Along channel flow and sediment dynamics at North Inlet, South Carolina

In May of 2005, an observational program was carried out to investigate the along channel hydrodynamics and suspended sediment transport patterns at North Inlet, South Carolina. Along channel variability, which is important in establishing sediment transport pathways, has not been characterized for this system. Measurements of water column currents, salinity, bed sediment, suspended sediment concentration, and particle size distribution were obtained over a complete tidal cycle along the thalweg of the inlet entrance. Along channel currents, shear stress and bed sediment distributions vary significantly in space and time along a 3 km section bracketing the inlet throat. Most of the variability is consistent with geomorphic controls such as bed elevation variability and channel width. The highest velocities, shear stresses, suspended sediment concentration and bed sediment grain size are observed in the narrowest section of the inlet throat. Magnitudes systematically decrease along the channel toward the marsh as changes in channel geometry and branching reduces flow energy. Due to tidal asymmetry, the ebb phase contains significantly higher currents and associated sediment transport. Over the complete tidal cycle, depth integrated transport is directed towards the marsh landward of the intersection of Town and Debidue Creek. In contrast, net transport is out of the inlet seaward of this intersection. Sediment grain size distributions show 35% more material less than 63 μm on flood, suggesting net landward transport of fines.     

Morphodynamic Processes of Tidal Inlet and Ebb-Tidal Delta of Shuidong Bay

Based on the observed data in 1982- 1985 at Shuidong Bay area, west Guangdong Province, the morphodynamic processes of tidal inlet and ebb-tidal delta in the barrier-lagoon system are presented, including the dynamic features of tides and in the tidal inlet, the regional dyanmics and longshore sediment transport in the ebb-tidal delta, the genesis of the entrance bar, the recent erosion and deposition in the ebb-tidal delta and so on. The paper attempts to answer two questions, i. e., the stability of the tidal inlet and the feasibility of dredging on the entrance bar for the course of Shuidong Harbour. The results show that the stability of the tidal inlet is ideal and that dredging action on the entrance bar may be successful if dredging is deep enough and the course position reasonable.     

Net Sand Transport Direction in a Tidal Inlet, using Foraminiferal Tests as Natural Tracers

The relationship between the flux of exotic benthic foraminiferal tests (i.e. tests which are supplied from open-sea sources alone) in a tidal inlet and that of bulk sediment was analysed, which can be expressed as two first-order linear equations. According to this relationship, in order to determine net sediment transport directions in the entrance, the test concentration in surficial sediments of the tidal basin can be compared against a ‘ critical level ’. The critical level is determined for the conditions that no net transport of bulk sediment is present within the entrance. If the observed concentration (averaged over the tidal basin) is higher than the simulated critical level, then the net sediment transport is directed to landward. This method is applied to the analysis of net sand transport at Christchurch Harbour, a tidal inlet system located in southern England. In this investigation, concentrations of exotic foraminiferal tests in the surficial sediments of the tidal basin and ebb tidal delta area were obtained from the analysis of sea-bed sediment samples. A series of probable critical levels were calculated based upon the data sets with regard to: (1) sediment discharge from the rivers; (2) magnitude of sediment discharge within the entrance during the ebb; (3) the test concentration outside the harbour; (4) the thickness of the moving layer; and (5) two parameters associated with dispersive processes. The results show that the concentration in the tidal basin sediment is higher than a number of simulated critical concentrations for representative cases. Consequently, the high level of the concentration of exotic benthic foraminiferal tests within the harbour should be explained as a result of landward net transport of sands within the entrance.     

