250701_42 ERW_Stormwater Report

Date: 7/1/2025 Project: Project No: ERW 24006174A Calculated By: MN Checked By: JF Conduit Outlet Protection Calculations Scour Hole # S1 Design Parameters: Design Storm Flow for 25 Year, Q . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vertical Dimension of Outlet Pipe, D o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Horizontal Dimension of Outlet Pipe, W o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tailwater Depth, TW 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scour Hole Depth, y (1/2 D o or D o ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Apron Dimension Calculations: Minimum Bottom Width, W 1 = 2W o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Minimum Bottom Length, L 1 = 3D o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Minimum Top Width (max side slope of 3:1) , W 2 . . . . . . . . . . . . . . . . . . . . . . . . Minimum Top Length (max side slope of 3:1) , L 2 . . . . . . . . . . . . . . . . . . . . . . . . Rip Rap Stone Size Calculations: Unit Dicharge, q = Q/D o = · W 1 = 4.00 ft L 1 = 6.00 ft W 2 = 10.00 ft L 2 = 12.00 ft 2.62 cfs per foot Case I: y = 1/2 D o 0.0125 q 1.33 = 1.08 in Therefore, use TW Apron Thickness, TH = 2×d 50 with filter fabric . . . . . . . . . . . . . . . . . . . . . Median Stone, d 50 = · 5.24 cfs 24 in 24 in 0.50 ft 12 in OFF d50 = 4 in TH = 8 in Case II: y = D o 0.0082 q 1.33 = TW Apron Thickness, TH = 2×d 50 with filter fabric . . . . . . . . . . . . . . . . . . . . . . . Median Stone, d 50 = Notes: 1. The side slopes shall be 3:1 or flatter. 2. The bottom grade shall be 0.0% (level). 3. There shall be no overfall at the end of the apron or at the end of the culvert. 4. Fifty (50) percent by weight of the rip-rap mixture shall be smaller than the median size stone designated as d 50. The largest stone size in the mixture shall be 1.5 times the d50 size. The rip-rap shall be reasonably well graded. 5. The thickness of the rip-rap apron may be two (2) times the median stone diameter provided that the apron is constructed on a bedding of four (4) inches of 3/4 inch clean stone on approved filter fabric material. 6. Rip-rap and filter

may be two (2) times the median stone diameter provided that the apron is constructed on a bedding of four (4) inches of 3/4 inch clean stone on approved filter fabric material. 6. Rip-rap and filter fabric shall meet the standards of the governing Soil Conservation District as well as the requirements of the local municipality. 7. Where the scour hole is to be placed within an existing or proposed waterway: a. The scour hole sidewalls should be eliminated to maintain a smooth hydraulic line along the waterway bottom to avoid inviting turbulent flow from a sudden depression in the waterway. b. If the flow in the waterway is greater than the flow from the proposed outlet, the rip-rap used to construct the scour hole should be sized based on the greater flow value according to the standard rip-rap. Footnote: 1. Tailwater depth shall be the 2 year storm if discharging into a detention basin. For areas where tailwater cannot be computed, use TW = 0.2D o . 1