Jökull


Jökull - 01.12.1980, Page 31

Jökull - 01.12.1980, Page 31
abnormal ones mentioned earlier. The very high content of material almost identical to the Grímsvötn volcanic ash was observed in this peak. Fig. 7 shows sediment transport curves for fine and coarse silt in 1976. There is only a minor quantitative difference between the sampling stations except, as already stated, the Sl-samples from the bridge yield greater quantities of the coarse sediment. No abnor- mal sediment peak was observed. SEDIMENT DEPOSITED IN THE FLOOD CHANNELS After the jökulhlaup of 1972 most of the flood channels were strewn with black, fine- grained material (see Tómasson 1974, Fig. 7.). This layer consisted mainly of the sand and coarse silt grain size group (see histograms of layer 1 in Fig. 8). The layer was generally about 10 cm thick. No such layer could be found in the flood channels after the jökul- hlaup of 1976. The deposition of such a material in a channel with velocity exceeding 5 m/s is against hydraulic rules. It should take place at only l/100th of the actual velocity of the jökulhlaup (Hjulström 1935). After this jökulhlaup the channels at the outlet were also strewn with deposits of ice fragments partly hidden in black sand and still finer material. In the field this was assumed to be glacier ice. A few samples were collected and laboratory analysis of one of them revealed that it contained identical Fig. 7. Coarse and fine silt in suspended load in Skeiðará jök- ulhlaup 1976. Key: 1. coarse silt at glacier outlet; 2. coarse silt.at bridge, Sl-samples; 3. coarse silt at bridge, S3-samples; 4. coarse silt 10 km downstream of bridge; 5. fine silt at glacier outlet; 6. fine silt at bridge, Sl-samples; 7. fine silt at bridge, S3-samples; 8. fine silt 10 km downstream of bridge. — Mynd 7. Aurrit mós og mélu í Skeiðarár- hlaupi 1976. 1. Mór við Útfall. 2. Mór við brú Sl. 3. Mór við brú S3. 4. Mór 10 km neðan brúar. 5. Méla við Útfall. 6. Aléla við brú S/. 7. Méla við brú S3. 8. Méla 10 km neðan brúar. kg/s JÖKULL 30. ÁR 29

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