Our last blog was about the soil moisture characteristics of the beach. Besides our measurements for the soil moisture characteristics we have been preparing the aeolian equipment and ourselves for windy days with aeolian sand transport. At first this did not seem to happen, but wednesday 7 October was our lucky day! So today's blog post is about measuring aeolian sand transport.
The conditions required for aeolian sand transport on the beach are very specific. Besides a wind power of about 4 to 5 or higher on the scale of Beaufort, the wind angle, tide (beach width) and the soil moisture content do play a role as well. The beach of our field site is quite a narrow beach and therefore the extent of the fetch, the length on the beach surface over which the wind blows, is determined by the tide and the wind angle. In other words, during low tide the beach is relatively wide and thus it is more likely for wind transport to occur. Adding winds from the NW to N or SW to S results in a large fetch and a high potential for sediment transport.
On wednesday 7 October we arrived at the beach at 7:30 in the morning, just after dark. At this time there was no transport (yet) so we had some time to setup our measuring equipment. To measure the amount of sand transport, we use the Modified Wilson and Cooke sediment catchers. These instruments are just poles containing a wind vane and a set of plastic bottles. The wind vane ensures that the opening of the plastic bottles (which catch the sand) is directed towards the wind direction. So we first set up the poles and when the transport started we added the plastic bottles. The bottles were replaced every hour to measure the amount of transport in 1 hour. Unfortunately due to some small rain events we had to remove some of the bottles during the measuring period to prevent clogging of the inlet tubes. In addition to the sand catchers, we also measured sediment transport on small timescales (seconds). Here fore we use home-built saltation sensors which measure the intensity of saltation. We arrived home after a fieldwork day of almost 11 hours. We were tired and hungry because we were running around at the beach all day. But.... we got some data! The day after the event we stayed home to weigh and clean all the bottles with sand (270 in total) so we are prepared for the next wind event!
Unfortunately the event of wednesday 7 October was the only main event we could measure in the past 3 weeks because the wind was most of the time coming from the east. During this condition the dunes act as a kind of wind shield reducing wind speeds and thus aeolian transport on the beach as well. Additionally, if the wind is strong enough to transport sand in westerly direction, the transport does not contribute to dune growth.
Now our hopes are up for coming wednesday or tuesday (21 and 22 October) when wind forces of 6 and wind gusts of about 7 on the scale of Beaufort are forecasted. However, if rain is forecasted the possibility of measuring transport decreases and because the wind is coming from the west we need low water (ebb) conditions for a long fetch. We hope Aeolus will fulfill our wishes and Poseidon will leave us alone these days...
The conditions required for aeolian sand transport on the beach are very specific. Besides a wind power of about 4 to 5 or higher on the scale of Beaufort, the wind angle, tide (beach width) and the soil moisture content do play a role as well. The beach of our field site is quite a narrow beach and therefore the extent of the fetch, the length on the beach surface over which the wind blows, is determined by the tide and the wind angle. In other words, during low tide the beach is relatively wide and thus it is more likely for wind transport to occur. Adding winds from the NW to N or SW to S results in a large fetch and a high potential for sediment transport.
On wednesday 7 October we arrived at the beach at 7:30 in the morning, just after dark. At this time there was no transport (yet) so we had some time to setup our measuring equipment. To measure the amount of sand transport, we use the Modified Wilson and Cooke sediment catchers. These instruments are just poles containing a wind vane and a set of plastic bottles. The wind vane ensures that the opening of the plastic bottles (which catch the sand) is directed towards the wind direction. So we first set up the poles and when the transport started we added the plastic bottles. The bottles were replaced every hour to measure the amount of transport in 1 hour. Unfortunately due to some small rain events we had to remove some of the bottles during the measuring period to prevent clogging of the inlet tubes. In addition to the sand catchers, we also measured sediment transport on small timescales (seconds). Here fore we use home-built saltation sensors which measure the intensity of saltation. We arrived home after a fieldwork day of almost 11 hours. We were tired and hungry because we were running around at the beach all day. But.... we got some data! The day after the event we stayed home to weigh and clean all the bottles with sand (270 in total) so we are prepared for the next wind event!
Unfortunately the event of wednesday 7 October was the only main event we could measure in the past 3 weeks because the wind was most of the time coming from the east. During this condition the dunes act as a kind of wind shield reducing wind speeds and thus aeolian transport on the beach as well. Additionally, if the wind is strong enough to transport sand in westerly direction, the transport does not contribute to dune growth.
Now our hopes are up for coming wednesday or tuesday (21 and 22 October) when wind forces of 6 and wind gusts of about 7 on the scale of Beaufort are forecasted. However, if rain is forecasted the possibility of measuring transport decreases and because the wind is coming from the west we need low water (ebb) conditions for a long fetch. We hope Aeolus will fulfill our wishes and Poseidon will leave us alone these days...