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[[File:Gravel_diagram.jpg|thumb| 75 mm drop from the impervious surface onto the gravel diaphragm]]
[[File:Gravel diaphragm.png|thumb|Note the fall from the impervious surface to the gravel surface]]
[[File:Gravel diaphragm.png|thumb|Note the fall from the impervious surface to the gravel surface]]
==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===
===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}}
{{:Geotextiles}}
*[http://vwrrc.vt.edu/swc/april_22_2010_update/DCR_BMP_Spec_No_2_SHEET_FLOW_Final_Draft_v1-8_04132010.htm VIRGINIA DCR STORMWATER DESIGN SPECIFICATION No. 2]
*[http://vwrrc.vt.edu/swc/april_22_2010_update/DCR_BMP_Spec_No_2_SHEET_FLOW_Final_Draft_v1-8_04132010.htm VIRGINIA DCR STORMWATER DESIGN SPECIFICATION No. 2]
*[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]]