Sedimentary processes operating around the entrance to a river mouth port,Westport, New Zealand

The paper presents investigations of wave climate, tidal inlet hydraulics, and sand sediment bypassing at the entrance to Westport Harbour, South Island, New Zealand. The results complement and extend those of studies of bar morphologies and sediment characteristics already published. Longshore transport of about 1 × 10⁶ m³/year is directed in a net eastward fashion across the inlet because of an in‐built misalignment of the harbour training walls. Approximately 90% of the drift is bypassed, and has been since 1921, by deflection and splitting of the main sediment streams through the inner and outer bars and a transverse channel aoross the entrance. The outer bar appears to be the submarine, downdrift extension of Carters Breach and river load appears to contribute an order of magnitude less sediment to the complex than annual littoral drift. River sediments and littoral drift are mixed off the harbour and a declining proportion over time is recirculated to cause progradation of North Beach. The tidal compartment contributes little to scour of the entrance because of the predominance of bar bypassing. Contrary to the recommendations of several past studies, it is argued that improvements in navigation depths at West‐port are more likely to be obtained through modification of the littoral drift system than they are from tidal compartment enlargement.     

Sediment dynamic processes of the Yuehu inlet system, Shandong Peninsula, China

In order to investigate the sediment dynamic behavior of the Yuehu, a small inlet system characterized by abundant sediment supply and rapid sediment infilling, measurements and sampling were undertaken to obtain data sets of tidal water levels, current velocities, suspended sediment concentrations, grain size parameters, deposition rates and organic carbon contents. Sediment budget and the time–velocity asymmetry patterns of the inlet system were analyzed. The results show that the deposition rates are relatively high within the tidal basin. The total sediment flux cannot be balanced by the input from the open sea, the aerosol and biological production; rather, the material from land (which has been intensified by agricultural activities over the past several decades) represents a major component for the balance. Thus, the denudation rate must be reduced to protect the Yuehu as a natural reserve. Furthermore, it is found from the present study that the Yuehu inlet system exhibits all of the four time–velocity asymmetry patterns with varied frequencies of occurrence, compared with the two asymmetry patterns identified for larger inlet systems; such phenomena are partly due to the adjustment of entrance channel geometry. This behavior may be representative of the small tidal inlets at their late stage of morphological evolution and, therefore, may be utilized to prolong the lifespan of small inlet systems.     

山东荣成月湖潮汐汊道的时间-流速不对称特征

月湖是山东半岛东端的一个小型潮汐汊道.在月湖进行了潮位、潮汐观测及地形测量,运用两种方法从潮位记录推算出口门落潮主干道垂线平均流速和断面平均流速.发现除涨潮优势型和落潮优势型外,月湖还表现出另外两种时间-流速不对称,即涨潮历时大于落潮历时且涨潮流速大于落潮流速、涨潮历时小于落潮历时且涨潮流速小于落潮流速.月湖的时间-流速不对称特征与不规则潮汐、涨(落)潮的流速跟潮位的匹配关系、汊道口门的断面形态、口门特殊的水流结构及循环过程等因素有关.时间-流速不对称类型的研究,对于理解潮汐汊道系统的沉积动力行为及进行人工整治有一定意义.     

Mean water level setup/setdown in the inlet-lagoon system induced by tidal action-a case study of Xincun Inlet, Hainan Island in China

With the tides propagating from the open sea to the lagoon, the mean water level （MWL） in the inlet and lagoon becomes different from that at the open sea, and a setup/setdown is generated. The change of MWL （setup/setdown） in the system imposes a great impact on regulating the development of tidal marshes, on determining the long-term water level for harbor maintenance, on the planning for the water front development with the flood control for the possible inundation, and on the interpretation of the historical sea level change when using tidal marsh peat deposits in the lagoon as the indicator for open sea＇ s sea level. In this case study on the mechanisms which control the setup/setdown in Xincun Inlet, Hainan in China, the 2-D barotropic mode of Eulerian - Lagrangian CIRCulation （ELCIRC） model was utilized. After model calibration and verification, a series of numerical experiments were conducted to examine the effects of bottom friction and advection terms, wetting and drying of intertidal areas, bathymetry and boundary conditions on the setup/setdown in the system. The modeling results show that setup occurs over the inlet and lagoon areas with an order of one tenth of the tide range at the entrance. The larger the bottom friction is, a larger setup is generated. Without the advection term, the setup is reduced clue to a decrease of water level gradient to compensate for the disappearance of the advection term. Even without overtides, a setup can still be developed in the system. Sea level rise and dredging in the inlet and tidal channel can cause a decrease of setup in the system, whereas shoaling of the system can increase the setup. The uniqueness of the Xincun Inlet with respect to MWL change is that there is no evident setdown in the inlet, which can be attributed to the complex geometry and bathymetry associated with the inlet system.     

