Showing papers on "Siltation published in 1981"

Estuarine dredge and fill activities: A review of impacts

Abstract: Dredge and fill activities in estuaries have many environmental effects, most, although not all, of them deleterious. These effects include reduced light penetration by increased turbidity; altered tidal exchange, mixing, and circulation; reduced nutrient outflow from marshes and swamps; increased saltwater intrusion; and creation of an environment highly susceptible to recurrent low dissolved oxygen levels. Coral, oysters, and barnacles are particularly vulnerable to the effects of siltation. Both estuarine flora and fauna may be harmed by contaminants released into the water column by dredging operations. Ways to mitigate the effects of dredge and fill operations include careful pre- and post-construction environmental studies; use of bridging to create roadbeds where coastal wetlands cannot be avoided; use of a turbidity diaper and other means to control turbidity; dredging during periods of low benthic populations or during tides that would carry coarser sediments away from productive areas such as oyster reefs; and thoughtful disposal of spoil, such as locating spoil sites on the uplands with proper diking.

Channel geometry and flow estimates for two small mountain streams in the middle hills, nepal

Abstract: As part of a hazard mapping and evaluation study of the Kathmandu-Kakani area in the Middle Hills, Nepal, conducted as a United Nations/Unesco project, channel conditions at 43 stations on Thulo Khola and Ghatte Khola have been measured and peak discharges for the 1979 monsoon (a season of approximately normal precipitation) have been estimated. The results show that variations in the channels and the flows they carry in this tropical, high-relief area conform to the patterns established by other studies. This should allow hydrologic concepts and models that are derived from the long records and widespread studies of mid-latitude regions to be used in tropical mountains, where there are few direct observations. Flood hazards in the Kathmandu-Kakani area appear to be limited to those riparian zones which consist of constructed agricultural terraces. Damage to these terraces by channel expansion could provide a large influx of sediment to the fluvial system with consequent downstream siltation.

Erosion Control Along Transportation Routes in Northern Climates

Abstract: The widespread occurrence of permafrost in northern climates dictates the use of specialized measures to protect the terrain from adverse impacts associated with the construction of transportation facilities. Through the adoption of appropriate construction techniques and by providing proper drainage and erosion control facilities within the right-of-way, serious environmental degradation can be avoided. The most appropriate type of erosion control measure is selected according to local terrain and drainage conditions, including a Soil Erosion Code (SEC). Erosion control begins with the provision of effective drainage across the right-of-way. Typical erosion control measures include the use of permanent linings such as granular blankets, rock riprap and gabions and temporary linings, such as plastic sheets and fiberglass rovings. Ditch checks, rock aprons, energy dissipators and siltation basins may be used to minimize erosion of exposed soils along the route and attendant siltation of streams. Cut slopes may be protected with insulation in ice-rich, fine-grained soils to ensure stability and prevent slump material from reaching the drainage system. Ultimately, long-term erosion protection along transportation corridors is provided through appropriate revegetation techniques, periodic surveillance and regular maintenance of drainage and erosion control facilities. Key words: erosion, erosion control, permafrost engineering, revegetation, transportation corridors