Analysis on Coast Sediment Environment and Calculation of Erosion and Deposition for Bayah Beach, Indonesia

Transcription

1 9:2 (217) Analysis on Coast Sediment Environment and Calculation of Erosion and Deposition for Bayah Beach, Indonesia GAO Feng 1, ZHAO Peng 1, ZHANG Hua-ping 2 1. Tianjin Research Institute for Water Transport Engineering, Key Laboratory of Engineering Sediment of Ministry of Communications. China 2. CCCC Second Harbor Consulatants Co., Ltd. China Seal326@163.com Abstract: The proposed project situated in the vicinity of the village of Bayah, which situated in Banten Province on the south-west coast of the island of Java, Indonesia. It faces to the Indian Ocean, the wave direction is mainly concentrated between S and SSW. There is hidden reef and river near the proposed harbor site. During the rainy season, the sea s color is turbid nearshore. The obvious sandbank lies in both sides of the estuary, and there is a turbidity zone near the estuary. Therefore, the proprietors worry that the risk of sediment deposition exists in the harbor after the implementation of this project. Under this background, the hydraulic and sediment numerical model testing must be carried out in order to understand the erosion trend and the silting intensity in this area under the combined effects of coastal dynamics, which will provide a scientific basis for determining the port planning design. The project site is bedrock-sandy coast type, the main bed-load is sand. Hydrodynamic force is weak in the project area, and current velocity is less than sediment entrainment velocity in different isobaths. But wave from SSW-WSW is strong and frequent. The headland of bedrock coast type alternates with the bay of sandy coast type, and the shoreline twists and turns. Because the surrounding sand is relatively limited, so many sites facing to the open area appear erosion features. By analyze different remote sensing images since 26, the beach is under slightly erosion due to lack of sediment source. According to the result of sediment numerical model, suspended sediment concentration varying with time during December, 213, and the deposition intensity is.16m/a, the deposition volume of harbor district and channel is 9 14m 3 /year. Combinations of adverse conditions continue for 24 hours, the maximal thickness of sudden deposition in the harbor district range from 1.3 to 5.cm/day. Keywords: sediment; coast; erosion; deposition. 1. Introduction Bayah beach faces to the Indian Ocean(6 57 S and E, as shown in Figure 1), situated in Banten Province on the south-west coast of the island of Java, Indonesia. The wave direction is mainly concentrated between S and SSW. There is hidden reef and river near the coast. During the rainy season, the sea s color is turbid nearshore. The obvious sandbank lies in the both sides of the estuary, and there is a obvious turbidity zone near the estuary. The proposed project intends to be developed and operated. Therefore, the clients worry that the risk of sediment deposition exists in the harbor after the implementation of this project. Under this background, the hydraulic and sediment research work will be carried out in order to understand the erosion trend and the silting intensity in this area under the combined effects of coastal dynamics, which will provide a scientific basis for determining the port planning design. 2. Meteorological hydrological conditions 2.1. Wind Indonesia has a tropical climate, with two distinct monsoonal wet and dry seasons. Between May and October is dry season, while the remaining is rainy season. The project area is a typical equatorial oceanic climate, which is humidity, hot and rainy all year round, and annual variation of meteorological factors is not significant. The seasonal variation of wind in Indonesia is shown in Figure 2. We can find that the project site has obvious seasonal changes. In rainy season, the wind of project site is mainly from W and NW direction, while the dry season is opposite. According to the data from BMKG (Badan Meteorologi Klimatologi dan Geofisika), the average wind speed is between 5~1 knot, and the maximum wind speed can reach to 1~2 knot in July or August. The statistics wind data (from 1992 to 21) referring to the statistical data of ECMWF(European 61

