# Short-Term Morphological Responses of Adjacent Intertidal Flats to the Construction of Tidal Gates in an Estuarine Tributary

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

^{3}/s) that account for 6% of the total number of tidal gates, of which more than 70% were built after the year 2000. As a water control structure, tidal gates prevent inflows from seasonal tides and storm-surging events, while allowing the downstream movement of freshwater freely under normal conditions [2,3]. However, the construction of tidal gates causes adverse physical, chemical, and biological effects that change the connectivity between the estuary and the flood plain [4].

^{3}/y, leading to severe problems in terms of flood control [16]. Moreover, the construction of a large-scale tide gate/barrage has led to the downstream acceleration of the sedimentation area by transforming the estuarine circulation and, thus, the sedimentation process [17,18]. After constructing a barrage in the upper reaches of the Keum river estuary in Korea, the sediment deposition rate in the estuary increased from 3.5 million to 6.7 million m

^{3}/y [19]. The Vilaine estuary, on the Atlantic coast of France, is another excellent example of the influence of human activities on natural evolution. Since the dam was built in 1970, the fall of the tidal prism and the increase in sediment deposition rates have led to a decrease in the tidal current, which played a decisive role in expanding the downstream tidal flats [20]. In the downstream areas of many tidal gates in China, increased sedimentation rates caused by tidal waves have been observed with closed gates [21].

## 2. Materials and Methods

#### 2.1. Study Site

^{2}. The OR is a mountain river with a tidal reach of 78 km (Figure 1b). The average annual discharge and runoff of the OR are 443 m

^{3}/s and 14 billion m

^{3}, respectively. The Nanxi River (NR) is the largest left-bank tributary of the lower OR, with a length of 142 km and a basin area of 2436 km

^{2}. The proposed NR tidal gate (NRTG) is located at the mouth of the tributary (Figure 1c). The average annual discharge and runoff in the section where the gates are located are 85 m

^{3}/s and 2.68 billion m

^{3}, respectively.

^{3}in a flood tide and 0.7–1.5 kg/m

^{3}in an ebb tide. The riverbed comprises medium and fine sediments with a diameter of 0.125–0.250 mm, whereas the suspended medium and fine silts have a diameter of 0.006–0.030 mm.

#### 2.2. Model Scope

#### 2.3. Model Scales

_{H}, then the horizontal coordinates x and y are at the same scale λ

_{L,}as follows:

_{u}and λ

_{v}are the velocity scales in the x- and y-directions, respectively; λ

_{t}

_{1}is the kinematic time scale, and δ is the distortion rate. By combining Equation (8), additional scales may now be derived from the right side of Equation (2):

_{C}and λ

_{R}are the Chezy coefficient and hydraulic radius scale, respectively. The estuarine area is usually broad and shallow, and the wall resistance can be ignored, yields

_{n}and λ

_{Q}are Manning roughness coefficient and flow discharge scale, respectively.

_{∗}is the averaged sediment-carrying capacity.

_{α}= 1 yields:

_{0}is a dimensionless coefficient to be determined by experimental or in situ data, γ is the specific weight of seawater, γ

_{s}is the specific weight of sediment particles, and V is the streamwise velocity. Equation (14) has been validated by data sets obtained from laboratory experiments and studies of large rivers and estuaries, such as the Yellow River and the Yangtze River, and α

_{0}is estimated to be 0.023 [28].

_{0}= 1, λ

_{V}= λ

_{u}= λ

_{v}, and λ

_{γ}= 1; the following scale may now be derived from Equation (14):

_{s}are the density of seawater and sediment particles, respectively. The continuity equation of the suspended load reflects the change in suspended sediment transport from unsteady flow, but it does not reflect the incipient motion of sediment particles at the bottom. Therefore, to make the suspended sediment movement similar, it is necessary to satisfy the similarity of sediment incipient motion, viz.:

_{∗}is the incipient velocity.

_{2}is the morphological time, and yields:

_{ρ}

_{’}is the dry density scale and λ

_{t}

_{2}is the morphological time scale.

