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Impacts of vegetation changes on the hydraulic and sediment transport characteristics in Guandu mangrove wetland

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dominated by Cyperus malaccensis Lam. and Phragmites communis (L.) Trin., have changed into a swamp habitat, dominated by Kandelia candel (L.) Druce. The coverage area of K. candel has increased from 0.04 ha in 1978 to 20.75 ha in 1994. The

Kandelia habitat was more salty and located at a higher ground surface elevation than was the P. communis habitat. Variations

of the water surface elevation and reduction of the channel conveyance due to increase of the coverage area of K. candel (L.) Druce were also obtained in this study.

A horizontal two-dimensional model, TABS-2, was applied in this study to simulate the hydraulic and sediment transport characteristics of this estuary wetland. Four cases with different removal ratios show that water surface elevation decreases as the removal ratio increases. When the removal ratio of Kandelia reaches 20%, variations of the water surface elevation in the wetland became insignificant. Significant sediment deposition occurs due to the extensive root network of Kandelia. The average deposition is about 33 mm during a 200-year return period flood event. Removal of Kandelia reduces the sediment deposition rate. When the removal ratio reaches 20%, the reduction in sediment deposition is about 5 mm. Considering the factors of flood protection and sediment deposition, the optimal removal ratio is between 10 and 20%. It also found that mangrove removal improves the ecological restoration of Uca (Thalassuca) Formosensis Rathbun, an endemic species of the fiddler crab in Taiwan. © 2004 Elsevier B.V. All rights reserved.

Keywords: Kandelia candel (L.) Druce; Mangrove; Wetland; Removal ratio; Hydraulic simulation; Sediment routing; Ecological restoration

Corresponding author. Tel.: +886 2 23917278;

fax: 886 2 23917278.

E-mail addresses: leehy@ntu.edu.tw (H.-Y. Lee), d90521017@ntu.edu.tw (S.-S. Shih).

1 Tel.: +886 2 23630231 2408x36; fax: +886 2 23631558.

1. Introduction

Coastal wetlands are widely distributed in estuar-ies throughout the world. Because of the tremendous volumes and varieties of inhabiting plants, they are re-garded as being among the most productive ecosystems

0925-8574/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2004.07.003

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Fig. 1. The location map of the Guandu mangrove wetland. The study site located near the confluence of Keelung River flowed to west direction and Tanshui River flowed to north direction. The northern and southern parts of the tidal creek were the Guandu Nature Park and Guandu mangrove wetland, respectively.

(Teal, 1962; Day et al., 1989). They are known to pro-vide food sources and diverse habitats for large num-bers of resident and migratory organisms. Wetlands have also been shown to be very efficient in removal of nutrients from agricultural runoff (Com´ın et al., 2001). The Guandu Natural Reserve (25◦07N, 121◦027E) lies in the west of the Guandu floodplain, which is located in the confluence of the Keelung and Tanshui Rivers, approximately 10 km from the river mouth, near Taipei, Taiwan. The Guandu wetland near the Keelung River is a semi-diurnal tidal regime with tidal ampli-tude of approximately 1–2 m. Water temperatures at Guandu range from 18◦C in February to 28◦C in July. The Guandu mangrove wetland is a typical coastal wet-land, with Kandelia candel (L.) Druce mangrove devel-oping as a result of the tidal action (see alsoFig. 1). The vegetation in the reserve comprises K. candel, Phrag-mites communis (L.) Trin. and Cyperus malaccensis Lam. Recently, due to dike construction and other en-vironmental impacts associated with human activity, K. candel spreads progressively and becomes the main specie in this wetland, forming the mangrove swamps (Lin et al., 2003). The increase of K. candel cover has some adverse effects on the river hydraulic character-istics, including velocity retardation, increase of water surface elevation and sediment deposition.

The Water Resources Agency in Taiwan widened the river cross-section right downstream of the wetland in 1964 and constructed the Guandu dike in 1968. This dike blocks the tidal influx of salt water and therefore soil salinity in the northern part of the Guandu wetland decreased, where ecological development has diverged from the surrounding catchments and affected only by fresh water. In 1986, the Council of Agriculture (COA) in Taiwan declared the Guandu mangrove wetland, an area of around 55 ha to the south of the Guandu dike, to be a nature conservation area. However, the afore-mentioned spreading of mangrove has some negative impacts: (1) increased mangrove cover limiting water-fowl usage of this habitat. The study ofFroneman et al. (2001)points out that vegetation diversity in and around ponds is important in determining their usage by waterbirds. (2) Increased roughness coefficient of the river leading to a higher mean water surface eleva-tion (Chow, 1973; Lee and Shih, 2003). (3) Reduceleva-tion of the flow velocity in the wetland causing aggrada-tions in the mangrove wetland (Li and Shen, 1973). The higher water elevations pose a threat to the resi-dents and property along the river.

