Zoek binnen publicaties
NCK-days 2016 : book of abstracts (Inspiration Centre Grevelingen, Ouddorp, 16-18 March 2016)
NCK-days 2014 : preparednesss (UNESCO-IHE, Delft, 27-28 March 2014)
NCK-days 2017 : book of abstracts (Den Helder, 15-17 March 2017)
Erodibility of soft fresh water sediments : the role of bioturbation by meiofauna
Markermeer is a large and shallow fresh water lake in The Netherlands. It has a 680 km2 surface and a 3.6 m mean water depth. Markermeer is characterized by its high turbidity, which affects the lake ecosystem seriously. As part of a study that aims to mitigate this high turbidity, we studied the water bed exchange processes of the lake’s muddy bed. The upper cm’s – dm’s of the lake bed sediments mainly consist of soft anoxic mud. Recent measurements have proved the existence of a thin oxic layer on top of the soft anoxic mud. This oxic layer is believed to be responsible for Markermeer high turbidity levels. Our hypothesis is that the oxic layer develops from the anoxic mud, and due to bioturbation. In particular we will refer to bioturbation caused by meiobenthos. The objective of this study is to determine the influence of the development of the oxic layer on the water-bed exchange processes, as well as the role of bioturbation in this processes. This is done by quantifying the erosion rate as a function of bed shear stresses, and at different stages of the development of the oxic layer. Our experiments show that bioturbation increases the erosion rate of Markermeer sediments, and therefore affects the fine sediment dynamics of the lake.
Turning off the DRIP (data-rich, information-poor) : rationalising monitoring with a focus on marine renewable energy developments and the benthos
Marine renewable energy developments (MREDs) are rapidly expanding in size and number as society strives to maintain electricity generation whilst simultaneously reducing climate-change linked CO2 emissions. MREDs are part of an ongoing large-scale modification of coastal waters that also includes activities such as commercial fishing, shipping, aggregate extraction, aquaculture, dredging, spoil-dumping and oil and gas exploitation. It is increasingly accepted that developments, of any kind, should only proceed if they are ecologically sustainable and will not reduce current or future delivery of ecosystem services. The benthos underpins crucial marine ecosystem services yet, in relation to MREDs, is currently poorly monitored: current monitoring programmes are extensive and costly yet provide little useful data in relation to ecosystem-scale-related changes, a situation called ‘data-rich, information-poor’ (DRIP). MRED - benthic interactions may cause changes that are of a sufficient scale to change ecosystem services provision, particularly in terms of fisheries and biodiversity and, via trophic linkages, change the distribution of fish, birds and mammals. The production of DRIPy data should be eliminated and the resources used instead to address relevant questions that are logically bounded in time and space. Efforts should target identifying metrics of change that can be linked to ecosystem function or service provision, particularly where those metrics show strongly non-linear effects in relation to the stressor. Future monitoring should also be designed to contribute towards predictive ecosystem models and be sufficiently robust and understandable to facilitate transparent, auditable and timely decision-making.
Sensitivity analysis and model type evaluation for subsidence above offshore gas reservoirs
This paper describes the results of a sensitivity study conducted to understand how the prediction of subsidence due to gas extraction from offshore gas fields depends on a few key parameters, such as the connection to the adjacent aquifers and the material mechanical properties. The analysis has been performed using an axi-symmetric Finite Element model. A specific gas production field, the Naomi-Pandora gas field in the Northern Adriatic basin, has been assessed in detail. For each of the layers considered in the sand/shale stratification a low value, an intermediate and a high value for the soil stiffness were applied, as determined from oedometer tests, radioactive markers and vertical seismic profiling respectively. The reservoir constitutive behavior has been modeled using different approaches, namely: a linear-elastic, a power law and a Modified Cam Clay model. The study has been performed for different pressure scenarios representing different levels of interaction with adjacent aquifers. The results show the sensitivity of the subsidence bowl as a result of the imposed conditions. These results are compared with the predictions obtained using 3D non-linear elastic and hypoplastic subsidence models of the same gas fields, demonstrating a good agreement. The stiffness of the reservoir is the main factor affecting the surface subsidence. For a gas pressure reduction less than 50 bars the observed seabed subsidence hardly varied for different reservoir material models. The maximum extent of the predicted subsidence bowl (2 cm contour) in 2030 remains far from the Po di Goro parallel and far from the coastline.
