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1.
Thick sequences of sediment surround the Whitsunday Islands on the middle shelf of the Great Barrier Reef (GBR) Platform. Much of this sediment is siliciclastic material deposited since the sea‐level highstand at around 6·5 ka. This raises a mass balance dilemma because modern terrigenous discharge to the GBR Platform is restricted to the inner shelf. Shallow seismic profiles and sediment samples were collected over 450 km2 around the Whitsunday Islands to quantify the mass of siliciclastic sediment for a dynamic model of the shelf. The sea floor and pre‐Holocene surfaces were mapped using 4584 stations along the seismic profiles and a graphical computer program. The total volume of sediment between these two surfaces is 3·67 ± 0·45 × 109 m3. This volume is composed of buried reefs (0·13 ± 0·01 × 109 m3), medium‐ (0·70 ± 0·30 × 109 m3) and fine‐grained shoals (2·84 ± 0·35 × 109 m3). The volume estimates combined with measurements of carbonate concentration, density and porosity indicate that 1850 ± 380 Mt of Holocene siliciclastic sediment surround the Whitsunday Islands in medium‐ (510 ± 225 Mt) and fine‐grained shoals (1340 ± 155 Mt). The total mass of siliciclastic material is 1·7–2·6 times that stored in Cleveland Bay, a similar sized repository on the inner shelf. A simple numerical model has been constructed to explain this large quantity of Holocene siliciclastic sediment. The model results in the appropriate siliciclastic mass next to the Whitsunday Islands by integrating regional shelf processes over time. Unlike the present day, rivers discharged sediment to the middle shelf during the early Holocene. This material was subsequently focused by northward transport into the vicinity of the islands, a geomorphologically complex region that serves as a sediment trap. Although direct riverine inputs to the middle shelf have stopped during the present sea‐level highstand, previously deposited siliciclastic sediment is continually being winnowed from the middle shelf and redeposited next to the Whitsunday Islands. The transport and distribution of siliciclastic sediment on the GBR Platform is thus influenced significantly by storage around islands on the middle shelf. 相似文献
2.
A study was conducted between April 2004 and September 2005 to estimate groundwater and nutrient discharge to the Neuse River estuary in North Carolina. The largest groundwater fluxes were observed to occur generally within 20 m of the shoreline. Groundwater flux estimates based on seepage meter measurements ranged from 2.86?×?108 to 4.33?×?108 m3 annually and are comparable to estimates made using radon, a simple water-budget method, and estimates derived by using Darcy’s Law and previously published general aquifer characteristics of the area. The lower groundwater flux estimate (equal to about 9 m3 s?1), which assumed the narrowest groundwater discharge zone (20 m) of three zone widths selected for an area west of New Bern, North Carolina, most closely agrees with groundwater flux estimates made using radon (3–9 m3 s?1) and Darcy’s Law (about 9 m3 s?1). A groundwater flux of 9 m3 s?1 is about 40% of the surface-water flow to the Neuse River estuary between Streets Ferry and the mouth of the estuary and about 7% of the surface-water inflow from areas upstream. Estimates of annual nitrogen (333 tonnes) and phosphorus (66 tonnes) fluxes from groundwater to the estuary, based on this analysis, are less than 6% of the nitrogen and phosphorus inputs derived from all sources (excluding oceanic inputs), and approximately 8% of the nitrogen and 17% of the phosphorus annual inputs from surface-water inflow to the Neuse River estuary assuming a mean annual precipitation of 1.27 m. We provide quantitative evidence, derived from three methods, that the contribution of water and nutrients from groundwater discharge to the Neuse River estuary is relatively minor, particularly compared with upstream sources of water and nutrients and with bottom sediment sources of nutrients. Locally high groundwater discharges do occur, however, and could help explain the occurrence of localized phytoplankton blooms, submerged aquatic vegetation, or fish kills. 相似文献
3.