The original types of plant in this area are shrubs and trees adapted to the harsh seashore environment, having large and smooth leaves. Moreover, the thick

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in-environment. Some of these plants are oceanic drift plants, whose lightweight and fibrous fruits ride ocean currents and take root in faraway places. Analysis of aerial photographs taken from 1978 to 1994 revealed that the marsh habitat has changed into a forested wet-land habitat. The area covered by K. candel has in-creased from 0.04 ha in 1978 to 20.75 ha in 1994 (see alsoFig. 2).

2. Numerical simulation

A horizontal two-dimensional numerical model, TABS-2, is used to simulate the hydraulic and sedi-ment transport characteristics in this study.

2.1. Horizontal two-dimensional model, TABS-2 The TABS-2 model is a two-dimensional, depth-averaged, finite element hydrodynamic numerical model developed by the Waterways Experiment Station of U.S. Army Corps of Engineers. It computes water surface elevation and horizontal velocity components for subcritical, free-surface flow in two-dimensional flow fields. TABS-2 also computes a finite element solution of the Reynolds form of the Navier–Stokes equations for turbulent flows. Friction is calculated ac-cording to the Manning or the Ch´ezy equation, and eddy viscosity coefficients are used to define turbu-lence characteristics. Both steady and unsteady state problems can be analyzed.

∂v ∂t + u ∂v ∂x + v ∂v ∂y− 1 ρ  εyx∂ 2v ∂x2+ εyy 2v ∂y2+  + g∂a ∂y + g∂h ∂y+ τy= 0 (2) ∂u ∂t + u ∂u ∂x + v ∂u ∂y − 1 ρ  εxx∂ 2u ∂x2 + εxy 2u ∂y2  + g∂a∂x + g∂h ∂x + τx= 0 (3)

where u and v are the horizontal and vertical velocities, respectively,ρ is the density of the fluid, a is the bed elevation, h is the water depth andτxandτyare shear stresses in the x and y directions, respectively. 2.1.2. Equations of sediment routing

Ariathurai et al. (1977) developed the basic convection-diffusion equation: ∂C ∂t + u ∂C ∂x + v ∂C ∂y = ∂x  Dx∂C∂x  +∂y  Dy∂C∂y  + α1C + α2 (4)

where C is the concentration, Dxand Dy are effective

diffusion coefficients in the x and y directions, respec-tively,α1is the coefficient of the source term andα2 is the equilibrium concentration portion of the source term =−α1Ceq.

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Fig. 3. TABS-2 mesh grid of the study site. The TABS-2’s mesh grid of study site. The above and below plots are the Tanshui River System and Guandu mangrove wetland, respectively. The 12 cross-sections site of Guandu mangrove wetland which were used to calculate the bed elevation changes. The numbers of 12 cross-sections were assigned orderly sequentially from upstream to downstream of Keelung River.

Please refer toSMS 7.0 User’s Manual (2000)for more details.Fig. 3illustrates the generated grid. 2.2. Determinations of TABS-2’s parameters 2.2.1. River bed roughness coefficient

The Manning’s coefficient, n, is the most important parameter in open-channel flow computations. This co-efficient was originally developed in 1889 by an Irish engineer, Robert Manning, and was later modified to its present, well-known, form:

V = k nR

2/3S1/2

f (5)

where V is the mean velocity, R is the hydraulic radius, Sf is the slope of the energy grade line and k = 1 in

Metric Units or =1.486 in English Units.

The Manning’s n represents degree of the energy loss caused by the riverbed and riverbank roughness. Many studies investigated this roughness coefficient (Chow, 1973; Fukuoka and Fujita, 1991). In this study, some findings ofWang’s (2001)study were used to

es-timate the values of n and then the numerical model pa-rameters were calibrated and verified using the experi-mental data conduced by theWater Resources Agency in Taiwan (1996).

The Manning’s n increase with increasing return pe-riod flood and reaches a maximum value of 0.283 when the return period flood equals 20 years. The details are provided inTable 1.

