Estimating long term sediment yields from sediment core analysis

Sediment cores from lakes and reservoirs can be used to estimate sediment yields. In proglacial lakes, the bed sediment typically accumulates as varves, reflecting changes in seasonal and annual sedimentation. This report compares the results from two different methods of measuring suspended sediment transport in a Norwegian glacial river and lake. Sediment yields obtained from a study of sediment cores in the proglacial lake Nigardsvatn along with measurements of the delta topset were compared with a 25-year record of sediment transport based on automatic water sampling and water discharge measurements at a monitoring station at the inflowing river. During the period from 1980 to 2005, analyses of sediment cores taken from the lake bed along with measurements and grain size distribution analysis of the delta indicated that a volume of 175,670 to 202,697 m3 was deposited in the lake Nigardsvatn and corresponding river delta. The year 1980 was selected as a convenient starting point because a large-magnitude flood with a 100-year recurrence interval occurred at the end of 1979, leaving an easily recognizable sediment layer and accurate reference point. Sediment cores were taken at a total of 24 locations throughout the lake and 25 locations in the delta. The densities of the sediment cores were found to vary between 1.3 and 1.5 g/cm3 during the period examined in this study, giving a total suspended sediment load of between 175,670 and 202,697 tons. The measurements carried out at the sediment station in the glacier melt-water river gave a value of 294,800 tons during the same period. A final value of 211,100 tons is calculated from the monitoring station results after deduction of the sediment fraction which passes through the lake without being deposited. This gives a discrepancy between the two methods of 8,403 to 35,430 tons (4–20%). This can be partially attributed to the difficulties of measuring the water discharge in the unstable glacier meltwater river. In conclusion, sediment cores may be used to extrapolate or correct measurements from sediment monitoring stations over longer periods but caution should be made when considering single years.

W pracy przedstawiono wyniki badań prowadzonych według dwóch różnych metod pomiaru ilości rumowiska unoszonego, dostarczonego w latach 1980–2005 do jeziora Nigardsvatn w Norwegii, które zasilane jest wodami roztopowymi z lodowca Jostedalsbreen. Wielkość dostawy rumowiska, określoną na podstawie analizy rdzeniowej z sondowania geologicznego osadów dennych jeziora, porównano z wynikami uzyskanymi na podstawie danych z 25-cio letniego ciągu pomiarów zmącenia w rzece zasilającej jezioro. Stwierdzono, że różnica w wielkości dostawy rumowiska do jeziora określona dwiema metodami wynosi 4–20%, przy czym mniejsze wartości uzyskano na podstawie analizy rdzeniowej osadów. Autorzy uważają, że metoda ta może być stosowana do oszacowania lub korygowania wyników pomiarów zmącenia w rzekach zasilających jeziora i zbiorniki wodne w dostatecznie długich okresach.