Long-term riverbed response of lower Tedori River,Japan, to sediment extraction and dam construction
Effects of sediment extraction and dam construction on changes of riverbed characteristics over yearly to decadal scales in the lower Tedori River of Japan are clarified. Over 1950–1991, the riverbed degraded in excess of 0.5–3.5 m. Concurrently, riverbed sediment volume of the 0–16 km reach decreased by 12.7 × 106 m3. Intensive sediment extraction was the dominant cause of riverbed degradation during the period. During 1991–2007, an increase in riverbed sediment volume of 0.6 × 106 m3 resulted in accretion of the riverbed by average depth 0.04 m. The cessation of sand and gravel mining (SGM), coupled with Tedorigawa Dam operation since 1980, was responsible for that accretion. Temporal change in riverbed elevation during 1950–2007 indicates that there were five phases of vertical adjustment. Combination of nonlinear regression models described four of these phases well. During 1950–1979, the first four modes of empirical orthogonal function analysis successfully captured temporal and spatial responses of the riverbed to natural and anthropogenic impacts. That is, the first mode explained the mean riverbed profile and temporal variation in riverbed sediment volume. The second through fourth spatial eigenfunctions reflected spatial variation in vertical adjustment rate for phases II, III and I, respectively. The corresponding temporal eigenfunctions explained the respective effects on the riverbed of SGM, of imbalance between sediment transport capacity and sediment supply, and of dredging activity. 相似文献
4.
Variability of dissolved reactive phosphate flux rates in nearshore estuarine sediments: Effects of groundwater flow 总被引：1，自引：0，他引：1
Theoretical diffusive flux rates for dissolved reactive phosphate (DRP) were determined for sediments in a small area of the Indian River, Florida for the period March–May 1982. Flux rates from the sediment varied from 29 to 70 × 10?6g per m2 per day in seagrass associated sediments to 3–25 × 10?6g per m2 per day for an area devoid of seagrass. Simultaneous measurements of groundwater seepage velocities indicated greater velocities in seagrass associated sediments (1.03 × 10?6m per sec) than an area devoid of grass (0.77 × 10?6m per sec). Measured seepage flux accounted for more than 99% of the combined estimated diffusive and seepage flux of DRP for nearshore seagrass sediments. Also noted was an apparent direct relationship between tidal height, DRP and seepage velocity in nearshore sediments (25 m from shore) which further demonstrates the importance of hydrogeologic variables to these areas. 相似文献
5.
Assessment of the wave climate at near coast is vital for estimation of morphological changes, such as growth of sand spit
and associated siltation of tidal inlets. Vellar (bar-built) estuary is one of the prominent estuaries along the southeast
coast of India, located at 11°30′N and 79°46′E, less studied in terms of its morphological features. The inlet of Vellar is
exposed to high energetic waves, inducing large sediment transport rates and shoreline changes. Local wave characteristics
are not accurately defined and the available wave information at near coast is limited (point based observations). In the
present study, three decoupled numerical models are employed to derive the monthly nearshore wave climate at Vellar by transforming
waves from deep water to nearshore. These models are independently validated with buoy observations in deep water and wave
gauge data at nearshore. Based on the nearshore wave data, littoral drift along the coast was estimated and compared with
the spit growth at Vellar inlet. The estimated average littoral drift along this coast from February to October is 1.93 × 106 m3 toward north and from November to January it is 1.52 × 106 m3 toward south, resulting in a net northerly drift. Results indicated that increase in the wave energy during the period of
July to September is responsible for the maximum growth of the sand spit observed in the field. 相似文献
6.
海南岛北部浅海海砂资源丰富,以砂砾质沉积为主,集中于琼州海峡东口潮流沙脊区、河口三角洲和沿岸水下岸坡等海域。本文通过对海南岛北部海域的地形地貌、地层、底质类型等方面的调查,分东、西两个区域研究了海砂资源的分布、范围、规模和类型等地质特征,并估算了海砂资源量,结合沉积环境,评价了各区域海砂的资源潜力。研究区海砂分布广泛,但东、西区特征和规模具有较大差异,西区海砂以砂质砾和泥质砂为主,厚度5~10 m;东区海砂以砂、砾质砂和砂质砾为主,厚度10~50 m,规模远超西区。研究区共圈定出9处资源潜力区,资源量为91.4亿m3,其余区域为远景区,其资源量为71.5亿m3。根据地形地貌、水动力和沉积环境等因素,研究区海砂可分为潮流沙脊堆积体系、河口水下三角洲堆积体系、岬湾海岸水下岸坡堆积体系和侵蚀残留体系几种类型。其中位于琼州海峡东口浅滩区的潮流沙脊堆积体系海砂连续分布面积达328 km2,平均厚度超过25 m,资源量83.9亿m3,最具潜力。 相似文献
7.
