Detailed Stormwater Management Practice Design - Tauranga City ...

More documents

Recommendations

Info

The following points require some discussion: a) Residence Time: There have not been many studies that relate water quality performance in swale design. The most recognised work has been done in the U.S. (Metropolitan Seattle, 1992). That study recommended a residence time of 9 minutes for flow to pass through the swale and provide approximately an 80+% removal of total suspended solids. Most governmental agencies in the U.S. have adopted that criterion. More recently, the recommendation has been recommended upward to 22 minutes due to the uncertainty of performance (Washington State Department of Ecology, 2001). That change in residence time is a significant change from the 9-minute criterion but it is not recommended that the time be increased until further investigation of swale performance is done in New Zealand. b) Manning’s Coefficient of Roughness: Determining roughness coefficients is more art than science. Many design handbooks provide one value for Manning’s coefficient of roughness of 0.2 (Metropolitan Seattle, 1992) or 0.25 (California Stormwater Quality Association, 2003). The Auckland Regional Council (ARC) funded a swale study (Larcombe, 2003) where dye tests were done on a swale to determine "n" by measuring flow times through the swale. In all of the test trials the values of Manning’s coefficient of roughness varied from 0.18 - 0.30. In reality the range is very consistent with the recommendations provided in the literature. The ARC recommended a series of equations for determination of "n" and using those equations provides values lower than Larcombe found in his study. It is recommended that a standardised value for Manning’s coefficient of roughness be set at 0.25. It is a mid-point in the Larcombe study and agrees favourably with the California recommendation. It is not felt that using the equations would provide necessarily a better result in design. For the 10-year storm analysis, it is assumed that the vegetation is submerged so the coefficient of roughness is reduced accordingly. The value selected is 0.03 (Chow, 1959). c) Lateral Inflow: A common concern with swales is lateral inflow from site areas to a point where the flow does not achieve the 9-minute residence time. Always strive to meet the 9minute residence time. An example of this is SMG-9 Figure 2 Swale with Check Dams and Diversion of Lateral Inflow that, in addition to check dams, shows a lateral flow diversion that directs the lateral flow to the head of the swale. Where lateral inflow cannot meet the nine-minute residence time for part of the alignment, the normal approach is to accept that the average flow through the swale does take nine minutes. There will be areas in the upper part of the swale that may exceed the required residence time so the average is appropriate in light of the benefits that swales provide. Page 8 Updated 01/11/2012
SMG-9 Figure 2 - Swale with Check Dams and Diversion of Lateral Inflow SMG-9.4.1.2 Detailed Design Procedure Please refer to DS-5 Stormwater of the Infrastructure Development Code for more design details. The design approach takes the designer through a series of steps that consider swale performance for water quality treatment and consideration of larger flows to ensure that scour or resuspension of deposited sediments does not occur. a) Estimate runoff flow rate from the water quality storm. Use the 2-year 1-hour storm as the water quality storm and use the rational formula for calculation of flows. One difference between swale and filter strip design and other stormwater management practices is that they are designed by flow rate, which is why the 2-year 1-hour storm is used in lieu of the 90% storm. Other practices are designed by calculation of the water quality volume. b) Establish the longitudinal slope of the swale. c) Select a vegetation cover. It should be grass and would generally be either perennial rye or fescue. d) The value for Manning’s coefficient of roughness is 0.25 Page 9 Updated 01/11/2012
