May-June 2006

Effect of urban soil compaction on infiltration rate

J.H. Gregory, M.D. Dukes, P.H. Jones, and G.L. Miller

ABSTRACT: Inadvertent soil compaction at the urban lot scale is a process that reduces infiltration rates, which can lead to increased stormwater runoff. This is particularly important in low impact development strategies where stormwater is intended to infiltrate rather than flow through a traditional stormwater network to a detention basin. The effect of compaction on infiltration rates on sandy soils in North Central Florida was measured with a double ring infiltrometer on urban construction sites and across various levels of compaction. Average non-compacted infiltration rates ranged from 377 to 634 mm hr-1 (14.8 to 25.0 in hr-1) for natural forest, from 637 to 652 mm hr-1 (25.1 to 25.7 in hr-1) for planted forest, and 225 mm hr-1 (8.9 in

hr-1) for pasture sites. Average infiltration rates on compacted soils ranged 8-175 mm hr-1 (0.3-6.9 in hr-1), 160 to 188 mm hr-1 (6.3 to 7.4 in hr-1), and 23 mm hr-1 (0.9 in hr-1) for the same respective sites. Although there was wide variability in infiltration rates across both compacted and non-compacted sites, construction activity or compaction treatments reduced infiltration rates 70 to 99 percent. Maximum compaction as measured with a cone penetrometer occurred in the 20 to 30 cm (7.9 to 11.8 in) depth range. When studying the effect of different levels of compaction due to light and heavy construction equipment, it was not as important how heavy the equipment was but whether compaction occurred at all. Infiltration rates on compacted soils were generally much lower than the design storm infiltration rate of 254 mm hr-1 (10.0 inches hr-1) for the 100-yr, 24-hr storm used in the region. This implies that construction activity in this region increases the potential for runoff and the need for large stormwater conveyance networks not only due to the increase in impervious area associated with development but also because the compacted pervious area effectively approaches the infiltration behavior of an impervious surface.

Keywords: Compaction, cone index, double ring, infiltration, LID, low impact development, penetrometer, stormwater

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Soil carbon pools in central Texas: Prairies, restored grasslands, and croplands

K.N. Potter and J.D. Derner

ABSTRACT: Establishment of perennial grasses on degraded soils has been suggested as a means to improve soil quality and sequester carbon in the soil. Particulate organic carbon may be an important component in the increased soil carbon content. We measured particulate organic carbon [defined as organic carbon in the 53 to 2000 µm (0.002 to 0.08 in) size fraction] and mineral associated organic carbon (defined as the less than 53 µm (0.002 in) size fraction) at three locations in central Texas. Each location had a never-tilled native grassland site, a long-term agricultural site and a restored grassland on a previously tilled site. Organic carbon pool sizes varied in the surface 40 cm (16 in) of native grassland, restored grasslands and agricultural soils. The native grasslands contained the largest amounts of total organic carbon, while the restored grasslands and agricultural soils contained similar amounts of total organic carbon. Both particulate organic carbon and mineral associated carbon pools were reduced beyond the depth of tillage in the restored grass and agricultural soils compared to the native grassland soils. The restored grassland soils had a larger particulate organic carbon content than the agricultural soils, but the increase in particulate organic carbon was limited to the surface 5 cm (2 in) of soil. Trends in particulate organic carbon accumulation over time from nine to 30 years were not significant in this study.

Keywords: Particulate organic carbon (POC), native grassland, soil quality, mineral associated carbon (MAC), total organic carbon (TOC)

Integrated bioengineering and geotechnical treatments for streambank restoration and stabilization along a landfill

K. Barrett, W. Goldsmith, and M. Silva

ABSTRACT: Vegetative-based bioengineering treatments were integrated with geotechnical treatments to provide effective, affordable and environmentally sound restoration and stabilization of an eroding streambank [8 m (27 ft) high and 145 m (470 ft) long] along a sanitary landfill adjacent to Mill Creek in Cincinnati, Ohio. Bank stabilization design featured the use of dormant live brush in several configurations: live poles installed through interstices in a rock toe; brush layers laid between geogrid-wrapped soil lifts; live stakes through a coir blanket, and a brush fascine staked in a trench near the top of bank. Vegetation has grown vigorously and no erosion problems have been observed. Bank stability was confirmed by a topographic survey in 2004 showing no observable difference from a post-construction survey in 1999. A quantitative vegetation survey in April 2004 determined essentially 100 percent grass coverage on the banks. Seventy-four percent of stakes/poles installed in the upper bank survived, while 39 percent in the lower bank survived. Overall, percent coverage by the shrub/tree layer was visually estimated at about 75 percent.

Keywords: Bank erosion, bank stabilization, biotechnical engineering, erosion control; phytoremediation, phytostabilization, soil bioengineering

 
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Infiltration rate in andisols: Effect of changes in vegetation cover (Tenerife, Spain)

C.C. Jiménez, M. Tejedor, G. Morillas, and J. Neris

ABSTRACT: Soil erosion and infiltration are influenced by chemical, mineralogical, and physical soil properties. Typically, erosion increases with decreasing infiltration. Undisturbed Andisols are considered to be highly stable and resistant to water erosion. A wide variety of Andisols can be found on the island of Tenerife (Canary Islands). The objective of the present paper was to determine the infiltration rate of five Andisols and assess the influence of deforestation and a change in vegetation on the infiltration rate. Apparent hydraulic conductivity was measured using a double ring infiltrometer in soils from the Vitrixerands, Hapludands, Haplustands and Fulvudands great groups. The Infiltration rates in the natural soils were very high and in all cases were greater than 130 mm hr-1 (5.1 in hr -1). Harvesting the natural vegetation appeared to increase the bulk density, while also reducing organic matter and the apparent saturated hydraulic conductivity values. Under these conditions erosion can be stimulated.

Keywords: Andisols, Canary Islands, infiltration rate, land use change, Tenerife

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