Time-lapse photography of sandbar displacements for describing riverine morphodynamics

Created by: Nones Michael

The tracking of the edge-of-water lines displacements of a sandbar can be performed using a video camera fixed on a bridge pier and an ad-hoc developed Matlab tool. The methodology was already tested using the Po River in Italy as a case study, evaluating the effects of a flood happened in December 2017.

The outcomes showed that this video-based methodology is: i) economically attractive if compared to more traditional monitoring systems; ii) a valuable system to monitor long-term fluvial processes providing detailed indications on how to better plan river management activities.

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Remote imagery is frequently adopted in continuously monitoring the fluvial behaviour given the advantages of using non-invasive techniques and, combined with numerical modelling tools, this methodology can offer the opportunity to forecast the future evolution of riverine morphodynamics and eventually prevent hazards to hydraulic infrastructures located within the channels (e.g., bridges).

 

Methodologies: video-based monitoring and numerical modelling

The edge-of-water line changes were monitored with a fixed camera Mobotix MX-M15D-SEC installed on a pier of the railway bridge between the cities of Revere and Ostiglia in Italy (coordinates 45°03'13.4"  N, 11°08'16.2" E). The station consisted of a single camera having two day/night sensors with a resolution of 12.5 megapixels and a router remotely connected, allowing for a real-time monitoring and configuration. Images were acquired every 12 h, but only daily images were compared to recognize the edge-of-water line displacements of the sandbar because of the better light conditions. The Matlab image processing toolbox was used to correct the image distortion due to the camera lens, while the edge-of-water line features were extracted using an own-developed procedure.

The numerical simulations were performed using the iRIC 3.0 suite (March 2018 version) and its 2-D solver MFlow_02, which adopts an unstructured grid for computing the vertically averaged unsteady flow and the variations of the riverbed elevation through a finite element method[2] and was already tested in this reach[1].

 

Results

Despite the differences in simulating the sandbar changes, this preliminary, qualitative comparison between time-lapse photography and modelling results pointed out the importance of using continuous video monitoring to be further applied for the calibration of existing codes. In particular, the camera-based method captured the dynamics of the emerged sandbars along the Po River, indicating the need for a specific implementation in a CFD model to properly represent the sandbar dynamics considering both dry and wet conditions.

To overcome the usual limitation of available field data regarding the emerged parts of a river, the monitoring of edge-of-water line displacements with non-invasive techniques like the photogrammetric method can be very helpful. Indeed, once installed, the camera acquires images for long periods (years), covering all the water level conditions and therefore allowing for an evaluation of the long-term river morphodynamics.

 

Conclusions

Aiming to investigate the sandbar dynamics and the consequent variations of the riverbed, a simple video-based method was applied, which continuously acquired the edge-of-water line positions of a large and semi-exposed sandbar in the Po River in Italy. The occurred morphological change was evaluated and qualitatively compared to numerical modelling results obtained with the iRIC code.

Both the time-lapse photography and the numerical results showed that, in this region, flood waves can easily remove sediments that accumulated at bars during low flow conditions: floods redistribute the flow into a wider cross-section, reshaping the deepening and narrowing of the main channel typically observed during dry periods.

The research demonstrates that monitoring the riverine edge-of-water line displacements with a fixed camera can be an economical and reliable method for reproducing the river morphodynamics band for providing additional evidence for CFD models’ calibration, requiring a limited effort[3].

References

  1. Nones, M.; Pugliese, A.; Domeneghetti, A.; Guerrero, M.. Po River morphodynamics modelled with the open-source code iRIC; Kalinowska, M., Mrokowska, M., Rowinski, P., Eds.; Springer: Cham, 2018; pp. 335-346.
  2. Nelson, J.M.; Shimizu, Y.; Abe, T.; Asahi, K.; Gamou, M.; Inoue, T.; Iwasaki, T.; Kakinuma, T.; Kawamura, S.; Kimura, I.; et al. The international river interface cooperative: Public domain flow and morphodynamics software for education and applications. Advances in Water Resources 2016, 93, 62-74, 10.1016/j.advwatres.2015.09.017.
  3. Nones, M; Archetti, R.; Guerrero, M.; Time-Lapse Photography of the Edge-of-Water Line Displacements of a Sandbar as a Proxy of Riverine Morphodynamics. Water 2018, 10(5), 617, 10.3390/w10050617.