Loading...
10-006 2017 03 16 Basis of Design - RMB.pdfGZA March, 2017 Abbreviated Basis of Design Discussion Roberts Meadow Brook Channel Rehabilitation Project As shown on the Project drawings, rehabilitation of the Roberts Meadow Brook channel segment at the Site is extensive in scope and character. The affected channel segment is not a natural channel. In 1871, Roberts Meadow Brook was relocated during the construction of the Lower Roberts Meadow Reservoir and its associated dam, which is at the upstream end of the Project Site. The realigned, man-made channel has conveyed flow fromthe dam’s spillwayto the Roberts Meadow Brook downstream of Reservoir Road. During the 1955 flooding resulting from Hurri- caneDiane, thedamwas breachedandtheReservoir Roadbridgewaswashedout. Thesubsequent repairs involved additional channel stabilization in the brook between the dam and bridge, and a new bridge constructed in a slightly different location. Since that time, the brook planform and profile have shown signs of instability. In recent years, the brook has exhibited extreme erosion of the left bank, threatening private property and downstream areas including Reservoir Road and the bridge. Project Goals A careful examination of the affected channel segment indicates that the reach is characterized by a very steep gradient (2.7%), with little in the way of stable bedform materials and structure to provide positive grade control. Recognized needs include: Establish grade control Reduce streambank erosion Enhance fish habitat Maintain width/depth ratio Maintain river stability Dissipate excess energy Withstand large floods Maintain channel capacity Be compatible with natural channel design, and Be visually acceptable to the public. Structural Measures Inorderto accomplishthe above goals, the channel rehabilitationofthis reachofRoberts Meadow Brook has been designed in accordance with the tenets presented in these and other references: Rosgen, D.L. 2001 The Cross-Vane, W-Weir and J-Hook Vane Structures…Their Description, Design and Application for Stream Stabilization and River Restora- GZA March, 2017 tion Proceedings of the 2001 Wetlands Engineering & River Restoration Confer- ence held in Reno, Nevada, August 27-31. USDA– NRCS Part 654StreamRestorationDesign– NationalEngineering Hand- book, August, 2007; Chapter 11: Rosgen Geomorphic Channel Design. The primary structure to be utilized in the Roberts Meadow Brook Channel Rehabilitation Project is the Cross-Vane structure. The Cross-Vane is a grade control structure that decreases near-bank shear stress, velocity, and stream power, but increases the energy in the center of the channel. The structure can establish grade control, reduce bank erosion, create a stable width/depth ratio, and maintain channel capacity, all while maintaining sediment transport capacity and sediment com- petence. The Cross-Vane also provides for secondary circulation patterns corresponding with channelpatterns, but with high velocitygradients and boundarystress shifted away fromthe banks of the channel. The Cross-Vane is also recognized as a stream habitat improvement structure. The vane arm portion of the Cross-Vanes for the Roberts Meadow Brook rehabilitation have been specified with a relatively shallow angle of about 20 degrees to the bank, which provides for a long vane length and protects the greatest length of streambank. The vane slope is approximately 7 percent and provides for a 2’-0”± drop from “bankfull height” to the center, deepest portion of the vane. Where the valley width is sufficient, a “Bankfull Bench” has been provided adjacent to the existing higher banks, and the Cross-Vane structure is integrated into the bench with a sill of large boulders. The size of the individual boulders which make up the Cross-Vane structure was determined by an empirical relationship corresponding with bankfull shear stress, calculated for the existing stream channel. The bankfull shear stress of about 3.4 lb/ft 2 (16.5 kg/m 2) indicates a boulder size of at least 43” (1.1 m) is required; thus, a minimum boulder size of 48” in anydimension has been specified. The channel bottom will consist of the small boulders, cobbles, and gravel present throughout the affected stream reach. At each Cross-Vane, the invert boulders will have a surface protrusion of about 12” and the footer boulders will be embedded 3’-0” below the channel bottom. Except as described below for the large Cross-Vane boulders, the same existing channel materials will