Having some background in materials science, I look at gravel beds this way: There are at least three phenomena affecting drainage: 1. Percent porosity 2. Porosity size 3. Surface area If you want the best drainage, you must keep the particulate size constant to maximize percent porosity at about 50%. Layering a finer gravel on top of a coarser gravel drops the porosity to about 25% at the interface where the finer grit cascades into the coarser porosity. Mix in a finer grit still, and if the three meet, the porosity would drop to about 12.5%. Eventually it’s concrete. Porosity size and surface area are related. When you double the particle diameter, you halve the surface area per unit volume. This has some obvious effects: 1. It allows the water to flow faster due to less friction 2. It retains less water due to less surface area for wetting to occur. Wetting is an interaction between water and the surface polarity of the aggregate that retains a thin layer of water. 3. A third effect arising from 2 is that when the grain diameter approaches the thickness of the surface water layer, all the porosity changes from air porosity to water porosity. Having said all that, I don’t really know if xerophytic bulbs really grow in well-drained media or if that’s just a poor man’s substitute for a desert. Maybe a hardpan clay on a slope makes more sense. Tim <html xmlns:///v="urn:schemas-microsoft-com:vml/" o="urn:schemas-microsoft-com:office:office/" xmlns:///w="urn:schemas-microsoft-com:office:word/" m="http://…" xmlns="http://www.w3.org/TR/REC-html40/"><head><meta http-equiv=Content-Type content="text/html; charset=utf-8"><meta name=Generator content="Microsoft Word 14 (filtered medium)"><style><!-- /* Font Definitions */ @font-face {font-family:///Calibri/; panose-1:2 15 5 2 2 2 4 3 2 4;} /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {margin:///0in/; margin-bottom:.0001pt; font-size:///12.0pt; font-family:"Times New Roman","serif";} a:link, span.MsoHyperlink {mso-style-priority:99; color:///blue/; text-decoration:///underline/;} a:visited, span.MsoHyperlinkFollowed {mso-style-priority:99; color:///purple/; text-decoration:///underline/;} p.MsoListParagraph, li.MsoListParagraph, div.MsoListParagraph {mso-style-priority:34; margin-top:///0in/; margin-right:///0in/; margin-bottom:///0in/; margin-left:.5in; margin-bottom:.0001pt; 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mso-level-tab-stop:///none/; mso-level-number-position:///right/; text-indent:-9.0pt;} ol {margin-bottom:///0in/;} ul {margin-bottom:///0in/;} --></style><!--[if gte mso 9]><xml> <o:shapedefaults v:ext="edit" spidmax="1026" /> </xml><![endif]--><!--[if gte mso 9]><xml> <o:shapelayout v:ext="edit"> <o:idmap v:ext="edit" data="1" /> </o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='11.0pt;font-family:"Calibri","sans-serif";color:/…'>Having some background in materials science, I look at gravel beds this way:<o:p></o:p></span></p><p class=MsoNormal><span style='11.0pt;font-family:"Calibri","sans-serif";color:/…'>There are at least three phenomena affecting drainage:<o:p></o:p></span></p><p class=MsoListParagraph style='text-indent:-.25in;mso-list:l0 level1 lfo1'><![if !supportLists]><span style='11.0pt;font-family:"Calibri","sans-serif";color:/…'><span style='mso-list:///Ignore/'>1.<span style='font:///7.0pt "Times New Roman"'> </span></span></span><![endif]><span style='11.0pt;font-family:"Calibri","sans-serif";color:/…'>Percent porosity<o:p></o:p></span></p><p class=MsoListParagraph style='text-indent:-.25in;mso-list:l0 level1 lfo1'><![if !supportLists]><span style='11.0pt;font-family:"Calibri","sans-serif";color:/…'><span style='mso-list:///Ignore/'>2.<span style='font:///7.0pt "Times New Roman"'> </span></span></span><![endif]><span style='11.0pt;font-family:"Calibri","sans-serif";color:/…'>Porosity size<o:p></o:p></span></p><p class=MsoListParagraph style='text-indent:-.25in;mso-list:l0 level1 lfo1'><![if !supportLists]><span style='11.0pt;font-family:"Calibri","sans-serif";color:/…'><span style='mso-list:///Ignore/'>3.<span style='font:///7.0pt "Times New Roman"'> </span></span></span><![endif]><span style='11.0pt;font-family:"Calibri","sans-serif";color:/…'>Surface area<o:p></o:p></span></p><p class=MsoNormal><span style='11.0pt;font-family:"Calibri","sans-serif";color:/…'>If you want the best drainage, you must keep the particulate size constant to maximize percent porosity at about 50%. Layering a finer gravel on top of a coarser gravel drops the porosity to about 25% at the interface where the finer grit cascades into the coarser porosity. Mix in a finer grit still, and if the three meet, the porosity would drop to about 12.5%. Eventually it’s concrete.<o:p></o:p></span></p><p class=MsoNormal><span style='11.0pt;font-family:"Calibri","sans-serif";color:/…'><o:p> </o:p></span></p><p class=MsoNormal><span style='11.0pt;font-family:"Calibri","sans-serif";color:/…'>Porosity size and surface area are related. When you double the particle diameter, you halve the surface area per unit volume. This has some obvious effects:<o:p></o:p></span></p><p class=MsoListParagraph style='text-indent:-.25in;mso-list:l1 level1 lfo2'><![if !supportLists]><span style='11.0pt;font-family:"Calibri","sans-serif";color:/…'><span style='mso-list:///Ignore/'>1.<span style='font:///7.0pt "Times New Roman"'> </span></span></span><![endif]><span style='11.0pt;font-family:"Calibri","sans-serif";color:/…'>It allows the water to flow faster due to less friction<o:p></o:p></span></p><p class=MsoListParagraph style='text-indent:-.25in;mso-list:l1 level1 lfo2'><![if !supportLists]><span style='11.0pt;font-family:"Calibri","sans-serif";color:/…'><span style='mso-list:///Ignore/'>2.<span style='font:///7.0pt "Times New Roman"'> </span></span></span><![endif]><span style='11.0pt;font-family:"Calibri","sans-serif";color:/…'>It retains less water due to less surface area for wetting to occur. Wetting is an interaction between water and the surface polarity of the aggregate that retains a thin layer of water.<o:p></o:p></span></p><p class=MsoListParagraph style='text-indent:-.25in;mso-list:l1 level1 lfo2'><![if !supportLists]><span style='11.0pt;font-family:"Calibri","sans-serif";color:/…'><span style='mso-list:///Ignore/'>3.<span style='font:///7.0pt "Times New Roman"'> </span></span></span><![endif]><span style='11.0pt;font-family:"Calibri","sans-serif";color:/…'>A third effect arising from 2 is that when the grain diameter approaches the thickness of the surface water layer, all the porosity changes from air porosity to water porosity.<o:p></o:p></span></p><p class=MsoNormal><span style='11.0pt;font-family:"Calibri","sans-serif";color:/…'><o:p> </o:p></span></p><p class=MsoNormal><span style='11.0pt;font-family:"Calibri","sans-serif";color:/…'>Having said all that, I don’t really know if xerophytic bulbs really grow in well-drained media or if that’s just a poor man’s substitute for a desert. Maybe a hardpan clay on a slope makes more sense. <o:p></o:p></span></p><p class=MsoNormal><span style='11.0pt;font-family:"Calibri","sans-serif";color:/…'>Tim<o:p></o:p></span></p><p class=MsoNormal><span style='11.0pt;font-family:"Calibri","sans-serif";color:/…'><o:p> </o:p></span></p></div></body></html> _______________________________________________ pbs mailing list pbs@lists.pacificbulbsociety.net http://lists.pacificbulbsociety.net/cgi-bin/…