An extreme rainfall event on August 9, 2009, which was close to setting a world record for 48-h accumulated rainfall, induced the Xiaolin deep-seated landslide, which was located in southwestern Taiwan and had volume of 27.6?×?106?m3, and caused the formation of a landslide dam. The landslide dam burst in a very short time, and little information remained afterward. We reconstructed the process of formation and failure of the Xiaolin landslide dam and also inferred the area of the impoundment and topographic changes. A 5?×?5-m digital elevation model, the recorded water stage of the Qishan River, and data from field investigation were used for analysis. The spectral magnitude of the seismic signals induced by the Xiaolin landslide and flooding due to failure of the landslide dam were analyzed to estimate the timing of the dam breach and the peak discharge of the subsequent flood. The Xiaolin landslide dam failure resulted from overtopping. We verified the longevity of the Xiaolin landslide dam at about 2 h relying on seismic signals and water level records. In addition, the inundated area, volume of the impoundment behind the Xiaolin landslide dam, and peak discharge of the flood were estimated at 92.3 ha, 19.5?×?106?m3, and 17?×?103?m3/s, respectively. The mean velocity of the flood-recession wave front due to the dam blockage was estimated at 28 km/h, and the peak flooding velocity after failure of the dam was estimated at 23 km/h. The Xiaolin landslide provides an invaluable opportunity for understanding the mechanism of deep-seated landslides and flooding processes following a landslide dam failure. 相似文献
8.
The core of the turbidity maximum zone in the Saint-Lawrence Estuary is located in the North Channel and oscillates in front of the large (3×106 m2) intertidal flats and marshes of Cap Tourmente. It is shown that seasonal fluctuations in the intensity and the position of this core are mainly determined by suspended sediment exchanges between the channel and the marshes. Fine sediments, most of them found 20 km downstream in the channel off Cap Maillard in late winter and early spring, are advected upstream over the flats during the summer months by the tide. The deposition, favored by marsh plant growth, reaches 5×105 metric tons in three months. A period of intense erosion, at a mean rate of 4,500 metric tons per tide, coincides with the destruction of the plant cover by migratory geese. The material removed fills up the Chenal de l’Île d’Orléans upstream and is flushed back into the water column during the next spring freshette. This rotating seasonal sediment circulation, although very localized, exerts a major influence on the distribution and storage time of suspended particles in the upper estuary. 相似文献
9.
R. Verney J.-C. Brun-Cottan R. Lafite J. Deloffre J. A. Taylor 《Estuaries and Coasts》2006,29(4):653-664
Tidal currents and the spatial variability of tidally-induced shear stress were studied during a tidal cycle on four intertidal
mudflats from the fluvial to the marine part of the Seine estuary. Measurements were carried out during low water discharge
(<400 m3 s−1) in neap and spring tide conditions. Turbulent kinetic energy, covariance, and logarithmic profile methods were used and
compared for the determination of shear stress. The cTKE coefficient value of 0.19 cited in the literature was confirmed. Shear stress values were shown to decrease above mudflats
from the mouth to the fluvial part of the estuary due to dissipation of the tidal energy, from 1 to 0.2 N m−2 for spring tides and 0.8 to 0.05 N m−2 for neap tides. Flood currents dominate tidally-induced shear stress in the marine and lower fluvial estuary during neap
and spring tides and in the upper fluvial part during spring tides. Ebb currents control tidally-induced shear stress in the
upper fluvial part of the estuary during neap tides. These results revealed a linear relationship between friction velocities
and current velocities. Bed roughness length values were calculated from the empirical relationship given by Mitchener and
Torfs (1996) for each site; these values are in agreement with the modes of the sediment particle-size distribution. The influence
of tidal currents on the mudflat dynamics of the Seine estuary was examined by comparing the tidally-induced bed shear stress
and the critical erosion shear stress estimated from bed sediment properties. Bed sediment resuspension induced by tidal currents
was shown to occur only in the lower part of the estuary. 相似文献
10.
