Numerical simulations are used to study how countermeasure facilities for sediment‐laden flood should be located. To verify the validity of the numerical simulation, river survey data are used. However, these are often obtained every few years, e.g., by LiDAR surveying, and the results are expected to reflect sediment transport due to multiple heavy rainfall events. It’s difficult to assume riverbed fluctuations due to sediment‐laden flood from a single heavy rainfall event. Here, LiDAR surveys were conducted at high frequency in Yotagirigawa river, and the effect of surveying periods on the expression of riverbed change due to debris flow was analyzed. First, if the surveying wasn’t conducted before the debris flow, the decrease in riverbed‐height could not be captured. Second, if the survey was not conducted immediately after debris flow, it wouldn’t be possible to capture the riverbed rise. These suggest that high‐frequency surveying is necessary to validate the numerical simulation.