Gravel diaphragms

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75 mm drop from the imperviousA hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil. surface onto the gravel diaphragmA level spreading device placed at a runoff discharge location, perpendicular to flow, to maintain sheet flow and distribute runoff as evenly as possible across a pervious area or stormwater infiltration practice. A gravel diaphragm acts as a pretreatment device, settling out suspended sediments before they reach the practice.
Note the fall from the imperviousA hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil. surface to the gravel surface

Gravel diaphragms are simply gravel filled trenches which act as inlet structures by redistributing concentrated flow to sheet flow: reducing the erosive power of the water and promoting infiltration. They also act as a pretreatment by settling out sedimentSoil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls. particles before they reach any downstream structure or practice.

If the contributing drainage areaThe total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds. is solely turf (e.g., sports field), then the gravel diaphragms may be eliminated from the treatment trainAlso called the water cycle, this is the process of water evaporating condensing, falling to the ground as precipitation and returning to the ocean as run-off.A combination of lot level, conveyance, and end-of-pipe stormwater management practices..

Gravel diaphragms are ideal for distributing flow and providing 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. for

Planning

The gravel diaphragmA level spreading device placed at a runoff discharge location, perpendicular to flow, to maintain sheet flow and distribute runoff as evenly as possible across a pervious area or stormwater infiltration practice. A gravel diaphragm acts as a pretreatment device, settling out suspended sediments before they reach the practice. serves two purposes. First, it acts as a 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. device, settling out coarse particles before they reach an infiltration practice. Second, it acts as a level spreader, maintaining sheet flow as 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. flows over the filter stripa gently sloping, densely vegetated areas that treat runoff as sheet flow from adjacent impervious areas. They function by slowing runoff velocity and filtering out suspended sediment and associated pollutants, and by providing some infiltration into underlying soils.. If the contributing drainage areaThe total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds. is steep, then larger stone should be used in the diaphragm,

  • Diaphragms can be designed along curves as readily as straight lines,
  • As they are not a conveyanceA long narrow trench or furrow dug in the ground, as for irrigation, drainage, or a boundary line.Movement of water from one location to another. practice, they should be designed along a single contour line, promoting the level spreading function,
  • Flow should be directed into the device laterally.

Design

  • Where flow comes off an imperviousA hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil. surface, a drop of 75 mm or more to the surface level of the diaphragm is recommended,
  • Typical dimensions for the trench excavation are 300 mm deep and 600 mm wide. The trench may be any length,
  • The trench should be lined with 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. before filling with gravel,
  • River rock or decorative aggregateA broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations. may be included in a layer at the top of the gravel fill, for aesthetic value.

Materials

Gravel

The type of medium sized aggregatesA broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations. recommended for gravel diaphragms are described in choker layer. In some contexts it may be desirable to apply a surface layer of more decorative stone.

Geotextiles

The properties of geotextiles vary widely.

Geotextiles can be used to prevent downward migration of smaller particles in to larger aggregatesA broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations., and slump of heavier particles into finer underlying courses. The formation of biofilm on geotextiles has also been shown to improve water quality:

  • By degrading petroleum hydrocarbons[1]
  • By reducing organic pollutant and nutrient concentrations [2]
  • When installing geotextiles an overlap of 150 - 300 mm should be used.

Material specifications should conform to OPSS 1860 for Class II 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. fabrics [3].

  • Fabrics should be woven monofilament or non-woven needle punched.
  • Woven slit film and non-woven heat bonded fabrics should not be used, as they are prone to clogging.

In choosing a product, consider:

  1. The maximum forces that will be exerted on the fabric (i.e., what tensile, tear and puncture strength ratings are required?),
  2. The load bearing ratio of the underlying native soilThe natural ground material characteristic of or existing by virtue of geographic origin. (i.e. is the 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. needed to prevent downward migration of aggregateA broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations. into the native soilThe natural ground material characteristic of or existing by virtue of geographic origin.?),
  3. The texture (i.e., grain size distribution) of the overlying and underlying materials, and
  4. The suitable apparent opening size (AOS) for non-woven fabrics, or percent open area (POA) for woven fabrics, to maintain water flow even with sedimentSoil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls. and microbial film build-up.
Recommended criteria for selection 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. fabric
Percent soil/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. passing 0.075 mm (#200 sieve) Non-woven fabric apparent opening size (AOS, mm) Woven fabric percent open area (POA, %) Permittivity (sec-1)
>85 ≤ 0.3 - 0.1
50 - 85 ≤ 0.3 ≥ 4 0.1
15 - 50 ≤ 0.6 ≥ 4 0.2
5 - 15 ≤ 0.6 ≥ 4 0.5
≤ 5 ≤ 0.6 ≥ 10 0.5

Performance research

http://www.mdpi.com/2073-4441/7/4/1595/htm


External Resources


  1. Newman AP, Coupe SJ, Spicer GE, Lynch D, Robinson K. MAINTENANCE OF OIL-DEGRADING PERMEABLE PAVEMENTS: MICROBES, NUTRIENTS AND LONG-TERM WATER QUALITY PROVISION. https://www.icpi.org/sites/default/files/techpapers/1309.pdf. Accessed July 17, 2017.
  2. Paul P, Tota-Maharaj K. Laboratory Studies on Granular Filters and Their Relationship to Geotextiles for Stormwater Pollutant Reduction. Water. 2015;7(4):1595-1609. doi:10.3390/w7041595.
  3. ONTARIO PROVINCIAL STANDARD SPECIFICATION METRIC OPSS 1860 MATERIAL SPECIFICATION FOR GEOTEXTILES. 2012. http://www.raqsb.mto.gov.on.ca/techpubs/OPS.nsf/0/2ccb9847eb6c56738525808200628de1/$FILE/OPSS%201860%20Apr12.pdf. Accessed July 17, 2017