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− | 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_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]] |
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− | {{TextBox|1=Gravel diaphragms are ideal for distributing flow and providing pretreatment for | + | 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. |
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| + | 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 |
| *[[Bioretention cells]] | | *[[Bioretention cells]] |
| *[[Bioswales]] | | *[[Bioswales]] |
− | *[[Enhanced grass swales]] | + | *[[Enhanced swales]] |
| *[[Vegetated filter strips]]}} | | *[[Vegetated filter strips]]}} |
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− | ===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. |
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− | ===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, 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, | | *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, |
− | *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. |
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− | ===Materials=== | + | ==Materials== |
− | <h4>Geotextile</h4>
| + | ===Gravel=== |
− | {{:geotextiles}}
| + | 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]]. |
− | <h4>Gravel</h4>
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− | {{:Pea gravel}}
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− | ===References=== | + | ===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]
| + | {{:Geotextiles}} |
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− | {{:Feedback}}
| + | ==External Resources== |
| + | *[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] |
| + | ---- |
| + | [[category: pretreatment]] |