Hydrol. Earth Syst. Sci., 21, 3991–4010, 2017 https://doi.org/10.5194/hess-21-3991-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License. Has dyke development in the Vietnamese Mekong Delta shifted flood hazard downstream? Nguyen Van Khanh Triet1,4, Nguyen Viet Dung1,4, Hideto Fujii2, Matti Kummu3, Bruno Merz1, and Heiko Apel1 1GFZ German Research Centre for Geosciences, Section 5.4 Hydrology, 14473, Potsdam, Germany 2Faculty of Agriculture, Yamagata University, 1-23 Wakaba-machi, Tsuruoka, Yamagata 997-8555, Japan 3WDRG Water & Development Research Group, Aalto University, Helsinki, Finland 4SIWRR Southern Institute of Water Resources Research, Ho Chi Minh City, Vietnam Correspondence to: Nguyen Van Khanh Triet ([email protected]) Received: 4 March 2017 – Discussion started: 6 April 2017 Revised: 26 June 2017 – Accepted: 5 July 2017 – Published: 7 August 2017 Abstract. In the Vietnamese part of the Mekong Delta of 19–32 cm. However, the relative contributions of the three (VMD) the areas with three rice crops per year have been drivers of change vary in space across the delta. In summary, expanded rapidly during the last 15 years. Paddy-rice culti- our study confirms the claims that the high-dyke develop- vation during the flood season has been made possible by im- ment has raised the flood hazard downstream. However, it plementing high-dyke flood defenses and flood control struc- is not the only and not the most important driver of the ob- tures. However, there are widespread claims that the high- served changes. It has to be noted that changes in tidal levels dyke system has increased water levels in downstream areas. caused by sea level rise in combination with the widely ob- Our study aims at resolving this issue by attributing observed served land subsidence and the temporal coincidence of high changes in flood characteristics to high-dyke construction water levels and spring tides have even larger impacts. It is and other possible causes. Maximum water levels and du- recommended to develop flood risk management strategies ration above the flood alarm level are analysed for gradual using the high-dyke areas as retention zones to mitigate the trends and step changes at different discharge gauges. Strong flood hazard downstream. and robust increasing trends of peak water levels and du- ration downstream of the high-dyke areas are found with a step change in 2000/2001, i.e. immediately after the disas- trous flood which initiated the high-dyke development. These 1 Introduction changes are in contrast to the negative trends detected at sta- tions upstream of the high-dyke areas. This spatially different The Vietnamese Mekong Delta (VMD), the so-called rice behaviour of changes in flood characteristics seems to sup- bowl of Vietnam, encompasses an area of 4.0 million port the public claims. To separate the impact of the high- hectares, of which over 2.6 million hectares are used for dyke development from the impact of the other drivers – i.e. agriculture. It accounts for more than 52 % of the national changes in the flood hydrograph entering the Mekong Delta, food production and more than 85 % of the annual rice export and changes in the tidal dynamics – hydraulic model simu- (GSO, 2015). Being a low-lying coastal region, the VMD is lations of the two recent large flood events in 2000 and 2011 susceptible to both riverine and tidal floods, threatening agri- are performed. The hydraulic model is run for a set of sce- cultural production and the safety of people (Schumann et narios whereas the different drivers are interchanged. The al., 2008; Kuenzer et al., 2013; Le et al., 2007; Dung et al., simulations reveal that for the central VMD an increase of 2009; Dung et al., 2011; Van et al., 2012). Climate change 9–13 cm in flood peak and 15 days in duration can be at- and sea level rise are expected to increase the risk, not only tributed to high-dyke development. However, for this area due to flooding but also due to droughts and salinity intru- the tidal dynamics have an even larger effect in the range sion (Wassmann et al., 2004; Khang et al., 2008; Toan, 2014; Hak et al., 2016). Further, the region faces severe sediment Published by Copernicus Publications on behalf of the European Geosciences Union. 