Gravel diaphragms

75 mm drop from the impervious surface onto the gravel diaphragm

Note the fall from the impervious 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 sediment particles before they reach any downstream structure or practice.

If the contributing drainage area is solely turf (e.g., sports field), then the gravel diaphragms may be eliminated from the treatment train.

Gravel diaphragms are ideal for distributing flow and providing pretreatment for

• Bioretention cells
• Bioswales
• Enhanced swales
• Vegetated filter strips

Planning

The gravel diaphragm serves two purposes. First, it acts as a pretreatment device, settling out coarse particles before they reach an infiltration practice. Second, it acts as a level spreader, maintaining sheet flow as runoff flows over the filter strip. If the contributing drainage area 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 conveyance 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 impervious surface, an elevation drop of 75 to 150 mm to the surface of the diaphragm is recommended to dissipate energy, promote settling and allow for accumulation of sediment and debris between cleanings;
• 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 geotextile before filling with gravel; and,
• River rock or decorative stone may be included in a layer at the top of the gravel fill, for aesthetic value.

Materials

Gravel

The type of medium sized aggregates 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.

See Clogging for notes on their application in LID structures.

Geotextiles can be used to prevent downward migration of smaller particles in to larger aggregates, and slump of heavier particles into finer underlying courses. Geotextiles are commonly used on low strength soils (CBR<4). The formation of biofilm on geotextiles has also been shown to improve water quality:

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

Material specifications should conform to OPSS 1860 for Class II geotextile fabrics . Note when expansive clays are present, a non-infiltrating design may be necessary. If used, geotextile socks around perforated pipes should conform to ASTM D6707 with minimum water flow rate conforming to ASTM D4491 (12,263 L/min/m² at 5 cm head).

• 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 soil (i.e. is the geotextile needed to prevent downward migration of aggregate into the native soil?),
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 sediment and microbial film build-up.

Recommended criteria for selection of geotextile fabric

Percent soil/filter media 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

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External Resources

• VIRGINIA DCR STORMWATER DESIGN SPECIFICATION No. 2
• National Engineering Handbook Part 628 Dams Chapter 45 Filter Diaphragms

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

A 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.

Soil, 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.

The 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.

Stormwater management following the hierarchical approach: Source Control measures, Conveyance Control measure and End of Pipe treatment to achieve the water quality and water balance target for lot level development of the preferred strategy.

A combination of lot level, conveyance, and end-of-pipe stormwater management practices.

Initial 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.

The portion of water precipitated onto a catchment area, which then 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.

a 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.

A structural practice that is located within the drainage system where flows are concentrated and are being conveyed along corridor. Include but are not limited to pervious pipes, swales, and other similar systems.

Filter 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.

A 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.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

The natural ground material characteristic of or existing by virtue of geographic origin.

A 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.

The 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.

Gravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices.