The secondary permeability of "impervious" cover in Austin, Texas

Wiles, Thomas Jefferson, 1970-

The secondary permeability of "impervious" cover in Austin, Texas

dc.contributor.advisor	Sharp, John Malcolm, 1944-	en
dc.creator	Wiles, Thomas Jefferson, 1970-	en
dc.date.accessioned	2013-06-24T19:44:11Z	en
dc.date.available	2013-06-24T19:44:11Z	en
dc.date.issued	2007-12	en
dc.description	text	en
dc.description.abstract	The term "impervious" is commonly used in urban settings to describe the permeability of buildings, roads, and parking lots. When estimating recharge to an aquifer underlying an urbanized area, impervious cover becomes a prime consideration. It is commonly assumed that an increase in impervious cover leads to a decrease in precipitation recharge. However, even a cursory glance at most roads, sidewalks, or parking lots reveals that, far from being impervious, there are abundant fractures that may provide avenues of infiltration. For this study, method was developed to determine the secondary permeability of pavements using a double ring infiltrometer to measure the infiltration rate of water into fractured pavements. Linear extrapolation is employed to determine the infiltration rate as the water depth approaches zero, which is used as a proxy for hydraulic conductivity by assuming that the gradient is unity. Data were collected on concrete and asphalt pavements located in Austin, Texas, at each point a fracture or expansion joint intersected along 30-meter scanlines. By dividing the sum of the discharges for each fracture by the area represented by the scanline we are able to determine the equivalent-porous-media hydraulic conductivity. The equivalent hydraulic conductivities for discrete fractures were found to range at least three orders of magnitude, from >10⁻² to 10⁻⁵ cm/sec; scanline hydraulic conductivities range two orders of magnitude from >10⁻⁴ to 10⁻⁶ cm/sec; permeability along the scanlines tends to be dominated by one or two highly conductive fractures; and the hydraulic conductivity of the entire paved surface is 5.9·10⁻⁵ cm/s. Both apertures and point hydraulic conductivities were found to have logarithmic distributions but cross plots demonstrated no correlation, which indicated that a combination of the fill material and sub grade, not the fractures and expansion joints themselves, limit infiltration. By multiplying the paved surface hydraulic conductivity by the time the surface can be expected to be saturated, we find that 170 mm or 21 percent of mean annual rainfall is available as potential recharge. When coupled with an enhanced subsurface permeability structure resulting from the installation of utilities and the reduction of evapotranspiration from the reduction of vegetation, the net effect of roads and parking lots could be an increase in precipitation recharge.	en
dc.description.department	Earth and Planetary Sciences	en
dc.format.medium	electronic	en
dc.identifier.uri	http://hdl.handle.net/2152/20402	en
dc.language.iso	eng	en
dc.rights	Copyright is held by the author. Presentation of this material on the Libraries' web site by University Libraries, The University of Texas at Austin was made possible under a limited license grant from the author who has retained all copyrights in the works.	en
dc.subject.lcsh	Aquifers--Texas	en
dc.subject.lcsh	Concrete--Permeability	en
dc.subject.lcsh	Asphalt--Permeability	en
dc.title	The secondary permeability of "impervious" cover in Austin, Texas	en
thesis.degree.department	Geological Sciences	en
thesis.degree.discipline	Geological Sciences	en
thesis.degree.grantor	The University of Texas at Austin	en
thesis.degree.level	Masters	en
thesis.degree.name	Master of Science in Geological Sciences	en