the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Hydraulic response of a tropical urban estuary to a typhoon event: case study of Ho Chi Minh City and typhoon Usagi
Francisco Rodrigues do Amaral
Nicolas Gratiot
Thierry Pellarin
Abstract. We investigate the most severe rainfall event ever experienced in Ho Chi Minh City (HCMC), Vietnam. It occurred on November 25th, 2018 when typhoon (TY) Usagi directly hit HCMC. During this event, there was more than 300 mm in rainfall over 24 h which led to flooding and considerable material damages. We propose an in-depth study of the hydrological response of this urban estuary region at a short time scale by focusing on the days before and after typhoon Usagi. We use a set of data analysis and signal processing tools to characterize and quantify both coastal and continental effects on the hydrosystem. We found that TY Usagi made landfall without forming a significant storm surge. The extreme rainfall does not translate in immediate river discharge but presents a 10 hour time lag between peak precipitation and peak residual discharge. Nevertheless, increased water levels can be seen at both urban and upstream river stations with a similar time lag. At the upstream station, residual discharge represents 1.5 % of available rain water and evidence of upstream wide spread flooding was found. At the urban station, we assess the potential surface run off during the event to be 8.9 % of the upstream residual discharge. Hence, the Saigon river would be capable of evacuating the urban flood water after the event. However, a time lag in peak water level and peak rainfall was found and attributed to the combination of high tide and impervious streets which prevented the evacuation of rain water and resulted in street flooding of up to 0.8 m. Overall, it was found that despite not having a significant storm surge, the coastal tidal forcing is predominant in the hydrodynamics of the river even during severe heavy rainfall with tidal fluctuations of water level and respective discharge much larger than the residuals.
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Francisco Rodrigues do Amaral et al.
Status: closed
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RC1: 'Comment on egusphere-2023-396', Anonymous Referee #1, 23 Apr 2023
In this paper the authors describe the impact of Typhoon Usagi rainfall on the hydrology around Ho Chi Minh City, Vietnam on 25th November 2018. In particular they evaluate the impact of intense rainfall and storm surge, on the hydrological system using tools to characterize these coastal vs surface runoff (‘continental’) contributions. The authors contrast the river, surface runoff and coastal (tidal) responses at two river gauge stations upstream of the Typhoon-struck coastline. The preprint is well referenced and structured, with good quality figures and tables to support the key messages. The hypothesis is given, but objectives have not been stated clearly. References are occasionally missing to back up statements in the text. Some technical terms and conclusions should probably be explained further for a general audience to avoid confusion. Some of the technical aspects in the method (and conclusions drawn from this) were difficult to follow so perhaps would benefit from being clarified/rewritten (e.g. surface runoff assumptions downstream of a dammed area). The standard of English is good, the title reflects the contents of the manuscript also. While the abstract could more concisely describe method and results, in the main body of the paper the methodology, the presentation of results, and the conclusions reached are all satisfactory. I have not checked the statistics contained in the appendix tables - these are accepted ‘as is’. The manuscript requires some editorial assistance from NHESS (some grammar errors noted).
However, the approach taken in this paper is an interesting one, and therefore I believe the manuscript will ultimately contribute something new to the scientific discourse. I recommend accept subject to (quite a few) minor revisions as described below:
Minor revisions
While the Aim of this manuscript is clear (to investigate the precipitation and storm surge impacts from Typhoon Usagi on the local hydrology of the Saigon river, HCMC), the manuscript would benefit from having the objectives clearly stated in the introduction section too (L46-51).
It would benefit the paper to be clearer with terminology, from the beginning of the manuscript, and to use it consistently throughout. Some examples:
- a cleaner differentiation between river levels and sea levels (and river gauge vs tide/sea gauge stations). The phrase ‘water levels’ is a little generic even when discussing data from around a tidally influenced river /estuary.
- What is H / water level? It is not stage (with a datum from the river gauging station), seemingly. Is it depth of water above the (unknown) channel bed level, or head?
- in section 3.6: Water discharge is a phrase that doesn’t translate well - do the authors mean river (fluvial) discharge?
It would benefit the paper to support particular statements with more references. E.g.,:
- L 30 “Vietnam lies within the most active cyclogenesis regions in the world”.
- L38 & L74-76. HCMC is one of the most vulnerable coastal regions in the world to flooding: Why does it rank most vulnerable (More of a certain type of flood hazard than other LECZs? A greater population at risk? More likely to /higher frequency of flooding than other locations?)? It has already been stated that the probability of typhoon occurring in southern Vietnam is not large (L33).
- L47-48 Perhaps introduce the concept of/your meaning of the terminology “coastal and continental effects”.
