Dredged Areas Research Articles

Influences of meteorological conditions, runoff, and bathymetry on summer thermal regime of a Great Lakes estuary

To better understand the primary drivers of the thermal regime in a Great Lakes estuary, and their implications for local biota, water temperature variations in the Milwaukee Estuary of Lake Michigan were studied between July and October of 2019 using a network of 25 sensors at 18 locations. Like Lake Michigan, the estuary was thermally stratified July to October, and historically dredged channels with water depths greater than 6 m allowed for the free movement of cold lake water into the estuary. However, temperatures in the estuary fluctuated rapidly both spatially and temporally, reflecting lake temperature fluctuations driven by changing meteorological conditions. Lake-driven upwelling and downwelling events influenced water temperature more than tributary contributions. Periodic upwelling and downwelling events caused temperature changes by up to 15 °C in less than 24 h. Nearshore upwelling events occasionally allowed cold, hypolimnetic lake water to reach tributary portions of the estuary beyond dredged areas, while downwelling events disrupted thermal stratification and caused the deep, dredged portions of the estuary to exceed 20 °C. Thermal impacts from these events were especially noticeable in transition zones between dredged and not dredged channels less than 2 m deep. The warming effects from downwelling persisted longer inside the estuary – up to two weeks – than cooling effects from upwelling, which typically lasted a few days. The predominantly lake-driven, rapid summer water temperature fluctuations observed in the Milwaukee Estuary serve as an important consideration in habitat restoration activities happening in Great Lakes estuaries.

Effects of Sediment Dredging on the Reduction in Sediment Internal Loading of Lake Taihu and the Self-recovery Ability of Benthic Organism

Sediment dredging has a great effect on the control of lake internal loading and is one of the important methods for lake internal loading management. In this study, the dredged area of Taihu Lake was used as the main object. An estimation of the reduction in whole lake internal loading of Taihu Lake in decade years was carried out. At the same time, we evaluated the effect of sediment dredging on the control of internal loading in the northern area of Taihu Lake (Zhushan Bay and Meiliang Bay). The results indicated that a total of 42 million cubes of sediment was dredged from Taihu Lake, and the total nitrogen, total phosphorus, and organic matter in the dredged sediment was estimated to be approximately 6.26×104 tons, 1.83×104 tons, and 11.7×105 tons, respectively. This was roughly equal to the 20 years of external loading pollution accumulated in Taihu Lake. From a long-term perspective, sediment dredging could effectively increase the water quality of Meiliang Bay within five years and that where the external loading has been controlled effectively. However, the water quality of Meiliang Bay subsequently rebounded, but dredging still reduced the nitrogen and phosphorus content in surface sediment. On the contrary, dredging could not effectively control the internal loading of Zhushan Bay, which still had a large input of external loading. The amount of sediment internal loading recovered to the original level of the pre-dredging period. In a six-year-long period of continuous monitoring of the benthic organism community of Zhushan Bay, the results indicated that sediment dredging could cause negative effects on sediment dredging initially, but the density and biomass of the benthic organisms in the dredged area had later been recovered to the un-dredged level. There was no difference between the dredged and un-dredged areas with regard to the diversity indices of benthic organisms. The results of this study indicated that sediment dredging can effectively control the lake internal loading. However, the maintenance period of dredging effects was related closely to the input intensity of the external loading. In addition, sediment dredging did not have a large influence on the benthic organism community and could recover to the original level depending on self-recovery.

