REACH SCALE APPLICATION OF UAV+SFM METHOD IN SHALLOW RIVERS HYPERSPATIAL BATHYMETRY |
Paper ID : 1088-SMPR-FULL |
Authors: |
Omid Bagheri1, Masoud Ghosian *1, Mohammad Saadatseresht2 1Dept. of Civil and Environmental Engineering, Tarbiat Modares University, Al-Ahmad Ave., Tehran I.R. Iran P.O. Box 14115-143 2Iran - Tehran - North Kargar - after Jalal Ale Ahmad - 2 schools of Technical College, PoBox 14395-515 |
Abstract: |
Nowadays, many rivers across the world are highly regulated and changed either in flow or sediment regimes. Rehabilitation of these rivers is an increasing demand and needs sophisticated tools to assess the current condition of the river. Using influencing parameters derived from the assessment, the regulated river can be reengineered and rehabilitated. Such parameters are used in most approaches like SHIRA for spawning habitat rehabilitation. Hydrodynamic models are strong tools in process-based and ecohydraulics analyses of rivers which have been widely used in this realm and shown convincing results. The most important input of these models is the river bathymetry. Previously, due to lack of surveying technology, scientists tried to overcome the limitations in data collecting by using simplifying approaches. For instance, cross-sectional approach or reference reaches have been used, but these approaches failed to represent the real processes of the river even in simple systems. Their failure can be due to interpolation error among far apart crosssections or inaccurate approaches for extrapolating the result in reference reaches. In order to enhance the validity of such simplifying approaches to develop into spatially explicit approaches like near-census or hyperspatial, new surveying methods may be presented or improvements may be applied to the present methods. Due to complexity of river systems in all aspects such as shape, vegetation coverage, and hydraulics condition, using a single technique to provide such data is not applicable unless in limited parts of the system. Few studies have shown that it is feasible to use UAV+SfM technique in riverine environments to produce hyperspatial bathymetry data at the mesoscale (10 m to a few hundred meters) with the maximum error of 0.05 m. However, in rehabilitation scenarios we are dealing with scales from reach (100-1000 channel widths) to segment (1000-10000 channel widths). In such scenarios, UAV+SfM method is not feasible to use with current workflow. In the present study, we modified current UAV+SfM workflow by using S1000 multirotor autopilot. Defining an appropriate flying plan by autopilot option leads to a more effective control on flying height and precise control on photos coverage and overlap. Our suggested workflow made speed flying of 1m/s feasible, therefore reach scale applications of UAV+SfM were available. In addition, this technique was improved by having more control over image taking parameters. Furthermore, because of faster implementation, low illumination changes were observed that provided suitable and more effective conditions to use spectral-depth relationship method. We mapped 1400 meters of the Alarm River (Lar National Park, Iran) with 0.01 m resolution as a natural system having a very diverse morphological, hydrological and vegetation coverage condition. A simple correction algorithm was used in the submerged regions of the river. The spectral-depth relationship method can be used to correct the refraction effect in submerged regions where flow depth in river is greater than 0.4 m. However, more rtkGNSS and depth data points are required in this method and the results are site-specific and might be affected by scene illumination, flow and substrate condition. In this study, we observed few small regions with depths more than 0.4 m, so we did not use spectral-depth relationship. The results were compared with a dense rtkGNSS point cloud to evaluate the accuracy of the resulted DEM. In our work we found that observed errors in elevation of submerged regions were fluctuating between -0.046 and 0.047 m. Particularly, the mean error was approximately equal to zero and the standard deviation was 0.024 m. According to the result of this study, the UAV+SfM technique is an appropriate technique to map river systems such as Alarm and can be used as input data for reach scale hydrodynamic models. However, overhanging vegetation in small rivers may lead to underestimation of flow area, depending on channel and flow condition. The correction due to overhanging vegetation were considered manually. Meanwhile, several limitations for the temporal resolution of the developed method were considered that were due to the flow, vegetation and wind condition. UAV+SfM method provides hyperspatial resolution data over reach to segment scales in shallow rivers with short-spare vegetation considering the temporal resolution limitations. We should note that in spite of providing hyperspatial resolution, application of the aforementioned method in smaller scales leads to logistical challenges that may not be economical as compared to rtkGNSS. |
Keywords: |
UAV, SfM, Alarm River, Hyperspatial Resolution, Near-Census River Science. |
Status : Paper Accepted (Oral Presentation) |