- L48-50 the sentence beginning “For the first time in a data scarce region, satellite and in-depth measurements were gathered and jointly analyzed during an unprecedented extreme event ...” might require some qualification for two reasons. Firstly there are gauges and data as shown in Fig 1 (is data scarce because it is incomplete?). Secondly, more generally, there are a number of papers that have combined satellite data with (limited) data collected on the ground in areas which are considered ‘data-sparse’ and this is often explored through the lens of extreme flood events as case studies. E.g. Dung et al., 2011 (https://hess.copernicus.org/articles/15/1339/2011/), Kuenzer et al., 2013 (https://www.mdpi.com/2072-4292/5/2/687), Mohammed et al., 2018 (https://www.mdpi.com/2072-4292/10/6/885), Tegos et al., 2022(https://www.mdpi.com/2306-5338/9/5/93 ). Perhaps it is just sentence construction -i.e., it’s the first time this new method has been applied, in a “data scarce” region?
- L72-74. The Trinh et al., 2020 reference I believe refers to the wider Northwest Pacific Ocean being one of the most active regions of the world for Tropical Cyclones [TCs] (~30% of all annual tropical storms), not the South China Sea region. Many of the NWP TCs don’t travel into this smaller area. It would be beneficial to clarify/correct this statement.
– L 74. Please define a typhoon (e.g., wind speeds or category scale) vs a tropical storm.
– L127. Technically there are four categories in Table 2, not three.
– L168. Please define ‘low net discharge’ – i.e. low is relative to what/under what categorization?
– L216. dH is introduced to correct for an unknown datum. How was it derived/calculated?
- datums generally are unstated throughout this paper? What is the mean sea level reference datum - Is that local mean or global mean? Also, in L237-240 – the datum could be provided for the SRTM DEM; this is relevant if (river and coastal) flood levels are measured against these elevations.
- L221-224 – the introduction of K, dt and dz parameters is difficult to understand without context, perhaps rephrase this paragraph to clarify why they are important, what they mean and how they are used to optimise RMSE if this is important to your manuscript.
- Unclear about the statement that river slope explains seasonal variation in discharge rates (L344-346) and lack of discharge response after intense rainfall (L475-479). Perhaps explain a bit more the thinking in these sentences. [Hup-Hdn] should be relatively constant if levels at both locations change by approximately the same amount?
L559-560- “…high tide removing possibility of surface runoff to the river”. I don’t understand this reasoning/sentence. Please explain? Do you mean obscuring the response?
Fig A1 – colorbar units have been cut off.
Citation: https://doi.org/10.5194/egusphere-2023-396-RC1 -
AC1: 'Reply on RC1', Francisco Amaral, 27 Apr 2023
Dear Reviewer #1,
We would like to express our gratitude for the time and effort you invested in studying our manuscript and for the valuable feedback you provided. You may find our response letter attached to this comment.
Thank you and kind regards,
Francisco Rodrigues do Amaral
-
RC2: 'Comment on egusphere-2023-396', Anonymous Referee #2, 02 Jul 2023
- AC2: 'Reply on RC2', Francisco Amaral, 10 Jul 2023
Status: closed
-
RC1: 'Comment on egusphere-2023-396', Anonymous Referee #1, 23 Apr 2023
In this paper the authors describe the impact of Typhoon Usagi rainfall on the hydrology around Ho Chi Minh City, Vietnam on 25th November 2018. In particular they evaluate the impact of intense rainfall and storm surge, on the hydrological system using tools to characterize these coastal vs surface runoff (‘continental’) contributions. The authors contrast the river, surface runoff and coastal (tidal) responses at two river gauge stations upstream of the Typhoon-struck coastline. The preprint is well referenced and structured, with good quality figures and tables to support the key messages. The hypothesis is given, but objectives have not been stated clearly. References are occasionally missing to back up statements in the text. Some technical terms and conclusions should probably be explained further for a general audience to avoid confusion. Some of the technical aspects in the method (and conclusions drawn from this) were difficult to follow so perhaps would benefit from being clarified/rewritten (e.g. surface runoff assumptions downstream of a dammed area). The standard of English is good, the title reflects the contents of the manuscript also. While the abstract could more concisely describe method and results, in the main body of the paper the methodology, the presentation of results, and the conclusions reached are all satisfactory. I have not checked the statistics contained in the appendix tables - these are accepted ‘as is’. The manuscript requires some editorial assistance from NHESS (some grammar errors noted).