Acid-leachable trace metals in sediments from an industrialized region (Ennore Creek) of Chennai City,SE coast of India: An approach towards regular monitoring

The article presents the results for enrichment of acid-leachable trace metals (ALTMs) from Ennore Creek in north Chennai, a metropolis on the southeast coast of India. ALTMs Fe, Mn, Cr, Cu, Ni, Co, Pb, Zn and Cd along with sediment texture, OC and CaCO₃ were analyzed in surface sediments collected during two different seasons, pre-monsoon (PRM) and post-monsoon (POM) seasons to identify and observe the input of trace metals in the creek from various sources in the city limits. The most prominent feature of the ALTMs is the enrichment of Fe, Cr, Cu, Ni, Pb and Zn in the sediments, which is mainly attributed to the intense industrial activities around Chennai, and to the rapid industrialization policies. The ALTMs also indicate their association with the finer fractions, OC and Fe–Mn oxyhydroxides. The enrichment is very well supported by the correlation, grouping and clustering of ALTMs in statistical analysis. The differential behavior of ALTMs in POM season compared to PRM season is possibly due to the excess level of industrial effluents in the channel feeding Ennore Creek. Comparative results of ALTMs with other estuarine regions also indicate that the study area has been enriched with trace metals during the past two decades. The results of the present study suggest the need for a regular monitoring program which will help to improve the quality of Ennore Creek.     

Long-term morphological modeling of a tidal inlet: the Arcachon Basin,France

The Arcachon Lagoon on the French Atlantic coast is a triangular shaped lagoon of 20 km on a side connected to the ocean by a 3-km wide inlet between the mainland and an elongated sand spit. This tidal inlet exhibits a particularly active morphology due to locally strong tidal currents and rough wave conditions. During the past 300 years, minimum and maximum spatial extents of the Cap Ferret sand spit have varied by 8 km while one or two channels have alternately allowed circulation between the lagoon and the ocean. These impressive morphological changes have never prevented regular flushing of the lagoon, eventhough the spit came as close as 300 m from the coast during the 18th century. According to Bruun's concept of tidal inlet stability [Theory and Engineering (1978), 510 pp.], the balance between longshore littoral transport and the tidal prism ensures the perpetuity of the inlet.Process modeling was believed to give better insight into the respective roles of tides and waves in driving the long-term morphological changes of the inlet. A two-dimensional horizontal morphodynamic model was therefore developed, combining modules for hydrodynamics, waves, sediment transport and bathymetry updates. The use of process models at a scale of decades requires a schematization of the input conditions. We defined representative mean annual wave and tide conditions with respect to sediment transport, i.e. conditions that induce the same annual transport as measured in the field. Driven by these representative conditions, simulations run from the 1993 bathymetry show that the tide is responsible for the opening of a new channel at the extremity of the sand spit (where tidal currents are the strongest), while waves induce a littoral transport responsible for the longshore drift of sand bodies across the inlet. One particular simulation consisted in running the model from a hypothetical initial topography where the channels are filled with sand and the entire inlet is set to a constant depth (3 m). The results show the reproduction of a channel and bar system comparable to historical observations, which supports the idea that the lagoon is unlikely to be disconnected from the ocean, provided tide and wave conditions remain fairly constant in the following decades.     