2.2.2. Diffusion coefficient and eddy viscosity coefficient

According to the data provided by the User’s Man-ual of TABS-2, the eddy viscosity coefficient in the mangrove wetland and rest of the study sites have been

Table 1

Lists the Manning’s n in four different return period of flood

Return period (year) Manning’s n

20 0.283

10 0.281

5 0.233

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Dy= 0.23 Du∗ (7) where h is the water depth, and u∗is the shear velocity. 2.2.3. Fall-velocity of sediment

Rubey’s (1933) formula is applied to calculate the fall velocity of sphere particles:

w = F  dg  γs− γ γ 1/2 (8) where F = 0.79 for particles greater than 1 mm, g is the acceleration due to gravity, d is the particle size, r is the specific gravity of water and rs is the specific gravity

of sediment particles. For smaller grain sizes

F =  2 3 + 36ν2 gd3(γs/γ − 1) 1/2 −  36ν2 gd3(γs/γ − 1) 1/2 (9) Table 3

The boundary conditions of upstream and downstream in hydraulic simulations Return period (year) Tanshui River (m3/s) Keelung River (m3/s) Hsin-Dan Creek (m3/s) Da-Han Creek (m3/s) Water surface elevation of river mouth (m) 2 6200 1200 2800 2200 1.21 5 10400 1780 4800 3820 1.23 10 13400 2120 6300 4980 1.27 20 16000 2400 7500 6100 1.29

Source: Water Resources Agency, Taiwan, 1996.

2.3.2. Boundary conditions of steady-state hydraulic simulations

Under the steady-state hydraulic simulation case, the upstream and downstream boundary conditions, in-cluding 2, 5, 10, and 20 years return period floods and the river mouth water surface elevation, were cited from the investigations conducted by theWater Resources Agency, Taiwan (1996), as listed inTable 3.

2.3.3. Boundary conditions of sediment transport simulations

The upstream and downstream boundary conditions used in TABS-2’s sediment transport simulations are provided in Figs. 4–6, respectively. The 200 years return period flood hydrograph which has 46 dura-tion hours was set as the upstream boundary condi-tion, as shown in Fig. 4 (Water Resources Agency, Taiwan, 1996). Furthermore, the corresponding river mouth water surface elevation is 2.3 m, during a de-signed 200 years return period flood event. The 200

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Fig. 4. The 200-year flood hydrograph of Tanshui River System (Water Resources Agency, Taiwan, 1996).

Fig. 5. Temporal variations of the water surface elevations at the river mouth during a 200 years flood event (Water Resources Agency, Taiwan, 1996).

Fig. 6. The upstream sediment supplies of the Tanhsui River System, which includes: Keelung River, Hsin-Dan Creek, and Dan-Han Creek, respectively.

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Qs= 4.5235Q1.1392 : Da-Han Creek (10) Qs= 1.7472Q1.4515 : Hsin-Dan Creek (11) Qs= 0.3836Q1.5841 : Keelung River (12)

where Qsis the sediment supply rate and Q is the

dis-charge.

2.4. Mangrove removal cases

Four different removal ratios, namely 0, 20, 30, and 50%, were simulated to investigate the impacts of the mangrove removal ratio on the hydraulic and sediment transport characteristics. The locations of the suggested removal area and 12 cross-sections which were used to calculate the bed elevation changes were shown inFigs. 3 and 7. The spatial variations of the wa-ter surface elevations under four different return period floods were provided inFigs. 8–11. No significant vari-ations were observed when the removal ratio exceeded 20%.

The western part of the mangrove swamp is older than the others, hence the mangroves in this area are taller than those in the rest of the area. Taller trees tend to induce a higher water surface elevation and raise the risk of flooding. Therefore, the mangroves at the western and southern part of the swamp are suggested to be removed first. The suggested removal areas are shown inTable 4.

Spatial variations of the riverbed elevations are pro-vided inFig. 12. The sediment transport simulations revealed that sediment deposition occurred in most of the 12 cross-sections, and thus the riverbed elevation increased after 46 h of simulation. The sediment de-position depth ranged between 10.45 to 122.65 mm. We can conclude that mangroves will cause significant sediment deposition.