Page 1: SMG-9 - Detailed Stormwater Managem
Page 4 and 5: In the same regard reducing total s
Page 6 and 7: As an example, a stormwater managem
Page 8 and 9: SMG-9 Table 2 Stormwater Management
Page 12 and 13: e) Select a swale shape. Two shapes
Page 14 and 15: SMG-9.4.1.3 Flows in Excess of the
Page 16 and 17: SMG-9.4.1.6 Swale Case Study Refer
Page 18 and 19: SMG-9.4.2.1 Basic Design Parameters
Page 20 and 21: ) Once the peak discharge is determ
Page 24 and 25: SMG-9 Figure 11 - Diversion Weir SM
Page 26 and 27: tf = time required for runoff to pa
Page 28 and 29: c) Infiltration (depending on soils
Page 30 and 31: Rain Garden design differs only sli
Page 32 and 33: SMG-9.4.4.1.1 Filter Media Filter m
Page 34 and 35: SMG-9.4.4.1.5 Transition Layer The
Page 36 and 37: SMG-9 Table 11 Grasses, Ground Cove
Page 38 and 39: Infiltration practices are used for
Page 40 and 41: SMG-9 Figure 19 Schematic of a Soak
Page 42 and 43: Any impermeable soil layer close to
Page 46 and 47: SMG-9 Figure 21 Soil Textural Trian
Page 48 and 49: e) Calculate the practice depth and
Page 50 and 51: SMG-9.4.6.1 Types of Pond Ponds are
Page 52 and 53: A study in the U.S.A (DNR. 1986) in
Page 54 and 55: Where there are downstream flooding
Page 56 and 57: There may be mitigation requirement
Page 58 and 59: SMG-9.4.6.12 Pond Aesthetics and La
Page 60 and 61:
SMG-9 Figure 25 Schematic of a Stor
Page 62 and 63:
When an extended detention orifice
Page 64 and 65:
The forebay should meet the followi
Page 66 and 67:
Requiring, during design, safety fe
Page 68 and 69:
Until recently, the filling and dra
Page 70 and 71:
The most common design priority for
Page 72 and 73:
SMG-9.4.7.1.2 Water Quality Volumes
Page 74 and 75:
The design steps are the following:
Page 76 and 77:
For the purposes of this guideline,
Page 78 and 79:
SMG-9.4.8.7 Detailed Design Procedu
Page 80 and 81:
This is indicated on SMG-9 Figure 3
Page 82 and 83:
SMG-9 Figure 37 - Waitakere City Co
Page 84 and 85:
SMG-9.4.9.7.7 Stormwater Management
Page 86 and 87:
SMG-8 Figure 8b - Information on ra
Page 88 and 89:
SMG-9.4.10.5 Advantages of Water Ta
Page 90 and 91:
SMG-9 Figure 39 Calculation of Rood
Page 92 and 93:
The 15% wastage factor accounts for
Page 94 and 95:
Commercial and industrial sites wil
Page 96 and 97:
SMG-9 Table 24 Stormwater Managemen
Page 98 and 99:
SMG-9.4.11.2 Separator Size The siz
Page 100 and 101:
VH = Q/1.125 Calculate the surface
Page 102 and 103:
When an outfall is sited in a coast
Page 104 and 105:
SMG-9.5.1.3 Riprap Aprons Outlet pr
Page 106 and 107:
SMG-9 Figure 46 Angled Entry of Out
Page 108 and 109:
SMG-9 - Appendix A Case Studies SMG
Page 110 and 111:
SMG-9 Apx A.2 Filter Strip Case Stu
Page 112 and 113:
SMG-9 Apx A.3 Sand Filter Case Stud
Page 114 and 115:
SMG-9 Apx A.4 Rain Garden Case Stud
Page 116 and 117:
SMG-9 Apx A.5 Infiltration Case Stu
Page 118 and 119:
SMG-9 Apx A.6 Ponds Case Study SMG-
Page 120 and 121:
SMG-9 Table A.6.3 Stage-Storage Rel
Page 122 and 123:
Outflow from ED orifice = Q = 0.62A
Page 124 and 125:
= (Qn/d 1.67 s 0.5 ) - Zd b = ((.04
Page 126 and 127:
SMG-9 Apx A.8.3 Design Water qualit
Page 128 and 129:
SMG-9 Apx A.9 Green Roof Case Study
Page 130 and 131:
Extended detention orifice sizing -
Page 132 and 133:
The minimum width and depth require
Page 134 and 135:
SMG-9 Table B.1.2a Particle Charact
Page 136 and 137:
Particle Diameter (µm) SMG-9 Table
Page 138 and 139:
SMG-9 Figure B.1.3 Wetland Process
Page 140 and 141:
Plants do take up nutrients or meta
show all

Detailed Stormwater Management Practice Design - Tauranga City ...

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?