Adaptive management in Kushiro Wetland in the context of salt wedge intrusion due to sea level rise

Climate change has been revealed to increase the occurrence of natural disasters and to damage ecological systems. In coastal regions, there are many environmentally significant areas from the viewpoint of biodiversity, such as areas registered under the Ramsar Convention. The Kushiro Wetland is the largest wetland and one of the most significant natural systems in Japan, remaining mostly unaffected by human activity. In the wetland, there are many species, which do not tolerate saltwater. When the sea level rises, the communities of freshwater plants and fish may die out due to the extension of salt wedge intrusion along the Kushiro River. Therefore, in this study we make an attempt to understand the impact of sea level rise on the ecological system of the wetland by using a three-dimensional numerical model, which allows us to evaluate the effect of sea level rise on salt wedge intrusion. It is revealed that the National Park area may be affected by salt water when sea level rise reaches about 0.15 m. Also, the maximum sea level rise may induce saltwater intrusion into the Ramsar area. As a result, we proposed an adaptive management strategy for protecting the ecological system of the wetland from salt wedge intrusion.


INTRODUCTION
The third and forth assessment reports from the Intergovernmental Panel on Climate Change (IPCC) revealed that the ocean temperatures and average sea-levels have increased globally (IPCC, 2001(IPCC, , 2007)).Also, climaterelated issues have been revealed to have significant implications for coastal and natural resource management (NOAA, 2006;Nakayama et al., 2010).Solomon et al. (2007) and Dutta et al. (2005) found that climate change increases the occurrence of natural disasters, such as storm surges or flood events, leading to environmental damage, which can in turn increase the impact of natural disasters (Belfiore, 2003;Hirabayashi and Kanae, 2009;Kwak et al., 2012;Ma et al., 2010;Walsh, 2004;Watson, 2008).Sea level rise (SLR) is one result of climate change, with the A1F1 scenario predicting the maximum increase in SLR of 0.59 m and the B1 scenario the minimum increase of 0.18 m (IPCC, 2007).Creel (2003) demonstrated that about 75% of the world's population live within 200 km of a coastline.In Japan, 46% of the total population reside in coastal areas covering only 32% of the nation's land area.Therefore, the severity of impacts from increasing floods or storm surges is expected to be greater due to the large coastal population.Also, from the viewpoint of biodiversity, in the coastal region there are many environmentally significant areas, such as those which are registered by the Ramsar Convention and designated as World Heritage.
The Kushiro Wetland is the largest wetland and one of the most significant natural systems in Japan, remaining mostly unaffected by human activity.It was the first area in Japan to be registered under the Ramsar Convention (Kushiro Wetland Restoration Committee, 2005;Miyake, 2010).Therefore, the Kushiro Wetland requires protection.However, before the Kushiro Wetland was registered under the Ramsar Convention, much of the coastal area had been developed and human activities begun to affect the ecological system.As an example, the 154 km long Kushiro River that flows through the wetland was straightened and the downstream portion protected by dikes in consideration of the surrounding human population, which numbers about 230,000 within the 2,510 km 2 river basin (Figure 1).In its present condition, salt wedges cannot intrude into the Kushiro Wetland and thus do not affect the ecology of the system.However, SLR is expected to lead to the enhancement of salt wedge intrusion and, consequently, to serious damage to the ecological system of the wetland, such as the loss of endangered fresh-water species (Dutta et al., 2012).
This study thus aims to clarify the influence of SLR on salt wedge intrusion along the Kushiro River due to climate change.A three-dimensional numerical model, Fantom3D, was applied to evaluate salt wedge intrusion when SLR occurs (Maruya et al., 2010;Shintani and Nakayama, 2010;Nakayama et al., 2012).Fantom3D is an object-oriented hydrodynamic simulator that through the use of parallel computing is able to clarify the potential for salt wedge intrusion along the Kushiro River due to SLR.Finally, adaptive management was proposed for protecting the region registered by the Ramsar Convention.

