The Intergovernmental Panel on Climate Change (IPCC 2007) reported that during the 21st century, the sea level will rise another 18 to 59 cm due to global warming. The most recent report of IPCC [1
] has shown that global mean sea level will rise between 0.43 m (0.29–0.59 m, likely range; RCP2.6) and 0.84 m (0.61–1.10 m, likely range; RCP8.5) by 2100 (medium confidence) relative to 1986–2005. Sea level rise (SLR) poses one of the greatest threats to the stability of coastal structure and human settlements along coastlines [2
Climate change and its impact on the coastal zone of Vietnam are very serious issues. Results of recent investigations suggest that climate change tends to accelerate geohazards. Therefore, adaptation to climate change has rapidly become an urgent issue. The sea level in the north coast is rising about 2 mm/year [10
]. The number and intensity of tropical cyclones have complicated this change with a tendency of having been much more severe in recent years (2004–2014). Climate change-related geohazards on the coast include erosion, inundation and flooding, saline intrusion, and sea dike instability. Sea level rise, land subsidence, and tropical cyclones are strengthening the coastal erosion [12
]. Each year the exceeding rate of erosion due to sea level rise is 0.1–0.3 m/y. A typhoon landing at high spring tide can cause a disaster of inundation, flooding, and saline intrusion on the coast. The inundation and coastal flooding can lead to the immigration of hundreds of thousands of people. Sea level rise causes larger wave pressure on the sea dikes, making them more unstable in typhoons and storm surges. As a result, a possible compound geohazard can be also caused by climate change. In the near future, coastal dike failures may be expected to occur almost annually due to budget constraints, a lack of information on sea boundary conditions, such as water levels and wave heights, and a lack of suitable design methods [2
]. Therefore, countermeasures must be developed to mitigate the influences of extreme weather events and persistent SLR for coastal protection. Among the alternatives available for countering these threats, applying soil stabilization and soil improvement with geosynthetics are promising strategies for coastal structures that must sustain severe wave action and storm surges [14
The Nam Dinh coast is a part of the Red River Delta, northern Vietnam (Figure 1
). The coast has experienced significant nearshore topographical changes, including both severe sediment erosion and accretion [11
]. Erosion of this coast has been well documented, for example, by Pruszak et al. (2002), Le et al. (2007), and Duc et al. (2012, 2017) [11
]. The coastal erosion in Hai Hau was considered to have started from the beginning of the 20th century (1905) [18
]. During the period 1930–1965, the maximum lateral retreat rate was 22 m/y in the Hai Ly and Hai Chinh communes. The Hai Ly coast was significantly eroded from 1965 to 1985. The average rate during that period was 21 m/y. In the same period, the rates were 5 m/y on the Hai Dong coast and 11 m/y on the Hai Chinh–Hai Thinh coast. The south part of Thinh Long town was accreted. Currently, the shoreline in Hai Dong has been accreted. However, the erosion continues to increase in other segments. The most severe erosion segment is now shifting to the southwest. The erosion is very significant on the coast of Thinh Long town. The shoreline was retreating up to 40–50 m/y from 2003 to 2005 [11
]. Beach relief in the south part of Thinh Long town was lowered by approximately 1.7 m in the period from July 2010 to September 2011.
The erosion in Hai Hau has a close relation to the degradation of the Ha Lan river mouth (the former main river mouth of the Red River system at that time) [17
]. The evidence of Ha Lan mouth degradation can be found at Giao Long and Giao Phong shorelines, which were continuously accreted with rapid rates (reaching up to 100 m/year in some segments during the period 1905–1930). The main river mouth was then shifted to the Red river mouth and the shoreline in Hai Hau was eroded. Alongshore sediment transport resulted in southward transport in winter and northward transport summer [22
]. In total, sediment at the coast is dominantly transported south-westward by the northeast and east waves with the volume of about 0.6–0.8 mil. m3
]. The longshore sediments are deposited in the southward river mouth (Ninh Co river mouth or Lach mouth), causing serious changes of navigation channel. Annually, a large amount of dredging was deployed to maintain the channel. Moreover, lowering of the seabed in front of the dikes in Hai Hau coast resulted in a reduction of sediment transport capacities for all locations, causing serious problem to the sea dike stability [17
Recently, the coastal zone in Nam Dinh province has experienced more intensive sea attacks due to severe typhoons, climate change, and human activities (sand exploitation and engineering measures). This paper defines characteristics and reasons of nearshore topographical changes in Nam Dinh province and its impacts on coastal stability.
