Sediment Basin Design Example

Transcription

1 Sediment Basin Design Example

2 Given: K value (per APWA ): K2 = 1 K25 = 1.1 Rational Method Runoff Coefficient, C = 0.51 (per APWA ) Drainage Area, A = 18 acres Time of Concentration, Tc = 5 minutes (per APWA ) Rainfall Intensity, I (per APWA Table ): 2 year/50% storm I = 5.4 in/hr 25 year/4% storm I = 8.5 in/hr Bottom of Sediment Basin Elevation: Basin Volume Data: Rectangular Basin 100 wide x 180 long with 3:1 side slopes: Stage/Storage Chart: Elevation Area (SF) Volume (CF) Volume (CY) Cumulative Volume (CY) ,000 18, ,700 20, , ,500 22, , , ,300 24, ,190 Riser: Perforated CMP pipe Emergency Spillway: 10 foot wide 20 foot long spillway with 3:1 side slopes. Stabilized with a TRM. Spillway longitudinal slope of 20:1.

3 Task Design the Sediment Basin: Step 1 - Determine Design Flows: Q2 = KCIA (per APWA ) = 1.0 x 0.51 x 5.4 x 18 = 50 cfs Q25 = 1.1 x 0.51 x 8.5 x 18 = 86 cfs Step 2 Determine required Sediment Storage: From Page 6 of the design standards: The storage volume shall be 134 cubic yards per tributary acre to the basin. The tributary area shall include all areas that drain to the basin, including any offsite areas or undisturbed areas. Minimum Sediment Storage = 18 acres x 134 cy/acre = 2,412 CY* * Note Storage Volume is based on Total Tributary Area to basin not just the area being disturbed. Step 3 Determine Top of Primary Riser Elevation: Required Storage Volume = 2,412 CY Interpolating from stage/storage chart Top of Primary Riser Elevation =

4 Step 4 - Determine Required Riser Pipe Perforations: Tributary Drainage Area Table A Drawdown Mechanism Sizing Chart Single Column Riser (inches) Perforated Riser Hole Diameter (1) 4-Column Riser (inches) Use a 4 column riser with 0.8-inch holes at 6 inch spacing (vertically), OR a 12 column riser with 0.5-inch holes. For simplicity, specify 5/8 diameter holes (.625 ) for a 4 column riser. This will draw down the basin in approximately 48 hours. Step 5 Determine Size of Primary Spillway: 12-Column Riser (inches) Skimmer (inches) 3-5 acres 1.0 N/A N/A to 10 acres N/A to 15 acres N/A 0.7 N/A to 20 acres N/A to 25 acres N/A to 30 acres N/A to 35 acres N/A to 40 acres N/A Notes: (1) Perforated riser hole diameters based on 6-inch on center spacing vertically. (2) Minimum diameter ½ inch, Maximum diameter 1-1/2 inch. (3) Source of data - Interpolated from Penn State University Fact Sheet #F253 Controlling Dewater of Sediment Basins. (4) Drawdown based on 134 CY/acre storage volume and 48 hour target drawdown period. From Page 7 of the design standards: The principal spillway shall be designed to pass the 50% (2 year) storm without overtopping the emergency spillway. To aide the designer, Table B can be used for determining principal spillway capacity. From Table B on Page 8 of the design standards: A 36 inch riser will pass 50 CFS at a ponded depth of 2.25 feet.

5 Use a 36-inch riser/barrel, therefore r = 1.5 feet. Step 6 Determine Required Size of Concrete Anchor: Compute displaced volume of riser/barrel: Riser Height = L = = 3.1 feet. Riser Volume = πr^2 x L = πx 1.5^2 x 3.1 = 21.9 CF Buoyant force: 21.9 CF x 64 lb/cf x 1.25 Factor of Safety = 1,752 lbs Minimum Required Concrete Anchor = 1,752 lbs/150 lbs/cf = 11.7 CF 11.7 CF of concrete required round up to 1/2 CY of concrete Note if the barrel section of pipe is not entirely buried in the dam, its volume must also be added to the above computations. Step 7 Determine Cleanout Elevation: Cleanout Elevation = ½ of distance from bottom of basin to top of principal riser Bottom of Basin= Top of Principal Spillway = Cleanout Level = Step 8 - Determine Minimum Emergency Overflow Elevation; Set Emergency Overflow at or above the 2 year design storm elevation no extra freeboard is required. Emergency Spillway Elevation = Primary Spillway Elev. + 2 year design flow depth = feet =