The “San Giacomo di Galizia” Warship Galleon (1597)—Building Narratives through an Archaeological and Historical Reading of the Ribadeo I Shipwreck

Early November 1597. After an intense combat with four enemy ships, San Giacomo di Galizia (also known as Santiago), a just over 1000-ton galleon, enters the Ribadeo harbour in a terrible state, where it wrecks. This war vessel had been built in Naples in 1590 and sailed the Mediterranean and the Atlantic until it sank. In late November 2011, during an archaeological survey of the dredge area to improve the navigation of the ports in Galicia, a large ship was found and identified as the San Giacomo, which wrecked 414 years prior to its discovery. Several archaeological campaigns permitted a thorough record of the wreck and the recovery of hundreds of objects which this ship carried on its final journey. These artefacts included ceramics, metalwork, and wood, objects which reflected the activities that occurred on board during its short life. Combining an interdisciplinary approach based on artefacts, documents, and chemical analysis, the aim of this paper is to, on the one hand, attempt to reconstruct the sailing itinerary of the ship over its period of use and, on the other, to discuss how these commodities can help to write new narratives about the activities which occurred on board.

An Attempt to Study Foundation Anchoring Conditions in Sedimentary Estuaries Using Integrated Methods

The search for and knowledge of the best conditions for anchoring the foundations of certain structures such as bridges, tunnels and quays in sedimentary estuaries is a challenge, for both scientists in general and engineers in particular. Indeed, wharves are structures that receive a lot of stresses and therefore require anchoring to avoid tilting and to guarantee their stability during service. This work, based on the analysis of data from seismic refraction methods, mechanical soundings and laboratory tests, characterises the terrain of the Wouri estuary in Central Africa. The objective is to determine and present the subsurface layers encountered as well as their characteristics, in order to define the best conditions for anchoring the foundations to ensure the stability of the quays to be built there. The seismic refraction campaign shows that the study area is relatively heterogeneous over the first 25 m, with velocities measured in the range 1520–1750 m/s; modulated in two distinct ranges, between 1520–1580 m/s characteristic of mud and loose sediments (alternating layers of clay, sand, loose silt) and the range 1580–1750 m/s corresponding to the signature of sandy-silty or compact clays. The mechanical tests show sedimentary soils, with alternating layers of sandy clay and clayey sand over the 42 m drilled, loose over the first 30 m in the bank area and over the first 15 m in the canal or dredge area, with a limit pressure of less than 1 MPa. Similarly, the soil samples taken and tested in the laboratory show that the soils are clayey over the first 30 metres, plastic and liquid with respect to their water content, respectively, below and above the liquidity limits, confirming their loose character. The results of seismic refraction, mechanical soundings and laboratory tests show that, in estuarine areas characterised by alternating sandy clay and clayey sand, there are not always hard formations in the first 25 metres of depth but, from a depth of 30 m, the soils become moderately compact and begin to form an anchoring layer sufficient to guarantee the stability of the quays against earth pressure forces.

MATAGORDA SHIP CHANNEL AND ITS ENTRANCE'S NAVIGATION EFFICIENCY AND SAFETY IMPROVEMENT VIA ADCIRC AND CMS MODEL SIMULATION

The present study investigated alternatives including deepening/widening of Matagorda Ship Channel in the south central coast of Texas to improve navigation safety through entrance inlet channel by using ADCIRC and CMS numerical models. The alternatives modify bayside channel depths from 38 ft to 50 ft, referenced to Mean Lower Low Water (MLLW), and widths from 200 ft to 600 ft, and deepen the entrance channel from 38 ft to 55 ft MLLW and widen the channel from 300 ft to 600 ft. The alternatives include seven new dredged material placement areas along the ship channel. Model results show the proposed ship channel dimensions will slightly increase flow efficiency and current magnitude in Matagorda Bay. Current velocities in and around ship channel tend to increase with alternatives, large river input and future sea level rise in the region. However, the current effect becomes more pronounced during tropical storms.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/YAugdJe9dIw

Sediment Transport Model Quantifies Plume Length and Light Conditions From Mussel Dredging