JON TUNNICLIFFE MICHAEL CHURCH RANDOLPH J. ENKIN 《Boreas: An International Journal of Quaternary Research》2012,41(1):84-101
Tunnicliffe, J., Church, M. & Enkin, R. J. 2012 (January): Postglacial sediment yield to Chilliwack Lake, British Columbia, Canada. Boreas, Vol. 41, pp. 84–101. 10.1111/j.1502‐3885.2011.00219.x. ISSN 0300‐9483. Seismic records and evidence from sediment cores at Chilliwack Lake provide the basis for a long‐term (postglacial) sediment budget for a 324‐km2 Cordilleran catchment. Chilliwack Lake (11.8 km2 surface area), situated in the North Cascade Mountains, near Chilliwack, British Columbia, was formed behind a valley‐wide recessional moraine in the final phase of post‐Fraser alpine glaciation. Seismic surveys highlight the postglacial lacustrine record, which is underlain by a thick layer of sediments related to deglacial sedimentation. Sediment cores provide details of grain‐size fining from the delta to the distal lake basin. The cores also show a record of intermittent fire and debris flows. Magnetic measurements of lake sediments provide information on grain size, as well as a dating framework. The total postglacial lake‐floor deposit volume is estimated to be 397 ± 27 × 106 m3. Including estimates of fan and delta deposition, the specific postglacial yield to the lake is calculated to be ～86 ± 13 Mg km2 a?1. The sediment volume in the uppermost (Holocene) lacustrine layer is 128 ± 9 × 106 m3, representing ～41 ± 4 Mg km2 a?1 in the Holocene. Compared with other Cordilleran lakes of similar size, particularly those with glacial cover in the watershed, Chilliwack Lake has experienced relatively modest rates of sediment accumulation. This study provides an important contribution to a growing database of long‐term (postglacial) sediment yield data for major Cordilleran lakes, essential for advancing our understanding of the pace of landscape evolution in formerly glaciated mountainous regions. 相似文献
11.
In the lower delta of the Paraná River, at the head of the Río de la Plata estuary (Argentina), we compared net aboveground
primary production (NAPP) and soil properties of the dominant macrophyteScirpus giganteus (Kunth) in a floating and an attached marsh community. Both marshes are tidally influenced but in different ways. The floating
marsh site is relatively isolated from tidal influences because its ability to float makes it resistant to overland flow and
to sediment inputs from the estuary. The attached marsh lacks the capacity to float and receives sediment supplies from the
estuary through overland flow. These hydrologic differences are reflected in lower mineral content in sediments of the floating
marsh. Using a leaf tagging technique, estimated NAPP was 1,109 ± 206 g m−2 yr−1 for the floating marsh and 1,866 ±258 g m−2 yr−1 for the attached marsh. We attribute the lower NAPP of the floating marsh to isolation from sediment input from overland
flow. 相似文献
12.
The Mfolozi Estuary on the KwaZulu-Natal coast of South Africa is the most turbid estuary in Natal due to poor catchment
management, leading to large quantities of suspended particulate matter (SPM) entering the estuary from the Mfolozi River.
This paper quantities some of the solute and sediment dynamics in the Mfolozi Estuary where the main documented environmental
concern is the periodic input of SPM from the Mfolozi Estuary to the St. Lucia system, causing reduction of light penetration
and endangering biological productivity in this important nature reserve. Synoptic water level results have allowed reach
mean bed shear stresses and velocities to be calculated for an observed neap tidal cycle. Results indicate that ebb velocities
dominate the sediment transport processes in the estuary when fluvial input in the Mfolozi River is of the order of 15–20
m3 s–1. Observed and predicted flood tide velocities are too low (<0.35 m s–1) to suspend and transport significant amounts of SPM. Observed results indicate that although the SPM load entering the estuary
is dominantly from the Mfolozi River, the Msunduzi River flow plays a major role in the composition of the estuary's salinity
and velocity fields. It is calculated that the Mfolozi Estuary would fill with sediment in 1.3 years if it was cut off from
the sea. The major fluvial flood events help maintain the estuary by periodically pushing sediment seawards (spit progrades
seawards 5 m yr–1) and scouring and maintaining the main flow channel in the estuary. During low fluvial flow conditions, tidal flow velocities
will become the dominant control on sediment transport in the estuary. Interchange of SPM between the St. Lucia and Mfolozi
estuaries under present conditions is complicated by the strong transverse velocity shear between the two systems at their
combined mouth. This is creating a salinity-maintained axial convergence front that suppresses mixing of solutes and SPM between
the systems for up to 10 h of the tidal cycle during observed conditions.
