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.
Highlights
Flash floods are a type of natural disaster which is quite common worldwide and very dangerous [1]
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
The inclined Lidar technique is based on the fact that a near-infrared Lidar mounted with a large incidence angle can detect suspended particles that are slightly below the surface, provided that water is turbid enough [16]
Summary
Flash floods are a type of natural disaster which is quite common worldwide and very dangerous [1]. It is difficult to obtain reference stage data during the passing of a flash flood In this case, a simple (one-point) calibration procedure was considered: (1) the Lidar incidence angle (θ) is not adjusted but rather measured (using an inclinometer); and (2) only the Lidar position above the river bottom (H0 ) is adjusted, so that the Lidar stage estimations coincide with one reference stage data taken during a low-water period (H ≈ 0). A simple (one-point) calibration procedure was considered: (1) the Lidar incidence angle (θ) is not adjusted but rather measured (using an inclinometer); and (2) only the Lidar position above the river bottom (H0 ) is adjusted, so that the Lidar stage estimations coincide with one reference stage data taken during a low-water period (H ≈ 0) This is equivalent to estimating the stage as:
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