Mussel dredging causes resuspension of sediment particles that reduce water clarity and potentially leads to reduced eelgrass growth. In order to study the impact of resuspension from mussel dredging on light conditions in the water column, field experiments were conducted at two sites in the Limfjorden. Light loggers were placed in two circular arrays around the dredge area. Vertical profiles of current velocity were measured by an ADCP and the sediment particle size composition was obtained from sediment core samples. The field data was used to force, calibrate and validate a sediment transport model developed in the FlexSem model system. Changes in sediment concentrations during and after mussel dredging were modelled for the two sites and for seven scenarios. We found that the distance and direction of the plume in the model was in good agreement with light logger data. The plume duration was less than 1 hour, and the impact range was between 260-540 m. The scenarios showed that fishing intensity and current speeds were most important for shaping the sediment plumes. Changes in suspended sediment concentrations were 0.62 to 1.79 mg/l on median average and 1.22 to 11.61 mg/l for the upper quantile of the plume, which were on the same order of magnitude as background values in the Limfjorden. The amount of fishing days during the eelgrass growth season was 6-8% in Lovns Bredning and 16-35% in Logstor Bredning and less than 1-2% of the total area was dredged per season. Even though there are substantial changes in the light conditions from the sediment plumes, the overall spatio-temporal impact in the study area is considered low. We recommend that management plans in other areas could sustain a shellfish fishery by limiting fishing intensity and frequency near eelgrass beds. The presented approach combines observational data, sediment transport modelling and reported fishing activity. It is a step forward within sediment transport modelling and could be incorporated into environmental impact assessments. The results have recently been used as scientific background for recommendations to improve the management plans according to the Danish Mussel Policy and relevant EU Directives.

Dredging Impacts on Soil Properties of the Kankakee River System 150 Years after Perturbation

Freshwater marshes provide ecosystem services such as improving water quality by storing water and filtering nutrients from upland runoff, minimizing erosion and flooding by reducing stream and river velocity and peak flows, and carbon sequestration by storing organic matter for long extended periods of time. These ecosystem services have increased interest in the protection and restoration of marshes and soil properties and are an important consideration for successful restoration. This study was conducted in Indiana, USA, within the historical extent of the Grand Kankakee Marsh that once encompassed 202,343 ha, until the dredging of the Kankakee River from 1852 to 1917 to convert the marsh to cropland. Current (in 2019) soil properties (organic matter mass and concentration, bulk density, moisture content, and carbonate concentration) were compared between (1) marshes that were dredged and presently in other land-use categories such as cropland and forests, and (2) remnant marshes that were not impacted by dredging. On average, organic matter mass and concentration were not significantly different between dredged areas and marshes (p > 0.05). However, marshes tended to have lower bulk densities, greater moisture contents, and greater carbonate concentrations compared to dredged areas. The soil properties of remnant marshes in this study can be used to evaluate the success of marsh restoration efforts in regions with similar soils, climate, and land-use history.

Dredging activity and associated sound have negligible effects on adult Atlantic sturgeon migration to spawning habitat in a large coastal river.

Dredging is considered a major threat/impedance to anadromous fish migrating to spawning habitat. Due to the perceived threat caused by dredging, environmental windows that restrict dredge operations are enforced within many rivers along the east coast. However, it is generally unknown how anadromous fish react to encountering an active dredge during spawning migrations. Atlantic sturgeon (ATS) are an endangered, anadromous species along the Atlantic slope of North America. To determine if and how an active dredge may affect ATS spawning migration, a Vemco Positioning System array was deployed around an active hydraulic-cutterhead dredge that adult ATS must traverse to reach spawning habitat in the James River, VA. Telemetry data showed that all ATS that entered the study area survived. ATS that migrated upstream during dredge operations (N = 103) traversed the dredge area and continued upstream to spawning habitat. Many ATS made multiple trips through the study area during dredge operations. There was no noticeable difference in swim behavior regardless of whether the dredge was absent or working within the study area. We suggest that dredging in the lower James River does not create a barrier for adult ATS migrating to spawning habitat or cause adults to significantly modify swim behavior. This is the first study to utilize fine-scale telemetry data to describe how an organism moves in relation to an active dredge. This methodology could be used to describe dredge-sturgeon interactions on different life stages and in other locations and could be expanded to other aquatic organisms of concern.

