Dramatical hydro-sedimentary changes induced by bamboo fences over mangrove tidal flat of the largest delta in Beibu Gulf, southwestern China
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Abstract: Mangrove forest is one of the most important ecological and environmental resources by effectively promoting tidal flat deposition and preventing the coastal region from typhoon. However, there have been mass loss of mangrove forests due to anthropogenic activities. It is an urgent need to explore an effective way for mangrove restoration. Here, three rows of bamboo fences with hydro-sedimentary observation set over Aegiceras corniculatum mangrove tidal flat of the Nanliu Delta, the largest delta of Beibu Gulf, China, were conducted to analyze the hydro-sedimentary variations induced by bamboo fences. Results identified that the mean horizontal velocity Um per burst (20 min) decreased by as much as 71% and 40% in comparison with those without bamboo fences in March and November, respectively, when the tidal current entering the bamboo area during flood. The maximum of mean horizontal flow velocity Um-max at bamboo area was 50%–75% of that without bamboo fences during ebb tide. The suspended sediment concentration of bamboo area suggested a maximum reduction of 57% relative to bare flat during flood, and was 80% lower than bare flat at ebb peak. Moreover, the turbulent kinetic dissipation ε at flood tide was significantly higher than that at ebb tide, while the bamboo fences greatly increased the turbulent kinetic dissipation ε by 2 to 5 times relative to bare flat, resulting in an increase of the bed elevation by inhibiting the sediment incipient motion and intercepting suspended sediment. The siltation rate at the bamboo area was 140% and 29.3% higher than that at the bare flat and the region covered with A. corniculatum, respectively. These results highlight that bamboo fences can effectively attenuate tidal current and thus promote siltation over mangrove flat, which contribute great benefit to mangrove survival.
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Key words:
- mangrove tidal flat /
- bamboo fences /
- hydro-sedimentary process /
- bed siltation
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Figure 1. Study area and instrument deployment. a. The location of Beibu Gulf; b. the location of Nanliu Delta and Qixing Island; c. the study area of Qixing Island with the bamboo fences, hydro-sedimentary instruments and GPS-RTK profile deployment; d. the parameters of bamboo strip; e. field pictures of instrument deployment in March and November. ADV: Acoustic Doppler Velocimeter; OBS-3A: Optical Backscattering Sensor; ASM: Argus Surface Meter; CTD: conductivity, temperture, depth; HR: high resolution.
Figure 4. Time series of a, e and i water depth h; b, f and j mean horizontal flow velocity Um; c, g and k mean direction of the flow Dc; d, h and l mean horizontal velocity perpendicular to the bamboo fences Upb at normal area and bamboo-inserted area in March and November, respectively. Stations A3, S3 and S4 were between the bamboo fences. T1–T4 refer to the first tide to the fourth tide in March and November.
Table 1. Instrument setup and related obtained parameters during the observation
Instrument Related obtained parameter Sample rate/Burst
