Towards Affordable 3D Physics-Based River Flow Rating: Application Over Luangwa River Basin
Abstract. Unmanned aerial vehicles (UAVs), affordable precise Global Navigation Satellite System hardware, echo sounders, open-source 3D hydrodynamic modelling software, and freely available satellite data have opened up opportunities for a robust, affordable, physics-based approach to monitor river flows. In short, the hardware can be used to produce the geometry. 3D hydrodynamic modelling offers a framework to establish relationships between river flow and state variables such as width and depth, while satellite images with surface water detection methods or altimetry records can be used to operationally monitor flows through the established rating curve. Uncertainties in the data acquisition may propagate into uncertainties in the relationships found between discharge and state variables. Variations in acquired geometry emanate from the different ground control point (GCP) densities and distributions which are used during photogrammetry-based terrain reconstruction. In this study, we develop a rating curve using affordable data collection methods and basic principles of physics. The specific objectives were to: determine how the rating curve based on a 3D hydraulic model compares with conventional methods; investigate the impact of geometry uncertainty on estimated discharge when applied in a hydraulic model; and investigate how uncertainties in continuous observations of depth and width from satellite platforms propagate into uncertainties in river flow estimates using the rating curves obtained. The study shows comparable results between the 3D and traditional river rating discharge estimations. The rating curve derived on the basis of 3D hydraulic modelling was within a 95 % confidence interval of the traditional gauging based rating curve. The physics-based estimation requires determination of the roughness coefficient within the permanent bed and the floodplain using field observation as both the end of dry and wet season. Furthermore, the study demonstrates that variations in the density of GCPs beyond an optimal number (9) has no significant influence on the resultant rating relationships. Finally, the study observes that it depends on the magnitude of the flow which state variable approximation (water level & river width) is most promising to use. Combining stage appropriate proxies (water level when the floodplain is entirely filled, and width when the floodplain is filling) in data limited environments yields more accurate discharge estimations. The study was able to successfully apply low cost technologies for accurate river monitoring through hydraulic modelling. In future studies, a larger amount of in-situ gauge readings may be considered so as to optimise the validation process.
Hubert T. Samboko et al.
Status: final response (author comments only)
- RC1: 'Comment on gi-2022-21', Anonymous Referee #1, 10 Feb 2023
- RC2: 'Comment on gi-2022-21', Anonymous Referee #1, 10 Feb 2023
RC3: 'Comment on gi-2022-21', Anonymous Referee #1, 10 Feb 2023
AC1: 'Reply on RC3', Hubert Samboko, 13 Feb 2023
- RC4: 'Reply on AC1', Anonymous Referee #1, 20 Feb 2023
- AC1: 'Reply on RC3', Hubert Samboko, 13 Feb 2023
RC5: 'Comment on gi-2022-21', Anonymous Referee #2, 24 Feb 2023
- AC2: 'Reply on RC5', Hubert Samboko, 21 Mar 2023
Hubert T. Samboko et al.
Python Scripts https://doi.org/10.4121/21557148
Hubert T. Samboko et al.
Viewed (geographical distribution)
This paper proposed a method for estimating river flow rate using a 3D flow model and field data. The objective is clear and important. However, the methods and composition of methods are quite common and practical, so a scientific novelty required for an original scientific paper is not enough in the present form.
I am afraid I misunderstand some details, but in my opinion, main scientific and engineering weaknesses of the hypothes of the manuscript are as follows:
1) Use of one rating curve for a channel consisting of a low-flow channel and floodplain (river engineers use two or more rating curves for such a river section better to explain the water level/flow rate relation)
2) Superiority of 3D model compared with 1D model sounds not fair (HEC-RAS had overestimated in Figure 9 for high flood but not sure how the 1D model was calibrated. In the Annex, it was said that the roughness was calibrated for high flow. If so, why was it overestimated in Figure 9? I think a 1D model can account for combined roughness parameters (not confident HEC-RAS can do, but accounting for the combined roughness of a compound channel with 1D flow model is standard.)
Point-to-point (mainly minor) comments:
L15: suggest adding 'multi-beam' before echo sounders (if you want to
include single beam one, use parenthesis)
L24: 'determine how .... methods' incomplete sentence?
L25: 'the' hydraulic model?
L29: Meaning of 'physics-based' should be defined in the abstract if use it in the abstract. (Of course, we can guess that author is thinking 3D flow modelling is physics-based, but 1D flow model can be said physics-bases since the shallow-water equations are based on the Navier-Stokes equations which is used for 3D flow model.)
L30: permanent -> stable or immobile (or some others?) ('permanent' sounds something like (very rigid) bedrock but the target site seems sand bar)
L34: 'is most promising to use' A bit vague and logical flow of the sentence is not clear.
L36: remove 'b' before 'hydraulic'
L44: 'implement' would come prior to 'validate'?
L45: flow rate would be one of the most important inputs for the flow model (maybe authors see the flow rate as output, but such standpoint is not explained yet).
L59: 'It is within this technological gap....' hard to read?
L65: 'The process of applying' -> 'Distributing and surveying'?
L81: The sentence seems incomplete?
L91: 'within' ->'for' (not confident)
L92: 'a number of times until' -> 'with different flow rates'?
L99: Combining DEM (obtained by LiDAR or photogrammetrically) and bathymetry (obtained by echo-sounding) are quite common in river engineering, so maybe introducing not only authors' output but other works would be good.
L122, 'the 2 other sites' a bit vague? 'the two sites discussed in previous works (one or two citations)'?
L228: add 'software' before 'D-Flow Flexible...'?
L132: 'in thickness' -> 'their thickness'?
L162: 'point cloud' suddenly appeared.
L164: 'does not affect the water levels' is it realistic? (For subcritical, non-uniform, varied flow, local water level is affected by the downstream flow, I think. Of course, I understand what you want to say, but you can say it with different expressions)
L164: 'A small selection ... is taken' not clear.