Final Design
Final Design Components:
Overview
The pond shape was designed to fit the surrounding topography. The eastern edge is the embankment structure, so a straight centerline was chosen to improve soil stability and water retention. The resulting shape occupied a 7.4 acre surface area, with a sunny-day average depth of 8.7 ft and volume of 64.6 ac-ft. To protect aquatic life from winter freeze events, a large section of the pond was designed to reach a maximum depth of 12 ft. The effective height of the dam was 18 ft, and the resulting product of storage and effective height was 1163 ac-ft2. Outer slopes for both cut and fill were set at 4:1, and inner slopes were set to 3:1.
The embankment was designed to utilize on-site material to create a homogeneous earthfill structure. A filter diaphragm consisting of ASTM C-33 size media would be required in the center of the embankment around the principal spillway conduit. A concrete cradle was also included in the design to stabilize the conduit along the length of the embankment.
To design the pond, a number of hydrologic and hydraulic considerations first had to be made. Using GIS, SSURGO soil data and NLCD land cover data were used to determine overall curve number via the SCS curve number method. Watershed slope and length were also delineated in GIS. It should be noted that potential error may exist from the application of the SCS method in this project due to violations in infiltration assumptions from a high water table. However, because the drainage area is small (1.5 mi2) it was decided that this error would not significantly alter peak flow calculations. The WinTR-20 (v. 3.20) program was used to determine peak inflows for the 10, 25, and 50 year-24 h storms. Rainfall data for these storms was obtained from NOAA Technical Paper 40 and 49, and NRCS TR-60.
The images below display the final design in a topographic plan view (left) and a 3D conceptual view (right). The four major design components are listed in detail below, as well as the project's economic analysis.
1. Creek Diversion
The creek diversion was designed to convey a 10 year 24-h storm based on CPS 582. Using Manning’s Continuity Equation, the width, depth and side slopes were calculated as 3 ft, 2.5 ft and 1.5:1 ft, respectively. An 8 in. river cobble liner was chosen for the diversion rather than a grassed liner because vegetation would not be able to sustain constant flow.
2. Principal Spillway
The principal spillway system is displayed in below. The culvert was the first component of the principal spillway to be designed. A 60 in. diameter corrugated metal pipe (CMP) was determined to be capable of passing the 25 year, 24-hr storm. The outflow ditch was designed with the same standards as the creek diversion to direct water into Back Creek after exiting the pond. The ditch was lined with 8 in. of river cobble to dissipate energy and prevent erosion downstream of the impoundment.
A riser structure diameter of 90 in. was selected to reduce excessive noise and vortexing. A trash rack and anti-vortex device were also included to prevent the system from becoming clogged. A baseflow orifice was also included at the sunny-day water elevation (1796 ft) to pass the calculated 0.9 cfs streamflow from Shuffle Branch. The last component of the principal spillway system was the pond drain. This was added to the bottom of riser structure to minimize the number of pipes passing through the embankment. A 24 in. diameter CMP with a slide gate would be capable of draining the pond in approximately 7 days. This design would also require a grate to prevent the loss of stocked fish when the pond is drained.
3. Emergency Spillway
The emergency spillway was designed to pass a 50 year, 24-hour peak flow. A table provided in the Virginia Stormwater Management Handbook provided the emergency spillway dimensions based on the calculated maximum discharge value of 270 cfs. The emergency spillway dimensions were calculated to be 38 ft for bottom width, 92 ft in length, 3:1 side slope, and 2.3% slope.
4. Cattle Watering System
A watering system for the cattle on the farm was created to provide an ample amount of water to the cattle daily. The mix of Holsteins, Angus, and Wagyu need about 15 gal of water per day. It was assumed that the herd of 150 cattle can drink in three separate events per day, each lasting approximately 60 min. To design this system, the Virginia Livestock Watering Systems spreadsheet was used. A SCH 40 PVC pipe with a nominal diameter of 1.5 inches was chosen based on efficiency and lower cost. While a gravity-fed system was more desirable due to low operating cost, a pressure system with a pump was chosen to ensure the cattle would not have to come anywhere near the dam.
Economic Analysis
The economic analysis for the farm pond can be found in Tables 1-4 below. The final adjusted cost estimate for this project is $1,426,560. The majority of this cost (70%) was primarily attributed to the cost of hauling the leftover cut volume of dirt from excavation activities. It should be noted that a safety factor of 10% was added to the initial cost to account for taxes and shipping costs not included in the economic analysis.