Subsidence from geodetic measurements in the Ravenna area
The derivation of subsidence due to a specific cause from geodetic measurements is, in principle, simple, but assumptions implied in the standard approach are never fully correct in practice. Over or underestimation by a factor of up to two may occur. Geodetic measurements alone cannot differentiate between different causes of subsidence. This article describes a modified approach that avoids assumptions on reference point stability and exploits a-priori knowledge of spatial and temporal subsidence patterns. The present integral approach recognizes that geodetic measurements reflect differential, not absolute, vertical displacement of the benchmarks, not of the 'surface', within the area surveyed and recognizes errors that are, or are not correlated in time and/or subsidence in the Ravenna area from the Comune di Ravenna for the period 1982-2002 were revisited.
Environmental effects of land subsidence induced by gas withdrawal along the Ravenna Coast, part I : available information
An overview is given of the expected environmental impact related to predicted land subsidence due to planned gas-withdrawal in the Ravenna coastal zone. This region is a typical example of a deltaic area with soft sediments, susceptible to land subsidence caused by tectonic movements and natural compaction, water withdrawal from shallow aquifers, and gas withdrawal from deep reservoirs. Subsidence may lead to effects on the natural and built environment and on the local economy. This paper presents Part I of the findings of an international multi-disciplinary team of experts, which has performed a comprehensive study to investigate the gas-withdrawal effects on land subsidence in the Ravenna area. It focuses on relevant and available information. For their implication and final conclusions reference is made to Part II of the combined paper.
Environmental effects of land subsidence induced by gas withdrawal along the Ravenna Coast, part II : environmental impact
An overview is given of the expected environmental impact related to predicted land subsidence due to planned gas withdrawal in the Ravenna coastal zone. This region is a typical example of a deltaic area with soft sediments, susceptible to land subsidence caused by tectonic movements and natural compaction, water withdrawal from shallow aquifers, and gas withdrawal from deep reservoirs. Subsidence may lead to effects on the natural and built environment and on the local economy. This paper presents part II of the findings of an international multi-disciplinary team of experts, which has performed a comprehensive study to investigate the effects of gas withdrawal on land subsidence in the Ravenna area. These findings were supported by site visits to inspect coastal and inland environments. This paper discusses the impact of subsidence particularly on coastal dynamics and on mitigation measures. A comparison is made with a similar situation in The Netherlands. Effects of gas withdrawal induced subsidence on the coastal dynamics in the Ravenna Province are marginal. In the area above the Angela-Angelina gas field additional shore protection measures are recommended in order to suppress potential beach erosion. Public safety and risk of damage is assessed and it is concluded that the subsidence due to proposed gas production will cause no risk to public safety and no disproportionate increase in the risk of damage.
Subsidence induced by gas production : an integrated method
An integrated subsurface modelling study was performed in order to predict the amount and the lateral extent of subsidence in an area of the Northern Adriatic region. A number of selected gas fields were independently modelled using an approach in three steps. First, static geological and petrophysical subsurface models were built that were large enough to include the wider area surrounding the gas fields. Secondly, the pressure changes were modelled using a 3D subsurface reservoir simulator, through history matching using production and pressure data. Modelling extended to a few decades after the expected end of the gas production in order to capture all significant effects in the reservoirs and aquifers. The third phase of modelling consisted of finite element geomechanical modelling, taking into account the geometry, vertical heterogeneity and geomechanical rock properties of the subsurface and the pressure fields obtained in the second phase. Finally, the resulting subsidence grids obtained for the individual fields were added together to provide regional contour maps representing the combined effects.