Dredging method effects on sediment resuspension and nutrient release across the sediment-water interface in Lake Taihu, China.

Environmental sediment dredging is one of the most common methods for the remediation of contaminated sediments in lakes; however, debate continues as to whether the effectiveness of dredging methods contributes to this phenomenon. To determine sediment resuspension and nutrient release following dredging with a variety of dredging methods, four dredging treatments at wind speeds of 0-5.2 m/s were simulated in this study, namely suction dredging (SD), grab dredging (GD), ideal dredging with no residual sediments (ID), and non-dredging (ND). Field sediments from suction and grab dredging areas (including post-dredged and non-dredged sediments) of Lake Taihu were used to assess the release abilities of soluble reactive phosphorus (SRP) and ammonia nitrogen (NH4+-N) from the sediment-water interface. The effects of residual sediments on nutrient concentrations in water were also evaluated. The results reveal that inhibition of resuspension of particulate matter and nutrients released through sediment dredging decreases with increasing levels of residual sediment. Total suspended particulate matter content in the mean water columns of ID, SD, and GD under wind-induced disturbance (1.7-5.2 m/s) decreased by 67.5%, 56.8%, and 44.3%, respectively; total nitrogen and total phosphorus in ID (SD) treatments were 19.8% (12.9%) and 24.5% (11.2%) lower than that in ND treatment. However, there were ~ 1.6 and 1.5 times higher SRP and NH4+-N in the GD treatment compared with the ND treatment at the end of the resuspension experiment (0 m/s). A significant increase in the SRP and NH4+-N release rates at the sediment-water interface was also observed in field sediments from a grab dredging area, indicating that GD may pose a short-term risk of nutrient release to the water body. Hence, dredging methods with less residual sediments both during and after dredging improves the dredging quality.

Morphological changes due to marine aggregate extraction for beach nourishment in the German Bight (SE North Sea)

Facing the predicted rise in global sea level, sandy shorelines are under increasing pressure. In order to counteract the loss of material at eroding coastlines, beach nourishment is considered to be an environmentally friendly approach worldwide. This has resulted in a rising demand for aggregates, which are frequently extracted from the seafloor near the coast. In order to explore the long- and short-term morphological changes of such mining on the seabed, the largest extraction area in the German Bight (Westerland Dredging Area, established in 1984) was investigated in this study. Several measurement campaigns were conducted between the years 1994 and 2017 using a set of hydroacoustic techniques. The measurements revealed that up to 20-m-deep pits with diameters of more than 1 km were dredged into the seafloor. The depressions caused by this sand mining are still detectable more than 30 years later. Because of slope failures that mainly consist of fine sand, the formerly steep rims at fresh dredging pits smoothed within a few months. However, after approximately 1 year, muddy sediments dominated the deposition. Since the sedimentation rates are slow, a complete backfill of the post-dredging pits is likely to take many decades. A natural regeneration towards the former seafloor conditions is only visible at the shallow rims of the oldest dredging pits.

Assessment of Dredging/Dumping Scenarios for Figueira da Foz Coastal Region (W Portugal)