3.1. Nearshore Topographical Change
Based on the data of cross-shore profiles (cross sections) were measured over the periods as stated in the above section, six typical cross sections representing the coast of Quat Lam, Thinh Long, and Rang Dong areas are used for analysis of nearshore topographical changes. Each area has two representative sections. The topographical changes of the cross sections are shown in Figure 3
and the positions of the six typical cross sections are shown in Figure 6. The results of overlapping analysis of cross-sections that had the same positions from topographic measurements in August 2009, May 2016, and December 2019 as shown in Figure 3
indicate the following.
In Quat Lam area: The beach has a gentle slope and relatively stable. In the northern part of the revetment (CS1 cross section), the beach is locally eroded right the revetment foot to the elevation of −0.7 m and −1.03 m (NCD). In the southern part of the revetment (CS2 cross section), the beach has gentle slope, right next to the revetment without erosion.
In Thinh Long area: The beach was strongly eroded, especially the beach area adjacent to the revetment in the period of 2009–2019, with the average erosion level between 0.3 m and 2.0 m (CS3 and CS4 cross sections). In the period of 2009 to 2016, the whole section had a strong accretion trend with an average accretion level from 0.35 m to 1.0 m. However, in the period from 2016 to 2019 the beach was eroded on a large scale (almost overwhelmed all of the previous accretion) and especially strongly in the beach area from the edge, with average erosion level ranged from 0.4 m to 1.2 m.
In Rang Dong area: This area had greater and more complicated level of erosion and accretion than in Thinh Long area. The general trend shows the cross sections all eroded, in which the beach 150 m away from the revetment was strongly eroded with average level of 0.7 m to 1.2 m. Specifically next to the revetment foot were eroded the strongest with the level of erosion from 2.5 m to 3.5 m. The width of the more than one-meter-deep eroded beach was about 100 m from the revetment foot to the outside (CS5 and CS6 cross sections).
As recognized from Figure 3
, the coast at some cross sections (CS5, CS6) were silted up to 1.5 m at the distance from the shoreline seawards of about 250 m during the period 2009–2016. One of the main reasons is the shift southwards of Ninh Co river flow direction. The change of sediment flow caused intensive siltation in Rang Dong area during 2009–2011 (Figure 4
b). However, this issue needs more detailed study, which unfortunately cannot be covered by this research. Some data variation may also derive from the topographical interpolation. The cross sections of CS5 and CS6 for this year 2009 were interpolated from the state published topographical map of the scale 1:5000.
Morphological change of seabed around Ninh Co estuary was accessed by comparing seabed topography data between 2009, 2011, and 2019, which is demonstrated in Figure 4
and Figure 5
. There is a slight erosion at Hai Thinh side in period from 2009 to 2011, which was seen by comparing satellite data of 2009 and 2011 (Figure 4
a) and also was found when comparing measured bathymetry data of 2009 and 2011 (Figure 4
b). The change to the submerged morphological seabed was different at the inside estuary area, in front of estuary area, and in front of Rang Dong area, with little deposition, erosion, and huge deposition at these areas, respectively (Figure 4
b). The waterway access channel was constructed with jetties in the year of 2015 and other activities such as sand mining from 2018 to 2019; sand dredging at the access channel have made changes to the morphology of the seabed at area in front of Ninh Co estuary and Rang Dong site. Sand accumulation occurred at the corner made by the jetty with the bank at Hai Thinh site and little accumulation at emerged beach at Rang Dong (Figure 5
a); however, huge erosion happened at the submerged beach of Rang Dong and inside the access channel (Figure 5
b). The bathymetry change would cause change to the hydrodynamics (wave and current) in the area, thus affecting the stability of bank protection work there.
3.2. Hydrodynamic Changes
Four simulation scenarios were set up in which bathymetry of the study area for two time periods of 2009 and 2019 and waterway construction have been used to investigate how waves and currents change when approaching the shore. Boundaries and bathymetry data of scenarios are presented in Table 3
. The purpose of each scenario is given as following: scenario SC1 is used to assess the seasonal change of wave characteristics along Nam Dinh coast in the Northeast monsoon and Southwest monsoon seasons. The two scenarios SC2 and SC3 have been used for comparison of hydrodynamic conditions to assess the impact of the waterway structure and the beach lowering. Scenarios SC3 and SC4 will be compared with each other to assess the impact of sea level rise (SLR) on the hydrodynamic characteristics along the coast.
