UAV-based monitoring of river bed morphodynamics for multi-hazard vulnerability assessment of bridges

Accurate extraction of high-resolution digital elevation models (DEMs) is critical for many flood-sensitive rivers regarding landform change monitoring, hydraulic modeling, sediment transport tracking, and evaluation of river channel morphodynamics. Multi-temporal repeat monitoring of flood-vulnerable rivers is crucial due to rapid alteration of morphological properties of in-channel landforms. Thus, in this study the three-dimensional (3D) DEMs of the study region were acquired by unmanned aerial vehicle (UAV) based surveys in order for continuous tracking of stream channel morphology for the rivers sensitive to floods. Repeated high-resolution topography of the Bogacay basin, Antalya, Turkey was obtained in this study by means of UAV-based Structure from Motion (SfM) photogrammetry. The acquired topography during two consecutive years allows analysis of the relations between the main geomorphic processes related to landform alterations and their role in sediment transfer. In conjunction with the flood simulation, the scour depths at bridge piles after a probable flood (Q500) were predicted by HEC-RAS software. The flood analysis was conducted for a maximum runoff of 2560 m³/s with a calculated return period of 500 years (Q500). The results of HEC-RAS flood and scour analyses indicated that a maximum scour depth of 2.49 m could be expected during a probable flood with a maximum water depth of 8.2 m measured from the scoured depth. This water level corresponded to a 0.46 m of submersion of the bridge deck since the pier height was 5.25 m and the maximum flood velocity was predicted as 5.7 m/s. The results indicated that the alterations in the river channel after an expected flood event, allowed reliable evaluation of riverbed morphodynamics, while verifying that UAV-SfM and Dem of Difference (DoD) are useful tools in geomorphological dynamic mapping and in change monitoring studies.