Fernandez-Fernandez, S.; Silva, P.A.; Ferreira, C.; Ribeiro, A.S.; Abreu, T.; Romao, S.; Baptista, P.; Fontan-Bouzas, A.; Dias, J.M..; Coelho, C., and Bernardes, C. 2018. Assessment of Dredging/Dumping Scenarios for Figueira da Foz Coastal Region (W Portugal). In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 1266–1270. Coconut Creek (Florida), ISSN 0749-0208.Figueira da Foz coastal region (W Portugal), that comprises sandy beaches and the Mondego estuary-inlet, is exposed to the high energetic wave climate of the North Atlantic Ocean, which induces important local morphological changes. These changes entail shoaling problems as the formation of a submerse sandbar at the inlet which can block navigation towards local harbor. Therefore, coastal dredging is a non-structural temporal solution for this problem. Nevertheless, dredging activities have an impact on local hydrodynamic processes that should be known beforehand. The aim of this study is to clarify this issue using modeling tools dedicated to help stakeholders in decision making process. For that purpose, Delft3D-WAVE module propagates the most representative regional wave conditions under four dredging scenarios including the respective dump areas. Additionally, Delft3D-FLOW module simulates tidal flows along spring and neap tides. The hydrodynamic parameters, significant wave height, orbital velocity, wave-induced currents, tidal current velocity and total sediment transport are analyzed in comparison with numerical solutions obtained in reference scenario (no dredging and no dumping). The results highlight changes in these hydrodynamic parameters in dredging and dumping areas as well as in surrounded areas. Different scenarios draw a similar change pattern for different parameters, namely a reduction of flow velocity intensity within dredged areas and their increase in adjacent areas. Furthermore, hydrodynamic alterations intensify in response to the increase in significant wave height and wave direction considered. These findings are the basis for understanding the influence of dredging and dumping operations on the hydrodynamic of this coast.

Environmental dredging residual generation and management.

The presence and magnitude of sediment contamination remaining in a completed dredge area can often dictate the success of an environmental dredging project. The need to better understand and manage this remaining contamination, referred to as "postdredging residuals," has increasingly been recognized by practitioners and investigators. Based on recent dredging projects with robust characterization programs, it is now understood that the residual contamination layer in the postdredging sediment comprises a mixture of contaminated sediments that originate from throughout the dredge cut. This mixture of contaminated sediments initially exhibits fluid mud properties that can contribute to sediment transport and contamination risk outside of the dredge area. This article reviews robust dredging residual evaluations recently performed in the United States and Canada, including the Hudson River, Lower Fox River, Ashtabula River, and Esquimalt Harbour, along with other projects. These data better inform the understanding of residuals generation, leading to improved models of dredging residual formation to inform remedy evaluation, selection, design, and implementation. Data from these projects confirm that the magnitude of dredging residuals is largely determined by site conditions, primarily in situ sediment fluidity or liquidity as measured by dry bulk density. While the generation of dredging residuals cannot be avoided, residuals can be successfully and efficiently managed through careful development and implementation of site-specific management plans. Integr Environ Assess Manag 2018;14:335-343. © 2018 The Authors. Integrated Environmental Assessment and Management Published by Wiley Periodicals, Inc. on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Impact of Sand Dredging Activities on Ecosystem and Community Survival in Ibeshe Area of Lagos Lagoon, Nigeria

This study investigates the impact of sand dredging activities on coastal ecosystem and community survival around Ibeshe area of the Lagos Lagoon, Nigeria. Surface water and bottom sediment samples were collected from locations partitioned into Dredged Area (DA) and Undredged Area (UA) in the lagoon for laboratory analyses. Questionnaires were administered to community dwellers and sand miners to further analyze perception of locals. Significant differences in water quality parameters and sediments properties from Dredged and Undredged areas, and highly significant relationship between sand dredging and unsustainable ecological practices in Ibeshe were observed. One way ANOVA test indicates that all water quality parameters measured with exception of turbidity and nitrate concentration were not significantly different (P > 0.05) between Dredged and Undredged area. With exception of moisture content other sediment parameters measured were not significantly different (P > 0.05) between Dredged and Undredged areas. The project also determined the impact of dredging activities on the fisheries and fishing business in the environment. Turbidity values revealed that fishes will be negatively affected due to dredging activities; moreover dwellers pay little or no attention to conservation of the area due to unawareness. Sand dredging operations negatively influence ecological resources and water composition of the area. This project is designed to randomly sample the opinion of sand mining operators and residents; and questionnaires were administered to obtain data. Strict regulation and enforcement will assist to control ecological destruction and solve problems facing the lagoon environment.

