Application of an automated LSPIV system in a mountainous stream for continuous flood flow measurements

Abstract

Abstract:An imaging-based automated large-scale particle image velocimetry (LSPIV) system for flash flood monitoring is developed and deployed in a mountainous stream in the Longchi Catchment, Chengdu, China. This system is built from a low-cost Raspberry Pi board-level computer with a camera module, which can acquire continuous images/videos automatically at programmed intervals. The minimum quadratic difference algorithm tracks surface patterns as flow tracers to estimate the distribution of surface velocities. Meanwhile, a stereo imaging-based 'virtual pole' method has been developed to reconstruct the threedimensional topography with a stereo digital camera, and a cross-sectional bathymetry has been generated without manual surveying. The varying water stage and water surface gradient, which are critical parameters that affect image rectification and surface velocity measurements, can also be directly resolved by applying the two imaging modules together. Discharge can then be estimated with the velocity-area method through selected cross sections. A flash flood that occurred between 24 July 2014 and 25 July 2014 is selected for analysis. The water surface level reconstructed from image processing was validated with marked water levels, and a good agreement was found with a root mean square error of 3.7 cm. The discharge recorded during the flood recession process ranged from approximately 3.5 to 27 m 3 /s. The rating curve obtained can be well described by a power function, and the linear regression suggested a Manning's n roughness coefficient of 0.18 of one specific cross section. Some limitations of the presented large-scale particle image velocimetry system are also put forward, and possible solutions are provided for future improvements. With these proposed upgrades, the system can provide valuable datasets of flash floods in steep mountainous streams, which are critically needed for improving our understanding and modelling of many hydrological processes associated with flood generation, propagation and erosion, as well as for real-time forecasting.

Steep Bedload‐Laden Flows: Near Critical?

“…While the cascade bed in the steep reach generated huge friction and dissipation by hydraulic jumps, the milder plane bed was much smoother and allowed higher Fr to occur. Continuous field LSPIV measurement on a steep stream confirmed globally subcritical or near‐critical flows (Ran et al, ). These observations demonstrate that in laterally confined channels, flow types and channel bed roughness can adjust quite effectively to limit supercritical flow occurrence.…”

Photogrammetric discharge monitoring of small tropical mountain rivers: A case study at Rivière des Pluies, Réunion Island

“…A similar idea was recently brought forward by Ran et al . [] but not pursued with the development of an automatic technique.…”

“…The catchment of the Riviè re des Pluies extends over 46 km 2 from 2270 m to sea level and receives an average annual rainfall of 3400 mm [Pouget and Garcin, 2003]. The river is 18.5 km long and the study site is located at kilometer 16 with an upstream catchment of 38.1 km 2 .…”

“…This branch has an average slope of 2.7% and the width of the flow varies between approximately 2 and 45 m from low discharge to flood events. The discharge does typically not exceed a few m 3 s 21 throughout most of the year, whereas it has been suggested that the peak discharge can exceed 1000 m 3 s 21 [Pouget and Garcin, 2003] during large flash floods. The combination of environmental factors comprising a tropical climate, a mobile and complex riverbed and a large discharge variability make Riviè re des Pluies a challenging study site for the development and application of photogrammetric discharge measurements.…”

Technical note: Stage and water width measurement of a mountain stream using a simple time-lapse camera

“…In relation to flow characteristics, water surface width can be measured from ground-based cameras (Ashmore and Sauks, 2006;Gleason et al, 2015) and correlated with discharge to establish a width-discharge curve in some types of rivers, and local flow velocity has been measured using particle image velocimetry (Creutin et al, 2003;Hauet et al, 2008;Tsubaki et al, 2011;MacVicar et al, 2012;Ran et al, 2016;Stumpf et al, 2016). Direct measurement of stage is less well developed, although Young et al (2015) obtained a water level and discharge record using manual image processing on a small, steep channel using inexpensive groundbased cameras combined with channel geometry and rough-ness assumptions.…”