2 9:2 (217) Advances in Sciences and Engineering Centre for Medium-Range Weather Forecasts), which is about 27 km southwest from the project. The wind direction is given priority to with S to E (12 ), and the maximum wind speed is 18 m/s from WNW (27-3 ). Figure 1. Location of Karang Taraje cement port Figure 2. Wind rose 2.2 Tides and Current Hydrographic survey had been conducted on offshore area of the project. The survey included wave, tide and sediment observation, the distribution of stations is shown in Figure 3. The start time and duration of tide measurement on two stations is different, on T1 station it is from 12/12/213 to 1/16/214(36 days), on T2 station it is from 12/14/213 to 12/3/213(16 days). The tidal constituent in project area is obtained from T1 station, which is shown in Table 1, using the harmonic analysis method. Statistical analysis shows that, the tidal type in project area is the mixed tide which is dominated by the semidiurnal tide. Based on the measured tidal range and flood and ebb lasted, the maximum tidal range for T1 station is 1.44m, the minimum tidal range is.16m, and the average data is.73m. The average flood tidal duration is 6.5h, and the mean ebb tide duration is 5.9h. Figure 3. Location of survey stations Table 1. The tidal constituent in project area Tide M2 S2 N2 K2 K1 O1 P1 M4 MS4 A(cm) P(º)

3 9:2 (217) A tidal hydrology observation is conducted during the rainy period in December 213, including neap tide (Dec.12~13th), medium tide (Dec.16~17th) and spring tide (Dec.2~21th). And the main characteristics are as follows, seeing Table.2. Table 2. The statistical results of measured average vertical value(unit: m/s) Station SPRING Tide MEDIUM Tide NEAP Tide Average Maximum Average Maximum Average Maximum C C C C C During the site measurement, the wave power in this area is relatively strong. For example, during spring tide the current measurement was broken down by strong wave. The result of the current was measured in rainy season, and it can be used as a reference for rainy season. If the current of dry season is needed, the current measurement in dry season is recommended. 2.3 Wave According to the statistical data of BMKG (Badan Meteorologi Klimatologi dan Geofisika) from 25 to 211, the average wave height is between 2 m and 3m from January to March, and from June to December, and the maximum wave height is 2m in April or May. Based on the statistic result that collected ECMWF data from , the occurrence percentage of SSW wave direction is about 45%, which is the strong wave direction and prevailing wave direction. S wave direction is about 36.42%; it is the strong wave directions. The percentage of SE and SSE wave is 2.43%. The wave type is typically swell. In 214, the wave observation at Bayah Sea was held for 3 months with time series method. Observation data showed the wave condition at west monsoon season. Waves that occur in these waters is a combination of long waves generated in the Indian Ocean and the local waves. The dominant wave propagates from SSW with the significant wave height (Hs) ranged between meters with a significant period (Ts) seconds. While the maximum wave height and peak wave period (Hmax & Tp) are mostly found at the height between meters with a period of seconds. The maximum wave height is 3.46 meters by a period seconds and propagates from SSW. Frequency(%) Figure 3. Wave rose (28 ~ 212) Figure 4. Waves in the west of estuary area are significantly stronger (mirror to W direction) During the field observation, another wave relatively large area is near the estuary in the north of the proposed project, especially the beach of estuary west is relatively flat and broad. Because it is not covered with cape bay, the SW direction of sea waves effect directly. The waves are strong all the year, and the maximum width of surf zone can reach above 4m. Bayah beach is located within the cape bay, which can be covered by the cape head. But the waves near the cape head and the north shore are strong, especially in the west of estuary (Seeing Figure 4.). So the cape bay waters where the proposed project located will also have a certain degree of influence, which can t be ignored in the analysis of wave power. 63

