Prediction On Erosion And Accumulation Of The Seabed By The Project Of Western Jetty Of Lanshan Harbor

The Project of Western Jetty of Lanshan Harbor (PWJLH) is scheduled to be located in the south of Lanshan Fishing Port (LFP). This project maybe change the local tidal current, suspended sediment concentration (SSC), or the pattern of the erosion and accumulation of the seabed due to the changes of coastal boundaries. Numerical model is one of the best methods to quantitatively predict the effects of the PWJLH on the erosion and accumulation of the seabed. Considering complicated conditions of the coastal dynamic environment, sediment dynamics, and together with the immature sediment itself, the predicted results will be somewhat limited if only with numerical model. So, in order to obtain microcosmic quantitative assessment and macroscopically understanding, numerical model is combined with field observations in the thesis. Firstly, the tidal current, SSC distribution, benthal deposited materials were basically analyzed by field observations. Then, the tidal current, SSC distribution, and the erosion and accumulation of the seabed were simulated with the numerical model of ECOMSED.The main results in the thesis were drawn as follows:1. Observed background around the scheduled projectThe tidal current is dominant by half daily tidal current with the directions of flood and ebb tidal currents of SW~SSW and NE～NNE, respectively. The largest current velocity of flood tide ranges from 40 cm/s to 55 cm/s, larger than that of ebb tide ranging from 20 cm/s to 40 cm/s. The tidal current is generally reversal current with the direction basically parallel to the coast.The SSC at the surface in the area of PWJLH and its adjacent area ranges from 11.7 mg/L to 28.4 mg/L with an average of 18.6 mg/L. The SSC at the bottom in the area of PWJLH and its adjacent area ranges from 8.7 mg/L to 48.9 mg/L with an average of 22.1 mg/L. Generally speaking, the SSC at both the surface and the bottom in the study area increases from the coast to the deep waters.The benthal deposited materials are mainly sand and sandy-silty clay. The sand is mainly located in the southern and northern areas of the scheduled project. Generally, the benthal deposited materials near the coast are mainly medium-fine sand, gradually becoming silty sand, sand-silt-clay, clayey silt, and silty clay. The different grains of the deposited materials are basically parallel to the isobaths, sand lower than–3m isobath and silty clay deeper than–3m isobath. The rating of the benthal materials between–3m and–5m isobaths is better than that deeper than–5m isobath, while it is not as good as that lower than–3m isobath.2. Numerical modelThe numerical model is the best method to predict the effects of filling projects on the erosion and accumulation of the seabed. Based on the field investigation, the 3D model-ECOMSED was modified to simulate tidal current, SSC, and erosion and accumulation of the seabed around PWJLH. The modified ECOMSED is expected to be useful for the simulation of hydrodaymics, SSC, and erosion and accumulation of the seabed around PWJLH and similar coastal waters.3. Changes of tidal current before and after the projectThe tidal current with both directions and the values will not change largely before and after the scheduled project, while the tidal current will become slightly stronger near the project due to the decreased drowned area, which is to be caused by the scheduled project. There will be a small cyclonic circulation to the north of the dyke of PWJLH affected by the scheduled project. Blocked by the scheduled project, the tidal current near the coast will move around the project.4. Changes of SSC before and after the projectThe general characteristics of the SSC both before and after the scheduled project are as the following: the SSC is lower in the waters of the project ranging from 11 mg/L to 19 mg/L; the SSC isolines are generally parallel to the isobaths, decreasing towards the coast; the SSC in flood tide is higher than that in ebb tide, so are high tide and spring tide than low tide and neap tide, respectively.Compared with that before the scheduled project, the same SSC isoline near the project circles the project due to the disturbance of the project on the tidal current. Generally, the SSC will not change largely after the construction of the project, although the SSC distribution will slightly change, i.e. the SSC near the scheduled project will be slightly smaller than that before the project.5. Changes of erosion and accumulation of the seabed before and after the projectThe general characteristics of the pattern of erosion and accumulation of the seabed near the scheduled project both before and after the scheduled project are as the following: accumulation occurs at the areas lower than–10 isobath; some suspended sediment is deposited near the coast due to the relatively small tidal current with a monthly accumulation rate of 2-6 mm; while a slight erosion occurs in the areas deeper than–10 isobath with a monthly accumulation rate of <0.2 mm.Compared with those before the scheduled project, the accumulation of the seabed of the nearly enclosed area by the scheduled project and LFP will be intensified due to the lowered tidal current velocity; and at the same time, the southwestern areas of the project will suffer slightly erosion due to scouring by the ebb current, so will do the southern and northern areas of the PWJLH's dyke.Under the effects of tidal current, the monthly accumulation rate of the nearly enclosed area by PWJLH and LFP before the scheduled project ranges 2 mm to 3mm, in contrast to the monthly accumulation rate of 2 mm to 6mm after the project. So, the yearly accumulation rates of the nearly enclosed area by PWJLH and LFP before and after the scheduled project are 3.0 cm and 4.8 cm respectively in terms of the averaged monthly accumulation rates of 2.5 mm and 4 mm. The above simulated results show that the scheduled project would weaken the dynamic conditions near the project, and therefore, increase the suspended sediment deposition of the nearly enclosed area by PWJLH and LFP, whereas, the accumulation is slight and will not significantly affect the normal navigation of the fishing ships.The observed and simulated results as mentioned above could be useful to give some scientific evidences to decision-makers for better management of the PWJLH, LFP and Lanshan Harbor, and if so, the PWJLH will bring the best benefits to the local economy, society and eco-environment.