Final Design
Design Plan
The final design consists of three main components:
Two redesigned ford stream crossings
Two constructed riffles surrounding the crossings with an accompanying riffle particle distribution
Two grade control structures
Each detail of the design will be described in further detail below.
Ford Stream Crossings
Two ford stream crossings were designed in order to allow Mr. Shorter to transport cattle and machinery over Tom's Creek between adjacent fields. Both crossings were designed in accordance with USDA-NRCS Standard 578 and accompanying VA-NRCS Design Note 578. The crossings have design slopes of 8:1 slopes extending from the channel bottom until it meets natural topography. The crossings will be constructed of three layers of stone. The bottom layer of stone will be VDOT #1 stone (D50 =2”, D100 =4”) in order to armor the stream crossing. The middle layer will be 3 inches of VDOT #57 (D50 =0.5”, D100 =1.5”) in order to further armor the crossing and fill void spaces at the bottom of the channel. Lastly, a top layer of 3 inches thick and contain crushed limestone and local alluvial stone will be added as a hoof contact zone to protect cattle as they cross. Both crossings were designed to pass a bankfull flow of 740 cubic feet per second.
Civil 3D drawings of the upstream and downstream ford crossings. More design details and considerations can be found in the Final Report in the Resources Tab.
Constructed Riffles
Civil 3D drawings of the upstream and downstream constructed riffles. More design details and considerations can be found in the Final Report in the Resources Tab.
Surrounding both the upstream and downstream crossings will be constructed riffles. These redesigned riffles will improve stream functioning and provide necessary habitat to juvenile hellbender salamanders and other aquatic organisms. The geometry of the riffles was designed to match the bankfull area of the reference reach, 150 square feet, and different Rosgen ratios including: including width-to-depth, maximum depth-to-average depth, low bank height-to-maximum depth, flood prone width-to-bankfull width, riffle length-to-bankfull width, and riffle slope-to-average slope were calculated in order to create an idealized riffle geometry. The constructed riffles were designed to have lower side slopes of 3:1 and upper side slopes of 4:1. This design choice serves two different purposes. First, it provides a gradual transition between the riffles and the floodplain. Second, it constricts flow during low-flow conditions to within the lower section.
Constructed Riffle Particle Recipe
A riffle recipe was designed for both constructed riffles in order to minimize riffle migration and create suitable habitat for juvenile hellbenders. Class A1 Rip-Rap was included in the distribution as it will aid in the armoring of the riffles. The riffle particle distribution was designed with a higher concentration of large cobble than what was found in the stream in order to support juvenile hellbender burrowing habits. Gravel and sand/silt/clay were included in order to more closely replicate the particle distribution found in the reference reach. More details regarding design choices can be found in the Final Report.
Grade Control Structures
The cross vane structure was designed to be placed at the tail of the riffle at the upstream constructed riffle. The cross vane functions by condensing the riffle grade change in a hydraulic jump. This structure will mitigate the over steepening of the riffle over time, reducing the risk of riffle migration and damage to the crossings. The cross vane structure was chosen for the upstream riffle as the upstream section of stream is straight. This allows the cross vane structure to allow a scour pool to form in the center of the channel, away from the channel banks. The cross vane structure will tie into the banks at half the bankfull height of the stream channel. The design specifications can be seen in the construction drawing on the right.
Both grade control structures will be constructed out of 3' x 2' x 2' boulders. This boulder size was designed using a variety of different rock sizing methods found in the NRCS Engineering Handbook and NRCS Rock Sizing Technical Supplement. The particle distribution of the reference riffle was also considered when choosing a boulder size for both the vane arms of the structures and the footers. Further design specifications can be seen in the construction drawing on the right.
The J-hook structure was designed to be placed at the tail of the riffle at the downstream constructed riffle. The J-hook structure controls the grade of the riffle in the same way as the grade of the cross vane structure. A J-hook structure was chosen instead of a cross vane at the downstream riffle as the stream bends at the downstream riffle. The bended geometry of the J-hook structure diverts the flow away from the bank of the stream and creates a scour pool in the center of the channel, reducing bank erosion. The J-hook structure will also tie into the banks at half the bankfull height of the stream channel. The design specifications can be seen in the construction drawing on the left.
Cost Analysis