METHOD
The three-dimensional numerical model, Fantom3D, was applied to investigate the influence of salt wedge intrusion due to SLR on the Kushiro Wetland.Fantom3D is an objectoriented hydrodynamic model for the analysis of environmental fluid mechanics (Maruya et al., 2010;Shintani and Nakayama, 2010;Nakayama et al., 2012).The computational scheme is similar to Nakayama et al. (2006), where attention was given to analysis of the stratified flow field.One of the key features of the present object-oriented design is the division of a computational domain into smaller domain objects, which enables one to carry out parallel computing and thereby save run-time cost.When we conduct a numerical computation in an enclosed bay or a lake, we need to connect the computational domain to a river or the ocean as a boundary condition.In many previous models, a discharge rate or the change in tidal level was given as a simple boundary condition.However, to model the precise interaction between a river and a lake or the ocean and an enclosed bay, simple boundary conditions, such as vertically uniform salinity, cannot provide sufficient accuracy.Although previous types of hydrodynamic models developed with traditional procedure-based languages, i.e., FORTRAN (77,90,95) and C can realize a specific connection between a river and a lake or the ocean and an enclosed bay, the present object-oriented design can provide generalized connection between any water bodies having different sizes and resolutions without modifications in the source code.Therefore, to evaluate the intrusion distance of the salt wedge in this study, thirteen domains were simply interconnected to express the river system and used with a horizontal mesh size of 10 m and a vertical mesh size of 0.2 m over river widths of 50 m to 200 m and water depths of 1 m to 3 m (Figure S1).
The large eddy simulation model was used for evaluating the vertical eddy diffusivity with a constant horizontal eddy diffusivity of 0.01 m 2 s −1 .Because of the large numerical diffusion due to the advection term, the horizontal eddy diffusivity is generally given as 0 m 2 s −1 .However, since the present computation uses a small horizontal mesh size, 10 m, and the ULTIMATE QUICKEST scheme is applied which can reduce numerical diffusion, we decided to carry out the computation with a horizontal eddy diffusivity of 0.01 m 2 s −1 .The validity of Fantom3D in the Kushiro Wetland was investigated by comparison to field observations (Nakamoto et al., 2011(Nakamoto et al., , 2012)).Also, the validity of Fantom3D was confirmed in the application to stratified flow field (Maruya et al., 2010;Shintani and Nakayama, 2010;Nakayama et al., 2012).It should be noted that salt wedge intrusion was observed in field observations to reach the river mouth only in 2009 (Nakamoto et al., 2011).
Since the main purpose of this study was to investigate the maximum intrusion distance of the salt wedge, we used the minimum discharge at the upstream end from 1980 to 2009 and the largest tidal change observed at Hirosato station from 2002 to 2010.In general, there are two ways to account for tidal conditions in numerical simulations of an estuary.One is to choose the maximum tidal component, such as M1 in Japan, and the other is to use the largest observed tide.As our purpose was to investigate the influence of salt wedge intrusion, we decided to use the largest observed tide, which results in the larger salt wedge intrusion compared to the maximum tidal component.In consideration of SLR of 0.59 m, a low pressure effect was included, which corresponds to a sea-level increase of 0.93 m measured at Kushiro Port on the 8th of October, 2006.Therefore, 1.52 m was chosen as the maximum SLR in this study.Since the periodical change in the salt wedge intrusion was formed 33 hours after the initial condition (corresponding to three days), the computation was carried out for five days by forcing with periodical tidal conditions (Figure S2).The other computational conditions are shown in Table I, in which Case 1 and Case 8 correspond to the present and the maximum SLR conditions, respectively.Present water temperatures and salinity were given as boundary conditions by computing ten-year averages from 2001 to 2010.The main factor that decides the distance of salt wedge intrusion is the difference in density due to salinity in the Kushiro Wetland.However, it should be noted that annual average water temperature may decrease the intrusion distance compared to the water temperature in summer.Therefore, we may need to consider the computational results as the minimum distance of salt wedge intrusion when SLR occurs.

RESULTS AND DISCUSSION
The influential area of salt wedge intrusion along the Kushiro River was divided into three categories (Figure S3).The area where saltwater always exits is expected to affect aquatic plants along the Kushiro River.Therefore, the fully influential area of saltwater, shown as red lines in Figure S3, must be protected.As the distance of salt wedge intrusion fluctuates due to the tidal effect, there is an area where saltwater only intrudes when the tidal level is high.This partial influential area of saltwater, shown as orange lines in Figure S3, is considered to be less protected than the red line area.The blue lines indicate the area that is not affected by saltwater.
In the present condition, Case 1, the 20 psu water was found to reach up to 4 km from the river mouth, and the 10 psu water to 5.4 km (Figure S3 and Figure S4).However, when the minimum tidal level occurs (during ebb tide), all the salt water was found to withdraw from the river mouth, excluding a small amount in a deep pocket located 1 km from the river mouth.In Case 2, the 20 psu and 10 psu water intrude up to 5.5 km and 6.2 km, respectively, which indicates the potential for salt water to reach to the National Park area.In Case 7, the 20 psu and 10 psu water intrude up to 8.5 km and 9.2 km, and salt water remains more than 5 km from the river mouth even during the ebb tide.Therefore, in Case 7 and Case 8, salt water was found to reach the Ramsar area and may affect the National Park area.Interestingly, the type of mixing associated with salt wedge intrusion around the river mouth was the same in all computations, since the water is deep enough relative to the tidal amplitude for salt-wedge type intrusion.However, since the river suddenly narrows at 3.5 km from the river mouth, strongly-mixed intrusion was found to occur in the upstream region from 3.5 km.Also, the width suddenly becomes narrower again at 8 km from the river mouth, which suppress the salt wedge intrusion.
In the Kushiro Wetland, major species along the Kushiro River are Sparganium erectum ssp.stoloniferum, Sparganium simplex, Calamagrostis langsdorffii, Phragmites australis, Phalaris arundinacea and Artemisia montana, respectively.The above mentioned species, including trees, make nine major communities along the Kushiro River (Table II).As Calamagrostis langsdorffii, Phragmites australis, Phalaris arundinacea and Artemisia montana are known to be salt tolerant, salt wedge intrusion due to SLR may not affect them.In contrast, Sparganium erectum ssp.stoloniferum and Sparganium simplex do not have tolerance to salt water, and thus may not be able to adapt to changes in the environment and may die out under maximum SLR.Since Phragmites australis and Phalaris arundinacea in particular have high tolerance to salt water, they may be selected naturally instead of Sparganium erectum ssp.stoloniferum and Sparganium simplex due to SLR.Trees also do not have high tolerance to salt water.The maximum SLR may thus lead to a less biodiverse vegetation with the loss of significant species in the Kushiro Wetland, especially along the downstream portions of the Kushiro River.
Therefore, among the nine major species along the Kushiro River, five community types may die out due to the salt water intrusion (Table II).The expected dominant species are shown in Figure 2 when the maximum SLR occurs (Case 8).The decrease in aquatic plant diversity is expected to reduce fresh-water fish from the river mouth to about 9 km.Therefore, to protect the present environmental conditions, it may be necessary to suppress salt wedge intrusion.There are several ways to protect the wetland, such as a protection dam for salt wedge intrusion or cut-off Table I.Discharge, salinity, water temperature and tide at the downstream and upstream ends of the computational domain for Case 1 to Case 8.It should be noted that discharge relates to the upstream boundary and water level relates to the downstream boundary.

