the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Outburst flood scenarios and risks for a rapidly growing high-mountain city: Pokhara, Nepal
- 1Institute of Environmental Science and Geography, University of Potsdam, Potsdam, Germany
- 2Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Potsdam, Germany
- 3Département de Géographie, Université Paris-Cité, Paris, France
- 4Institute of Geosciences, University of Potsdam, Potsdam, Germany
- 1Institute of Environmental Science and Geography, University of Potsdam, Potsdam, Germany
- 2Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Potsdam, Germany
- 3Département de Géographie, Université Paris-Cité, Paris, France
- 4Institute of Geosciences, University of Potsdam, Potsdam, Germany
Abstract. Pokhara (c. 850 m a.s.l.), Nepal’s second largest city, lies at the foot of the Higher Himalayas and has more than tripled its population in the past three decades. Rapidly expanding built-up areas are high in demand for construction materials and several informal settlements cater to unregulated sand and gravel mining in the Pokhara valley’s main river, the Seti Khola. This river is fed by the Sabche glacier below Annapurna III (7,555 m a.s.l.), some 35 km upstream of the city, and traverses one of the steepest topographic gradients in the Himalayas. In May 2012 an outburst flood caused > 70 fatalities and intense damage along this river and rekindled concerns about flood-risk management. We estimate the flow dynamics and inundation depths of outburst flood scenarios using the hydrodynamic model HEC-RAS. We simulate the potential impacts of peak discharges from 1,000 to 10,000 m3 s-1 on land cover based on high-resolution Maxar satellite imagery and OpenStreetMap data (buildings and road network). We also trace the dynamics of two informal settlements near Kaseri and Yamdi with high potential flood impact from RapidEye, PlanetScope, and Google Earth imagery of the past two decades. Our hydrodynamic simulations highlight several sites of potential hydraulic ponding that would largely affect these informal settlements and sites of sand and gravel mining. These built-up areas grew between three and twentyfold, thus likely raising local flood risk well beyond changes in outburst hazard. Besides these drastic local changes, about 1 % of Pokhara’s urban built-up area and essential rural road network is in the highest hazard zones highlighted by our outburst simulations. Our results stress the need to adapt early-warning strategies for locally differing hydrological and geomorphic conditions in this rapidly growing urban watershed.
Melanie Fischer et al.
Status: final response (author comments only)
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RC1: 'Comment on nhess-2022-64', Adam Emmer, 14 Apr 2022
General comments:
This study deals with the identification of exposed elements (buildings, roads) to (virtual) outburst flood scenarios in the city of Pokhara, Nepal. The authors exploited OpenStreetMap and manual mapping of built-up areas from remote sensing images and overlaid these layers with spatial extents of outburst flood scenarios simulated in HEC-RAS software. The authors repeatedly mention that the city is threatened by an outburst floods, but (nowadays) there are no lakes which could burst located upstream in the Sabche cirque. Therefore, the reader may wonder what could be the source of such flooding? I see a good argumentation with the 2012 flood, but it was also not an outburst flood according to the description provided by the authors (rather a highly mobile ice-rock avalanche which transformed into the hyper-concentrated flow; perhaps somewhat similar to the 2021 Chamoli disaster; 10.1126/science.abh4455). Overall, the flood scenarios used are not well-justified and would benefit from better linkages to the past floods and potential future flood sources. For instance, why using 1,000 to 10,000 m3/s (1,000 m3/s step) and not e.g. 1,000 to 5,000 m3/s (500 m3/s step) if the 2012 flood corresponds to 3,700 m3/s in Kharapani. And it was likely less in Pokhara I guess – to put your flood scenarios in the context, could you also use HEC-RAS to estimate the 2012 peak in the city? If it was the largest recent flood, it could provide good guiding value for comparison and scenarios justification. Future flood scenarios should be better connected to potential sources of these floods in my opinion. Possible flood sources are very briefly touched in only one paragraph of discussion section 5.2, but I’m convinced it deserves more attention. My suggestion to the authors is to elaborate bit more on the potential source(s) of their otherwise virtual flood scenarios (sub-glacial outburst (?), glacial surge-indiced damming of the valley (?), outbursts of possibly landslide-dammed lakes (?), transformed ice-rock avalanche (?)). Could (some of) these processes lead to the impoundment / generation of enough water for 10,000 m3/s in the 30 km far city? The latter seems the most likely to me (also in the light of the 2012 event), but than it is not an ‘outburst’. And so I suggest to re-consider and check the use of the word ‘outburst’ in this context as it could be terminologically misleading (similarly, the use of the word ‘risk’). Considering the actual content of the manuscript, my suggestion for possibly revised title would be ‘(Extreme) flood scenarios and exposed areas in a rapidly growing … ’, or similar.
