疏浚土倾倒后悬浮泥沙扩散输移的数值模拟

Numerical simulation of the dispersion and transportation of suspended sediment from dredged soil matter

疏浚土的处置是航道疏浚整治中需要解决的问题。疏浚土在倾倒区倾倒之后,大部分疏浚土会沉积在抛泥区附近,其余泥沙将在抛泥点的局部区域形成高浓度悬沙,在重力、波浪、潮流、风生流等因素作用下扩散输移。建立了平面二维潮流、悬沙浓度增量输移扩散的数学模型,采用有限单元法离散求解,研究了虾峙门口外航道整治工程中疏浚土倾倒后悬浮泥沙在水体中的扩散输移,以此分析抛泥后不同悬沙浓度增量的悬浮泥沙的扩散范围。虾峙门口外外海处设置了1#、2#、3#倾倒区,模型计算结果表明,各悬沙浓度增量的悬浮泥沙扩散最大面积的最小值及包络线面积的最小值均发生在3#倾倒区抛泥的工况,原因在于3#倾倒区位于外海,水深明显大于1#、2#倾倒区,其倾倒区容积最大。可见,在水动力条件相当的情况下,水深条件形成的各倾倒区容积对悬浮泥沙的扩散和输移是有影响的。倾倒区容积越大,对悬浮泥沙扩散和输移越有利。从经济因素来看,疏浚船的运输距离所发生的运费是不可忽略的,疏浚航道至1#、2#倾倒区的距离要短于至3#倾倒区的距离,把疏浚土运至1#和2#倾倒区的运行费用明显低于运至3#倾倒区的运行费。研究结果建议,选择1#和2#倾倒区为疏浚土倾倒区,并根据潮流方向灵活安排施工作业,可将1#倾倒区作为落潮时疏浚土的倾倒区,2#倾倒区作为涨潮时疏浚土的倾倒区。

According to the plan, 25 × 10^4 t to 30×10^4 t docks will be built in Ningbo-Zhoushan port. The large ships over ten-thousand tonnage from the open ocean to the northern port area of Ningbo and southern the Xiazhimen waterway. But the smallest natural depth of the waterway 2 m, which only meets the demand for ships of 15 × 10^4 t to 20× 10^4 t ship nt. That is to say, the construction of the deep waterway lags behind the which restricts the capability of getting larger ships. To match the construction and regulation is imperative for the waterway outside Xiazhimen mouth. How to deal with the dredged soil is matter to be solved in the dredge and regulation of waterway. Three dumping areas named 1 # , 2 # and 3# far away from the land are set in open sea outside Xiazhimen mouth. Most of the dredged spoil matter dumped in the area will deposit in the dumping area, the rest will form suspended sediment of high-concentration around the dumping point and he dispersed and transported by gravity, wave, tidal current and wind-driven current. A 2D mathematical model for tidal current and dispersion ＆ transportation of suspended sediment is built up and solved with finite element discrete method. The dispersion ＆ transportation of dredged soil from the waterway outside Xiazhimen mouth is simulated after being dumped, and the effect of the increased suspended sediment on ocean environment near the engineering area is analyzed. The modeling shows that both the smallest value and the smallest enveloping curve area of the largest area of different increased suspended sediment are in 3 # dumping area. It is because that the 3 # dumping area is located in the open ocean where the water depth is larger than that of 1 # and 2# dumping area, leading to the largest volumetric capacity there. It is thus evident that under the same hydraulic conditions, the volumetric capacity resulted from water depth has the effects on the dispersion and transportation of the suspended sediment. The larger the capacity is, the better for the dispersion and transportation of suspended sediment. But from the economic point of view, the operational cost caused by the transportation distance from the dredge site to the dumping area couldn＇t be ignored. The cost is much lower for 1 #and 2 #than 3 # So 1 # and 2# dumping areas are suggested for the dredge project, and according to the direction of the tidal current, 1 # dumping area could be used for dumping during ebb tidal while 2# dumping area during flood tide.