#### 2.4. Choice of the Model Sediments

_{50}and density ρ

_{s}. Using natural sediment as the model sediment in a suspended sediment model is difficult because the model sediment must satisfy the criteria of similar sedimentation and incipient motion. The choice of model sediment is also governed by practical questions, such as the level of investment and the test site. Generally, it is necessary to select lighter materials for the prototype sand. After using a light model sediment, the morphological time scale ${\lambda}_{{t}_{2}}$ is always greater than the kinematic scale ${\lambda}_{{t}_{1}}$. Therefore, in one model tidal cycle, we obtain the morphological evolution corresponding to θ prototype tidal cycles, resulting in:

^{2}. Due to the limited choice of laboratory, the horizontal and vertical scales of the model were 800 and 100, respectively, with a distortion rate of 8. According to the field-measured sediment data, the median particle size d

_{50}was 0.0067–0.0087 mm, the settling velocity was 0.023–0.025 cm/s at 20 °C, and the incipient velocity was 1.12–1.81 m/s at a water depth of 2–10 m. According to the similarity of settling velocity, the settling and incipient velocity of the model sediment were required to be 0.018–0.02 and 11.2–18.1 cm/s, respectively.

^{3}, a settling velocity of 0.018 cm/s, an incipient velocity of 14 cm/s, and a dry density of 0.45 g/cm

^{3}was used as the model sediment. The SSC and morphological time scale were empirically determined by relating the time intervals, in the prototype and the model, that corresponded to an actual morphological evolution in the past [29]. Calculated and selected scale values of the experimental model are shown in Table 1. According to several validation test results, the model sediment concentration, along with the amplitude and distribution of topography erosion and deposition, were in good agreement with the actual situation at λ

_{S}= 0.38 and ${\lambda}_{{t}_{2}}=547.5.$

#### 2.5. Control System and Measurement Technique

## 3. Results

#### 3.1. Model Calibration and Verification

^{3}and the deviation was 5%, compared with the measured total volume of 8.332 million m

^{3}.

#### 3.2. Effect of the NRTG on the Surrounding Hydrodynamics

^{3}/s and 73.6 m

^{3}/s, respectively. The selected lower estuary boundary condition was the water-level process of an ordinary spring tide. The flow stream lines can describe the general current patterns of the model zone. To characterize the flow stream lines, a large number of floats made of finely shredded white paper, with an equivalent diameter of 1 to 2 cm, were sparsely distributed over the observed area. The trajectories of the floats can be extracted from the photographic sequences to reflect the stream line distribution. As shown in Figure 6, after the project was implemented, the flood tide affected by the NRTG that initially entered the NR flowed past the gate site and then turned to the upstream of the OR; during this phase of the ebb tide, the main stream was closer to the south bank of the OR.

#### 3.3. Morphological Change of Intertidal Flats

#### 3.4. Evolution of the Tidal Channel Network

## 4. Discussion

#### 4.1. Variation of Tidal Flat Elevations

#### 4.2. Change of Drainage Capacity of the Tidal Channel Network

#### 4.3. Distribution Changes of the Tidal Channel Network

## 5. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## References

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**Figure 1.**Study area: (

**a**) Zhejiang Province in China, (

**b**) the Ou River (OR) estuary, and (

**c**) the Nanxi River tidal gate (NRTG) on the mouth of the Nanxi River (NR). A dashed box in Figure 1c marks the observed intertidal flat in the model test.

**Figure 2.**Model layouts: (

**a**) a sketch map of the experimental model, (

**b**) the scheme of the tidal maker and data acquisition system, and (

**c**) the model site. RE: Roughness element.

**Figure 3.**Model calibration: tidal level (

**a**) W1 and (

**b**) W2; flow velocity and direction (

**c**) C1, (

**d**) C2, and (

**e**) C3. The locations of the calibration points are shown in Figure 2a.

**Figure 4.**Model verification: (

**a**) flow velocity and direction, and (

**b**) SSC. The verification points were selected at the proposed tidal-gate position (NRTG in Figure 2a).

**Figure 6.**Model characterization of the flow stream lines via photographic sequence: peak floods (

**a**) without and (

**b**) with NRTG, and peak ebb (

**c**) without and (

**d**) with NRTG. The red line encloses the intertidal zone with plants. Blue stars mark the positions of observation points (P1, P2, and P3).

**Figure 7.**Comparison of the tidal levels at (

**a**) P1, (

**b**) P2, and (

**c**) P3, and the flow velocities and directions at (

**d**) P1, (

**e**) P2, and (

**f**) P3 before and after the construction of the NRTG, during the spring tide. The shading represents the exposure period of the intertidal flats.

