Flow through media

From LID SWM Planning and Design Guide
Jump to: navigation, search

Practices which infiltrate surface runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. through an engineered soil or filter media, and discharge through an underdrain include stormwater planters and some forms of bioretention. The maximum flow rate from the BMPBest management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls. may be limited by the hydraulic conductivityA parameter that describes the capability of a medium to transmit water. of the medium, or by the properties of the perforated pipe.

The maximum flow rate through a bed of filter mediaThe engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles. (Qmax, media, m3/s) may be calculated as\[Q_{max, media}=\frac{K_{m}\times A_{p}\times \left (\frac{h_{max}}{d_{m}} \right )}{3.6\times 10^{6}}\]

Where:

  • Km is the hydraulic conductivityA parameter that describes the capability of a medium to transmit water. of the filter mediaThe engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles. (mm/hr),
  • Ap is the area of the practice (m2),
  • hmax is the total head of water within bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. components over the perforated pipe (m) (i.e. ponding + mulch + filter media), and
  • dm is the depth of the filter mediaThe engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles. (m).

Example calculations

A stormwater planter with footprint of 8 x 1.5 m is planned to receive runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. from an adjacent rooftop. The initial design for the planters includes 750 mm depth of filter medium, 50 mm rock mulcha top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales., and a further ponding of 300 mm. The underdrainA perforated pipe used to assist the draining of soils. pipe will be embedded into high performance bedding or similar, with a strip of geotextileFilter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils. over the top to prevent migration of the filter mediaThe engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles. into the pipe. The lab test states that the medium has a hydraulic conductivityA parameter that describes the capability of a medium to transmit water. of 25 mm/hr. The maximum flow through the medium is calculated and then a comparison made with the maximum flow through the pipe to see if the planter will drain freely\[Q_{max, media}=\frac{25 mm/hr\times 12\ m^{2}\times \left (\frac{1.1\ m}{0.75\ m} \right )}{3.6\times 10^{6}}=0.00012\ m^{3}/s\]


A bioretention cell with footprint of 30 x 10 m is planned to received runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. from adjacent roadways and parking facilities. The design includes 600 mm depth of filter medium, 75 mm wood based mulcha top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales., and ponding of 300 mm. Two underdrainA perforated pipe used to assist the draining of soils. pipes will be embedded at the base of the storage reservoir. These will connect together and then have an upturn within a manhole at the downstream end to prevent discharge until the head of water reaches the top of the storage reservoir within the cell. The lab test for the filter medium state that it has a hydraulic conductivityA parameter that describes the capability of a medium to transmit water. of 80 mm/hr. The downstream pipe in the manhole can convey 0.002 m3/s on a 1% slope.\[Q_{max, media}=\frac{80 mm/hr\times 300\ m^{2}\times \left (\frac{0.975\ m}{0.6\ m} \right )\times 2}{3.6\times 10^{6}}=0.0022\ m^{3}/s\] As the maximum flow rate exceeds the downstream maximum permissible flow, the design must be amended. In order of preference, some options include:

  1. Reformulating the bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. filter mediaThe engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles. in collaboration with the soils provider, to reduce the hydraulic conductivityA parameter that describes the capability of a medium to transmit water.,
  2. Reducing the depth of the filter mediaThe engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles. bed in agreement with a Landscape Architect, and increasing pretreatmentInitial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms. (as filter bed < 600 mmm will provide less treatment),
  3. Increasing the size of the downstream pipe to accommodate a greater flow rate.