Once the model was calibrated and verified with the water level, wave, and current measurements at Ba Lat, Quat Lam, and Thinh Long, the wave and current information at the nearshore −5.0 m isobaths was extracted at selected points (P1–P3 in Figure 6
and Figure 7
), which are the observation points for analysis. Statistics of wave characteristics for three observation points during the two seasons of 2018–2019 (in which slope of revetment was broken at Rang Dong, Thinh Long, and Quat Lam) are shown in Table 4
. Wave rose of these points are presented in Figure 6
and Figure 7
A large variation of coastal wave characteristics in the study area can be found both in space and time. According to the space along the coast, the waves in Rang Dong and Thinh Long areas are much higher than in Quat Lam area. Over time of the year, the Northeast monsoon season has larger waves than the Southwest monsoon. During the Northeast monsoon season, in Rang Dong and Thinh Long areas, the mean of significant wave height was 0.45–0.46 m, and the maximum significant wave height was 1.51–1.65 m. In Quat Lam area, the mean was 0.43 m, the maximum was 1.21 m. During the Southwest monsoon season, in Rang Dong and Thinh Long areas, the mean was 0.32–0.35 m, the maximum was 1.17–1.21 m; in Quat Lam area, the mean was 0.27 m, the maximum was 1.08 m.
During the Northeast monsoon season, wave height was large, the percentage of waves was greater than 0.5 m in the Rang Dong and Thinh Long areas was 40.1–42.7%, and Quat Lam area was 19.6%; the angle between the mean wave direction and the mean coastline was about 60 degrees causing large longshore sediment transport leading to erosion. Whereas with the Southwest monsoon, wave height was small, and the percentage of waves greater than 0.5 m in the Rang Dong and Thinh Long areas was 41.5–39.2%, and in the Quat Lam area it was 17.7%; the angle between the mean wave direction and the mean coastline was about 90 degrees (almost perpendicular to the coast) so it did not cause longshore sediment transport, and the coast this season was less changed.
Effects of waterway construction and beach lowering on changing hydrodynamic characteristics were investigated by comparing modeling results between scenarios SC2 and SC3. Wave height at Quat Lam (at point P1) did not change much, where the maximum wave height was 1.36 m in SC3, and 1.35 m in SC2 (0.7%). Wave height at Thinh Long, located in the north site of Ninh Co river mouth (point P2), increased significantly with 1.66 m in SC3 and 1.28 m in SC2. The rise was 38 cm, which was equal to 29.7%. The situation of Rang Dong at south of Ninh Co river mouth (point P3) was quite similar to Thinh Long where the wave height in SC3 increased more when compared to SC2, with 1.74 m and 1.49 m, the difference was 0.25 m equal to a 25% increase (Figure 8
and Table 5
The maximum flow velocity Quat Lam calculated with SC3 scenario was 0.26 m/s compared to SC2 scenario of 0.23 m/s, an increase of 3 cm/s, equivalent to 13%. In Thinh Long area, the maximum flow velocity for SC3 scenario was 0.38 m/s compared to SC2 for 0.32 m/s, an increase of 6 cm/s, equivalent to 18.8%. The maximum flow velocity in Rang Dong for SC3 scenario is 0.40 m/s compared to SC2 for 0.33 m/s, an increase of 7 cm/s, equivalent to 21.2% (Figure 9
and Table 6
Although the protective measures have been intensively carried out in the coast, nearshore topographical changes and coastal erosion are still a seriously occurred in Nam Dinh province. The site investigation of nearshore topography shows the recent changes on the coast. In the north area the nearshore topography is quite stable. Meanwhile, the shoreline in the south part has experienced intensive changes with severe coastal erosion. In Thinh Long area, during the period from 2016 to 2019, the beach was seriously eroded, the cross-section was lowered from 0.4 m to 1.2 m.
The construction of jetties in 2015, and other activities such as sand mining in 2018–2019, and sand dredging at the access channel have made changes to the morphology of the seabed at the area in front of Ninh Co estuary and Rang Dong site. Accretion areas formed nearby the jetty at the Hai Thinh site and at the emerged beach at Rang Dong; however, serious erosion has occurred at submerged beach of Rang Dong and inside the access channel.
Hydrodynamic models show the changes of the nearshore bathymetry increase wave heights, wave pressures on sea revetments, and sea dikes. Therefore, the coast should be reinforced with properly constructed revetment, and nearshore sand exploitation should be halted.