4 9:2 (217) Advances in Sciences and Engineering 3. Sediment Environment 3.1 Main Sediment Source The project area lies in Bayah of west Java and is characterised by bedded units of quartz-rich sands and conglomerates, marls, calcareous sandstones, limestones and coals [1]. But, it is bedrock-sandy coast, and the surrounding sand is relatively limited. It is mainly gotten from the runoff sediment discharge of neighboring rivers and the offshore sediment transport under the action of wave and current. There are different forms during dry and rainy season in this area near the Cimadur estuary, which is shown in Figure 5. In rainy season, the wave power is relatively weak, and due to the rainfall the runoff effect is obvious, so the water is turbidity and spreads outward. However, the phenomenon in dry season is the exact opposite of rainy season, and the wave action is significant. Due to the lack of relevant information of this river runoff, during the study period, it is very difficult to find any relevant data and information on the record of the river, which brought a certain difficulty for the study. As the sediment runoff is difficult to determine, research team tried their best to collect information. According to Witteveen+Bos s report, the data of sediment discharge from Cimadur river is about 6~ m 3 /a. Figure 5. Situation maps of Cimadur River in dry and rainy season Figure 6. The sandbar-dotted spit in Cimadur estuary in different period On the other hand, this area is located in the tropical monsoon climate zone, due to the good vegetation along the coast and humid climate, so the carrying capacity of wind on continental sediment is limited. But for offshore and seabed beach, erosion and transport under the combination action of wave and current is also one of a significant sediment source. Because Bayah coast is located in the area under the action of SW swell, surrounding the offshore area of proposed project, such as the south of cape and the west coast the waves are all strong. There will be a certain transport movement for shore sand under the combination action of rip current and alongshore current induced by breaking waves, such as long shore transport and transverse transport of sediment. The sandbar-dotted spit in Cimadur estuary is the combined result of this type, which is shown in Figure 6. 64

5 9:2 (217) 3.2 Sediment Concentration The waters in proposed project is relatively clear, only after the rain it appears turbidity with diffusion of water suspended abrasive belt in Cimadur estuary, but in fact the measured concentration value is relatively limited. In 214, five sampling site sediment concentration measurement synchronized with the tide observation, the locations of sampling points are the same as the current measuring point. The statistical results of vertical average sediment concentration are shown in Table 3. Table 3. The statistical results of vertical average sediment concentration(unit: kg/m 3 ) Station Spring tide Medium tide Neap tide Average Maximum Average Maximum Average Maximum C C C C C It can be concluded from the results that, these change is small for the value of each station, and C3 is relatively smaller, C5 and C1 are slightly greater. And the average value of vertical average sediment concentration is between.32 kg/m 3 and.118 kg/m 3.The maximum value of vertical average sediment concentration is.147 kg/m 3 (C5 spring tide).the minimum value of vertical average sediment concentration is.12 kg/m 3 (C3 medium tide). As the measurement time is in December, so the above values can be used as a reference for rainy sediment concentration. 3.3 Sediment Characteristics The field the measurement is in December 213, there are 29 sampling points from the south of cape head to the north of Cimadur estuary. The sample analysis results show that, the sediments in project area are clay, slit, sand and gravel from fine to coarse in the order. The main ingredient is sand, and higher content in some sediments sample is gravel or silt individual. In all samples clay content is small to near zero. The median diameter is between.88mm and 8.472mm, in addition to the samples of 7-1 and 8-1 are greater than 1.mm. The median diameter in measured area is between.88mm and.414mm, and the average value is about.9mm, the main contents are fine sand and medium sand. The particle size of each measured section equally distributes. Results are similar with other stations, except the content near the south of cape head is slightly coarse. 4. Coastal Evolution and Sediment Movement Characteristics 4.1 Coastal Morphology and Beach Evolution Field reconnaissance was carried out twice in December 213 and January 214 in the proposed project area, including Rocky Beach in the east of cape, headland, proposed breakwater, wharf along the coast and both sides on Cimadur estuary. And the whole distance is about 4. km. Geomorphologic types in project area include low mountain, gentle hill, alluvial fan, alluvial plain, marine deposition terrace and beach etc. The modern coastal shore is mainly composed by bedrock coast and sandy coast. The headland of bedrock coast type alternates with the bay of sandy coast type, and the shoreline twists and turns, and many sites facing to the open area appear erosion features. Because of short of bathymetric data in project area to analyze beach evolution, satellite remote sensing image has been employed to study the change of coastline. Five remote sensing images (resolution ratio<.6m) from 26 to 213 are used. It can be seen that the coastline in the project area keep stable on the whole with slightly erosion, but there is a distinct change with coastline in mouth of the Cimadur River. On the combined effect of wave and runoff variation, the major flume in estuary oscillates, and morphology of mouth bar changes frequently. From the coastline change in recent years, the beach is under slightly erosion due to lack of sediment load from river and other sediment source. Estuary morphology is difficult to reach an equilibrium status, barrier spit and mouth bar will keep the change on the effect of runoff variation. The coastline where project locate is mainly comprise of bedrock and reef, and no large change occur in its morphology. When the equilibrium status is broken, such as build of jetty, both hydrodynamic conditions and sediment transport rate will change, so the erosion and deposition after project implement need to further analysis. 65

