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[[File:Gravel_diagram.jpg|thumb| 75 mm drop from the impervious surface onto the gravel diaphragm]]
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.
[[File:Gravel diaphragm.png|thumb|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.
{{textbox|Gravel diaphragms are ideal for distributing flow and providing pretreatment for
{{textbox|Gravel diaphragms are ideal for distributing flow and providing pretreatment for
*[[Bioretention cells]]
*[[Bioretention cells]]
*[[Bioswales]]
*[[Bioswales]]
*[[Enhanced grass swales]]
*[[Enhanced swales]]
*[[Vegetated filter strips]]}}
*[[Vegetated filter strips]]}}
==Planning considerations==
==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,
*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,
*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,
*Flow should be directed into the device laterally.
==Design==
==Design==
*Where flow comes off an impervious surface, a drop of 75 mm or more to the surface level of the diaphragm is recommended,
*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,
*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,
*The trench should be lined with geotextile before filling with gravel; and,
*River rock or decorative aggregate may be included in a layer at the top of the gravel fill, for aesthetic value.
*River rock or decorative [[stone]] may be included in a layer at the top of the gravel fill, for aesthetic value.
==Materials==
==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===
{{:Geotextiles}}
{{:Geotextiles}}
*[https://directives.sc.egov.usda.gov/OpenNonWebContent.aspx?content=17751.wba National Engineering Handbook Part 628 Dams Chapter 45 Filter Diaphragms]
*[https://directives.sc.egov.usda.gov/OpenNonWebContent.aspx?content=17751.wba National Engineering Handbook Part 628 Dams Chapter 45 Filter Diaphragms]
==References==
[[category: pretreatment]]
[[category: pretreatment]]