Economic Burden of Sand Dredging on Artisanal Fishing in Lagos State, Nigeria

Increasing demand for sand for construction purposes has made river/sea sand dredging a major threat to aquatic habitat and fishing. The study compared the cost and returns of artisanal fishermen in river sand dredging and non-dredging in selected fishing communities in Lagos state. The study showed that there were no significant differences in the average ages and experience in fishing of fishermen in sand dredging and non-dredging areas. However, there were significant differences in the average turbidity of river water and quantity of fish caught per labour hour by fishermen in dredging and non-dredging areas. The low productivity in sand dredging area is attributed to the negative effect sand dredging. Fishermen in non-sand dredging areas earned higher gross profit per day. The need for government to control the activities of the sand dredgers in fishing communities is recommended for the sustainability of the environment and fishing in the study area.

Habitat selection and utilization of white croaker (Genyonemus lineatus) in the Los Angeles and Long Beach Harbors and the development of predictive habitat use models

White croaker (Genyonemus lineatus) are a sentinel fish species for contamination due to their direct interaction with contaminated sediments through benthic foraging. White croaker within the Los Angeles (LA) and Long Beach (LB) Harbor exhibited hierarchical habitat selection: avoiding dredged areas while selecting for areas of high sediment total organic carbon (4.8–8.1%), high polychaete density (406–700 polychaetes/0.1 m2), and small sediment grain size (<23.5 μm). Model results suggest that these fish are moving into shallower waters at night, which may be to forage and refuge more during the day presumably to avoid predation. The predictive model for white croaker habitat use indicated three important areas of use within the LA-LB Harbor: Consolidated Slip, Inner Long Beach Harbor, and Fish Harbor. The areas containing the most frequently selected habitats by white croaker are also often areas of high sediment contamination, and thus are likely locations where these fish are acquiring contaminants. While many sediment mitigation alternatives exist, identifying and remediating sediments in key white croaker habitat may lead to a decrease in white croaker tissue concentrations over time if the fish continue to utilize these areas post remediation.

Diversity and spatial and temporal variation of benthic macroinvertebrates with respect to the trophic state of Lake Figueira in the South of Brazil

Benthic macroinvertebrates are widely used in evaluations of environmental impact and recommended in biomonitoring, but little used in Brazilian lentic environments. One of the main objectives has been to explain and predict the distribution of those species according to environmental characteristics. Thus, the study aims to characterize the predominant fauna of the Lake Figueira sediment and its relationship with organic matter and depth, analyzing seasonal variation in communities and aiming to select bioindicators of the trophic state. Sampling was carried out from January 2008 to January 2009, along the lagoon fetch, with an Ekmann-Birge dredge (area 225 cm²). Taxon richness was not significantly related with depth and organic matter content, but those variables were highly correlated (r = 0.962; r² = 0.926 and p &lt; 0.001). The constant oxygenation of the whole water column allows the occurrence of organisms, independent of depth and organic matter content. Chironomidae was the most abundant taxon and from the frequency of occurrence, abundance and clustering analysis it was possible to select Larsia sp. , Goeldichironomus maculatus, Xenochironomus sp. , Aedokritus sp. Cladopelma forcipis, Cryptochironomus brasiliensis, Nilothauma sp.1 and Caladomyia sp. C, Tanytarsus sp. , Tanytarsus rhabdomantis and Chironomus gr. salinarius as potential indicators related with the spatiotemporal faunal distribution of Lake Figueira.

Hydrodynamic and Sediment Transport in the Headland of Caofeidian after Reclamation of the Industrial Zone

Caofeidian sand island unique shoal-channel system provides construction conditions for deep-water port.By comparing the underwater topography changes before and after Caofeidian industrial construction to understand the sediment transport and erosion-deposition evolution. Bathymetry results in 2006 and 2012 showed that the deep channel is in good stability. 2 to 3 years later after construction, excepting artificial dredging area, erosion occurs in front of the headland. The sediment is transported to the west of harbor 1 entrance. Seabed sediment controlled by wave comes from the front of the southeast seawall inflection point is transported to the deep channel, some deposits in the east slope and the most is taken away by the deep channel water. All processes above become more weakly now, in the future the sea area in front of the headland will erode weakly, the sea area in the west slope will deposit weakly while in the east slope will erode weakly, and the sea area near the harbor entrance will erode.