Received: 22 May 1995 · Accepted: 31 July 1995 相似文献
13.
Vertical distribution and feeding types of nematodes from Chetumal Bay, Quintana Roo, Mexico 总被引：1，自引：0，他引：1
The vertical distribution and feeding type of nematodes in sediments of Chetumal Bay, Mexico, were studied in five intertidal transects along the urbanized zone in June and December 1995. Sediments were collected with a PVC corer to 6-cm sediment depth and cut immediately into three equal 2-cm depth fractions. Nematode density varied from 7.4 × 103 to 5.3 × 105 m?2 in June and from 1.7 × 104 to 7.2 × 105 m?2 in December. In June, the epistrate feederPseudochromadora sp. was the most abundant in the deepest sediment fraction (4–6 cm), whereas epistrate feeders,Neotonchoides sp.,Desmodora sp., and the deposit feederBathylaimus australis were dominant in the top most sediment (0–2 cm). In December, deposit feeders,Desmolaimus zeelandicus, Parodontophora sp., and the epistrate feederOncholaimus oxyuris were the most abundant in the deepest sediment, whereasNeotonchoides sp. andPseudochromadora sp. dominated the first 2 cm of sediments. Highest nematode density was recorded in the uppermost sediment layer (0–2 cm). Feeding types showed different abundance among transects and between months. There was a seasonal change in vertical distribution of nematodes, with the highest abundance in the deepest sediment layer in December, possibly due to the effect of wind waves on sediments of Chetumal Bay. The trophic composition of the nematode fauna in Chetumal Bay showed a dominance of deposit feeders and epistrate feeders, most likely in response to organic enrichment that is typical of eutrophic environments. 相似文献
14.
Abdalla E. M. Elsheikh Khalid A. Elsayed Zeielabdein Ibrahim A. A. Babikir 《Hydrogeology Journal》2009,17(8):2075-2082
The Khor Arbaat basin is the main source of potable water supply for the more than 750,000 inhabitants of Port Sudan, eastern Sudan. The variation in hydraulic conductivity and storage capacity is due to the heterogeneity of the sediments, which range from clay and silt to gravely sand and boulders. The water table rises during the summer and winter rainy seasons; it reaches its lowest level in the dry season. The storage capacity of the Khor Arbaat aquifer is estimated to be 21.75?×?106 m3. The annual recharge through the infiltration of flood water is about 1.93?×?106 m3. The groundwater recharge, calculated as underground inflow at the ‘upper gate’, is 1.33?×?105 m3/year. The total annual groundwater recharge is 2.06?×?106 m3. The annual discharge through underground outflow at the ‘lower gate’ (through which groundwater flows onto the coastal plain) is 3.29?×?105 m3/year. Groundwater discharge due to pumping from Khor Arbaat basin is 4.38?×?106 m3/year on average. The total annual groundwater discharge is about 4.7?×?106 m3. A deficit of 2.6?×?106 m3/year is calculated. Although the total annual discharge is twice the estimated annual recharge, additional groundwater flow from the fractured basement probably balances the annual groundwater budget since no decline is observed in the piezometric levels. 相似文献
15.
