Jökull - 01.01.2004, Page 53

Sediment thickness and erosion rates within Hvítárvatn, central Iceland
Table 1. Calculated sediment volume and sediment mass derived from the seismic survey of Hvítarvatn.
– Setrúmmál og þyngd reiknað út frá upplýsingum af endurvarpsmælingum á Hvítárvatni.
Seismic Survey area Whole lake estimates
Seismic Whole-
Hvítárvatn Seismic Seismic Seismic Seismic Survey Southern lake
Unit 1 Unit 2 Unit 3 Unit 4 Total Shallows LIA Scour total
Area (m2) 9.3x106 9.3x106 9.3x106 9.3x106 9.3x106 13.7x106 6.3x106 28.9x106
Vol. (m3) 11.7x107 2.6x107 9.7x107 8.1x107 32.1x107 13.3x107 19.1x107 64.1x107
Mass (g) 19.9x1013 4.4x1013 16.5x1013 13.6x1013 54.6x1013 22.6x1013 32.5x1013 109.0x1013
Table 2. Holocene erosion rates for the Hvítarvatn catchment based on sediment accumulation within the lake
and estimates of suspended sediment loss through the outflow stream Hvítar. – Rofhraði á vatnasviði
Hvítárvatns á nútíma byggt á setupphleðslu innan vatnsins og mati á svifaur sem tapast við útstreymi
Hvítárvatns í Hvítá.
Sediment fill in Sediment fill in Sediment fill in Total fill Suspended Total sediment
Seismic Survey Southern LIA Scour regional sedim. lost from lake delivered by
shallows deglaciation catchment
Volume (m3) 20.4x107 8.5x107 12.2x107 32.9x107
Mass1 (g) 34.7x1013 14.4x1013 20.8x1013 70x1013 51x1013 121 x1013
Erosion (min. rate) (max. rate)
rate2 1.6 cm ka−1 3.1 cm ka−1 5.3 cm ka−1
1Based on an average bulk density of sediment fill in is Hvítárvatn of 1.7 g cm−3.
2Based on an average bulk density of bedrock in the Hvítárvatn catchment of 2.7 g cm−3.
Sediment delivered to Hvítárvatn by the receding
Icelandic Ice Cap (Unit 1) was not derived from the
modern catchment of the lake. Consequently, to gen-
erate a realistic estimate of Holocene erosion rates
requires that we remove this sediment from the to-
tal sediment volume in the lake. This is straightfor-
ward within the seismic survey zone, where Unit 1
accounts for 36% of the total sediment volume. How-
ever, for those areas outside the seismic survey, we
lack seismic data that allow differentiation of Unit 1
sediment from catchment-derived sediment. Conse-
quently, we remove the same percentage of the to-
tal estimated sediment volume as observed within the
survey area (Table 2).
We estimate the sediment lost from Hvítárvatn via
its outlet stream Hvítá from actual measurements of
suspended sediment loads and discharge records at
the outlet gauging station. Total suspended sediment
loads were measured once each summer from 1965
to 1999 (Pálsson and Vigfússon, 1996). The average
suspended load was 35 mg L−1 (range: 10 to 100 mg
L−1; Vatnamælingar Orkustofnunar, 2004), a figure
that is independent of river discharge (Figure 13).
Discharge has been monitored or estimated monthly
since 1959 (Vatnamælingar Orkustofnunar, 2003); the
average annual discharge (1959–2001) is 46 m3 s−1.
At an average suspended load of 35 mg L−1, the mass
lost each year as suspended load is 5.1 x 1010 g. If
this rate is typical of the long-term average, the to-
tal loss is estimated to be 51 x 1013 g, similar to the
total volume of catchment-derived sediment retained
in the lake (Table 2). This calculation probably over-
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