Design features of ancient agricultural terrace walls in the Negev desert: human-made geodiversity.

摘要

Thousands of stone terrace walls were constructed by past societies in the dry valleys of the central Negev highlands to capture runoff/floodwaters from local rainfall to enable agriculture in the desert. Human intervention in geomorphic processes led to human-made geodiversity, as significant differences developed between terraced and natural stream channels. The former are characterised by soil aggradation, as terraced fields also captured sediment during each rainfall/flood event. The natural stream channels, on-the-other-hand, often experienced erosion and desertification. The ancient agricultural terraces were abandoned about 1000 years ago, but many stone terrace walls are still intact. On the basis of fieldwork, we report new findings of sophisticated design characteristics of terrace walls. (1) Mutual friction-hold of rectangular building stones was increased by placing the longest axis parallel with the flow direction of the floodwater and orienting the second longest axis vertically in the terrace wall. (2) The gradual addition of stone layers in the terrace walls, necessary to keep the wall above the aggrading field surface, was sometimes performed in a way that resulted in a staircase construction. Thus, the floodwaters would gradually cascade down to the next terraced field, thereby preventing undercutting and erosion. (3) In larger stream channels of 4th and 5th orders, the longitudinal axis of terrace walls was sometimes constructed with an oblique angle. This was apparently performed for two reasons: (i) to lessen the impact of the floodwaters on the terrace wall and (ii) to steer the water in diversion systems further onto the floodplain.