Environmental Engineering Reference
In-Depth Information
Alterations in Sediment Transport
Humans have altered fl uvial transport of sediment to the coast, mainly
through agricultural practices for food production. The cultivation of land
makes soil more susceptible to erosion, and some of the eroded soil reaches
the coastal zone. Deforestation and cultivation may cause up to eight fold
increase in sediment loads in small rivers downstream, and 3.5 fold increase
in larger rivers (Douglas 1990), and the exponential growth of human
population has led to an increased downstream transport of sediments. At
the same time, the expansion of human settlements and more sophisticated
agricultural practices required an increasing management of fresh water,
and triggered widespread construction of dams, dikes, levees, and canals.
The net result of waterworks has been that sediments now accumulate
behind dams, instead of being exported to the coastal zone.
The human-mediated changes in sediment transport do have the
potential to locally alter coastal environments. Before 200 BC the Huanghe
(Yellow River) drained a forested steppe, and the sediment load was about
an order of magnitude lower than during the 20th century (Milliman et al.
1987). As agricultural pressure grew, soil eroded from fi elds increasingly
entered the river, and the coastline expanded seaward 50 km across an
interval of 130 years. The sediment loads were high and varied seasonally
and interannually drastically altering the course of the river, but delta
expansion continued.
The delta of the Nile River shows a similar pattern. Natural and human
alterations of the course of the Nile created the Rosetta Branch through the
delta between 500 and 1000 AD, which brought an increased fl ow of Nile
water through the area, and silty sediments. The increased input of silt, and
subsequent trapping within the delta, caused the Rosetta Promontory to
advance into the Mediterranean Sea during the years 1500 to 1900 (Fanos
1995). After 1900 irrigation of fi elds on the Nile fl ood plain lowered the fl ow
of water through the Nile delta, what resulted in a steady recession of the
Rosetta Promontory and a shoreline that was back to where it might have
been between 1700 and 1800.
In the catchment area of the Mississippi River, intensive agriculture
expanded through the 1800s and into the 1900s, and the enhanced soil
erosion encouraged the formation of the Soil Conservation Service in
1935 (Turner and Rabalais 2003). In the early 20th century, intensive river
engineering, especially reservoir construction, greatly reduced sediment
transport within the basin (Meade and Moody 2010), and dikes built after
World War 2 to control fl oods also contributed to trap suspended sediments
and lowered the sediment transport downriver. Changes in area of the
