LABORATORY EXPERIMENT OF CHANNEL HEAD BIFURCATION BY RADIAL OVERLAND FLOW

Many studies have shown that incipient channelization can be explained by the interaction between flow and an erodible bed. Our recent theoretical study has shown that an eroding channel head by overland flow can maintain its shape without bifurcation if the flow depth is sufficiently large comparing to the channel head radius. Here, we performed nine experimental runs using three sediment sizes to provide better understanding of the bifurcation. A circular pan was filled with sand except at the center, where there were a sink hole and a drain installed at the pan bottom. When water had been filled over the sand to a desirable level, the drain was opened and unsteady radial overland flow accelerated toward the hole. After a while, water formed a downward-concave profile, leading to strong bed erosion in the vicinity of the hole and, as a result, the hole size expanded. In the initial development where flow depth was sufficiently large, the hole maintained its circular shape without channel incision while expanding As the flow depth and discharge reduced and the hole radius increased, the hole started to lose its circular shape. Thus, the experimental results confirmed the condition of channel bifurcation proposed by our previous study. Main incised channels with large spacing were firstly initiated and elongated upstream in the radial direction while small rills were generated later between main channels. Using the network circularity as an indicator, we propose the new four phases of channel network development (unchannelization, initiation, extension and abstraction).