Phytoplankton Investigation and Environmental Assessment for Dredged Area in Daqing Estuary Section of Dianchi Lake, China

Daqing River is one of the major rivers flowing into the Dianchi Lake. In this paper, phytoplankton investigation and environmental assessment for the dredged area in Daqing Estuary Section of Dianchi lake in Kunming city was reported. After microscopic identification and counting of 6 water samples(two of them is control plot), the water environment were evaluated by the phytoplankton species and quantity which were assessed by the biological diversity index, including Maglarf, Shanon-Wiener, Simpson diversity index, Pielou index and Berger-Parker dominance index. It’s found that there are mainly 5 species of algae, and among them, Microcystis spp. et al., are the dominant species. So the water is assessed as eutrophication, Heavy sewage. Based on the diversity index analysis, the dredged area sampling sites were relatively higher than the non-dredged areas in biodiversity of phytoplankton. Summarily, the water quality after dredging has been improved to some extent. Phytoplankton biodiversity indicators should be the important water environmental assessment method.

The Effects of Estuary Dredging on Removing Nitrogen and Phosphorus in Dianchi Lake, China

Sediment dredging is currently the most commonly selected option for getting rid of contaminated sediments. In this study, the effects of estuary dredging on removing nitrogen and phosphorus were investigated by comparing the different vertical nutrient content between the dredged and un-dredged areas in Dianchi Lake. The results showed that the contents of total nitrogen and total phosphorus in the un-dredged areas were relatively higher than that in dredged areas. Besides, the contents of bio-available nitrogen and phosphorus represented the similar results, only a few dredged spots showed a higher potential releasing capacity. Therefore, with the reduction of internal nutrient loading, it indicated that sediment dredging might be an effective and reliable way to improve such eutrophic lakes.

Morphodynamic consequences of dredging and dumping activities along the lower Oka estuary (Urdaibai Biosphere Reserve, southeastern Bay of Biscay, Spain)

Dredging and dumping in the lower Oka estuary (southeastern Bay of Biscay) during 1973–2003 have modified its pattern of sedimentary transport and morphology. An analysis of these activities through time and morphodynamic response of the estuarine system is presented. The relationships between both processes have been established.A Geographical Information System (GIS) has been used to create a temporal cartographic series of the changing patterns of estuarine sedimentary environments, identify the anthropogenic changes generated and observe the consequent responses of the estuarine system. The GIS has proven to be a very useful tool to monitor and evaluate the natural and human induced morphological evolution of the lower Oka estuary during the last 50 years. In the absence of dredging and dumping (1957–1973), the estuary had a distinct pattern of flood and ebb channels. Flood channels are deeper than ebb channels at their mouth, and progressively becomes shallower in the direction of the flooding tide. On the other hand, ebb channels form a seaward extension of the fluvial main channel. Both types of channels are prone to be evasive and braiding was common. During the period of study, the channels showed low natural variability relative to their location and spatial extension. The dredging and dumping carried out between 1973 and 2003 altered the natural flood/ebb channel distribution and modified sedimentary dynamics. Dredging caused the isolation of meanders of the ebb channel, the rapid infilling of the new dredged areas, and affected the flood channels thus producing the necessity for regular dredging, approximately every 5 years.Observation of the natural response of the estuarine system after the dredging and dumping makes it possible to state that, at present, the lower Oka estuary is not in a state of morphodynamic equilibrium and has a tendency to lose its capacity gradually. Dredging and dumping have accelerated this process and have increasingly unbalanced sedimentary regime.