egusphere-2023-2458 (NHESS)

Title:   Detection of flooding by overflows of the drainage network: Application to the urban area of Dakar (Senegal)

Authors :   Laurent Pascal Malang Diémé et al.

 GENERAL COMMENTS

The paper presents the development of a simple modelling system for the detection of urban drainage network overflow.  The drainage system consists of artificial drainage and storage basins. The model has been developed and applied on the urban outskirts of Dakar, Senegal. It aims to propose a method capable of modeling flows at fine resolution (5m2), over the entire area, and providing a rapid diagnosis of how the drainage network

is operating for design rainfall intensities The steps followed to the development of the modelling system are: i) determination of drainage directions, ii) application of a hydrological model for estimating flows at the outlets of elementary catchments, iii) the implementation of a hydraulic model for propagating these flows through the drainage network and iv) application of a simple storage model for the simulation of retention basins. The network overflow points are calculated as the difference between the calculated flows and the network’s capacity to evacuate them.

My major and minor comments are presented in the next paragraphs.

Major Comments

There are many points that should be clarified before considering the paper for publication.

1. A flow chart of the methodology should be used to present the methodology.   This will help the reader to understand the proposed modelling system.
2. It is not clear to me whether the drainage system (i.e. stormwater drainage network and retention basins) is constructed or it is planned. It is strange to me that the drainage network is a network of open channels of orthogonal cross section. Stormwater drainage network is usually underground and consists of pipes. If the network exists then the dimensions are set and known otherwise the dimensions of the drainage network elements (i.e. pipes and cannals) is a matter of design.  The authors should clarify this issue.
3. In continuation of the previous comment, more information about the study area should be presented, e.g. climate, historical extreme rainfall and flood events, hydrology, DEM, etc.
4. More information about the Kinematic Wave (KW) flow routing model should be given in Section 3.4. The governing equations of KW to be solved should be presented. 
5. Section 3.5. Why a simple linear storage model is not used for water retention structures?
6. All areas have the same soil characteristics as found in the experimental site. It would be more realistic to have a soil map of the area or CN maps to estimate the parameters of SCS rainfall abstractions (or effective rainfall) model.
7. What is the basin response time (Tr)? Is it the time of concentration or the time lag? In Equation 11, please explain what is Tm (transfer time).  Why Tr is not estimated by widely used common and typical equations?
8. Give the general equation of IDF curves as i=CTⁿD^-k
9. How and why a 4-hour rainfall is selected? Is 4 hours the critical duration of a storm? Please explain.
10. Why the spatial distribution of design rainfall is not considered? The same design hyetograph is applied over the study area.
11. A major drawback of the study is that the methodology has not been validated against historical flood events. The results presented are purely theoretical and could be fictional and not representative. The authors should simulate one or two events for validating the method and the modelling system.
12. The authors correctly write the deficiencies of the methodology but they should outlined and discussed these deficiencies earlier in the paper.
13. There are many awkward hydrological terms. Proper hydrological terms should be used.  Some of them are indicated in the minor comments bellow. 
14. English language needs improvement.  In some paragraphs, the English writing is poor.

Minor comments

1. There many improper hydrological terms. For example:
   1. Line 93. “…hydrological production ….” Please revise to “…..flow generation….”
   2. Line 97. “……injected in the model…..” Please revise to “……used as input data to the model…”
   3. Line 199. “…production model…” Please revise to “….hydrological model….”
   4. Line 308. “…..project…” Use the term “design”

And others. 

1. Equation 5. Not “si”. It is “if”
2. Line 180 and elsewhere. What is the OC model? It has not been described.
3. Table 2. It is not understandable.  Use the equation of reservoir level-storage volume-outflow curves.

The presented study falls within the scope of NHESS.  However, the paper is not ready for publication and needs at least major revisions before it would be acceptable for publication in the journal of NHESS. 

1. General comments

This study aims to model a fine-scale run-off model of the urban area and assessment of the response of the storm drainage network (canals and retention basins) to different rainfall events. The methodological approach is based on a preliminary reconstruction of the drainage directions modified by urbanization and the implementation of combined hydrological and 1D hydraulic models calibrated to the city’s urban conditions. My minor and major comments for this manuscript are as follows.

1. Minor comments

Grammatical errors

Line 42 – 44: There is a repeated sentence here.

Figure 1. Please insert the north arrow into the map

1. Major comments

- The title does not match the main objectives of this study. The authors should confirm the aim of this study focusing on the detection of floods or modeling of drainage networks.

- Please insert a flow chart of data processing

- The study should integrate the validation of flood simulation results using accurate reference data.

- Section 3.1. please give a brief explanation about ATHYS modeling that was applied in your study

- What is the source of the DTM data and soil map used in this study? Please explain in detail about the resolution as well as the accuracy of these data.

- Line 155: how the model calculates the average velocity of the flow (Vo) is not specified.

- Section 5 – Results and discussions part also does not match the main objectives of this study. Section 5.1 should robustly specify the results of flood simulation using the ATHYS modeling method as three methodological steps shown in the abstract.