Specific comments:
Fig. 1: please consider displaying topography (basic contour lines or cross-profiles at selected locations) in this figure
L99-100: please provide more details on your field mapping of sediment traces; how was it integrated with the overall workflow (Fig. 2)?
L107-108: I wonder what is the justification for using the steady flow HEC-RAS mode while it also offers unsteady flow mode which may be more suitable for this type of events characterised by limited though suddenly released total flood volume and substantial attenuation?
L126-127: this is not clear – did you use your field cross-profiles to enhance (manipulate) ALOS DEM? Or how these two are integrated in the study? Please provide more details on your methodology
L138-146: please consider summarizing these LC classes in table rather than in the text
L156-158: this is confusing; why don’t you name the hazard classes according to the peak discharge?
L164-165: it would be good to map sediment extent also before the 2012 event, so you could display the change of sediment extent associated with the 2012 flood in your Fig. 3
L171-172: simulated peak discharge in Kharapani is 3,700 m3/s, but on L41 you mentioned peak discharge 8,400 m3/s; please comment on this discrepancy
Fig. 6: please show the complete SE bank in this figure (so the flood extents do not terminate in the air)
Fig. 7: the inset map seems to display something different to the main map (or some LULC classes are not shown for some reason (?))
Table 1: it is a little mystery why there is a fluctuation (not gradual increase) in exposed areas for some of the LULC classes (for instance developed-medium: 153,446 m2 for HC1; 30,214 m2 for HC5; 50,432 m2 for HC6; 18,408 m2 for HC8 and 49,981 for HC10); in other words – why is the impact area of certain LULC not always the highest for the largest peak discharge scenario? Or does it show a difference to the previous peak discharge scenario (hazard class)? Please clarify.
Fig. 8: this is confusing and hard to read and I’m not sure this is the correct graph type to be used; what is the link between this graph and Table 1? What is the actual meaning of the area on y axis (part (a))? For instance, when I sum up the areas for exposed grassland and all scenarios (Table 1), the total area is something about 2,4 km2; here the total area of exposed grassland approx. 17 km2 which is confusing; the largest peak discharge scenario is shown at the bottom (I would expect starting with the 1,000 m3/s, so it can be deduced how much area is exposed for individual scenarios);
Table 2: please unify the naming of your hazard classes with the rest of the manuscript
Figs. 10 and 11: what are those strange geometric linear clusters of buildings exposed to hazard class 1-3? What about that cluster located far (and disconnected) from the river?
L282: ok, here you mention that you manipulated the DEM – please provide more details in the methods section
L289-290: ok, here is the possible answer to geometric clusters, but would that really be captured by HEC-RAS in this way (can it capture subsurface drainage in this mode)?
L337: the recommendations outlined here are very general (probably applicable anywhere) and not really providing a solution for the maintenance failure issue with the previous EW system; moreover, they are in detail elaborated by Thapa et al. (2022); I suggest revision or removal
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To sum up, I think this study might be of interest for the readers of NHESS, but I also believe it would benefit from some (rather major) revisions.
