| The sediment budget of Norwegian rivers and the flux of particle bound chemical
elements
Jim Bogen1, Truls E. Bønsnes1, Patricia Kennie1, Rolf Tore Ottesen2 and Ola Anfin
Eggen2
1Norwegian Water Resources and Energy Directorate, e-mail jbo@nve.no
2Geological Survey of Norway,
Abstract
The river basins of Norway are of extremely varying character and conditions differ
greatly between regions and from high to low latitudes. Sediment transport
monitoring programmes have revealed that there are five important sediment-source
areas. These are the gullied marine clay areas, the forest areas, the glacier
areas, the mountain areas outside the glacier covered areas and the cultivated
areas.
The mean value of measured sediment yield supplied from the areas classified as
“forest” is 2.88 t km-2, giving a total annual suspended sediment yield of 0.427 * 106 t
y-1. This includes a very large part of the country, also marshland and areas with
sparse vegetation are included in this category. A mean sediment yield of 5.0 t km-2
y-1was found for the mountain areas in mainland Norway outside the glacier areas.
Their total area is 152 819 km2, giving a total sediment supply of 0.764 * 106 t
y-1.
The soil loss from agricultural areas of the entire country has been estimated as
1.874* 106 t y-1 corresponding to a mean sediment yield of 37 t km-2 y-1. The
sediment yield of gullies incised in clay areas outside agricultural land has been
estimated as 155 t km-2 y-1and the correspondingerosion activity is 0.155*106 t y-1.
Different sediment yields have been calculated for different groups of glaciers varying
from 116 t km-2 y-1for smaller cirques and plateau glaciers to a mean value of 528 t
km-2 y-1for larger valley glaciers and outlet glaciers from ice caps. The total estimate
of sediment derived from glacial erosion was found to be 1.052 * 106 t y-1. The sum of
the sediment supplied from all the different areas in mainland Norway amounts to a
total of 4.272*106 t y-1. In Svalbard, the temperature regime is also important. One
third of the area of Svalbard is occupied by small glaciers believed to be cold and
frozen to the bed and hence do not erode. An erosion rate of 586 t km-2y-1
has been regarded as representative for the polythermal type of glaciers. A
yield of 82.5 t km-2 y-1is assigned to the non-glacial area giving rise to an
estimated annual flux of sediment from Svalbard in the order of 16*106 t yr-1
.
Eight river basins have been selected for further study of sediment budget and
calculation of the flux of the particle bound chemical elements Al, As, Cd, Cr, Cu, Hg,
Ni, Pb and Zn based on the analysis of overbank sediments on floodplains. The
range of the inorganic sediment flux and the percentage of organic material in
the total load amounts to 95 – 141Â 000 t y-1 and 4 – 14 % in Glomma, 23 –
143Â 000 t y-1 and 4-20% in Numedalslågen, 195 – 1319 t y-1 and 32 – 56% in
Vikedalselv, 45 – 127 000 t y-1 and 0.9 – 3.2% in Jostedøla, 46 – 155Â 000 t
y-1 and 1.5 – 3.2% in Blakkåga, 1500 t y-1 – 90Â 000 t y-1 and 0.9-25% in
Beiarelv and 5 - 23000 t y-1 and 1.5 -5.1% in Bayelva. Higher concentrations of
arsenic were found in the rivers of Svalbard in comparison to mainland Norway.
Sediment storage on floodplains has been observed at 35% of the total suspended
load, but construction of flood protection walls and bed level lowering have
prevented the sediment load from being deposited and decreased overbank
sedimentation in later years. In some of the rivers the diversions associated with
hydropower development have caused an increase in sediment delivery to the sea.
It is also assessed that climate change will lead to a substantial increase in
sediment delivery due to the predicted meltdown of glaciers, the increase in
precipitation and the increase in the number of flash floods of extreme magnitude. |