海南岛万泉河口海岸动态及其整治对策

通过应用沉积动力学方法计算了万泉河口口门均衡过水断面面积和沿岸输沙率．计算结果表明,波浪对万泉河口沿岸沉积物的输送起着控制作用,南北向输沙作用强烈,同时潮汐汊道目前处于不稳定阶段,口门断面的变化很大。在分析了万泉河口的自然地理条件以及基本演变规律后,还从增加纳潮量、扩大纳潮水域面积、整治沙美内海及其周边生态环境、建设导堤工程改善拦门沙的浅滩航道等方面提出了万泉河口潮汐汉道口门和航道治理的一些工程和生态措施。     

山东荣成湾月湖口门落潮干道的推移质输运

改进了Gao等 ( 1 994)方法 ,并用以计算潮汐汊道口门落潮干道的垂线平均流速。利用月湖潮汐汊道系统冬、夏季各一个月的潮位资料 ,计算落潮干道的潮流流速和推移质输运率。结果表明 ,流速的计算值和实测值呈良好的相关性 ,落潮干道内落潮流的输沙能力高于涨潮流输沙能力     

Morphodynamic behaviour of a mixed sand–gravel ebb-tidal delta: Deben estuary,Suffolk, UK

The morphodynamics of inlets and ebb-tidal deltas reflect the interaction between wave and tidal current-driven sediment transport and significantly influence the behaviour of adjacent shorelines. Studies of inlet morphodynamics have tended to focus on sand-dominated coastlines and reference to gravel-dominated or ‘gravel-rich’ inlets is rare. This work characterises and conceptualises the morphodynamics of a meso-tidal sand–gravel inlet at the mouth of the Deben estuary, southeast England. Behaviour of the inlet and ebb-tidal delta over the last 200 yr is analysed with respect to planform configuration and bathymetry. The estuary inlet is historically dynamic, with ebb-tidal shoals exhibiting broadly cyclic behaviour on a 10 to 30 yr timescale. Quantification of inlet parameters for the most recent cycle (1981–2003) indicate an average ebb delta volume of 1 × 10⁶ m³ and inlet cross-sectional area of 775 m². Bypassing volumes provide a direct indicator of annual longshore sediment transport rate over this most recent cycle of 30–40 × 10³ m³ yr^− 1. Short-term increases in total ebb-tidal delta volume are linked to annual variability in the north to northeasterly wind climate. The sediment bypassing mechanism operating in the Deben inlet is comparable to the ‘ebb delta breaching’ model of FitzGerald [FitzGerald, D.M., 1988. Shoreline erosional–depositional processes associated with tidal inlets, in: Aubrey, D.G., Weishar, L. (Ed.), Hydrodynamics and Sediment Dynamics of Tidal Inlets. Springer-Verlag Inc., New York, pp. 186–225.], although the scales and rates of change exhibited are notably different to sand-dominated systems. A systematic review of empirical models of sand-dominated inlet and ebb-tidal delta morphodynamics (e.g. those of [O'Brien, M.P., 1931. Estuary tidal prisms related to entrance areas. Civil Engineering, 1, 738–739.; Walton, T.L., and Adams, W.D., 1976. Capacity of inlet outer bars to store sand. Proceedings of 15th Coastal Engineering Conference, 1919–1937.; Gaudiano, D.J., Kana, T.W., 2001. Shoal bypassing in mixed energy inlets: geomorphic variables and empirical predictions for nine South Carolina inlets. J. Coast. Res., 17, (2), 280–291.]) shows the Deben system to be significantly smaller yet characterised by a longer bypassing cycle than would be expected for its tidal prism. This is attributed to its coarse-grained sedimentology and the lower efficiency of sediment transporting processes.     

海南岛洋浦港潮汐汊道口门的均衡过水面积

将汊道均衡与潮汐特征，纳潮量，淡水径流量及沉积物搬运格局相联系，确定了洋浦港的均衡条件，根据纳潮盆地的水面面积－水面高程曲线以及相关的特征潮位估算平均纳潮量，利用１９７７－１９７９年波浪观测资料和ＣＥＲＣ公式计算沿岸输沙率；此外，还用改进的Ｇａｄｄ公式确定口门涨、落潮流输沙率，计算中引入汊道口门流速频率分布函数的定义。由此而得洋浦港均衡过水面积为５８００ｍ＾２，与Ｏ＇Ｂｒｉｅｎ方法所得结果相比，本     