6 Step 9 Design Emergency Spillway: Use weir flow calculation and 25 year storm to determine depth. Do not reduce design flow for flows in primary spillway (assume primary spillway is plugged) Compute Flow Depth: Assuming Broad Crested Weir, L = 10 feet in length. Q = C x L x H ^ 3/2 Or H = (Q / (C x L)) ^ 2/3 For Q = 86 cfs: Assume C = 2.63 H = (86 / (2.63 x 10) ^ 2/3 H = 2.03 feet Flow Depth = = Step 10 Design Emergency Spillway Protection Material: The emergency spillway will need to be designed to resist velocities on the steepest portion of the spillway. This will nearly always require appropriately sized rip-rap and a bedding rock material. Maximum Allowable Discharge (cfs) Down Emergency Spillway Based on given Bottom Width and Size of Rip-Rap, using 3:1 side slopes of spillway sides and a maximum slope down the spillway of 20% (5:1 H:V) KDOT Nominal Bottom Width *** Typ. Flow Design Specification D50 (inch) V-ditch (0') 2 ft. 4 ft. 6 ft. 8 ft. 10 ft. Depth (ft) Depth (ft) 6" Stone ** 6" Riprap (Light 18") * 11" Riprap (Light 24") * 16" Riprap (1/4 Ton) * 22" * Riprap as per KDOT Specification Subsection 1116, Table 11, Stone for Riprap ** Stone as per KDOT Specification Subsection 1116, Table 12b, Stone for Aggregate Ditch Lining *** Maximum discharges for 0' through 6 ft. bottom widths taken from Charts 15 through 18 of Federal Highway Administration HEC-15 Manual "Design of Roadside Channels with Flexible Lining", April Discharge values for 8 ft and 10 ft widths were extrapolated based on the changes between the 4 ft and 6 ft widths.

7 The above chart may be used to determine the maximum allowable discharge down the emergency spillway. For Q = 76 cfs and W = 10 feet: Use KDOT Light 24 Rip Rap. Step 11 - Determine Minimum Top of Dam Elevation: Minimum Top of Dam = Emergency Spillway Design Depth plus 1 foot freeboard = = Minimum elevation for top of dam = Step 12 - Determine if flow baffles are required: Determine Effective Width (We) We = A/L where A is pond area when empty, L is flow path length through the pond. From Stage/Storage data, A = 18,000 SF L = 180 feet We = A/L = 18,000/180 = 100 feet Length/Width Ratio = L/We = 180/100 = 1.8 Since 1.8 < 2.0, interior baffles are required. Try increasing flow path length to 190 feet. We = 18,000/190 = 94.7 Length/Width Ration = 190/94.7 = 2.00 OK Arrange baffles to increase flow path length to 190 feet minimum.

8 Sediment Basin Data Design Summary Site Data: Tributary Drainage Area to Pond 18 acres 50% (2 year) Design Flow 50 cfs 4% (25 year) Design Flow 86 cfs Pond Data: Minimum Sediment Storage Volume 2,412 cy Bottom Elevation Sediment Cleanout Elevation Top of Riser Elevation Emergency Spillway Elevation Top of Dam Elevation Basin Shape Data: Area at Normal Pool (A) Flow Path Length (L) 18,000 SF 180 Ft Effective Width (A/L) 100 Ft Length to Width Ratio 1.8 Principal Spillway Data: Riser Pipe Diameter Barrel Pipe Diameter Riser Pipe Base Size Riser Pipe - # of Perforation Columns Riser Pipe Perforation Diameter Emergency Spillway Data: Design Flow Design Depth at Crest Design Width Riprap required 36-inch 36-inch ½ cy 4 column 5/8-inch 86 cfs 2.03 ft 10 ft KDOT Light 24 inch Tributary Drainage Area to Pond: 18 acres 50% (2 year) Design Flow: 50 cfs 4% (25 year) Design Flow: