the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Brief communication: the potential use of low-cost acoustic sensors in short-term urban flood warnings
- 1Institute of Earth Surface Dynamics, University of Lausanne, Lausanne, Switzerland
- 2Biostatistics and Spatial Processes (BioSP), INRAE, Avignon, France
- 3Department of Urban Water Management, Swiss Federal Institute of Aquatic Science and Technology, Dubendorf, Switzerland
- 4Institute of Atmospheric Sciences and Climate, National Research Council of Italy (CNR-ISAC), Bologna, Italy
- These authors contributed equally to this work.
- 1Institute of Earth Surface Dynamics, University of Lausanne, Lausanne, Switzerland
- 2Biostatistics and Spatial Processes (BioSP), INRAE, Avignon, France
- 3Department of Urban Water Management, Swiss Federal Institute of Aquatic Science and Technology, Dubendorf, Switzerland
- 4Institute of Atmospheric Sciences and Climate, National Research Council of Italy (CNR-ISAC), Bologna, Italy
- These authors contributed equally to this work.
Abstract. Floods in urban areas are one of the most common natural hazards. Due to climate change enhancing extreme rainfall, and cities becoming larger and denser, the frequency, magnitude and impact of these events are expected to increase. Pluvial floods can occur in urban areas within minutes. A fast and reliable flood warning system should thus be implemented in flood-prone cities to warn the public of upcoming floods and save lives and reduce damage. The purpose of this brief communication is to discuss the potential implementation of low-cost acoustic rainfall sensors in short-term flood warning systems.
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Nadav Peleg et al.
Status: final response (author comments only)
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RC1: 'Comment on nhess-2022-257', Anonymous Referee #1, 22 Nov 2022
Review comment for „Brief communication: the potential use of low-cost acoustic sensors
in short-term urban flood warnings“ by Nadav Peleg and Herminia Torelló-Sentelles et al.
The authors present several experiments with low-cost acoustic sensors for urban rainfall monitoring and propose their use in short-term urban flood warning. The experiments consist of one lab and two outdoor experiments testing the accuracy and possible advantages and limitations of these sensors. Overall, these sensors do not yield exact rainfall amounts but rather can complement existing observations by giving begin and end of rainfall events as well as spatial rainfall distribution when a dense network is employed.
The topic of this brief communication suits well into NHESS’ scope and is presented in a good quality. Despite the fact that acoustic sensors were introduced for rainfall measurements earlier as referenced by the authors, here these sensors are brought into perspective with other sensors and the real-world application of urban flood warning. I found some issues in the manuscript which should be addressed by the authors prior to publication which I recommend.
Specific comments:
- Figure 2 - lack of time series
This are two issues which could potentially solved at once. First, Figure 2 is somewhat unintuitive because the same symbols are used to depict different things. Even with the figure caption being informative the Figure would benefit from a revision. Suggestions would be to remove the correlation and rain drop count to (a) new subplot(s) showing e.g. the correlation to rain gauges over distance. Also, the rain gauges from meteoblue could be shown in the map.
Second, I’d really like to see time series of acoustic sensors compared to reference rain gauges. Such time series could illustrate both the correlation presented in Fig. 2 and the text as well as some issues why these sensors cannot be used to derived rainfall amounts directly. Time series plots could be shown as supplementary material, while I would encourage the authors to add them to Figure 2.
- More specific information on deployment
While the whole process chain from the acoustic sensor to a warning system is depicted in the manuscript some description on the acoustic sensors regarding pricing compared to traditional sensors, setup and the setup in a real-time, operational way with many sensors (as envisioned in l. 165) would make this manuscript more inspirational for researchers and stakeholders playing with the thought of experimenting or deploying such systems.
- Questions raised in the manuscript
Two questions are raised at the end of the introduction and answered in the following chapters while a third one which is a mix of both is raised in l. 137. The easiest way to make this more consistent would be to add this third question to the intro, but there might be other ways to solve this inconsistency.
Technical issues
- 24 You could also list examples of ground observations
- 68 I assume you mean 30 acoustic sensors?
