Gravel diaphragms

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, 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 geotextile before filling with gravel,
• River rock or decorative aggregate may be included in a layer at the top of the gravel fill, for aesthetic value.

Materials

Gravel

This rain garden in a school yard uses stone as both decorative edging and for erosion control.

This bioswale in a parking lot uses stone at the inlets and along the bottom of the swale to prevent erosion, as the sides are sloped.

For advice on aggregates used in underdrains, see Reservoir aggregate.

Stone or gravel can serve as a low maintenance decorative feature, but it may also serve many practical functions on the surface of an LID practice.

Stone for erosion control

Aggregates used to line swales or otherwise dissipate energy (e.g. in forebays) should have high angularity to increase the permissible shear stress applied by the flow of water. However, in some surface landscaped applications there may be a desire to use a rounded aggregate such as 'river rock' for aesthetic reasons. Rounded stones should be of sufficient size to resist being moved by the flow of water. Typical stone for this purpose ranges between 50 mm and 250 mm in diameter. The larger the stone, the more energy dissipation.

• Stone beds should be twice as thick as the largest stone's diameter.
• If the stone bed is underlain by a drainage geotextile, annual inspection and possible replacement should be performed as there is a potential for clogging of this layer to occur.

• 

  Stone lining the ponding zone of this rain garden. Image credit California Native Plant Society

• 

  Coarse angular stone laid onto a geogrid and geotextile. Image from wikimedia commons

Stone mulch

Finer inorganic mulch materials can be of value applied in areas with extended ponding times i.e. in the the centre of recessed, bowl shaped bioretention, stormwater planters, trenches or swale practices. Inorganic mulches resist movement from flowing water and do not float. Applying a thin layer of inorganic mulch over the top of wood based mulch has been shown to reduce migration of the underlying layer by around 25% . Inorganic mulches which may be available locally, include:

• Pea gravel
• River rock/beach stone
• Recycled glass
• Crushed mussel shells

---

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

---

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.

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.

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.

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.

A lot level bioretention cell designed to receive and detain, infiltrate and filter runoff, typically used for discharge from downspouts.

Includes the protection of soil from dislocation by water, wind or other agents.

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

(1) The wearing away of the land surface by moving water, wind, ice or other geological agents, including such processes as gravitation creep; (2) Detachment and movement of soil or rock fragments by water, wind, ice or gravity (i.e. Accelerated, geological, gully, natural, rill, sheet, splash, or impact, etc).

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.

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.

a top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales.

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

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.

a top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

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.