6 9:2 (217) Advances in Sciences and Engineering 4.2 Longshore Sediment Transport The coastline where project locate is mainly comprise of bedrock and reef, the area between river mouth and project is sandy coast which is dominated by longshore sediment transport and onshore-offshore sediment transport. From the direction of barrier spit and main wave direction(sw~ssw),the wave from SW~SSW is the major hydrodynamic factor for longshore sediment transport. The morphology of barrier spit on both side of river mouth is symmetrical. It indicated that the river is the dividing line direction of longshore sediment transport, in southern beach direction is from south to north, while in northern beach direction is from northwest to southeast. This characteristic is relate to the dominant wave direction(sw~ssw) and coastline trend. SMB formula [2], Xu Xiao formula [3], Chatu formula [3] and CERC formula [4] are employed to analyze the longshore sediment transport capacity. Table 4. Longshore Sediment Transport Rate in the Beach North of Project ( 14 m3 /a) Formula From south to north From north to south Net rate SMB (south to north) Xu Xiao (south to north) Chatu (south to north) CERC (south to north) It can be seen from the table, although there are some differences among the four formulas. Magnitude and change law is coincident. For beach north of project In natural conditions, longshore sediment transport capacity from south to north is about m 3 /a, the value from north to south is about m 3 /a, net transport rate of m 3 /a is from south to north. 4.3 Characteristics of Sediment Entrainment According to the analysis results for sediment sampling, entrainment condition under current are calculated employing Dou Guo-ren formula and Zhang Rui-jin formula [5]. (1) Dou Guo-ren formula (2) Zhang Rui-jin formula 1 2 ρ s ρ ghδ ε k (6) h + u.74lg 11 c = gd +.19 K s ρ d Where: h uc = d ρ s ρ 7 1 h (7) d ρ d h water depth d median diameter K s roughness height, for flat bed, if d.5mm, K s =.5mm, d>.5mm,k s=d; δ= cm; ε k=2.56cm 3 /s 2. Entrainment velocity under different water depths are listed in Table 5. Table 5. Entrainment velocity under different water depths(m/s) Isobaths(m) Dou Guo-ren formula Zhang Rui-jin formula Measured maximal velocity in spring tide maximal value is.3m/s in medium tide maximal value is.31m/s in neap tide value is maximal value.36 The results in table show that hydrodynamic force is weak in the project area, and current velocity is less than sediment entrainment velocity in different isobaths. Probability of bedload transport is very low, but for suspended sediment transport current is indispensable factor. Meanwhile, to further analyze sediment entrainment condition under waves, wave height formula for sediment entrainment named Sato-Tanaka formula (1962) is employed [6]. The results are listed in Table 6. (1) Surface entrainment 66