Setting value
Setting basis Target section KP 0.  wall, which can inhibit saltwater penetration from the Kushiro River to the Kushiro Wetland.However, as there is uncertainty in the prediction of the effects of SLR, the above-mentioned methods may not prove necessary.For an example of the uncertainty, SLR may only be 0.15 m, which corresponds to the minimum SLR (Case 2).Given the uncertainty, and the fact that the above-mentioned methods may cause other environmental problems such as the suppression of the upstream movement of seawater fish, we made an attempt to propose adaptive management for the protection of the ecological system of the Kushiro Wetland (Figure 3).Our main purpose was to protect the area registered by the Ramsar Convention.However, protection of the ecological system in the wider National Park area is also necessary from the holistic viewpoint of biodiversity protection in the wetland (Figure 1).It was found from the numerical computations under Case 2, which is likely to happen in the near future, that saltwater reached the National Park area but not the Ramsar area.Therefore, as a first step in the adaptive management, we suggest installing a minimal damage protection dam to prevent salt wedge intrusion into the wetland's ecological system (Figure 3).The characteristic of the minimal damage protection dam is that it would not change the environment very much compared to a fully installed protection dam or cut-off wall.In this first stage, therefore, the ecological system of the Kushiro Wetland could be protected from salt wedge intrusion without damaging the environment around the wetland.
If the sea level does not rise more than Case 3, no further protection of the Kushiro Wetland would be necessary.However, if the sea level rises more than Case 3, we would need to move to the second step: the protection of the Ramsar area, which is the main focus of our study.In this second stage, we need to stop the salt wedge intrusion into the Ramsar area by installing a full protection dam or a cut-off wall, which can inhibit saltwater penetration from the Kushiro River to the Ramsar area.Although the installation of the full protection dam or the cut-off wall may change the environment around the Kushiro River, it could protect the wetland area registered by the Ramsar Convention even if the maximum SLR eventuates.

CONCLUSION
To propose adaptive management in the Kushiro Wetland in the context of salt wedge intrusion due to SLR, the potential intrusion distance of the salt wedge was investigated using a three-dimensional numerical model, Fantom3D.Findings suggest: 1) The National Park area may be affected by salt water when SLR reaches about 0.15 m.
2) The maximum SLR may induce saltwater intrusion into the Ramsar area.Therefore, a full protection dam or a cut-off wall which can inhibit saltwater penetration from the Kushiro River to the Ramsar area would need to be installed if maximum SLR occurs.
3) The potential to ameliorate the influence of SLR on a wetland ecological system using adaptive management is shown.

Figure 1 .
Figure 1.Elevation around Kushiro Wetland and Kushiro River basin.Yellow and pink lines show Ramsar and National park areas, respectively.Green lines show Kushiro River basin.

Figure 2 .
Figure 2. Expected distributions of plants along Kushiro River under the largest SLR (Case 8).

Figure 3 .
Figure 3. Adaptive management in terms of salt wedge intrusion obtained from three-dimensional numerical computations.Phase 1: Protect National park area by installing minimal damage protection dam for preventing salt wedge intrusion.Phase 2: Change the area of conservation focus from the National park area to Ramsar area.No action is carried out.Phase 3: Protect Ramsar area by installing full protection dam or cut-off wall for preventing saltwater intrusion.

Table II .
Major community, classification and tolerance to saltwater of plants in the Kushiro Wetland.