Sedimentation in a river dominated estuary 总被引：2，自引：0，他引：2
J. ANDREW G. COOPER 《Sedimentology》1993,40(5):979-1017
The Mgeni Estuary on the wave dominated east coast of South Africa occupies a narrow, bedrock confined, alluvial valley and is partially blocked at the coast by an elongate sandy barrier. Fluvial sediment extends to the barrier and marine deposition is restricted to a small flood tidal delta. Sequential aerial photography, sediment sampling and topographical surveys reveal a cyclical pattern of sedimentation that is mediated by severe fluvial floods which exceed normal energy thresholds. During severe floods (up to 10x 103 m3 s?1), lateral channel confinement promotes vertical erosion ofbed material. Eroded material is deposited as an ephemeral delta in the sea. After floods the river gradient is restored within a few months through rapid fluvial deposition and formation of a shallow, braided channel. Over an extended period (approximately 70 years) the estuary banks and bars are stabilised by vegetation and mud deposition. Subsequent downcutting in marginal areas transforms the channel to an anastomosing pattern which represents a stable morphology which adjusts to the normal range of hydrodynamic conditions. This cyclical pattern of deposition produces multiple fill sequences in such estuaries under conditions of stable sea level. The barrier and adjacent coastline prograde temporarily after major floods as the eroded barrier is reformed by wave action, but excess sediment is ultimately eroded as waves adjust the barrier to an equilibrium plan form morphology. Deltaic progradation is prevented by a steep nearshore slope, and rapid sediment dispersal by wave action and shelf currents. During transgression, estuarine sedimentation patterns are controlled by the balance between sedimentation rates and receiving basin volume. If fluvial sedimentation keeps pace with the volume increase of a basin an estuary may remain shallow and river dominated throughout its evolution and excess fluvial sediments pass through the estuary into the sea. Only if the rate of volume increase of the drowned river valley exceeds the volume of sediment supply are deep water environments formed. Under such conditions an estuary becomes a sediment sink and infills by deltaic progradation and lateral accretion as predicted by evolutionary models for microtidal estuaries. Bedrock valley geometry may exert an important control on this rate of volume increase independently of variations in the rate of relative sea level change. If estuarine morphology is viewed as a function of the balance of wave, tidal and fluvial processes, the Mgeni Estuary may be defined as a river dominated estuary in which deltaic progradation at the coast is limited by high wave energy. It is broadly representative of other river dominated estuaries along the Natal coast and a conceptual regional depositional model is proposed. Refinement of a globally applicable model will require further comparative studies of river dominated estuaries in this and other settings, but it is proposed that river dominated estuaries represent a distinct type of estuarine morphology. 相似文献
16.
Supply limited sediment transport in a high-discharge event of the tropical Burdekin River, North Queensland, Australia 总被引：1，自引：0，他引：1
Kathryn J. Amos Jan Alexander Anthony Horn† Geoff D. Pocock‡ Chris R. Fielding§ 《Sedimentology》2004,51(1):145-162
Interactions between catchment variables and sediment transport processes in rivers are complex, and sediment transport behaviour during high‐flow events is not well documented. This paper presents an investigation into sediment transport processes in a short‐duration, high‐discharge event in the Burdekin River, a large sand‐ and gravel‐bed river in the monsoon‐ and cyclone‐influenced, semi‐arid tropics of north Queensland. The Burdekin's discharge is highly variable and strongly seasonal, with a recorded maximum of 40 400 m3 s?1. Sediment was sampled systematically across an 800 m wide, 12 m deep and straight reach using Helley‐Smith bedload and US P‐61 suspended sediment samplers over 16 days of a 29‐day discharge event in February and March 2000 (peak 11 155 m3 s?1). About 3·7 × 106 tonnes of suspended sediment and 3 × 105 tonnes of bedload are estimated to have been transported past the sample site during the flow event. The sediment load was predominantly supply limited. Wash load included clay, silt and very fine sand. The concentration of suspended bed material (including very coarse sand) varied with bedload transport rate, discharge and height above the bed. Bedload transport rate and changes in channel shape were greatest several days after peak discharge. Comparison between these data and sparse published data from other events on this river shows that the control on sediment load varies between supply limited and hydraulically limited transport, and that antecedent weather is an important control on suspended sediment concentration. Neither the empirical relationships widely used to estimate suspended sediment concentrations and bedload (e.g. Ackers & White, 1973) nor observations of sediment transport characteristics in ephemeral streams (e.g. Reid & Frostick, 1987) are directly applicable to this river. 相似文献
17.
St. Lucia Estuary is on the subtropical, predominantly microtidal Zululand coast of South Africa. Lake St. Lucia's surface area fluctuates between 420 and 215 km2 and has a mean depth of less than 1 m. The 21-km-long narrows connects Lake St. Lucia with the Indian Ocean. Tidal effects penetrate 14 km up the narrows. The St. Lucia system has changed substantially since the 1930s due to bad farming techniques within its catchment. Large amounts of sediment were deposited in the estuary mouth, resulting in relocation of the Mfolozi River mouth to the south at Mapelane. The St. Lucia catchment was subjected to two devastating floods in the last ten years: Cyclone Domoina during February 1984 and the September 1987 cutoff low flood. After floods scoured out the estuary, marine sand advanced up the estuary at a rate of 1200 m/y as a series of flood-tidal deltas. Over 600,000 m3 of sediment accumulated in the St. Lucia Estuary mouth from February 1988 to November 1989. Of this amount, 466,000 m3 of sediment was removed by dredging, although this has not stopped the shoaling. During high rainfall years, the estuary mouth is able to maintain an open outlet to the sea, but as lake levels drop, shoaling causes the mouth to constrict and eventually close. Without the dredging program the mouth would ultimately close during low rainfall years, causing management problems. 相似文献
18.