- 131 E-band CMLs are also in the magnitude of 10^0 km and can deliver data with sub-minute resolution
Further issues
The article type allows for 20 references while the authors cite 22 references. In my opinion 22 would be fine.
Data availability is not in agreement with NHESS data policy, e.g. “If the data are not publicly accessible, a detailed explanation of why this is the case is required.” The FAIR way of course would be a publication of acoustic sensors data accompanied by reference rainfall data but I can understand if the latter one is not possible due to meteoblue’s data policy.
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AC1: 'Reply on RC1', Nadav Peleg, 01 Dec 2022
We appreciate the reviewer's time and effort as well as their constructive and positive feedback.
As for the specific comments made by the reviewer: (1) we will revise Figure 2 based on the reviewer's suggestions and add another subplot to illustrate selected storms over time; (2) we agree that more information could be provided on the acoustic sensors regarding pricing and setup, and we will add this information to the text; and (3) in order to maintain consistency in the structure of the paper, we will revise the introduction to include the third question.
We are also grateful to the reviewer for pointing out the three technical issues - they will be addressed in the revised version. As suggested by the reviewer, our records of the acoustic sensors will be made public for other researchers to access.
Thank you again and best regards,
Nadav Peleg, on behalf of the co-authors
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RC2: 'Comment on nhess-2022-257', Anonymous Referee #2, 24 Nov 2022
The manuscript presents performance of a drop counting acoustic rain gauge which could according to authors in future contribute to the improvement of short-term flood warning systems. The authors are addressing two questions. First, how accurate are BIG-DRIP acoustic sensors, and second, what are their advantages/limitations in comparison to other rainfall monitoring devices.
The paper presents first results from a an extensive evaluation study of low-cost drop-counting acoustic sensors. The presented results are, unfortunately, insufficient to reach substantial conclusions, especially when it comes to the paper main topic - the evaluation of these sensors to flood early warning. My major criticism concerns both the contents of the paper and the level of detail of the evaluation analysis. Furthermore, the description of data used is not sufficient to allow reproduction and interpretation of the results. Finally, the advantages/limitations of the sensor are discussed only in very general way without stronger link to the presented results and without context with respect to already established acoustic sensors.
To the paper contents: the title and abstract of the paper do not unambiguously reflect contents of the paper. The presented results are insufficient for evaluating the sensors’ potential for flood early warning. The authors discuss this potential in a separate section; nonetheless, the discussion is very general and does not build on presented results. The authors conclude that i) acoustic sensors can significantly contribute to representation of rainfall spatial structure with binary (yes/no rainfall) information, ii) that they can support other devices, e.g. support reconstruction of spatial distribution of rain rate along microwave links, and iii) that they can be used directly in flood forecasting using some (not specified) machine learning technique. However, none of these topics (although being relevant) is specifically investigated in the study. The presented results showing correlations between acoustic-rain-gauge observations (drop counts/time) and near-by standard rain-gauge observations (rainfall intensities) cannot be considered an evaluation of rainfall spatial structure. The authors conclude they see the potential in the use of low-cost acoustic sensors based on their study. Such conclusion is unfortunately not supported by presented results.
To the evaluation analysis: The analysis evaluating accuracy of drop-counting sensors presents only very preliminary results showing i) correlation coefficients between detected rain drop counts and rain rates observed during one event at roof top of Authors’ hub, and ii) again correlation coefficients between detected raindrop counts and rain rates observed by near-by rain gauges during two case studies. Surprisingly the authors do not discuss at all how drop counts can be converted to rainfall intensity and how this is affected by drop sizes. The effect of drop size distribution is not discussed at all. The presented analysis is simply insufficient to soundly address research questions formulated by the authors in the introduction. How accurate is the transformation between drop counts and rain rate in terms of systematic deviation, or error mean square error? How does the accuracy relate to drop size distribution? What are detection limits of drop sizes, how well the sensors perform during heavy rainfall? How accurate is binary information (is rain / no rain)? How well can the sensor detect onset of an event (this is mentioned but not presented)? These (and other) questions would help to formulate more specific conclusion about accuracy of the sensors.
