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

Abstract

Abstract:To monitor the stage in turbid reservoirs with a sloping bank, it has been proposed to install a near-infrared Lidar on the bank and to orient it so that it points at the water surface with a large incidence angle (between ≈ 30 • and 70 • ). The technique assumes that the Lidar can detect suspended particles that are slightly below the water surface. Some laboratory results and the first long-term assessment (>2 years) of the technique are presented. It found that: (1) although the test Lidar provides erratic distance data, they can be easily filtered according to the intensity of the received signal; (2) the Lidar provides reliable data only when the water is very turbid (Secchi depth smaller than ≈ 1.0 m); and (3) the reliable data can be used to estimate daily stage values (after a simple field calibration) with an uncertainty better than ±0.08 m (p = 0.95). Although the present form of the technique is not very accurate, it uses an inexpensive instrument (≈1500 USD) which can be easily installed in a safe place (such as is the roof of a building). It is argued that the technique could be also used to monitor the stage and the sub-surface velocity in others turbid water bodies, such as some coastal areas (a recent field of application) and flooding rivers.

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

“…As shown in a previous study [16], both parameters can be obtained empirically through calibration when at least two reference stage data are available. However, it is difficult to obtain reference stage data during the passing of a flash flood.…”

“…As an example of application, let us consider: U(D) = 0.08 m and U(θ) = 0.01 rad (≈0.6 • ), which can easily be attained in practice [16]. In this case, and for a Lidar with an incidence angle of θ = 65 • , the uncertainty of the stage estimates should be 0.05 m during low-water periods and should increase up to ≈0.15 m if the stage reaches 6 m (Figure 1), which is an extreme case (return period > 50 years) for the river described herein.…”

“…The inclined Lidar technique is based on the fact that a near-infrared Lidar mounted with a large incidence angle (between ≈30 • and 70 • ) can detect suspended particles (due to the Tyndall effect) that are slightly below the surface, provided that water is turbid enough [16]. In this case, the distance to the detected particles can be measured with the "time of flight" (TOF) method, and this measurement can be easily used to compute a stage surrogate:…”

“…Nevertheless, there is still a need to better understand how a Radar responds to the roughness produced by the underlying current (something different than the roughness produced by the wind, which is well known) [15]. As an alternative, this study focuses on the "inclined Lidar" technique [16], which works only when water is very turbid: a situation likely to happen during flash floods (e.g., [2]), as discussed more in detail at the end of the study.…”

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

“…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.…”