Flood Hazard Analysis under Current and Future Climate Conditions for Ha Tinh Province, Vietnam

Floods draw strong attention globally due to their frequent occurrence and large damage to the environment, the economy, and the loss of human life. Urban and river flooding are the most common flood hazards in many countries worldwide. In addition, the changes on climate variables and hydrology can lead to serious water-related disasters and subsequently threaten the food and energy securities of Vietnam, especially in the coastal provinces where the climate change impacts are more obvious.

To assess the effects of climate change (CC) on local extreme rainfall intensity, urban and river flooding, global and regional climate models are necessary. These models do, however, have limitations in terms of spatial and temporal resolutions. To overcome that problem, statistical downscaling methods have become extremely useful and valuable, if validated. In addition to climate downscaling, numerical water system models are often used to assess flood risk as well as to understand the in-depth problems. These approaches do, nevertheless, require local observations on extreme rainfalls, river discharges, which are frequently inadequate, especially in data scarce regions. One of such data scarce regions is the Ha Tinh province of Vietnam. This research, therefore, explores insights into changes of hydrologically extreme, urban and river flood hazards due to changes in future climate, land use/land cover and sea level rise (SLR) in the Rao Cai river catchment, which is located in the Ha Tinh province of Vietnam.

The first part of this dissertation presents the statistical downscaling applied for Ha Tinh province, Vietnam. The Quantile Perturbation Method (QPM) was applied to a large ensemble of CMIP5-GCMs and SA-CORDEX-RCMs to derive future changes in climatic variables. The relative changes for the future period (2071-2100) compared to the historical period (1960-1990) were analysed for daily rainfall intensity, wet day frequency, temperature variables (Tmax, Tmin, Tmean) and potential evapotranspiration (ETo). The changes were calculated in seasonal/monthly scales and daily quantiles for different return periods. This study shows that in the rainy season, the extreme daily rainfall intensity was projected to increase in the range between 5 and 20%, whereas the wet day frequency was projected to decrease with some uncertainty. Under the 95% upper limit scenario for future rainfall intensities (2071-2100), a 20-year rainstorm in the current climate would become a 2-year storm in the future.  It indicates that future rainfall events will be more intense and there will be more extreme events, causing increased flood hazards in the study region. The ETo was also projected to increase due to the rise in temperature variables (Tmax, Tmin, Tmean) for all climate models. More precisely, the median change values increased between 5 and 10% for ETo and between 5 and 20% for temperature.

The second part of the research assesses the impacts of CC on the hydrological extremes including river peak and low flows. Two lumped conceptual hydrological models, NAM and VHM, were initially set up and calibrated using a stepwise method for the upstream catchment area. Future hydrological extremes of peak and low flows were projected using future rainfall intensity and ETo inputs after statistical downscaling as explained in the previous section. The results show that future peak flows are projected to rise due to an increase in rainfall intensity and a decrease in wet day frequency in the rainy season. This leads to an increase in flood risk, which is already a severe problem in the study area. More precisely, the median change values varied between +5% and +80% for future peak flows. The increasing trend was also observed for future low flow because of increased rainfall intensity during the dry season based on RCM projections. The expected increase in river discharge in the dry months may help to reduce the salt intrusion and water scarcity in the catchment.

The third part of the study analyses changes in river flood hazard due to climate change, SLR and land use change. An integrated hydrological and hydrodynamic model calibration strategy was proposed to overcome the shortage of data and to develop a coupled 1D/2D hydrodynamic MIKE FLOOD model in such a deficient database. The flood map results produced by the 2D hydrodynamic model were compared with satellite imagery-based flood maps and historical flood marks due to the unavailability of historical flood maps. After model validation, both urban (pluvial) and river (fluvial) catchment flooding were then evaluated using the MIKE FLOOD model under different upstream flows, downstream water levels, and SLR scenarios. The study results indicate that due to the increase of rainfall intensity and SLR, by the end of the 21st century the flood volume and inundation area were projected to increase significantly. The flood volume increases by approximately 37.7% and 46.1% for the 100-year return period of RCM RCP 4.5 and RCP 8.5 high scenario, respectively. The inundation area increases by 28% and 33% for the 100-year return period of RCM RCP 4.5 and RCP 8.5 high scenarios, respectively. By the end of the 21st century, under the combined impact of climate change, SLR and land use change, more than 55% of the total catchment is projected to be flooded, corresponding to approximately 800 km2 of inundation area.

The final part of this dissertation introduces the climate change impact analysis on the urban flood hazard in Ha Tinh city. An 1D/2D urban flood model was developed for the city. By simulating rainstorms for specific return periods based on intensity-duration-frequency (IDF) curves, current and future pluvial flash flood hazard maps were generated and compared. The results of this study show a significant increase in flood hazard under the CORDEX RCM RCP 4.5 and RCP 8.5 high scenarios. More precisely, the simulated inundation area for a 20-year return period was projected to increase from 33% for the current climate to 49% and 54% for the RCM RCP 4.5 and RCM RCP 8.5 scenarios, respectively.

Based on the study results, recommended adaptation strategies for flood mitigation for the Rao Cai river catchment and the Ha Tinh city were proposed. In addition to traditional end-of-pipe solutions, sustainable development strategies that apply innovative adaptation measures to integrate water into future spatial urban planning would be highly recommended.

The approaches described in this research are applicable to other study areas, particularly those with limited data availability. They make it possible to obtain more knowledge on climate variables and impacts on urban and river flooding. This knowledge could help water managers and provincial authorities in their decision-making process of spatial planning, disaster preparedness, adaptation, and mitigation measures under climate change conditions.