Bendway Weirs combined with Longitudinal Peaked Stone Toe Protection (LPSTP) were chosen as the protection structures for this project.  The Bendway Weirs designed for this project were intended to reduce erosion on the outer banks by reducing velocities near the bank and reducing the concentration of currents on the outer bank, while producing a better current alignment through the bend.  Additionally, the LPSTP is a protection method that allows stone to self-adjust or “self-launch” into any scour hole that may form on the stream side of the structure.  The lengths of the weirs were designed to intercept a large percentage of the flow and move the thalwag (the deepest section of the channel) away from the toe of the eroding bank and into a smoothed alignment streamward of the ends of the weirs.

Bendway Weirs are environmentally desirable as they improve aquatic habitat diversity and complexity (Shields et al., 1995).  The weirs will enhance the overall aquatic quality by increasing available habitat in this stream, in which rocky habitat is limited.  Some areas on the Mississippi River stabilized with Bendway Weirs have yielded anywhere from 2 to 13 times as many fish compared to similar sites using other types of bank protection (Davinroy et al., 1998).

Combining vegetation with the strength of the rock enhances the structural integrity of the weirs.  Once the high velocity flow vectors have been deflected away from the bank, less intensive and less costly biotechnical erosion control practices were used on the streambank areas between weirs. Incorporating appropriate and naturally occurring vegetation (poplar, willow, buckeye, native grass, etc.) with structural components (rock, turf reinforcement mats, geotextile materials, coir material, etc.) results in a more naturally maintaining system which enhances the aquatic and terrestrial ecosystem.  Some scour should be anticipated at the weir ends, immediately upstream and downstream.  The resulting scour pools will provide additional stream habitat and dissipate some of the erosive stream energy.
 

Figure 2 and 3.  The left photo shows Buckeye Creek before Bendway Weirs. 
The right photo shows the Bendway Weirs, during completion of the keyways landward of the LPSTP.

Construction began in October of 1998, and the contractor began by clearing, grubbing and reconsolidating the surface debris.  The stream was still dry at this time.  The equipment mobilized for this job included a dozer, scraper, loader, excavator and water truck.  The waste and eroding streambanks in the area of the exposed dump were “pulled back” and relocated in designated fill areas away from the stream.  The contractor then placed a 0.6 meter (two-foot) thick soil layer over the reconsolidated material and graded the area to drain.  A mid-channel island was also removed.  The erosion control consultant directed the contractor to salvage existing woody vegetation, when feasible, for future transplanting and stabilization of the completed streambank.  A mature toyon was subsequently re-established by transplanting with the loader landward of the LPSTP and “watered in” with the water truck.

The first phase of the streambank protection began with the construction of the peaked stone toe wall.  The alignment of the LPSTP was grade checked as per the plans to provide a smooth radius of curvature along this outer bend.  The LPSTP and Bendway Weirs were constructed using ½ T class rock.  The median rock size was approximately 0.6-1.0 meter (24”-36”) boulders.  The crest of the toe wall was 1.3 meters (4 feet) above the existing streambed.  LPSTP constructed 1 meter high requires about 3 tons per meter (1 ton per foot).  LPSTP applied at 6 tons per meter (2 tons per foot) would be about 2 meters tall. The LPSTP was easily constructed by using the loader to smooth the streambed prior to placement.  Keying into the bed was not required, but keying into the banks was important.  The loader was the primary piece of equipment used to build the LPSTP, while the excavator was only necessary to help “set” the rock and smooth the alignment.  Backfill, 3-4 meters wide, was placed immediately landward of the LPSTP in the immediate vicinity of the dump, from Weir 5 downstream to Weir 13, a distance of approximately 300 meters.  The backfill was compacted with water and 4 passes with the dozer, except for the last 0.6 meters, which were left uncompacted for future biotechnical plantings.

Biotechnical plantings are generally more successful, especially in harsh areas, if the plantings are done in conjunction with the construction instead of after.  Planting during construction often allows the cuttings to be placed deep into the soil, thus increasing survivability and geotechnical stability (McCullah, 1998).  It should be noted here that biotechnical plantings, such as pole cuttings of cottonwood and willow, should have been done prior to backfilling in the zone just landward of the LPSTP.  Use of the water truck and slightly compacted backfill would result in deeply planted cuttings.  However, because of scheduling problems and inexperience, the contractor was averse to incorporating planting during construction of the LPSTP.
One exception, however, was when a very large cottonwood tree was cleared from the streambed, upstream from Weir 1.  The tree had been deflecting flows into the eroding banks and would compromise the ability of the Bendway Weirs to control the flows through the reach.  The tree was cut up and the large limbs and branches were thrown behind the LPSTP with the terminal bud ends pointing up.  The excavator then removed the rootwad, with 2 meters of trunk attached, and also placed it behind the LPSTP.  Soil was placed over the branches and the rootwad, and then the backfill was moistened with the water applied by the water truck.

Thirteen Bendway Weirs were constructed at locations and angles designated on the plans using careful onsite analysis.  The onsite lay out, utilizing the advice of a Bendway Weir expert, is necessary because they have tremendous potential to affect stream processes and direction.  The most important weirs are the first two or three - where the flows enter the weir field and are thereby “captured”.  Also significant is the angle and length of the exit weir because it directs the flow downstream.  The Bendway Weirs were designed between 12-17 meters (35-50 feet) long (from the LPSTP out into the stream) and angled 10-20 degrees upstream from their radii.  Each weir was keyed into the bank to ensure the stream does not migrate or scour behind the structures.  The keys were of various length such that they extended through and landward of the LPSTP, into the bank until the crest of the key was 4 meters above the streambed, and the keyways were a uniform 1 meter thick.

The weirs were constructed at a level elevation, 1 meter above the streambed, with a crest width of 1.3 meters (4 feet).  Once again, the streambed was simply smoothed before placement of the stone.  The weirs were placed approximately 20-30 meters apart and angled upstream.  While the loader facilitated rapid construction of the weirs, the excavator was necessary to excavate the keyways and place the keystones.  As an additional precaution the LPSTP was extended for almost 100 meters above the first weir and 40 meters below the last downstream weir (Weir 13).  Another precaution was taken to ensure the opposite bank was protected:  a 150-meter long LPSTP was built along the left descending bank.  This LPSTP was also keyed into the bank at 35-meter intervals.  The upper end of the LPSTPs were keyed into the point bars by digging trenches 1.2 meters wide by 1.2 meters deep and filling those with sufficient stone to ensure the flows do not go around the LPSTP.

The Bendway Weir system combined with the LPSTP proved to be the most cost-effective stabilization technology considered for this site.  The project was completed ahead of schedule and about 30% under budget.

Construction costs for these stabilization techniques were approximately $200 per meter ($67 per foot) of streambank protected.  The entire project required almost 3600 tons of rock.  Over 600 meters of LPSTP was placed to protect the right bank adjacent to the weirs and the descending left bank.  The 13 Bendway Weirs and accompanying keyways required the placement of 260 meters (780 feet) of rock.  Therefore, the actual cost of building the weirs and LPSTP was only about $150 per meter ($50 per foot).