However, the approach taken in this paper is an interesting one, and therefore I believe the manuscript will ultimately contribute something new to the scientific discourse. I recommend accept subject to (quite a few) minor revisions as described below:
Minor revisions
While the Aim of this manuscript is clear (to investigate the precipitation and storm surge impacts from Typhoon Usagi on the local hydrology of the Saigon river, HCMC), the manuscript would benefit from having the objectives clearly stated in the introduction section too (L46-51).
It would benefit the paper to be clearer with terminology, from the beginning of the manuscript, and to use it consistently throughout. Some examples:
- a cleaner differentiation between river levels and sea levels (and river gauge vs tide/sea gauge stations). The phrase ‘water levels’ is a little generic even when discussing data from around a tidally influenced river /estuary.
- What is H / water level? It is not stage (with a datum from the river gauging station), seemingly. Is it depth of water above the (unknown) channel bed level, or head?
- in section 3.6: Water discharge is a phrase that doesn’t translate well - do the authors mean river (fluvial) discharge?
It would benefit the paper to support particular statements with more references. E.g.,:
- L 30 “Vietnam lies within the most active cyclogenesis regions in the world”.
- L38 & L74-76. HCMC is one of the most vulnerable coastal regions in the world to flooding: Why does it rank most vulnerable (More of a certain type of flood hazard than other LECZs? A greater population at risk? More likely to /higher frequency of flooding than other locations?)? It has already been stated that the probability of typhoon occurring in southern Vietnam is not large (L33).
- L47-48 Perhaps introduce the concept of/your meaning of the terminology “coastal and continental effects”.
- L48-50 the sentence beginning “For the first time in a data scarce region, satellite and in-depth measurements were gathered and jointly analyzed during an unprecedented extreme event ...” might require some qualification for two reasons. Firstly there are gauges and data as shown in Fig 1 (is data scarce because it is incomplete?). Secondly, more generally, there are a number of papers that have combined satellite data with (limited) data collected on the ground in areas which are considered ‘data-sparse’ and this is often explored through the lens of extreme flood events as case studies. E.g. Dung et al., 2011 (https://hess.copernicus.org/articles/15/1339/2011/), Kuenzer et al., 2013 (https://www.mdpi.com/2072-4292/5/2/687), Mohammed et al., 2018 (https://www.mdpi.com/2072-4292/10/6/885), Tegos et al., 2022(https://www.mdpi.com/2306-5338/9/5/93 ). Perhaps it is just sentence construction -i.e., it’s the first time this new method has been applied, in a “data scarce” region?
- L72-74. The Trinh et al., 2020 reference I believe refers to the wider Northwest Pacific Ocean being one of the most active regions of the world for Tropical Cyclones [TCs] (~30% of all annual tropical storms), not the South China Sea region. Many of the NWP TCs don’t travel into this smaller area. It would be beneficial to clarify/correct this statement.
– L 74. Please define a typhoon (e.g., wind speeds or category scale) vs a tropical storm.
– L127. Technically there are four categories in Table 2, not three.
– L168. Please define ‘low net discharge’ – i.e. low is relative to what/under what categorization?
– L216. dH is introduced to correct for an unknown datum. How was it derived/calculated?
- datums generally are unstated throughout this paper? What is the mean sea level reference datum - Is that local mean or global mean? Also, in L237-240 – the datum could be provided for the SRTM DEM; this is relevant if (river and coastal) flood levels are measured against these elevations.
- L221-224 – the introduction of K, dt and dz parameters is difficult to understand without context, perhaps rephrase this paragraph to clarify why they are important, what they mean and how they are used to optimise RMSE if this is important to your manuscript.
- Unclear about the statement that river slope explains seasonal variation in discharge rates (L344-346) and lack of discharge response after intense rainfall (L475-479). Perhaps explain a bit more the thinking in these sentences. [Hup-Hdn] should be relatively constant if levels at both locations change by approximately the same amount?
L559-560- “…high tide removing possibility of surface runoff to the river”. I don’t understand this reasoning/sentence. Please explain? Do you mean obscuring the response?
Fig A1 – colorbar units have been cut off.
Citation: https://doi.org/10.5194/egusphere-2023-396-RC1 -
AC1: 'Reply on RC1', Francisco Amaral, 27 Apr 2023
Dear Reviewer #1,
We would like to express our gratitude for the time and effort you invested in studying our manuscript and for the valuable feedback you provided. You may find our response letter attached to this comment.
Thank you and kind regards,
Francisco Rodrigues do Amaral
-
RC2: 'Comment on egusphere-2023-396', Anonymous Referee #2, 02 Jul 2023
- AC2: 'Reply on RC2', Francisco Amaral, 10 Jul 2023
Francisco Rodrigues do Amaral et al.
Francisco Rodrigues do Amaral et al.
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