be utilized for all work within the channel and up to three feet above the channel bottom. Thus, only the “natural” channel materials will be utilized below the Mean Annual High Water Line (MAHWL), which is judged to be approximately 2’-6” above the channel bottom. As indicated on the drawings, should additional “off-site” materials be required to attain the grades and dimen- sions shown, any materials used within three vertical feet of the channel bottom will consist of natural, rounded boulders, cobbles, and gravel consistent in character and appearance with the existing stream bed materials. As stated above, the Cross-Vanes will be constructed using boulders a minimum of 48” (4’-0”) in any dimension. Assuming a spherically-shaped boulder, this translates to a minimum weight of about 5,500 pounds (2.75 tons). Approximately 135 such boulders will be required to construct the four Cross-Vanes. The existing channel section is composed of boulders, cobbles, gravel, and sand, yet there does not appear to be sufficient large-boulder material to construct the required Cross-Vanes. The lack of such boulders in the existing stream channel may be an indicator of GZA March, 2017 inherent instabilityin the channel as originallyconstructed ca. 1871. As such, geologically-appro- priate boulders, rounded and natural in appearance, will be sourced from off-site areas and im- ported to the Site for the construction of the Cross-Vanes. The distance between Cross-Vanes was selected based on the overall channel slope, in percent, whichwasrevealed bytopographicsurveyto beapproximately2.7%. BasedonRosgen’s negative power function relationship between the ratio of pool/cross-vane spacing and bankfull width, a spacing of93 feet is indicated. As shownonthe drawings, spacing ofthe four (4) new Cross-Vane structures is 95 feet, indicated on the drawings by stationing along the construction baseline. At this reach of Roberts Meadow Brook, both banks are eroding due to confinement (lateral con- tainment) and entrenchment (vertical containment). The Cross-Vane is ideally suited for this ap- plication because the structure decreases the stream power and shear stress concurrently on both banks. This avoids lining or hardening both banks through the reach to provide protection. The areas of the left bank above the MAHWL will be armored with stone revetment to maintain pro- tection for up to and including the 100-year flood event. Cross-Vane structures can be implemented to maintain or enhance river stability and can facilitate multiple objectives. These structures have been successfully applied in natural channel design for river restoration, bank stabilization, grade control, irrigation diversions, fish habitat enhancement, bridge protection, and recreational boating. Cross-Vanes can be applied to meet the ever-increas- ing demand for environmentally “softer” structures that meet multiple objectives, including the goals of the Roberts Meadow Brook Channel Rehabilitation Project. Plantings Plantings have been specified for the “Bankfull Bench” areas and the upland areas above the re- vetment slopesadjacent totheundisturbedwoodedareas. Allspeciesofplantswereselectedbased on their prevalence within this reach of Roberts Meadow Brook and its immediately-surrounding areas. Theplantsselectedfor the“Bankfull bench” areasarethesame nativeplantsobservedalong the banks of the brook and at the cobble/gravel bars that have developed and include a mix of herbaceous and shrub species including Lurid Sedge, Fringed Sedge, Grass-Leaved Goldenrod, Winterberry, Silky Dogwood, and Speckled Alder. For the upland areas, Red maple, Red Oak, and Witch Hazel have been specified. Maintenance Plan To ensure that the implemented design is successful, a maintenance plan will be exercised to en- sure the following: Survival of the riparian vegetation. It is anticipated that volunteer species will help establish vegetation within the “Bankfull Bench” areas to supplement the planting spe- cifically included in the Project. These areas will be inundated several times during the year and survival may be spotty initially. The larger trees in the upland area will be nursery-grown and will carrya warrantythrough the construction procurement process. Structure stability. Post-storm inspections will be conducted of the Cross-Vane struc- turesfor grade control, bank stabilization, and habitat enhancement. Maintenance needs will be assessed and implemented to prevent future failures and secure proper function.