7 9:2 (217) (2) Completely entrainment H L H L D5 3 H = 1.35( ) 1/ sh(2πh1 / L) (8) L H D5 3 H = 2.4( ) 1/ sh(2πh2 / L) (9) L H Where, H and L are wave height and wavelength in deep water; D 5 is median diameter; h 1 and h 2 are critical depth for surface entrainment and completely entrainment; H and L are wave height and wave length at h 1 and h 2. Table 6. Wave Height for Sediment Entrainment under Different Water Depths isobath Wave height(m) (m) surface entrainment completely entrainment It can be seen from the calculation results, sediment on nearshore became completely entrainment under wave height of.85m, and sediments with smaller diameter are easier to be entrainment. The project is locate in the sandy beach with -16m water depth, it is need wave height of 2.m that sediment can be surface entrainment. Combing with the wave conditions is project area, sediment in the above water depth meet the entrainment condition under waves. In summary, sediment in project area is lifted by wave and transport by current. Especially in the wave breaking zone, the sediment is easier to transported under the effect of current and wave. There is no huge sediment sources, and sediment transport caused by the current and wave is relatively limited, therefore the whole coast in which the project locate is relatively stable, shoreline erosion and deposition can be basically equilibrium. 5. Numerical Modeling for sediment 5.1 Computational Domain and Mesh Generation Basing on the tidal current model, MIKE21 MT Module was employed to build sediment transport model with considering the wave effect. The computational domain used in sediment transport model is the same with it used in current model. In order to improve the computational efficiency and ensure enough resolution in project area, unstructured triangular mesh with local refinement is adopted. In open sea maximum mesh size is 1m and in project area mesh size is approximately 3m. A total of 37 compute nodes and 5699 grids are generated. Figure 7. The computational domain 67

8 9:2 (217) Advances in Sciences and Engineering 5.2 Calibration of numerical modeling Suspended sediment concentration varying with time at C1~C5 station during December, 213 are given by numerical model. Comparison results of measured and calculated date in spring tide, medium tide and neap tide. 5.3 Sedimentation results and analysis Deposition intensity in harbor is studied using the verified sediment transport model. And field of suspended sediment concentration in flood tide and ebb tide during the survey are depicted in Figure 8. In flood tide suspended sediment concentration round jetty head is relatively high because of high velocity of current bypassing. In ebb tide, due to jetty blocking ebb current, part of suspended sediment with river discharge cannot diffuse to open sea. Then in harbor district suspended sediment concentration during ebb tide is higher than the value during flood tide. Flood Tide Ebb Tide Figure 8. Field of Suspended Sediment Concentration in Flood Tide (project implementation) Deposition intensity of each part of the project had been calculated. The deposition intensity is.16m/a, the deposition volume of harbor district and channel is about m 3 /a. The method of estimation by empirical formula is also employed in the research. The harbor district is a semi open area, and a formula of estimating deposition for sandy coast which is proposed by Liu Jia-ju in 1988 is used [7]. K ωst 1 V2 d P = (1 + ψ ) γ 2 V1 d 1 2 (1) Where, P is annual averaged deposition intensity (m); ω is setting velocity; S is average suspended sediment concentration (kg/m 3 ), extracted from results of numerical model; t is deposition duration(s), (365 days) is used, in estimation of sudden deposition, 864(1 day) is used; r is the dry density of deposition(kg/m 3 ); K is the empirical coefficient; V1,V2 is the current velocity before and after project implement(m/s), extracted from results of numerical model. The results by empirical formula is shown that, in harbor district annual deposition intensity is.29 m/a, and averaged deposition volume is about m 3 /a. The results are close to the numerical model. Additionally, because the jetty blocking the current phenomenon of circumfluence exist in the project, the actual deposition volume should be 1.5~2 times as deposition volume without circumfluence. So, it needs to pay attention to monitor water depth. 6. References [1] Ben Clements, Robert Hall. Provenance of paleogene sediments in west java, Indonesia. proceedings, Jakarta26 international geosciences conference and exhibition. 26: 5-9. [2] Chen Shi-meng, etc. Coastal Dynamics (The second edition). Beijing: China Communications Press, [3] Xu Xiao. Directly calculating littoral transport rate with measuring wave data. The ocean engineering, 1996, 14(2):

9 9:2 (217) [4] U.S. Army Corps of Engineers. Shore Protection Manual. Coastal Engineering Research Center, U.S. Army Waterways Experiment Station, Corps of Engineers, Vicksberg, MS [5] Sediment Professional Committee of Chinese Hydraulic Engineering Society. Sediment Manual. Hebei Science and Technology Press [6] Shao Shou-liang. Formula Calculating for Sediment Critical Starting Depth and Starting Wave Height under Wave Loading. Port Engineering. 1993: [7] Code of Hydrology for Sea Harbor (JTJ223-98), Beijing: People Republic of China