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RC2: 'Comment on nhess-2022-64', Anonymous Referee #2, 15 Apr 2022
The manuscript by Fischer et al. reports on the flood impact assessment over Pokhara, Nepal by defining a set of peak-discharge scenarios as indicators of flood hazards. The authors derive a set of flood information maps by using the HEC-RAS model. Based on the inundation area, they overlay various categories of land-use class, and built-up area to determine the exposure. I believe the manuscript might be useful to the readers of NHESS, water experts, and those working on various aspects of disaster management. As I can see, a fair bit of work has been carried out, however, the current form of the manuscript lacks sufficient novelty and several vital information. Moreover, the usage of a few terminologies such as ‘outburst’, and ‘risk’ need justification. Details on the numerical aspects of the flood model set-up, which is vital in justifying the impact assessment are also missing in the text.
Below are my major and minor comments on the manuscript that the authors might consider for revision and re-submission-
1. Introduction- In the current form, the introduction projects more or less about the study region, and flood incidences. It is understandable that the focus of the study is on a mountainous region, however, following a generic (or top-down) approach to flood risk, and other flood-related issues may be desirable. A few statistics on concomitant climate change impacts may also be added here to show the severity of the flooding events.
2. Figure 1- Please add an appropriate legend to describe what the triangles (stations) represent. An inset map of the elevation\topography of the study area may be included within this figure as well.
3. Section 3.2- Not enough justification is provided on the selection of the ten peak discharge scenarios as inputs to the HEC-RAS model. Moreover, why did the authors consider a range between 1,000 and 10,000 m3/s? Please elaborate.
4. In continuation to the previous query, a major discrepancy arises with the class intervals (1000 m3/sec) between each peak discharge. What if there is a peak-discharge falling in the mid-way of two end values, which may not be incorporated appropriately within the flood model, but will add up the impacts on the communities.
5. Line 124: Vertical resolution of ALOS-DEM should be mentioned.
6. Line 127: The authors mention the consideration of around 572 cross-sections of the river channel. A separate figure providing these details may be provided, if possible in the supplementary material.
7. Line 141: The description of the land-use classes is not required to be added to the text. This may be provided in the form of a separate figure in the supplementary material.
8. Details of the time step of the HEC-RAS model simulation, final resolution of flood inundation maps, and courant number must also be added in section 3.2. Further, the justification of considering ALOS DEM (which is a freely available global product) as the bathymetry map for the study area may also be added, as sensitivity (if any) from the DEM will be reflected as inaccuracies in the set of flood inundation maps.
9. Line 164: How was the extent of sediment deposition quantified for the May 2012 flood event from the satellite imagery? How this piece of information was useful to the research addressed in this manuscript? Please justify.
10. The description of “Hazard” in the manuscript is ambiguous. Hazard indicates the severity of an event and is usually represented in terms of floodwater depth, velocity, the residence time of floodwater, etc. As a result of which, directly attaching the discharge scenarios to different levels of hazards is a very preliminary attempt. In another way, authors might consider terming them as low to high hazard classes rather than providing hazard classes as such.
11. Figure 7: The description of hazard classes within various land use classes is very difficult to locate. Some sort of different representation may be thought of here to locate the degree of hazard distinctly within land-use classes or create a separate figure for the same.
12. How did the authors carry out calibration and validation of the flood inundation outputs? Without this, the impact assessment over various land-use classes does not seem fitting.
13. At several places in the manuscript, the term ‘outburst’ flood appears misleading as there is no mention of the temporal dynamics of the flooding event. I request the authors to either justify or remove the ‘outburst’ term wherever it is mentioned in the manuscript.
14. The list of recommendations provided in the manuscript is very generic and applicable to any other case study. I suggest the authors be very specific and structure this section into possible structural and non-structural recommendations for flood management.
15. I am not fully convinced with the title of the manuscript over two points- ‘outburst’, and ‘risks’. The query regarding the usage of the former terminology is already mentioned in one of my earlier comments. The manuscript actually does not quantify ‘risk’, as it does not account for vulnerability as such. The impact assessment addressed in the work is more of an exposure assessment. Therefore, the usage of ‘risk’ terminology may be avoided in the title and elsewhere in the text. Moreover, the hazard is quantified as the extent of the inundated area, which is a very simple form of indicating a flood hazard.
Melanie Fischer et al.
Melanie Fischer et al.
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