The transformation of watersheds is a characteristic of human civilization. Humans transform natural landscapes into various kinds of "land use" that provide them with habitation and resources. Altered hydrology and soil erosion occur as a consequence of these transformations, which are problems that must be addressed. The main kinds of transformations include development of agriculture, urbanization, and alterations of streams, rivers, and coastlines. In all cases natural vegetation is removed or changed and land forms are simplified (usually leveled). Society generally accepts that these direct impacts must occur to accommodate human land use, but indirect impacts such as erosion are not acceptable and require engineering solutions and/or management.

Erosion is a major environmental impact that results in loss of agricultural productivity, aquatic pollution, and property damages among other problems. Although the impact of erosion has long been recognized (Bennett and Lowdermilk, 1938; Brown, 1984; Judson, 1968), it remains a challenge to society. Costs due to urban, shoreline, and agricultural erosion are tremendous, and a major industry of businesses and technologies has arisen for erosion control.

A set of ecological engineering techniques has evolved with the industry for erosion control; that is the subject of this chapter. This subdiscipline has been referred to as bioengineering, and it involves a combination of conventional techniques from civil or geotechnical engineering with the use of vegetation plantings (Table 3.1). It is an interesting field that is growing rapidly as a cost-effective solution to erosion problems. Most workers in the field are not concerned about (or perhaps not even aware of) problems with overlapping meanings of the term bioengineering, which is often used in other contexts (Johnson and Davis, 1990). Schiechtl and Stern (1997) provide some background discussion and end up suggesting the term water bioengineering for some applications. Gray and Leiser's (1982) use of the phrase "biotechnical slope protection and erosion control" is perhaps more appropriate but too long and awkward as a descriptor. Here, the field is referred to as soil bioengineering as a compromise term that is used by many workers.

The central basis of soil bioengineering from both a philosophical and a technical perspective is an understanding of the interface between hydrology, geomorphology, and ecology. Hydrology integrates the landscape, especially by water movements, and helps create an interactive relationship between landform and ecosystem. An old subdiscipline of ecology called physiographic ecology in part covered this topic. Physiographic ecology was a descriptive field analysis of vegetation and topography that flourished briefly around the turn of the 20th century (Braun, 1916; Cowles, 1900, 1901; Gano, 1917). These studies are detailed descriptions that convey a rich, though static, understanding of landscape ecology. Like many kinds of purely

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