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