Flash Flood Monitoring with an Inclined Lidar Installed at a River Bank: Proof of Concept

Abstract

Flash floods need to be monitored from a safe place, ideally with noncontact instruments installed at a riverbank and oriented so that they look obliquely at the water surface. The “inclined Lidar” technique could be useful for this purpose. It works based on the fact that a near-infrared Lidar mounted with a large incidence angle can detect suspended particles slightly below the surface, provided that the water is very turbid, something which is likely during flash floods. To check this hypothesis, an inexpensive “time of flight” (TOF) Lidar was installed during a rainy season at the Amacuzac River (Mexico), which was usually found to be extremely turbid (Secchi depth < 0.5 m). Under these circumstances, the Lidar had no difficulty detecting the water (sub) surface. Converting the measured distances into stage estimates through a simple (one point) calibration resulted in reasonable agreement with reference data (within ±0.08 m (p = 0.95) and always <0.5 m), especially during the passing of a flash flood. This is the first evidence that an inclined (TOF) Lidar can be used to monitor the stage during a flash flood. Indirectly, it also shows that a (Doppler) Lidar could be used to monitor water velocity during this type of event.

Stage Monitoring in Turbid Reservoirs with an Inclined Terrestrial Near-Infrared Lidar

“…However, this layer is unlikely to exist since: (1) as far as we know, in practice a near-infrared and low-power Lidar cannot detect suspended particles below a depth of a few decimeters (see Section 2.3); (2) three water column characterizations performed at Valsequillo at different times (November 2014 and April and June 2015; see triangles at Figure A1b) with a multi-parameter probe ("Exo2", Ysi, Yellow Springs, OH, USA) did not show any obvious stratification at a depth of ≈ 1.7 m (in terms of water temperature, oxygen demand, turbidity and chlorophyll-a concentration), and above all, (3) the same feature was observed later with the test Lidar pointing at a turbid river (the Amacuzac River [27]) when the stage was less than 1.0 metre. Therefore, the Lidar data suggesting the apparent detection of a layer below the water surface were likely to be due to a failure of the test Lidar under low signal strength conditions.…”

“…The proposed technique could also be used to monitor flash floods in rivers: as a matter of fact, the water is usually very turbid during this type of flood [4,27]; in this case, it is worth noting that "glint noise" should only slightly affect the response of a common Lidar pointing at a water surface with an incidence angle between ≈ 30 • and 70 • , unless the surface agitation is extreme [2,9].…”

High Frequency Field Measurements of an Undular Bore Using a 2D LiDAR Scanner

“…The typical height of the scanner above the mean water level prior to the passage of the tidal bore was 8 m. A recent description of the working principle of the LiDAR can be found in [10]. A description of the 2D scanner used in the present study is provided in [9].…”

“…In contrast to other remote sensing tools (e.g., RaDAR, video camera), 2D scanners are capable of accurately measuring surf zone wave geometry. In freshwater conditions where the presence of foam or air bubbles (required to scatter the incident laser) is scarcer, single point LiDAR has applications for steady water body monitoring [10] as well as for more dynamical systems such as flash floods [11]. In laboratory conditions, Martins, K. et al [12] demonstrated the potential of 2D LiDAR to describe the geometry of breaking waves at prototype scale.…”

“…Although tidal bores generally generate and propagate in quite turbid environments [1], there may not always be sufficient particle density at the surface to reflect the infra-red laser. Hence, the laser will often penetrate the water column and be scattered by particles floating at some unknown depth below the surface that can vary with location (Tyndall effect, e.g., [10]). This issue has also been observed in laboratory studies using the same scanner deployed in the present study (LMS511 SiCK commercial scanners, [13]).…”