INTERACTION BETWEEN HYDRODYNAMICS AND SALT MARSH DYNAMICS: AN EXAMPLE FROM JIANGSU COAST

Transcription

1 Proceedings of the Sixth International Conference on Asian and Pacific Coasts (APAC 211) Deceber 14 16, 211, Hong Kong, China INTERACTION BETWEEN HYDRODYNAMICS AND SALT MARSH DYNAMICS: AN EXAMPLE FROM JIANGSU COAST Z. HU, M.J.F. STIVE, T.J.ZITMAN Departent of Hydraulic Engineering, Delft University of Technology, Stevinweg 1 Delft, 2628 CN, the Netherlands Q.H. YE, Z.B.WANG, A. LUIJENDIJK Deltares, Rotterdaseweg 185 Delft, 2629 HD, the Netherlands Z. GONG State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Xikang road 1, Nanjing, 2198, China T. SUZUKI Ghent University / Flanders Hydraulics Research, Berchelei 115 Antwerp, B-214, Belgiu Salt arshes are distributed along ore than 4 k of the Jiangsu coast in Eastern China, which are regarded as iportant habitats and serve as coastal protection as well. Previous research has proven that salt-arsh vegetation can reduce current velocity and dapen waves by its stes and leaves. Reversely, hydrodynaic forces also have a significant influence on the growth of salt-arsh vegetation. To study the interaction between hydrodynaics and salt-arsh developent on the Jiangsu coast, a 2D scheatized odel has been built by using a new interactive structure between flow, wave and vegetation odules of the process-based odel Delft3D. In the hydrodynaic siulations, the ipact of vegetation on waves and currents is quantified. In the vegetation growth odule, the developent of salt arshes is influenced by inundation tie and shear stress fro hydrodynaic siulations. The feedback loop is copleted by hydrodynaic odules receiving the newly updated data of salt-arsh field fro the vegetation growth odule. The results show that wave height and current velocity are significantly influenced by vegetation. Reversely, the dynaics of arsh vegetation greatly rely on hydrodynaic conditions. Consequently, this interaction between hydrodynaics and salt arsh induces teporal variations of each other. In the odel, the salt arsh is especially sensitive to the waves. Though wave height is relatively sall on the Jiangsu coast, in ters of bed shear stress, waves ay be of great iportance to the developent of salt arsh. 1

2 2 1. Introduction In the eastern China, salt arshes cover about 41 k of the Jiangsu coast, with a axiu width of over 4 k [1]. Salt-arsh vegetation can reduce current velocity [2; 3] and wave energy through salt arsh canopy [4; 5]. Moreover, the vegetation-current feedback can strongly influence each other and the tidal landscape as well [6; 7]. To study the interaction between hydrodynaics and salt-arsh developent on the Jiangsu coast, an integrated wave-flowvegetation odel has been built. Specially, waves are included in the interaction odel to count the ipact of waves on the vegetation and vice versa. Wanggang wetland in the iddle part of the Jiangsu coast is picked as the study area, where salt arshes are widespread. Wanggang wetland is doinated by sei-diurnal tide. The tidal range is 3-4 and the slope of tidal flat is about.55 [8]. The significant wave height over the intertidal flat is.1.2 [9]. The doinant species of the salt-arsh vegetation is Spartina alterniflora [1], which is chosen as the representative vegetation in the odel. 2. Methods The interaction odel has been built within the process-based Delft3D syste in 2D [1]. Delft3D WAVE, FLOW and WAQ odules are eployed to siulate wave-current-vegetation feedback pattern Flow odule In the flow odule, arsh plants are scheatized as rigid cylinders. The effect of vegetation on flow is counted as extra resistance in oentu equation [11]: M uu (1) Where u is the current velocity [/s], λ is the resistance coefficient which is proportional to the product of ste density and diaeter of plant cylinders, [ -1 ] Wave odule In the wave odule, vegetation is also scheatized as rigid vertical cylinders. Wave energy dissipation due to vegetation v is given by [4]: gk sinh k h 3sinh k h 3 v C (2) DbvN H 3 rs 2 2 3k cosh kh Where ρ is the water density [kg -3 ], h is the water depth [], (ah) is the vegetation height [], H rs is root ean square wave height [], k is wave nuber [ -1 ], is angular frequency [s -1 ], b v is average diaeter of plants [], N is the nuber of plants per square eter [ -2 ], C D is averaged drag coefficient [-], v [N -1 s -1 ] is added in spectral action balance equation [12].

