimal erosion pro- tection if the weirs are too short or in- correctly spaced; potential navigational hazards; and the potential for inconsis- tent construction by an inattentive con- tractor (intensive construction manage- ment services will minimize this risk). The total construction costs for the both project sites, which included the ten bendway weirs, short sections of bank armoring, geotextile fabric; and deadfalU tree removal, was approximately $20,000.. The project was completed within one week. Armoring these two sections with Class II rip-rap would have increased the total project cost by nearly $10,000. Contact RCWD Engineer Derek Knapp, P.E., Montgomery Watson, (763) 595- 5261 at derek.knapp~mw.com or Kate Drewry, RCWD Administrator, (651) 766-4191 at drewr001 crtc.umn.edu for further information. r_ duce Creek Watershed District streambank erosion and stabilization project with bendway weir application.