Chapter 2 – Swales (incorporating Buffer Strips) Swales (incorporating Buffer Strips) WSUD Technical Design Guidelines for the Coastal Dry Tropics 2-1 Chapter 2 – Swales (incorporating Buffer Strips) Chapter 2 2.1 Introduction ............................................................................................................................................ 2-3 2.2 Design Considerations for Swales ........................................................................................................ 2-4 2.2.1 Landscape Design ........................................................................................................................... 2-4 2.2.2 Hydraulic Design ............................................................................................................................. 2-4 2.2.3 Vegetation Types ............................................................................................................................ 2-5 2.2.4 Driveway Crossings ........................................................................................................................ 2-5 2.2.5 Traffic Controls ................................................................................................................................ 2-6 2.2.6 Roof Water Discharge ..................................................................................................................... 2-6 2.2.7 Services .......................................................................................................................................... 2-7 2.3 Swale Design Process ............................................................................................................................ 2-8 2.3.1 Step 1: Confirm Treatment Performance of Concept Design ......................................................... 2-9 2.3.2 Step 2: Determine Design Flows .................................................................................................... 2-9 2.3.3 Step 3: Dimension the Swale with Consideration of Site Constraints ............................................ 2-9 2.3.4 Step 4: Determine Design of Inflow Systems............................................................................... 2-11 2.3.5 Step 5: Verify Design .................................................................................................................... 2-13 2.3.6 Step 6: Size Overflow Pits (Field Inlet Pits) ................................................................................... 2-13 2.3.7 Step 7: Make Allowances to Preclude Traffic on Measures ......................................................... 2-14 2.3.8 Step 8: Specify Plant Species and Planting Densities ................................................................... 2-14 2.3.9 Step 9: Consider Maintenance Requirements .............................................................................. 2-14 2.3.10Design Calculation Summary ........................................................................................................ 2-14 2.4 Landscape Design Notes ..................................................................................................................... 2-16 2.4.1 Introduction ................................................................................................................................... 2-16 2.4.2 Objectives ..................................................................................................................................... 2-16 2.4.3 Context and Site Analysis ............................................................................................................. 2-16 2.4.4 Streetscape Swales and Buffer Strips .......................................................................................... 2-17 2.4.5 Appropriate Plant Selection ........................................................................................................... 2-21 2.4.6 Safety ............................................................................................................................................ 2-22 2.5 Construction and Establishment ......................................................................................................... 2-23 2.5.1 Staged Construction and Establishment Approach ....................................................................... 2-23 2.5.2 Staged Construction and Establishment Method .......................................................................... 2-23 2.5.3 Horticultural Topsoils for Swales (and Buffer Strips) ..................................................................... 2-25 2.5.4 Sourcing Swale Vegetation ........................................................................................................... 2-25 2.5.5 Vegetation Establishment ............................................................................................................. 2-25 2.6 Maintenance Requirements ................................................................................................................ 2-26 2.7 Checking Tools ..................................................................................................................................... 2-27 2.7.1 Design Assessment Checklist ...................................................................................................... 2-28 2.7.2 Construction Checklist .................................................................................................................. 2-28 2.7.3 Operation and Maintenance Inspection Form ............................................................................... 2-28 2.7.4 Asset Transfer Checklist ............................................................................................................... 2-28 2.8 Engineering Drawings and Standards ................................................................................................ 2-33 2.9 Swale Worked Example ....................................................................................................................... 2-33 2.9.1 Step 1: Confirm Treatment Performance of Concept Design ....................................................... 2-36 2.9.2 Step 2: Determine Design Flows .................................................................................................. 2-36 2.9.3 Step 3: Configuring the Swale ...................................................................................................... 2-39 2.9.4 Step 4 Design Inflow Systems...................................................................................................... 2-41 2.9.5 Step 5: Verification Checks ........................................................................................................... 2-41 2.9.6 Step 6: Size Overflow Pits ............................................................................................................ 2-41 2.9.7 Step 7: Traffic Control ................................................................................................................... 2-42 2.9.8 Step 8: Vegetation specification .................................................................................................... 2-42 2.9.9 Calculation summary ..................................................................................................................... 2-42 2.10 References ............................................................................................................................................. 2-44 WSUD Technical Design Guidelines for the Coastal Dry Tropics 2-2 Chapter 2 – Swales (incorporating Buffer Strips) 2.1 Introduction Vegetated swales are used to convey stormwater in lieu of, or with, underground stormwater drainage systems, and to provide removal of coarse and medium sediments. They are commonly combined with buffer strips and bioretention systems (refer Chapter 3 - Bioretention Swales). Swales utilise overland flow and mild slopes to convey water slowly downstream. They provide a means of disconnecting impervious areas from downstream waterways, assisting in protecting waterways from damage by frequent storm events, by reducing flow velocity compared with piped systems. The interaction between stormwater flow and vegetation within swale systems facilitates pollutant settlement and retention. Even swales with relatively low vegetation height (such as mown grass) can achieve significant sediment deposition rates provided flows are well distributed across the full width of the swale and the longitudinal grade of the swale is kept low enough (typically less than 4 % grade) to maintain slower flow conditions. Swales alone cannot provide sufficient treatment to meet current stormwater treatment/ water quality objectives, but can provide an important pretreatment function for other WSUD measures in a treatment train enabling water quality objectives to be met. Swales are particularly good at coarse sediment removal as a pretreatment for tertiary treatment systems such as wetlands and bioretention basins. A typical sketch of a swale at-grade crossing is shown in Figure 2-1. Figure 2-1: Typical Arrangement of a Swale “at-grade” Driveway Crossing Buffer strips (or buffers) are areas of vegetation through which runoff passes while travelling to a discharge point. They reduce sediment loads by passing a shallow depth of flow through vegetation and rely upon well distributed sheet flow. Vegetation tends to slow velocities and coarse sediments are retained. With their requirement