(interval)Measurement location
above seabed/cmInstrument site
TimeADV turbulent velocity, flow direction, water depth 8 Hz/20 min 15 A2, A4 8–12, March ALEC flow velocity, flow direction, water depth 15 samples/20 min 10 A1, A3, B1, B2, B3 ADV turbulent velocity, flow direction, water depth 32 Hz/20 min 15 S1, S2, S3, S4, S6 HR-Profile flow velocity, flow direction, water depth 4 Hz/20 min 0 S5 7–10, November OBS-3A turbidity 10 Hz/1 min 10 S6 ASM-IV turbidity 4 Hz/20 min −10 S3, S5 RBR-CTD turbidity 0.1 Hz/consecutive 10 S1, S2, S4 Table 2. Hydrodynamic characteristic values for 4 tidal cycles in March
Site Tide U m-max/(m·s−1) hmax/m Range of Dc/(o) Upb-max/(m·s−1) A1 flood 0.04, 0.06, 0.04, 0.04 2.00, 2.06, 2.09, 1.92 100–300 0.05, 0.04, 0.03, 0.03 ebb 0.10, 0.12, 0.11, 0.10 180–230 −0.05, −0.06, −0.06, −0.05 A2 flood 0.33, 0.31, 0.32, 0.30 2.12, 2.17, 2.21, 2.04 30–75 0.04, 0.04, 0.04, 0.04 ebb 0.28, 0.25, 0.25, 0.26 205–240 −0.06, −0.07, −0.06, −0.06 A3 flood 0.18, 0.17, 0.17, 0.17 2.23, 2.49, 2.53, 2.35 35–330 0.14, 0.14, 0.13, 0.16 ebb 0.29, 0.29, 0.31, 0.30 170–210 −0.15, −0.16, −0.17, −0.15 A4 flood 0.55, 0.54, 0.53, 0.52 2.43, 2.49, 2.53, 2.35 40–80 0.04, 0.04, 0.04, 0.06 ebb 0.39, 0.40, 0.40, 0.39 210–220 −0.07, −0.07, −0.07, −0.07 B1 flood 0.21, 0.23, 0.28, 0.21 1.95, 2.00, 2.04, 1.87 280–340 0.18, 0.19, 0.16, 0.17 ebb 0.1, 0.18, 0.18, 0.08 110–130 −0.1, −0.08, −0.11, −0.08 B2 flood 0.24, 0.21, 0.11, 0.14 2.04, 2.11, 2.14, 1.97 230–90 0.11, 0.17, 0.09, 0.1 ebb 0.13, 0.14, 0.19, 0.17 230–290 −0.1, −0.09, −0.11, −0.08 B3 flood 0.17, 0.16, 0.15, 0.18 2.22, 2.27, 2.31, 2.14 330–40 0.14, 0.15, 0.14, 0.17 ebb 0.14, 0.29, 0.29, 0.28 210–260 −0.18, −0.13, −0.19, −0.14 Table 3. Hydrodynamic characteristic values for 4 tidal cycles in November
Site Tide U m-max/(m·s−1) h/m Range of Dc/(o) Upb-max/(m·s−1) S1 flood 0.14, 0.19, 0.20, 0.21 1.85, 1.29, 2.07, 1.76 30−90 0.025, 0.035, 0.04, 0.036 ebb 0.06, 0.05, 0.06, 0.06 140−230 −0.016, −0.014, −0.018, −0.016 S2 flood 0.08, 0.10, 0.09, 0.11 1.86, 1.37, 2.19, 1.90 40−120 0.014, 0.009, 0.015, 0.016 ebb 0.09, 0.05, 0.07, 0.06 150−250 −0.015, −0.003, −0.013, −0.008 S3 flood 0.25, 0.26, 0.31, 0.30 2.10, 1.57, 2.39, 2.09 30−70 0.045, 0.03, 0.049, 0.042 ebb 0.14, 0.16, 0.14, 0.13 180−240 −0.016, 0.012, −0.016, −0.018 S4 flood 0.19, 0.19, 0.25, 0.23 2.13, 1.62, 2.43, 2.12 40−110 −0.014, −0.008, −0.031, −0.023 ebb 0.14, 0.16, 0.14, 0.14 200−300 0.007, 0.003, 0.02, 0.005 S5 flood 0.38, 0.4, 0.49, 0.40 2.22, 1.71, 2.53, 2.22 30−80 0.064, 0.05, 0.07, 0.05 ebb 0.3, 0.27, 0.35, 0.38 210−265 −0.038, −0.015, −0.05, −0.033 S6 flood 0.22, 0.34, 0.49, 0.35 2.32, 1.80, 2.62, 2.32 20−70 0.064, 0.05, 0.08, 0.067 ebb 0.3, 0.3, 0.31, 0.26 215−250 −0.056, −0.05, −0.061, −0.047 Table 4. Variation of suspended sediment flux at bamboo-inserted area in November (unit: kg/(m2·s))
T1 T2 T3 T4 Total flood ebb flood ebb flood ebb flood ebb net difference S6 0.055 −0.061 0.10 −0.074 0.11 −0.15 0.11 −0.076 0.46 S5 0.027 −0.035 0.004 4 −0.23 0.000 89 −0.086 0.028 −0.15 S4 −0.019 0.010 −0.038 −0.001 9 −0.072 0.005 8 −0.032 −0.003 9 −0.29 S3 0.028 −0.13 0.14 0.11 0.17 −0.13 0.082 −0.092 −0.33 S2 0.0059 −0.007 9 0.007 6 0.000 54 0.008 5 −0.004 3 0.005 2 −0.005 6 0.17 S1 0.027 −0.016 0.059 −0.008 9 0.047 −0.016 0.037 −0.014 −0.089 -
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