**Figure 8.**Morphology of the intertidal flats at the time points of (

**a**) 0, (

**b**) 6, (

**c**) 12, and (

**d**) 18 months. The positions of the cross-profile transects (T1, T2, T3, T4, T5, and T6), as mentioned later, are marked by dashed lines in Figure 8a. Time = 0 months corresponds to the start of the experiment. The dashed box in Figure 1c marks the observed intertidal flat in the model test.

**Figure 10.**The typical morphological patterns used in the experiment. (

**a**) A tidal rill structure after 20 tidal cycles, (

**b**) the channel network structure at the end of the experiment, and (

**c**) the local morphology of the tidal channel network.

**Figure 11.**Tidal channel network formations after (

**a**) 3, (

**b**) 6, (

**c**) 9, (

**d**) 12, (

**e**) 15, and (

**f**) 18 months.

**Figure 12.**Elevation changes across (

**a**) T1, (

**b**) T2, (

**c**) T3, (

**d**) T4, (

**e**) T5, and (

**f**) T6. Positions of the cross-profile transects T1, T2, T3, T4, T5, and T6 are marked by dashed lines in Figure 4a.

**Figure 13.**Exceedance probability distributions of the unchanneled flow lengths after 6, 12, and 18 months.

**Figure 15.**Variation of the tidal channel (

**a**) mean sinuosity, (

**b**) junction number, and (

**c**) Hausdorff dimension.

Scales | Equations | Calculated Values | Selected Values |
---|---|---|---|

Horizontal length | λ_{L} | 800 | 800 |

Vertical height | λ_{H} | 100 | 100 |

Flow velocity | λ_{u} = λ_{v} = λ_{H}^{1/2} | 10 | 10 |

Flow discharge | λ_{Q} = λ_{L} × λ_{H}^{3/2} | 800,000 | 800,000 |

Current time | ${\lambda}_{{t}_{1}}$ = λ_{L} × λ_{u}^{−1} | 80 | 80 |

Roughness coefficient | λ_{n} = λ_{H}^{2/3} × λ_{L}^{−1/2} | 0.76 | 0.75 |

Settling velocity | λ_{ω} = λ_{H}^{3/2} × λ_{L}^{−1} | 1.25 | 1.35 |

Incipient velocity | λ_{V} = λ_{u} | 10 | 10 |

SSC | λ_{S} = λρ_{s}/λ_{(}ρ_{s} – ρ_{)} | 0.22 | 0.38 |

Morphological time | ${\lambda}_{{t}_{2}}$$={\lambda}_{{t}_{1}}$$\times {K}_{{t}_{2}}$× λρ_{’}/λ_{S} | 655 | 547.5 |

Segment ^{1} | Field (Million m^{3}) | Verification (Million m^{3}) | Relative Difference (%) ^{2} |
---|---|---|---|

L1–L3 | 1.469 | 1.702 | 15.9 |

L3–L5 | 2.468 | 2.176 | −11.8 |

L5–L7 | 2.876 | 3.430 | 19.3 |

L7–L8 | 1.125 | 1.024 | −9.0 |

Total | 7.938 | 8.332 | 5.0 |

^{1}The segment locations were marked by dashed lines in Figure 2a.

^{2}Relative difference = (value

_{Verification}− value

_{Field})/value

_{Field}.

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**MDPI and ACS Style**

Pan, D.; Li, Y.; Pan, C.
Short-Term Morphological Responses of Adjacent Intertidal Flats to the Construction of Tidal Gates in an Estuarine Tributary. *J. Mar. Sci. Eng.* **2022**, *10*, 882.
https://doi.org/10.3390/jmse10070882

**AMA Style**

Pan D, Li Y, Pan C.
Short-Term Morphological Responses of Adjacent Intertidal Flats to the Construction of Tidal Gates in an Estuarine Tributary. *Journal of Marine Science and Engineering*. 2022; 10(7):882.
https://doi.org/10.3390/jmse10070882

**Chicago/Turabian Style**

Pan, Dongzi, Ying Li, and Cunhong Pan.
2022. "Short-Term Morphological Responses of Adjacent Intertidal Flats to the Construction of Tidal Gates in an Estuarine Tributary" *Journal of Marine Science and Engineering* 10, no. 7: 882.
https://doi.org/10.3390/jmse10070882