3 Vegetation dynaic odule The siulation of the vegetation odule represents intrinsic growth and spatial spreading of vegetation [13]. It also counts ortality caused by inundation and shear stress [6]. Specially, as waves are included in the odel, the bed shear stress is enhanced by wave-current interaction [14]. The ortality due to shear stress is assued to be proportional to the axiu bed shear stress. Vegetation height and ste diaeter are assued to be proportional to the ste density. After each tie step, this odule provides the newly updated space-depended vegetation height, diaeter and density data for the next hydrodynaics siulations. As the dynaics of vegetation acts on longer tie scale than hydrodynaics [15], the vegetation data is updated every 2 days Model setting The flow doain covers an area of 8 k by 8 k, which is set upon a constant slope fro 4 to -12 (as is the ean sea level). Tide is coposed by M2, M4 and S2, with aplitudes of 1.7,.2 and.6 respectively. The wave boundary is set 4 k further seaward than the flow boundary. As sheltered by offshore sand ridges in the southeast [8], waves are prescribed to coe fro northeast (45 ). Locally generated waves by wind are neglected. A B 45 waves N Sea A' M2 S2 M4 B' Figure 1a. Top view of the doain. Figure 1b. Cross-shore profile of A-A'. 1-4 are observation points Obs.1-Obs.4. The initial vegetation patch is placed in the upper intertidal zone with an area of 7 by 2 (Fig 1a). The siulation is carried out fro May to October in the odel, because it is the tie that salt-arsh vegetation grows [16]. The base case is C V3 : 3 eaning 3 c H s fro boundary and V eaning vegetation being taken into account in this case. Siilarly, C N5 eans that in this case H s fro the boundary is 5 c and there is no vegetation. Cross sections A-A' and B-B' represents cross-shore and long-shore directions respectively (Fig. 1a). A-A' is the id-line of the tidal flat. There are 4 observation points (Obs.1 to Obs.4) on it. They are located at the ature arsh, fringe of the arsh, pioneer zone and bare tide flat respectively (Fig. 1b).

4 4 3. Results 3.1. Spatial variations of hydrodynaics Scenario Table 1. Spatial variations of H s and the agnitude e of current velocity ( u ) along cross section A-A' Mean significant wave height H s, Mean current velocity u (c/s) (c) and [standard deviation] (c) and [standard deviation] (c/s) Bare Tidal flat Pioneer zone Fringe of arsh Mature arsh Bare Tidal flat Pioneer zone Fringe of arsh Mature arsh C N3 9.4 [.8] 7.8 [1.2] 6. [2.3] 3.7 [3.] 14.8 [8.8] 1.9 [5.6] 1.3 [5.8] 7.3 [7.2] C V3 8.3 [.6] 6.8 [1.1] 4. [1.8] 1.3 [1.2] 13.8 [8.3] 8.4 [4.4] 4. [1.9] 3. [2.2] C N [1.3] 14.3 [2.2] 1.9 [4.1] 6.7 [5.5] 14. [8.4] 1.1 [5.3] 8.1 [4.1] 5.6 [3.6] C V [1.2] 13.4 [2.] 8.9 [3.5] 2.1 [2.] 13.2 [7.9] 9.1 [5.] 5.1 [2.3] 3. [2.1] C N [1.9] 21.4 [3.3] 16.2 [6.4] 1. [8.3] 13.4 [8.] 9.7 [5.] 7.7 [3.9] 5.6 [3.4] C V [1.5] 19. [2.9] 12.5 [5.] 2.1 [2.4] 12.8 [7.7] 8.7 [4.9] 5. [2.2] 3. [2.1] In the odel, sei-diurnal tides with spring-neap tide cycle are well represented. The ean tide range is 3.5. Flood currents run southward and ebb currents run northward (Fig. 2). The salt arsh is regularly flooded by high tides, which also delivers waves to the tidal flat (Fig. 3). Results show that the presence of the salt arsh induces spatial variations of hydrodynaic forces. The significant wave height (H s ) and the agnitude of current velocity ( u ) are averaged over all the flooding ties through half year. Rearkable reduction of both significant wave height and current velocity is shown in the cases with vegetation (Table 1). Salt arsh Salt arsh Figure 2. Flow velocity field during flood, C V3 Figure 3. H s [] distribution during flood, C V3 Salt arsh vegetation reduces wave energy efficiently. The salt arsh has a considerable ipact on the spatial distribution of H s on the tidal flat (Fig. 3). In the base run with vegetation, the ean significant wave height decreases by

5 5 85% fro bare tidal flat to the ature arsh, while the percentage of decrease is about 6% in the contrasting case without vegetation (Table 1). Moreover, the efficiency of salt arsh dapening waves increases with incident wave height. In case C V1, the H s reduces by 9% through the vegetation field. However, the percentage of H s reduction reains around 6% for all the cases without arsh (Table 1.). When the currents travel through the arsh canopy, the blockage effect of vegetation is significant (Fig. 2). During flood, the long-shore coponent of the ean current velocity along the B-B' is reduced fro 15.7 c/s to 7.3 c/s at the northern edge of the arsh. At the southern edge, it restores fro 4.8 c/s to 14.3 c/s (Fig. 2). Meanwhile, because of its sall agnitude, the crossshore coponent only reduces slightly and it even increases around the northern and southern edges. In the cross-shore direction, vegetation also leads to higher reduction of tie-averaged u fro seaward to landward (Table 1.) 3.2. Salt arsh dynaics Sea Sea Figure 4. Salt arsh developent after half year, the left one is C V3, the right one is C V3, the white dash line indicates the location of the initial arsh patch, and the legend indicates the ste density [ -2 ] The developent of the salt arsh is greatly influenced by the hydrodynaic force. Especially, the wave plays a crucial part in it. In the base run (C V3 ), the salt arsh expands fast, which is siilar to the reality of the Jiangsu coast [1] (Fig. 4). The ature salt arsh (ste density N 6% ax capacity) progresses 1 seaward, with a 2 pioneer zone in front of it. In the long-shore direction, the arsh expands 1 northward and 12 southward. By the end of October, the ean ste density of ature arsh increases fro 5 to about 9-2. Mean height and diaeter of S. alterniflora increases to 1.7 and.95 c respectively. These results are in agreeent with references [1; 17].

