Effects of construction on fluvial sediment, urban and suburban areas of Maryland

The equivalent of many decades of natural or even agricultural erosion may take place during a single year from areas cleared for construction. Areas undergoing rapid development near Baltimore, Maryland, and Washington, D.C., lie on the Coastal Plain and Piedmont, with slopes generally of 1-10% but sometimes of 20% and more. Soil is deep, and the annual precipitation of 42 inches (1100 mm) is evenly distributed, with high summer intensities. Average sediment yield is 200–500 t/mi2/yr (80–200 t/km2/yr), with predominantly wooded watersheds supplying sometimes even less. Intensive farming 50–80 years ago caused yields up to 1000 t/mi2/yr (400 t/km2/yr), but such high yields are no longer present owing to the continuous decline of farm acreage in the metropolitan periphery. Sediment concentrations from areas undergoing construction ranged from 3000 to over 150,000 ppm, whereas in natural or agricultural catchments the highest comparative concentration was 2000 ppm. In terms of annual values, yields from construction areas range from several thousand to a maximum of 140,000 t/mi2/yr (i.e., up to 55,000 t/km2/yr) from a small area. Total yield declines with increasing drainage area as a result of dilution from waters draining urban and other land not actually under construction. Observations demonstrate that sediment storage occurs on construction sites as well as in valley bottomlands. Actual yields from a given unit surface may be even larger than those derived from measurements in streams. Data on erosion from roadcuts in Georgia, when converted to soil loss per area, result in sediment yields similar to those from building sites: 50,000–150,000 t/mi2/yr (20,000–50,000 t/km2/yr), and local measurements indicate depths of erosion on roadcuts of 0.1–0.2 ft (3–6 cm) over time intervals of generally less than one year. Imposition of large quantities of sediment on streams previously carrying relatively small quantities of primarily suspended material produced deposition of channel bars, erosion of channel banks as a result of deposition within the channel, obstruction of flow and increased flooding, shifting configuration of the channel bottom, blanketing of bottom-dwelling flora and fauna, alteration of the flora and fauna due to changes in light transmission and abrasive effects of, sediment, and alteration of species of fish due to changes produced in the flora and fauna upon which fish depend. Analysis of building permit records showed that: (1) 50% of construction sites were open for eight months, 60% for nine, and 25% for more than one year; (2) contrary to expectations, construction activity is practically constant throughout the year, with about 84% of all sites being open in any one month; (3) the average size of a construction site for one permit-bearing building is 14,400 ft2 (1340 m2), a value that remained constant during the last decade. These findings, combined with statistics on highway construction, indicate that in four Maryland counties covering expanding metropolitan regions adjacent to Baltimore and Washington, at a minimum 7.2 mi2 (19 km2) of land are cleared at any one time for construction purposes. Housing and other buildings account for 5.7 mi2 (15 km2) and highways for 1.5 mi2 (4 km2). Sediment yield is roughly 700–1800 tons per 1000 increase in population. Progressive urbanization effects an initial rise in the total sediment, soon followed by a steady decline. The proportion of sediment from the construction source, however, will increase steadily and may overtake the total agricultural yield at a not very advanced stage of urbanization. Most economic evaluations of problems posed by urban sediment are subject to much uncertainty. The problem, however, is common to all conurbations, particularly where heavy earthmoving machinery is in use. Techniques for lessening sediment yield and delivery are available. Their widespread adoption presents complex issues of a local, legal, and social nature. (Key words: Sediment; urbanization)