Water and sediment movement in The Wash embayment has been determined from an extensive set of data, consisting of tidal current readings and suspended sediment concentration measurements. Instantaneous and residual currents in the embayment show a distinct lateral inhomogeneity, whereas vertically the water column is almost homogenous. The central deep water area (30–40 m) of the embayment is dominated by a residual landward water movement, whilst on the margins, the residual movement is seaward. Sediment is supplied predominantly in suspension from the north, through the northern extremity of Boston Deep. Suspended sediment pathways are coincident with the spring tide water movements and the subtidal channels act as the main conduits. Approximately 6·8 × 106 tonnes yr?1 of suspended sediments are supplied to the embayment from offshore areas. Bedload sediment supply is of lesser importance, ～ 1·4 × 104 tonnes yr?1. Whereas suspended sediment movement appears to be the dominant mode of transport throughout the embayment, bedload transport is important in reforming the sea bed into a variety of bedforms which are particularly well developed on the margins of channels and shoals. 相似文献
19.
J. J. Carrera-Hernández D. Carreón-Freyre M. Cerca-Martínez G. Levresse 《Hydrogeology Journal》2016,24(2):373-393
The city of Querétaro, located near the political boundary of the Mexican states of Querétaro and Guanajuato, relies on groundwater as it sole water supply. Groundwater extraction in the city increased from 21?×?106 m3/yr in 1970 to 104?×?106 m3/yr in 2010, with an associated drawdown of 100 m in some parts of the aquifer. A three-dimensional numerical groundwater-flow model has been developed that represents the historical evolution of the aquifer’s potentiometric levels and is used to simulate the effect of two scenarios: (1) a 40 % reduction in the extraction rate from public water supply wells in early 2011 (thus reducing the extraction to 62?×?106 m3/yr), and (2) a further reduction in 2021 to 1?×?106 m3/yr. The modeling results project a temporary recovery of the potentiometric levels after the 40 % reduction of early 2011, but a return to 2010 levels by 2020. If scenario 2 is implemented in 2021, the aquifer will take nearly 30 years to recover to the simulated levels of 1995. The model also shows that the wells located in the city of Querétaro started to extract water from part of the aquifer beneath the State of Guanajuato in the late 1970s, thus showing that the administrative boundaries used in Mexico to study and develop water resources are inappropriate, and consideration should be given to physical boundaries instead. A regional approach to studying aquifers is needed in order to adequately understand groundwater flow dynamics. 相似文献
20.
This paper examines the credibility and predictability of sediment flux of the Changjiang River that has discharged into the
seas on the basis of historical database. The assumption of the study stands on the lack of sufficient observation data of
suspended sediment concentration (SSC) during peaking flood period, which most likely results in the application of an inappropriate
method to the downstream-most Datong hydrological gauging station in the Changjiang basin. This insufficient method (only
30–50 times of SSC observation per year), that obviously did not cover the peaking SSC during peaking floods, would lead to
an inaccuracy in estimating the Changjiang sediment load by 4.7×108 t/a (multiyearly) into the seas. Also, sediment depletion that often takes place upstream of the Changjiang basin has, to
some extent, lowered the credibility of traditional sediment rating curve that has been used for estimating sediment budget.
A newly-established sediment rating curve of the present study is proposed to simulate the sediment flux/load into the seas
by using those SSC only under discharge of 60000 m3/s at the Datong station-the threshold to significantly correlate to SSC. Since discharge of 60000–80000 m3/s is often linked to extreme flood events and associated sediment depletion in the basin, unincorporating SSC of 60000–80000
m3/s into the sediment rating curve will increase the credibility for sediment load estimation. Using this approach of the present
study would indicate the sediment load of 3.3×108–6.6×108 t/a to the seas in the past decades. Also, our analytical result shows a lower sediment flux pattern in the 1950 s, but higher
pattern in the 1960 s–1980 s, reflecting the changes in landuse in the upstream of Changjiang basin, including widely devastated
deforestation during the middle 20th century. 相似文献