6 6 The arsh in the odel is very sensitive to the incident waves. If tieaveraged H s increases to 16.5 c on the bare tidal flat (Obs. 4), the arsh cannot expand seaward. When H s on the bare tidal flat increases to 3 c, the hydrodynaic forces becoe erosive to the initial arsh. With higher incident waves (e.g., 3 ), the arsh will even retreat further to the landward (Fig. 4) Teporal variations of hydrodynaics As the arsh is active, H s and current velocity are also influenced by the dynaics of the arsh over tie. This influence is significant especially on the fringe of the ature arsh and on the pioneer zone, where the interaction is ore intense and distinct. The dynaics of arshes acts on a longer teporal scale than hydrodynaics [15]. To copare the, the results are averaged over every three spring-neap circles, approxiately 45 days..6 Hs c.6.25 u Hs, c/s (c).2.25 u, (c/s).2 Hs c d u c/s dn Figure 5. Teporal variation of hydrodynaics on the arsh fringe, the left one is fro C V3, the right one is fro C V12 ; the green dash lines (dn) indicate the relative ste density of vegetation Hydrodynaic forces can either decrease due to the arsh expansion or they could increase due to arsh retreat (Fig. 5). Whether the arsh will expand or retreat is deterined by the strength of hydrodynaics. In the base run, the ean vegetation density increases by 4 % on the fringe of arsh (Obs. 2), which leads to 25% and 4% decline of H s and u respectively over half year. The decline of hydrodynaic forces leads to a ore favorable environent for the salt arsh. In the C V12, hydrodynaic forces are erosive to the salt-arsh vegetation on the fringe. The arsh density decreases by 8%, while H s and u increases by 35% and 4% respectively (Fig. 5). In this case, hydrodynaic forces are aplified due to the degeneration of the salt arsh Bed shear stress In the odel, the ortality of S. alterniflora is assued to be proportional to the ax shear stress fro current-wave interaction [14]. The critical shear stress is set at.26 Pa [18], beyond which soe vegetation will be wiped out (Fig. 6).

7 7 During high water level, shear stress due to currents alone is.2 Pa.1 Pa with a ean value of.45 Pa at the arsh fringe. As waves are included, the ax bed shear stress fro flow-wave interaction is.6 Pa.28 Pa in the base run on the fringe of arsh, with a ean value around.85 Pa. Though waves are relatively sall on the Jiangsu coast, field work shows that waves can enhance bed shear stress greatly [3; 9]. Moreover, bed shear stress on the arsh due to waves alone is higher than that fro currents along (Fig. 6). As incident wave height increases fro 3 c to 5 c, there is a higher chance that the vegetation on arsh fringe will be eroded..3.3 Bed shear stress due to waves alone, [Pa] Bed shear stress due to waves alone, [Pa] Bed shear stress due to currents alone, [Pa] Bed shear stress due to currents alone, [Pa] Figure 6. Coparison between bed shear stress induced by current alone and by wave alone at the fringe (Obs. 2) of the arsh. The left one is C V3, the right one is C V5, red dash lines indicate the critical shear stress (.26 Pa) of plant ortality 4. Discussion and Conclusion The results deonstrate a significant effect of vegetation on waves and currents. The base run (C V3 ) successfully reproduces the rapid developent of salt arshes on the Jiangsu coast [1; 17]. Furtherore, teporal variations of hydrodynaics due to the salt arsh developent are evident (Fig. 5). Hence, it is necessary to eploy dynaic arshes odule other than static vegetation patches for fairly long-ter hydrodynaic or orphodynaic prediction. The intertidal zone of the Jiangsu coast is very flat and the slope is very gentle [8]. Considering the hoogeneous topography, the interaction between vegetation and hydrodynaics can be very iportant to the evolution of the landscape [6]. Wave height on the Wanggang tidal flat is sall [8; 9], which ay be one of the reasons that salt arshes are widespread there. Field easureents have indicated that in ters of bed shear stress waves ay play a ore iportant role in arsh dynaics than currents [3]. Model results show that bed shear stress is enhanced by waves, which leads to a higher ortality of S. alterniflora. Moreover, the shear stress due to waves alone is greater than that due to currents alone [3; 9]. It is the reason why the developent of the arsh is sensitive to waves. Even a slight increase of H s (2 c) fro the boundary will restrain the

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