Laserfiche WebLink
B. 2D Hydraulic Modeling <br /> A 2D hydraulic model using HEC-RAS Version 5.0.3 was developed to identify hydraulic <br /> behaviors to support scour analysis. The model terrain was developed from post-2013 <br /> flood digital elevation models(DEM). The model generally uses 20 foot by 20 foot element <br /> sizes to represent the terrain and a Manning's n-value of 0.04 applied uniformly across the <br /> domain. <br /> Figure 2 HEC-RAS 2D Model: 2-Year Maximum Flow Depth shows the maximum flow <br /> depth for a 2-year design storm event. The Lyons Quarry is between cross sections 14567 <br /> and 15910. As seen at cross section 15910, flow breaks out of the main channel towards <br /> the Lyons Quarry. <br /> Figures 3 and 4, HEC-RAS 2D Model: 100-Year Maximum Velocity and HEC-RAS 2D <br /> Model: 500-Year Maximum Velocity, show maximum velocities during the 100- and 500- <br /> year design storm events, respectively. The maximum velocity in the 100-year storm event <br /> in 17.1 feet per second (ft/s) and occurs at the upstream portion of the study reach. The <br /> maximum velocity in the 500-year storm event in 23 ft/s and occurs at the upstream portion <br /> of the study reach. <br /> IV. Scour Analysis <br /> Scour analysis was completed to quantify scour depths along the proposed scour berm. Worst case scour <br /> would be caused by migration of St. Vrain Creek southwest and along the toe of the scour berm. Higher <br /> discharges in St. Vrain Creek have, at times, inundated the area along the proposed scour berm and <br /> hydraulic modeling indicates flows get close to the project area in storm events as low as the 2-year <br /> frequency. It also appears that the historic active channel was located in the area of the proposed scour <br /> berm and was realigned to the northeast along Highway 7. These hydraulic and historic behaviors indicate <br /> the need to consider potential migration of the main channel to the toe of the proposed scour berm. <br /> Potential scour mechanisms considered for the scour berm were: <br /> • Long term scour <br /> • General scour(regime scour) <br /> It was assumed that there is no contraction scour as St.Vrain Creek exits a confined upstream canyon and <br /> expands into the project reach. Lateral scour was considered, but was less than regime scour and was <br /> considered to not be additive to the general scour results. <br /> Long term scour and general scour were computed using methods outlined in Computing Degradation and <br /> Local Scour, by the US Bureau of Reclamation. Hydraulic parameters used for scour analysis were taken <br /> from the HEC-RAS 1D model to remain consistent with analyses completed by Matrix Design Group. <br /> General scour was completed using the 100-year storm event. The Neill, Lacey, and Blench scour <br /> equations were averaged to identify a general scour depth. Rock gradations used in the scour analysis <br /> were obtained from the Report channel samples for Reach 7(Sample R7). <br /> Long term scour analysis computed scour limited by armoring for the dominant flow; which was <br /> conservatively assumed to be the 5-year storm event. Stability analysis of the existing bed material used <br /> an average of the Meyer-Peter, Muller, Competent Bottom Velocity, Shield's Diagram, and Yang Incipient <br /> Motion methods. A stable particle size of 93 millimeters (mm)was computed for the 5-year storm event. <br /> The Reach 7 bed sample resulted in approximately 22%of the material being larger than 93 mm, indicating <br /> natural armoring would occur. Using these parameters, a depth to natural armor was computed. This <br /> depth was assumed to limit long term scour that would impact the scour berm. Additionally, downstream <br /> stream restoration efforts described in the Report were assumed to limit the potential for upstream migration <br /> of headcuts or overall channel lowering. <br /> hdrinc.com 1670 Broadway,Suite 3400,Denver,CO 80202-4824 <br /> (303)764-1520 <br /> 2 <br />