To the description of the data and evaluation methods: The level of detail provided about material and methods is insufficient to ensure reproducibility and enable comparison with other studies. For example, it is not clear how far are the reference rain gauges from drop counting sensors during case studies, what type of rain gauges these are, at which temporal resolution is the data evaluated. There is also no information about events occurring during the evaluated period, etc.
Advantages and limitations of the sensors are discussed in section 4, however, in very general manner. The reader can learn that authors identified relatively strong correlation link between recorded drop counts and rain rates and that the sensor can detect onset of events. The second claim is not supported by provided results (no such evaluation presented). Sensors characteristics related to its operation in longer term, such as energy consumption, are not discussed. Sensor’s performance with respect to already used acoustic sensors (e.g. https://www.vaisala.com/sites/default/files/documents/RAINCAP_Technology.pdf) might be also worth to discuss.
In a view of my criticism, I cannot recommend the manuscript for a publication. I am convinced that more extensive analysis and better evaluation framework is required to reach more specific and scientifically relevant conclusions. Also the scope of the manuscript has to be better defined. The assessment of sensor’s accuracy is a legitimate scope, however, it is not the evaluation of the sensor’s potential for flood early warning.
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AC2: 'Reply on RC2', Nadav Peleg, 01 Dec 2022
We appreciate the reviewer's time and effort as well as their constructive and positive feedback.
In the reviewer's opinion, the study's evaluation analysis and data description are lacking in detail, and the discussion of the advantages and limitations of using acoustic sensors in flood early warning systems remains rather general. We completely agree with the reviewer on this evaluation, but we have to point out that this article is submitted as a “brief communication”.
Our goal here is not to discuss the capabilities of a specific type of acoustic sensor, but rather to highlight to the natural hazard community the potential of using acoustic sensors in general to aid existing or planned urban flood warning systems. We complement our commentary with a short quantitative demonstration of the capabilities and current limitations of one type of acoustic sensor.
We specifically chose the “brief communication” format to provide our "personal" commentary on this specific Special Issue. In fact, as per NHESS criteria, “Brief communications are timely, peer-reviewed, and short (2–4 journal pages). These may be used to […] (b) report/discuss significant matters of policy and perspective related to the science of the journal, including "personal" commentary”, https://www.natural-hazards-and-earth-system-sciences.net/about/manuscript_types.html. We believe this description well fits our goal and our manuscript type.
The referee raises several relevant suggestions that could help us improve our manuscript. In our revisions, we will address many of the points raised by the reviewer while preserving the intended commentary angle and keeping the manuscript concise and within a "brief communication" format:
- We agree that the discussion on acoustic sensors' potential for flood early warning can be more meaningful by demonstrating how well the acoustic sensors record the rainfall spatial structure, which will be included in the revised version of the manuscript.
- We will briefly discuss possible approaches to convert drop counts to rainfall intensity. Apart from discussing the effect of drop size distribution (as suggested), we will discuss the possibility of segmenting rainfall events into rain types (e.g., convective/stratiform) and assigning individual drop-intensity relationships for each type.
- It is noteworthy to point out, however, that part of our message is that such a conversion is not necessarily needed to aid rainfall nowcasting algorithms for early flood predictions.
- The reviewer noted that there is insufficient detail provided about data and monitoring devices to ensure reproducibility and allow comparison with other studies. We will share the data collected by the acoustic sensors in an open repository accessible to all. We will also add additional information on the reference rain gauges (“meteoblue data”), for example, distances to the acoustic sensors and on the type of data recorded.
- We will elaborate on the advantages and limitations of using acoustic sensors, especially on the potential to use these sensors over a longer period (maintenance, energy consumption), as well as sensors available already on the market (such as by Vaisala), as suggested by the reviewer.
- We will revise the title and abstract to more accurately reflect the content of the manuscript.
We are confident that our revisions will strengthen the content of the short communication and deliver our key message, namely that acoustic sensors in general (and not just the specific type we are using) should be further developed as a viable alternative to e.g., expensive rainfall gauges to be used in rainfall nowcasting for flood warning systems.
Thank you again and best regards,
Nadav Peleg, on behalf of the co-authors
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AC2: 'Reply on RC2', Nadav Peleg, 01 Dec 2022
Nadav Peleg et al.
Nadav Peleg et al.
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