Furthermore, sediment transport simulations under four different mangrove removal ratios were also inves-tigated. Riverbed deposition ranged between 10.5 and 122.7 mm, with the average value equals to 33.0 mm. The reduction in sediment deposition is about 5 mm when the removal ratio reaches 20%. Long-term sedi-ment transport simulation is still under investigation. 3.3. Summaries

In summary, the optimum removal ratio is 10–20% according to the variations of the hydraulic and sed-iment transport simulation. The study of Lee et al. (2003) indicated that the spread of K. candel will not only imperil the marsh species, i.e. C. malaccensis Lam. and P. communis but also decrease the mud plate area where is the water birds habitat area.Froneman et al. (2001)points out that vegetation diversity in and around ponds is important in determining their usage by waterbirds. Meanwhile, the endemic specie of fiddler crabs in Taiwan, Uca (Thalassuca) Formosensis Rath-bun (Shih, 1994), has vanished from Guandu mangrove wetland (Chen et al., 2003). Removal of K. candel will

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Fig. 7. The location of mangrove removal area (aerial photograph of Guandu in 2002).

Fig. 8. Spatial variations of water surface elevation with four removal ratios under a 2-year flood event.

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Fig. 11. Spatial variations of water surface elevation with four removal ratios under a 20-year flood event.

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taller and, therefore, this part of mangrove swamp is suggested for removal first. The optimal removal ratio is between 10 and 20%.

3. Sediment transport simulations showed that sedi-ment deposition occurred in most of the 12 cross-sections. After 50 h of simulation, the sediment deposition depth ranged between 10 to 123 mm. 4. Sediment transport simulations with four removal

ratios were also investigated. The results show that riverbed deposition ranged between 10.5 and 122.7 mm, with the average of 33.0 mm.

5. Some mangrove removal is helpful in reducing flood threats and improving other habitat restoration.

Acknowledgement

The author would like to thank Chi-Seng Water Management Research for financial support. The au-thors of this work benefited greatly from discussions with Mr. A.M. Borghuis.

References

Chen, C.P., Hsieh, H.L., Lin, P.F., 2003. Studies of ecological con-servation strategy and investigation on mangroves in Hsin Chu

10, 251–270.

Lee, H.Y., Shih, S.S., 2003. The environment characteristic investiga-tions and the researches of operating tactics in Guandu Natural Reserve (II). No. 486 Report of Hydrotech Research Institute, National Taiwan University, Chi-Seng Water Management Re-search and Development Foundation, Taiwan.

Li, R.M., Shen, H.W., 1973. Effect of Tall Vegetations on flow and sediment. J. Hydraulic Division, ASCE, 99, 793–814. Lin, H.J., Shao, K.T., Chow, W.L., MAA, C.J.W., Hsieh, H.L., Wu,

W.L., Severinghaus, L.L., Wang, W.T., 2003. Biotic communi-ties of freshwater marshes and mangroves in relation to saltwa-ter incursions: implications for wetland regulation. Biodiversity Conserv. 12, 647–665.

Shih, S.D., 1994. Fiddler Crabs. National Museum of Marine Biol-ogy/Aquarium, Taipei, Taiwan, ROC.

Fukuoka, S., Fujita, K., 1991. Flow resistence due to the momentum transport caused by submerged vegetation in the river course, IAHR, pp. 9–13.

SMS 7.0 User’s Manual, 2000. Open-channel flow and sedimen-tation. Environmental Modeling Research Laboratory, Brigham Young University.

Taipei City Government, 2000. The management research report of Guandu Natural Park and Guandu Natural Reserve (II), Taiwan, ROC.

Teal, J.M., 1962. Energy flow in the salt marsh ecosystem of Georgia. Ecology 43, 614–624.

Wang, C.F., 2001. The study of flow resistance in Keelung River mouth. Master thesis, Department of Agricultural Engineering, National Taiwan University, Taiwan, ROC.

Water Resources Agency, Taiwan, 1996. The hydraulic model study of Tanshui River System and Taipei flood protection project, Tai-wan, ROC.

數據

Fig. 1. The location map of the Guandu mangrove wetland. The study site located near the confluence of Keelung River flowed to west direction and Tanshui River flowed to north direction
Fig. 3. TABS-2 mesh grid of the study site. The TABS-2’s mesh grid of study site. The above and below plots are the Tanshui River System and Guandu mangrove wetland, respectively
Fig. 5. Temporal variations of the water surface elevations at the river mouth during a 200 years flood event (Water Resources Agency, Taiwan, 1996).
Fig. 9. Spatial variations of water surface elevation with four removal ratios under a 5-year flood event.
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