Detailed Stormwater Management Practice Design - Tauranga City ...

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In the same regard reducing total site disturbance reduces the total amount of work required by erosion and sediment control practices during site development. Less site disturbance means less generation of sediments, which results in a lower potential for downstream sedimentation in streams and estuaries. Allowing existing vegetation to remain on sensitive areas such as steep slopes or upstream of wetlands which may exist on site will reduce adverse impacts to downstream resources. As recognition of downstream receiving water impacts has increased, the potential for mitigation requirements to address the sediment impacts becomes more likely. Reducing the potential for sediment delivery would correspondingly reduce mitigation requirements and associated costs. It is recognized however that mitigation practices will still be integral to site development in most cases, but prevention of impacts will lessen the reliance on mitigation practices to reduce or eliminate adverse impacts. SMG-9.2.4 Manage Stormwater as close to the Point of Origin as Possible From both an environmental and economic perspective, minimising the concentration of stormwater and its conveyance in pipes costs less money (by reducing pipe diameter or elimination of pipes) and helps to maintain natural hydrology. Pipes, culverts, and elaborate systems of inlets to collect and convey stormwater, work against these management objectives and generally make stormwater management more difficult as such systems concentrate flows and increase flow rates, with a result of worsening erosive potential. SMG-9.2.5 Rely on Natural Soil and Plant Processes The soil mantle offers critical contaminant removal functions through physical processing (filtration), biological processing (microbial action), and chemical processing (cation exchange capacity, other chemical reactions). Plants similarly provide substantial pollutant uptake/removal potential, through physical filtering, biological uptake of nutrients, and even various types of chemical interactions. Further discussion of these processes can be found in SMG-9 - Appendix B Stormwater Treatment Processes. SMG-9.3 Consideration of Devices / Practices in Series While these devices provide individual benefits for removal of contaminants, their use in series can provide greater benefit than those used only individually. A simplified equation for the total removal of a given contaminant for two or more stormwater management practices in series is the following (State of New Jersey, 2004): R = A + B – [(A x B)/ 100] Where: R = total removal rate Page 2 Updated 01/11/2012
A = Removal rate of the first or upstream practice B = Removal rate of the second or downstream practice The use of this equation is easiest when considering removal percentages rather than using effluent limits as data on performance of practices for effluent limits can be highly variable. Council has significant interest in sediment and metals as contaminants of greatest concern so practice performance for them is provided in the following SMG-9 Table 1 Removal Rates for Various Stormwater Practices for TSS and Metals. Using the removal rates provided in the table will allow for calculation of overall removal of the contaminant of greatest concern. SMG-9 Table 1 Removal Rates for Various Stormwater Practices for TSS and Metals Practice Removal Rates (%) TSS Zinc Copper Swales 70 75 60 Filter Strips 80 75 60 Sand Filters 80 90 90 Rain Gardens (normal) Rain Gardens (w/anaerobic zone) Infiltration Practices Dry Ponds (no extended detention) Dry Ponds (with extended detention) 90 90 Page 3 Updated 01/11/2012 40 50 60 80 90 80 70 40 10 20 60 20 30 Wet Ponds 75 50 40 Wetlands 90 80 80 Green Roofs Volume reduction only Water Tanks Volume reduction only Oil Water Separators Volume reduction only Volume reduction only Volume reduction only Volume reduction only 15 5 5
Page 1: SMG-9 - Detailed Stormwater Managem
Page 6 and 7: As an example, a stormwater managem
Page 8 and 9: SMG-9 Table 2 Stormwater Management
Page 10 and 11: The following points require some d
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
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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
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The forebay should meet the followi
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Requiring, during design, safety fe
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Until recently, the filling and dra
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The most common design priority for
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SMG-9.4.7.1.2 Water Quality Volumes
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The design steps are the following:
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For the purposes of this guideline,
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SMG-9.4.8.7 Detailed Design Procedu
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This is indicated on SMG-9 Figure 3
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SMG-9 Figure 37 - Waitakere City Co
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SMG-9.4.9.7.7 Stormwater Management
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SMG-8 Figure 8b - Information on ra
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SMG-9.4.10.5 Advantages of Water Ta
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SMG-9 Figure 39 Calculation of Rood
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The 15% wastage factor accounts for
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Commercial and industrial sites wil
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SMG-9 Table 24 Stormwater Managemen
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SMG-9.4.11.2 Separator Size The siz
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VH = Q/1.125 Calculate the surface
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When an outfall is sited in a coast
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SMG-9.5.1.3 Riprap Aprons Outlet pr
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SMG-9 Figure 46 Angled Entry of Out
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SMG-9 - Appendix A Case Studies SMG
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SMG-9 Apx A.2 Filter Strip Case Stu
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SMG-9 Apx A.3 Sand Filter Case Stud
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SMG-9 Apx A.4 Rain Garden Case Stud
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SMG-9 Apx A.5 Infiltration Case Stu
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SMG-9 Apx A.6 Ponds Case Study SMG-
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SMG-9 Table A.6.3 Stage-Storage Rel
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Outflow from ED orifice = Q = 0.62A
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= (Qn/d 1.67 s 0.5 ) - Zd b = ((.04
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SMG-9 Apx A.8.3 Design Water qualit
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SMG-9 Apx A.9 Green Roof Case Study
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Extended detention orifice sizing -
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The minimum width and depth require
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SMG-9 Table B.1.2a Particle Charact
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Particle Diameter (µm) SMG-9 Table
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SMG-9 Figure B.1.3 Wetland Process
Page 140 and 141:
Plants do take up nutrients or meta
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