Section 4.9 - Dry Swales - Sustainable Technologies Evaluation ...

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Low Impact Development Stormwater Management Planning and Design Guide be spaced far enough apart to allow access for maintenance equipment (e.g., mowers). Typical Details Figure <strong>4.9</strong>.3 Schematic of a dry swale Also see Figure 4.10 from the OMOE Stormwater Management Planning and Design Manual (OMOE, 2003). Pretreatment Pretreatment devices capture and remove coarse sediment particles before they reach the engineered soil (i.e., filter media) bed to prevent premature clogging and prolong effective function of dry swales. A two-cell design that incorporates a sedimentation 4-156 Version 1.0
Low Impact Development Stormwater Management Planning and Design Guide forebay is recommended as it provides the most-effective pretreatment. Several pretreatment measures are feasible, depending on the method of conveyance and the drainage area: • Sedimentation forebay (two-cell design): Forebay ponding volume should account for 25% of the water quality storage requirement and be designed with a 2:1 length to width ratio. This pre-treatment device is the most effective and has the easiest sediment-removal mechanism. • Grass filter strip (sheet flow): These grass strips should ideally be a minimum of three metres in width. However, space constraints at some sites prohibit this width. If smaller strips are used, more frequent maintenance of the filter bed can be anticipated. • Gravel diaphragm (sheet flow): A gravel diaphragm at the end of pavement should run perpendicular to the flow path to promote settling. The pea gravel diaphragm (a small trench running along the top of the dry swale) serves two purposes. First, it acts as a pretreatment device, settling out sediment particles before they reach the practice. Second, it acts as a level spreader, maintaining sheet flow into the dry swale. If the contributing drainage area is steep, then larger stone should be used in the diaphragm. A 50 to 150 mm drop from a hardedged surface into a gravel or stone diaphragm can be used to dissipate energy and promote settling. • Rip rap and/or dense vegetation (channel flow): These energy dissipation techniques are acceptable as pre-treatment on small swales with a drainage area of less than 100 square metres. Conveyance and Overflow <strong>Dry</strong> swales should be designed for a maximum velocity of 0.5 m/s or less for a 4 hour 25 mm Chicago storm event. The swale should also convey the locally required design storm (usually the 10 year storm) at non-erosive velocities with freeboard provided above the required design storm water level. Monitoring Wells A capped vertical standpipe consisting of an anchored 100 to 150 millimetre diameter perforated pipe with a lockable cap installed to the bottom of the facility at the furthest downgradient end is recommended for monitoring the length of time required to fully drain the facility between storms. Gravel Storage Layer • Depth: Should be a minimum of 300 mm deep and sized to provide the required storage volume. Granular material should be 50 mm diameter clear stone. 4-157 Version 1.0
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Section 4.9 - Dry Swales - Sustainable Technologies Evaluation ...

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