Project Progress

Progress: Spring Semester 2020

Bedrock Investigation

A bedrock investigation was performed at the site using a hand auger on the floodplains and low-terraces downstream of the existing crossing. From this investigation, we concluded that bedrock was located at an elevation of 77- 78 ft, or approximately 2 ft under the outlet of the crossing.

Hydrologic Analysis

Hydrologic analysis was conducted using StreamStats and TR-55. From the StreamStats website, the crossing was located and a report containing watershed characteristics and peak discharge was generated. TR-55 uses the SCS Runoff Curve Number method to calculate runoff based on land use and soil type (NRCS, 1986). TR-55 then adds the hydrographs produced by subwatersheds within a basin to compute peak discharge (NRCS, 1986). ArcGIS Pro was used to digitize stream channels from aerial imagery to determine their length (Figure _). Next, ArcGIS Pro was used to delineate the watershed and subwatersheds using a digital elevation model (DEM) of Blacksburg Virginia, located on the Town of Blacksburg website (Blacksburg, 2019) (pictured below, left). ArcGIS Pro and web soil survey were then used together to determine land use and hydrologic soil group composition so that curve numbers could be determined for each subwatershed for current and future land use conditions. Within TR-55, subwatersheds were routed as appropriate and the basin was modeled using a type II rainfall distribution and delmarva dimensionless unit hydrograph. The table (pictured below, right) contains the peak discharge results of StreamStats and TR-55.

Hydraulic Analysis

HY-8 is a culvert hydraulic analysis program that was used to analyze the flow capacity of culverts. Inputs for HY-8 analysis include a design discharge, tailwater channel geometry, roadway elevation data, culvert type, material, slope, geometry, and other inlet configurations. HY-8 was used to analyze potential crossing structures and was performed using a 10-year design discharge (522 cfs). Headwater depth at the design discharge and outlet velocity at low flows are the basis of the hydraulic performance score assigned to each structure (Table pictured below). Based on sufficient headwater elevation (no overtopping) and outlet velocity (allowing aquatic organism passage), both the open bottom arch structures scored high in hydraulic performance.

Grade Control

There was concern that removing the grade control provided by the existing crossing would lead to a hydraulic jump and a subsequent headcut. This would result in bed erosion that would migrate upstream also threatening bank stability. As a result, we designed a structure to hold the elevation of the streambed and ensure manageable energy dissipation over this drop in elevation.

A stone cross vane was developed as a grade control structure. First, stone size was determined using both USDA-NRCS “Technical Supplement 14C” and U.S. Army Corps of Engineers “Stability Thresholds for Stream Restoration Materials” (NRCS, 2007; Fischenich, 2001) . These rocks were sized using a 25-year discharge, to ensure structure stability as per the recommendation of our NRCS contacts, and the under-bridge cross section to determine water depths. Finally, the cross vane itself was designed based on the guidance of Hickman and Thompson (2010) which provides recommendations for the cross vane construction.

Progress: Fall Semester 2019

Site Survey & Pebble Count

We conducted a site survey with the other design team . The survey was of the entire flood plain, focused specifically on the longitudinal profile and the cross sections. Additionally, we conducted pebble counts to get a representation of the steam-bed material.

Survey Data in AutoCAD

The survey data was collected and inputted into AutoCAD civil 3D as a fieldbook file. The topographic map from the survey data is pictured on the right. We will tie in this data to the geographical data at a later date.