海南岛博鳌港洪水过程模拟

采用一个包括陆地淡水径流和风暴潮影响的模型计算了海南省博鳌港的洪水水位。模型中的外海潮汐考虑K^1和O^1分潮，风暴增水和河流流量的变化过程采用峰值函数的形式模拟，取得了与实测结果相近的计算结果。模拟实验表明，洪水水位峰值与河流洪峰流量，风暴增水峰值，系统的过水断面形状以及洪水历时的长短等因素有关。对于博鳌港口门通航条件整治，采用复合的梯形过水断面最为合适，因为相对于天然的口门过水断面，复合梯形断面能使湾内的最高洪水水位下降约20％。     

Process-based modeling of morphodynamics of a tidal inlet system

The morphodynamic evolution of an idealized inlet system is investigated using a 2-D depthaveraged process-based model,incorporating the hydrodynamic equations,Englund-Hansen’s sediment transport formula and the mass conservation equation.The model has a fixed geometry,impermeable boundaries and uniform sediment grain size,and driven by shore-parallel tidal elevations.The results show that the model reproduces major elements of the inlet system,i.e.,flood and ebb tidal deltas,inlet channel.Equilibrium is reached after several years when the residual transport gradually decreases and eventually diminishes.At equilibrium,the flow field characteristics and morphological patterns agree with the schematized models proposed by O’Brien (1969) and Hayes (1980).The modeled minimum cross-sectional entrance area of the tidal inlet system is comparable with that calculated with the statistical P-A relationship for tidal inlets along the East China Sea coast.The morphological evolution of the inlet system is controlled by a negative feedback between hydrodynamics,sediment transport and bathymetric changes.The evolution rates decrease exponentially with time,i.e.,the system develops rapidly at an early stage while it slows down at later stages.Temporal changes in hydrodynamics occur in the system;for example,the flood velocity decreases while its duration increases,which weakens the flood domination patterns.The formation of the multi-channel system in the tidal basin can be divided into two stages;at the first stage the flood delta is formed and the water depth is reduced,and at the second stage the flood is dissected by a number of tidal channels in which the water depth increases in response to tidal scour.     

Water quality impacts of stormwater discharges to Santa Monica Bay

Urban stormwater runoff is a major source of contaminants to southern California's coastal waters, yet little is known about the fate and effects of these discharges. A 3-year multidisciplinary project was conducted to investigate the dispersion of stormwater plumes in Santa Monica Bay and the resultant impacts on the water column and benthos. This paper describes the toxicity component of the study. Sea urchin fertilization toxicity tests were conducted on stormwater from the two largest discharges into the bay: Ballona Creek, which drains a highly urbanized watershed, and Malibu Creek, which receives runoff from a largely undeveloped watershed. Every sample of Ballona Creek stormwater tested was toxic (usually >5 toxic units), while Malibu Creek stormwater had a lower frequency and magnitude of toxicity (usually <4 toxic units). Surface water samples collected within the Ballona Creek stormwater discharge plume were always toxic whenever the concentration of stormwater in the plume exceeded 10%. The toxic portion of the Ballona Creek stormwater plume extended more than 4 km offshore on one occasion. Toxicity identification studies indicated that zinc was the primary cause of toxicity in both Ballona Creek stormwater and the discharge plume. No acute sediment toxicity (10-day amphipod survival) was present in the study area, although interstitial water toxicity was present at some stations located near the mouth of Ballona Creek. Differences in watershed characteristics likely were responsible for the greater toxicity of the Ballona Creek stormwater discharge plume. The Ballona Creek watershed contained a greater degree of urbanization (83% versus 12% for Malibu Creek) and the presence of a network of concrete flood control channels resulted in a stormwater plume containing elevated concentrations of toxins that received less initial dilution (compared to Malibu Creek) in the nearshore environment.     

