4.1.2. Environmental Capacity of CODMn

To figure out CODMn permitted emission at each catchment according to Linear Programming, this study took the maximum concentration during a whole tide as background concentration to estimate the capacity. Parameters about the permitted emission of CODMn are shown in Table 5.


**Table 5.** Parameter on CODMn (mg/L).

Linear programming solving was conducted on 7 catchment units based on the maximum remaining capacity. The maximum remaining emission permitted can be seen in Table 6. CODMn concentration of each control station and utilization rate can be seen in Table 7. Based on the result calculated with present source strength, concentration distribution was used to estimate the emission permitted which shows that only Puba and Shipu can still discharge. The result of solving the constraint conditions of each control station is concentrated in two stations, because the calculation of linear programming is carried out according to the mathematical conditions. When selecting the largest group of total capacity among all feasible solution, it is obviously reasonable for the catchment which is relatively close to the open sea.

**Table 6.** Emission permitted of COD (×10<sup>4</sup> *t*/*a*).



**Table 7.** Concentration of each control station concentration at the maximum environmental capacity of COD.

Table 7 shows Station 1 and Station 10 have reached the limited values. According to the prediction results of COD environmental capacity, without considering the uniformity principle, only from the aspect of maximum source strength increment, under the condition of meeting the control objectives, the maximum COD pollutant discharge capacity of Sanmen Bay is about 31.56 <sup>×</sup> <sup>10</sup><sup>4</sup> *t*/*a*, which is the maximum theoretical calculation result under the calculation conditions.

#### *4.2. Reduction of Main Pollutants*

As the nitrogen and phosphorus nutrients in Sanmen Bay have exceeded the standard, different emission reduction schemes for inorganic nitrogen and active phosphate should be analyzed. In this paper, the phased control method is used to calculate the pollutant reduction. Firstly, pre-calculation is conducted to analyze the influence of the variation of pollution source strength of each catchment on the distribution of the concentration field, so as to provide the basis for determining the formal calculation scheme and preliminarily determine the minimum reduction required to reach the target. Secondly, the reduction scheme is determined according to the phased control target. Finally, the results of each scheme were compared and selected, and the reduction of inorganic nitrogen pollutants in each catchment was determined on the basis of meeting the requirements of the phased control index for environmental capacity calculation of Sanmen Bay.

The water quality model was used to simulate a different reduction of inorganic nitrogen source strength, and the concentration of sea area under 0.65 mg/L was found to be relevant to the reduction of source strength [17,18]:

$$S = \mathfrak{Z36.43e}^{0.9146x} \tag{11}$$

*S* means area, *x* means percentage of source strength.

According to the model result, when the recent source strength reduction reaches 14.0%, the available sea area reaches 378.16 km<sup>2</sup> , which accounts for 60.28% of the total area in Sanmen Bay. Mid-term and long-term is long from the present, the natural conditions and socio-economic conditions may change significantly so that the prediction is likely to deviate (Figure 9). The water quality improvement target needn't be completed accurately. According to the model result, when the source strength reduction reached 30.0% and 44.0%, the available area can reach 441.72 km<sup>2</sup> in middle term and 497.62 km<sup>2</sup> in long term (Table 8).

port in the bay head has dropped below 0.1 unit.

30.0% and 44.0%, the available area can reach 441.72 km<sup>2</sup>

in long term (Table 8).

**Table 8.** Result of source strength reduction.

**Figure 9.** Result of source strength reduction in near term (**a**) mid-term (**b**) and long term (**c**). **Figure 9.** Result of source strength reduction in near term (**a**) mid-term (**b**) and long term (**c**).

area in Sanmen Bay. Mid-term and long-term is long from the present, the natural conditions and socio-economic conditions may change significantly so that the prediction is likely to deviate (Figure 9). The water quality improvement target needn't be completed accurately. According to the model result, when the source strength reduction reached

**Number Reduction Rate of Source Strength/% Area/km** 1 5 356.08 10 369.59 15 384.04 20 399.52 25 421.77 30 445.04 35 461.04 40 480.69 45 510.17 50 536.15

Figure 8d,e shows that the trend of concentration distribution and isoline trend in the bay are close, while the overall concentration decreases and the concentration contour continues to be extrapolated. After one and a half months, the exchange ratio of all water bodies in the bay reaches more than 85%; after two months, the exchange ratio of most water bodies in the bay reaches more than 90%, except for some waters at the top of the west of the Bay. It can be considered that the water exchange in Sanmen Bay has been

in middle term and 497.62 km<sup>2</sup>


**5. Conclusions Table 8.** Result of source strength reduction.

basically completed at this time.

#### **5. Conclusions**

A calibrated two-dimensional hydrodynamic model was built and fully validated to study the environmental characteristics of Sanmen Bay, including the tides, the residual currents, the tidal prism, and water exchange abilities.

Tides in the bay are regular semidiurnal tides, and the average tidal range is more than 4 m. The shallow water component has a certain influence on the tidal currents. The SSC in the bay is high, and is mainly caused by tidal current. The average tidal prism of the bay is about 20.78 <sup>×</sup> <sup>10</sup>8m<sup>3</sup> .

The distribution of semi-exchange capacity of water bodies varies greatly in different regions of the bay. Generally speaking, the water exchange capacity of the bay mouth and Shipu port is strong, and the water exchange in the west of the bay is slower than that in the East. The half exchange time of the whole bay is about 23 days, and the exchange time of 95% water body is about 60 days; the half-exchange time of relatively open sea area is less than 15 days, and 95% of water exchange time is about 50 days.

The concentrations of COD, inorganic nitrogen, and acid salt in Sanmen Bay showed a trend of being higher in the inner estuary and lower outside of the bay, and was higher in the western part and lower in the eastern part. The concentration of COD was lower than 0.60 mg/L in most areas of the eastern part of the bay, while was higher than 0.65 mg/L in the western part of the bay. The concentration of inorganic nitrogen was more than 0.70 mg/L near the west coast. The concentration of acid salt was lower in the outer bay, while was higher in the inner bay.

**Author Contributions:** Methodology, Y.Y. and J.Y.; software, J.Z.; validation, J.Z.; investigation, Y.Y.; resources, Y.Y.; writing—original draft preparation, J.Z.; visualization, J.Z.; supervision, L.L. and J.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Science Technology Department of Zhejiang Province (2020C03012, 2022C03044).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This research was partially supported by a grant from the Science Technology Department of Zhejiang Province. Data were provided by Qin Chen.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