3992 N. V. K. Triet et al.: Dyke development and flood hazard in the VMD starvation as a consequence of massive hydropower devel- Recently, Fujihara et al. (2016) and Dang et al. (2016) opment (Fu and He, 2007; Kummu and Varis, 2007; Fu et attempted to quantify the impacts of the high-dyke devel- al., 2008; Kummu et al., 2010; Xue et al., 2011; Gupta et opment on downstream water levels. Both studies derived al., 2012; Manh et al., 2015). In a recent study, Manh et high-dyke areas from Moderate Resolution Imaging Spec- al. (2015) estimated the delta sedimentation and the amount troradiometer (MODIS) satellite images and analysed histor- of sediment reaching the South China Sea to be diminished ical records to detect trends in water levels and to attribute by 40–95 % considering hydropower development scenarios, these trends to the dyke development. Fujihara et al. (2016) climate change, sea level rise and the deltaic land subsidence. investigated the period 1987–2006 at 24 stations in the VMD, Land subsidence becomes a notable problem in many cities while flooded areas were identified for 2000–2007. Dang et in the delta, with an estimated rate of 1–4 cm year−1 (Erban al. (2016) reduced the number of stations to 10, but included et al., 2014; Anthony et al., 2015). four stations in the Cambodian floodplains (CFP) and ex- The population in the VMD has extensive experience in panded the hydrological and satellite data to 2013/2014. Both living with floods. The first major human interventions were studies concluded that high-dyke development in the north- the construction of the Thoai Ha and Vinh Te canals in the ern part of the VMD and the increasing water levels down- 1820s, followed by several water works before and during stream are linked. Dang et al. (2016) also stated that the hy- the colonial period as well as after the reunification in 1975. draulic alterations in the VMD had the largest influence on However, large-scale flood control systems had not been im- water level changes, compared to dam construction in the plemented until the late 1990s (Käkönen, 2008). Starting Mekong Basin, sea level rise and deltaic land subsidence. from 1996, the government of Vietnam constructed a se- However, both studies were not able to quantify the contri- ries of waterways to divert part of the overland flood flow bution of the hydraulic alterations to the observed changes. from Cambodia towards the Gulf of Thailand, followed by Moreover, Dang et al. (2016) noted that the findings were the development of comprehensive dyke systems and hy- prone to considerable uncertainties, notably the uncertainty draulic structures according to the Five-Year Development of the water level data. Possible sources of these uncertain- Plan 1996–2000 for the region (Government of Vietnam, ties are land subsidence, change in reference datum, and in- 1996). The majority of dykes constructed within this pe- strument or observer errors. Building on these findings, our riod were so-called low dykes. Low dykes provide protection study has two objectives: (1) to re-analyse the water level against the early flood peak arriving around mid-July to mid- trends found in the previous studies under explicit consider- August, ensuring the farmers can grow two rice crops per ation of uncertainty, and (2) to quantify the changes caused year by keeping floodwater in the paddy fields after the sum- by the development of the dyke system on water levels in the mer crop. The so-called high dykes were mainly built after VMD by a detailed hydraulic modelling approach, as recom- the disastrous flood in 2000. They were designed according mended by Dang et al. (2016). to farmers’ needs and the demand of the provinces to pro- The first objective is achieved by comparing flood trends at tect the floodplains against a flood as high as in 2000. This the two major gauging stations downstream of the high-dyke protection standard along with the full control of the flow development areas with flood trends at stations upstream of into the floodplains nowadays allows the cultivation of three the development areas. This comparison includes an analysis crops per year in the provinces An Giang and Dong Thap in of the robustness against data errors. To obtain a better under- the upper part of the delta. standing of the trends, changes in flood peak and flood dura- The implementation of high dykes was claimed to benefit tion are analysed, which in combination define the severity the population by providing safety and additional income for of floods in the Mekong Delta (Dung et al., 2015). To quan- the farmers. However, Howie (2005), Käkönen (2008) ques- tify the relative contribution of high-dyke development, we tioned this claim, because impeding floodplain inundation re- update the quasi-2-D flood model developed by Dung et al. duces the input of sediments and thus of natural fertilizers to (2011) and Manh et al. (2014) using the latest comprehen- the paddy fields. This leads to reduced crop yields (Manh sive dyke survey data and high-resolution topographical data et al., 2014) and, consequently, intensified use of agrochem- from the newly available 5 × 5 m resolution lidar DEM for icals and higher costs. Howie (2005) and Käkönen (2008) the VMD.