Assessing typhoon-induced compound flood drivers: a case study in Ho Chi Minh City, Vietnam

Abstract

Abstract. We investigate the most severe rainfall event ever experienced in Ho Chi Minh City (HCMC), Vietnam. It occurred on 25 November 2018 when Typhoon (TY) Usagi directly hit HCMC. During this event, there was more than 300 mm of rainfall over 24 h which led to flooding and considerable material damage. We propose an in-depth study of TY-induced, compound flood drivers at a short timescale by focusing on the days before and after the event. We use a set of data analysis and signal processing tools to characterize and quantify both coastal and inland effects on the hydrosystem. We found that TY Usagi made landfall without forming a significant storm surge. The extreme rainfall does not translate into immediate river discharge but presents a 16 h time lag between peak precipitation and peak residual discharge. Nevertheless, increased river water levels can be seen at both urban and upstream stations with a similar time lag. At the upstream river station, residual discharge represents 1.5 % of available rainwater, and evidence of upstream widespread flooding was found. At the urban river station, we assess the potential surface runoff during the event to be 8.9 % of the upstream residual discharge. However, a time lag in peak river water level and peak rainfall was found and attributed to the combination of high tide and impervious streets which prevented the evacuation of rainwater 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 the predominant compound flood driver even during severe, heavy rainfall with tidal fluctuations in river water level and respective discharge much larger than the residuals.

Enhancing discharge estimation from SWOT satellite data in a tropical tidal river environment

“…Tidal fluctuations dominate the totality of the river Saigon from the confluence with the Dongnai river to the outlet of the Dau Tieng reservoir. In fact, at the seasonal scale the river water levels are controlled by the coastal water levels [20].…”

“…The instantaneous river discharge was estimated by applying a stage-fall-discharge (SFD) rating curve adapted from the general Manning-Strickler law (Eq 8), previously tested and validated by [49] and used to predict the total discharge of the Saigon river in [20,30]. The discharge is estimated as follows:…”

“…In addition, HCMC is home to almost 10 million inhabitants and its population grows at about 3% per year with these risks posing a threat to many livelihoods [15]. Several studies consider HCMC as a hotspot of vulnerability to climate change [16][17][18][19][20][21]. However, there has been little comprehensive studies regarding the potential of SWOT in an estuary setting such as the one in this study area.…”

“…As can be seen, tidal fluctuations govern the water level dynamics at all locations. In fact, the Saigon river's levels are influenced by tides up until the Dau Tieng reservoir (Fig1)[20]. The panels 0 to 3 illustrate the spatial spline interpolation (black points) and in-situ measurements (triangles) for the four SWOT passes that would occur during the time period of in-situ measurements.…”