Wave forces and moments on an intake well

A sea water intake well of size 20 m diameter and 15.5 m height in a water depth of 9.8 m is proposed north of the Visakhapatnam Port for a project to extract magnesia from sea water. A 1:25 scale model of the intake well was tested in the wave basin of the Ocean Engineering Centre, Indian Institute of Technology, Madras to measure the wave forces and moments on the intake well and the variation of water levels inside and outside the well. Accordingly, an intake well model of 0.8 m diameter and 0.62 m height was fabricated and fixed over a false bottom in a wave basin. The well model was subjected to the action of both regular waves for two test conditions, intake well inlet closed during installation and intake well inlet open. The experimental results on wave forces and moments were compared with the results of the Linear Diffraction Theory. The water level inside the well was measured to determine the submergence of suction pipes of pumps and location of the inlet opening of the intake well. The wave crest elevation in front of the well was also measured in order to fix the deck level of the well so as to avoid water overspill onto the deck. The salient results of the present study are presented and discussed in this paper.     

Random walk, zonation and the food searching strategy of Terebralia palustris (Mollusca, Potamididae) in Kenya

Terebralia palustris is a common mud-whelk present at a particularly high density in all Indo-West Pacific mangroves. Young snails feed on nothing but mud while larger specimens are able to feed on fallen leaves too. In Kenya (Mida Creek) under the canopy, competition for mangrove leaves can be very high due to the high density of Sesarmidae crabs. On open exposed muddy platforms, no Sesarmidae occur but the leaf density is very low because the leaves are only randomly present as they are deposited and removed twice a day by the tide. However, the snail density is always very high, raising the question as to whether the snails use a special searching strategy to optimize their resource finding rather than a purely random movement. By analyzing the snails' movements on a uniform area at different levels and comparing them with simulated random paths, we could show that the snails' movements are not purely random. The distribution of different size classes of T. palustris in Mida Creek was known to be quite odd: the same simulation approach suggests that the zonation asymmetry could reasonably be due to the stochastic recruitment of juveniles in space and time and maintained by a substantial long-lasting spatial inertia.     

Successful local marine conservation requires appropriate educational methods and adequate enforcement

MPAs and stakeholder education are marine conservation cornerstones, but data to assess adherence to regulations and the success of educational methods are missing. Local MPAs have been established to protect inter-tidal mudflats and shore users from bait collection which is a contentious worldwide issue. Video cameras monitored activity and confirmed if collectors adhered to the rules at three UK sites with different MPA systems. An educational approach (a voluntary code leaflet) was also assessed through stakeholder discussion and observation. Fareham Creek and Dell Quay supported a considerable number of collectors with none observed at Pagham Harbour. At Fareham Creek bait dug areas were evident in discrete patches in unprotected and protected areas, but observed collectors mainly used the latter. The failure to exclude collectors is due to the lack of enforcement. At Dell Quay virtually all dug areas were outside protected areas and was confirmed by the camera footage. Success is attributed to regular on-the-ground ‘unofficial’ enforcement by the managing NGO. Of the retailers, 75% had heard of the code and the majority stated they followed it. However, none of the 26 collectors observed followed a key rule (e.g. backfilling holes). Local marine conservation is relatively cheap and can be effective, but only if: management matches the actual pressure; scientific evaluation for all components (including education) is integrated from the beginning; adequate site enforcement is included; education methods are active, two-way and sustained.     

A Siltation Simulation and Desiltation Measurement Study Downstream of the Suzhou Creek Sluice, China

The Suzhou Creek Sluice is currently the largest underwater plain gate in the world, with a single span of 100 m. It is located in a tidal estuary at the junction of the Huangpu River and Suzhou Creek in Shanghai, China. In this study, physical and 2D vertical mathematical models were used to investigate and distinguish the mechanism of siltation downstream of an underwater plain gate from that of other gates types. According to quantitative data obtained by site investigation and the application of the physical hydrodynamic models, it was found that the characteristics of the tidal estuary as well as the fact that the sluice span is equal to the creek width are the major reasons contributing to siltation. A possible desiltation treatment system is proposed for the underwater plain gate. The system includes selection of a suitable location that allows the determination of a reasonable top elevation of the sluice floor, reserving sufficient space under the gate to accommodate siltation, setting up a mechanical desiltation system, and flushing silt along with overflow over the top of the gate. Furthermore, on-site hydraulic silt flushing experiments and a topography survey were conducted. These results showed that the measurement system is effective, and by maintaining this scheduled operation once a month, the downstream riverbed has been maintained in a good condition.