Thresholds for bed load transport and channel initiation in a chert area in Ashio Mountains, Japan: An empirical approach from hydrogeomorphic observations

We examined the effect of bed load transport on channel‐head locations where shallow landsliding is infrequent. Area‐slope thresholds for bed load transport estimated from hydrogeomorphic observations were compared with certain area‐slope relations at 24 channel heads in a chert area, the Ashio Mountains, Japan. The hydrogeomorphic observations include (1) spring discharge at 12 sites, (2) rainfall intensity, and (3) peak discharge and bed load transport immediately below two channel heads of differing gradient. We confirmed an increase of spring discharge in proportion to drainage area during a stormflow. We performed regression analyses involving the peak 4‐hour rainfall and the peak discharge per unit area to give the optimal rainfall‐runoff equation. The bed load yield increases abruptly when the peak discharge exceeds a critical discharge. We calculated area‐slope thresholds for bed load transport for 1‐year and 100‐year rainfalls, combining the rainfall‐runoff equation with the critical discharge. More than half of all channel heads are located where peak discharge produced by frequent rainstorms (return intervals of less than 100 years) is sufficient to transport bed load. In mountains where shallow landsliding is infrequent, bed load transport has a strong influence on channel‐head location.