2973 resultaten

  • Behaviour of subtidal sandbars in response to nourishments

    Auteurs: Radermacher, M.; Schipper, M.A. de; Price, T.D.; Huisman, B.J.A.; Aarninkhof, S.G.J.; Reniers, A.J.H.M. (2018)

    The behaviour of subtidal sandbars can be strongly influenced by the introduction of sand nourishments in the coastal system. This study focuses on the impact of nourishments on subtidal bar behaviour at spatiotemporal scales beyond a single nourishment project. It aims to determine the long-term behaviour of subtidal sandbars along an entire coastal cell, taking into account both the unnourished and nourished regime, and covering various types of nourishments. The analysis is based on over 50 years of sandbar evolution along the Delfland coast, a 17-km long coastal cell at the Dutch North Sea coastline protected by groynes and maintained with frequent sand nourishments. Observations reveal clearly different sandbar behaviour during the unnourished (first 20 years) and nourished periods of the dataset. Introduction of the first beach nourishments (nourished sand primarily placed at the subaerial beach) was found to stimulate sandbar development along previously unbarred sections of the coast. Shoreface nourishments (nourished sand placed at the seaward face of the pre-existing subtidal sandbar) tended to migrate shoreward rapidly at a rate of 20 to 60 m/year at this coast, thereby forcing the pre-existing sandbar to weld to the dry beach. An abrupt transition of sandbar dynamics was observed following a major nourishment operation (∼37.5Mm3 of nourished sand) that covered the entire coastal cell. A new, shallow sandbar formed with a degree of alongshore variability that was unprecedented at the Delfland coast over the full study period. These results imply that individual nourishments can influence the formation and migration of individual sandbars, while continued nourishments can fundamentally change long-term sandbar dynamics along an entire coastal cell.

  • Variation in the availability of metals in surface water, an evaluation based on the dissolved, the freely dissolved and Biotic Ligand Model bioavailable concentration

    Auteurs: Wijdeveld, A.J.; Schipper, C.A.; Heimovaara, T.J. (2018)
    Gepubliceerd in: Catena, volume 166 (2018), pagina 260-270

    In this study the spatial distribution of dissolved metals in surface water is studied at nine locations in Lake Ketelmeer (the Netherlands). The measured dissolved metal concentrations are combined with the local water quality parameters for salinity, pH, alkalinity and DOC to calculate a FIAM Free Ion Activity Model (FIAM) and the Biotic Ligand Model (BLM) based bioavailable metal concentration. The BLM model is used for Cu, Ni, Pb and Zn and the FIAM model for Cd, Cr, Cu, Ni, Pb and Zn. To be able to compare the dissolved metal concentration with the FIAM or BLM based bioavailable metal concentration, an accepted reference standard can be used which is also corrected for the bioavailable concentration. Here the Water Framework Directive (WFD) Annual Average Quality Standard (AA-QS) is used, corrected for the FIAM and BLM based bioavailable metal concentration under reference conditions. This yielded a site specific Risk Characterization Ratio (RCR-FIAM/RCR-BLM). The FIAM model shows an exceedance of the site specific AA-QS for Cu (RCR-FIAM of 1.8) and Pb (RCRFIAM of 1.5) in the northern middle part of the lake. This is due to a lower pH in this part of the lake. The BLM model was inconclusive with regard to spatial trends for Cu and Ni due to out of boundary conditions for the model. For locations where the BLM model was within the model boundary conditions, the RCR-BLM could be as high as 7.5 for Cu and 3.2 for Ni. The main water quality parameter causing the high RCR-BLM was the low DOC concentration. To establish if the locally increased RCR for Cu and Pb (FIAM) or Cu and Ni (BLM) poses an ecotoxicological risk to organisms the multi substances Potentially Affected Fraction (ms-PAF) model is used. The FIAM based ms-PAF indicates that the northern middle part of the lake has the highest chronic metal exposure risk, with an ms-PAF of 27%. The BLM based ms-PAF has a maximum of 45%, but lacks a spatial trend due to the missing BLM corrected Cu and Ni concentrations for some locations.

  • A model to assess microphytobenthic primary production in tidal systems using satellite remote sensing

    Auteurs: Daggers, T.D.; Kromkamp, J.C.; Herman, P.M.J.; Wal, D. van der (2018)

    Quantifying spatial variability in intertidal benthic productivity is necessary to guide management of estuaries and to understand estuarine ecological processes, including the amount of benthic organic carbon available for grazing, burial and transport to the pelagic zone. We developed a model to assess microphytobenthic (MPB) primary production using (1) remotely sensed information on MPB biomass and remotely sensed information on sediment mud content, (2) surface irradiance and ambient temperature (both from local meteorological observations), (3) directly-measured photosynthetic parameters and (4) a tidal model. MPB biomass was estimated using the normalized-difference vegetation index (NDVI) and mud content was predicted using surface reflectance in the blue and near-infrared, both from Landsat 8 satellite imagery. The photosynthetic capacity (maximum photosynthesis rate normalized to MPB chl-a) was estimated from ambient temperature, while photosynthetic efficiency and the light saturation parameter were derived from in situ fluorometry-based production measurements (PAM). The influence of tides (submergence by turbid water) on MPB production was accounted for in the model. The method was validated on several locations in two temperate tidal basins in the Netherlands (Oosterschelde and Westerschelde). Model based production rates (mg C m-2 h-1) matched well with an independent set of in situ (PAM) measurement based production rates. The relationship between photosynthetic capacity and temperature shows considerable variation and may be improved by using sediment surface temperature instead of ambient temperature. A sensitivity analysis revealed that emersion duration and mud content determine most of the variability in MPB production. Our results demonstrate that it is possible to derive a satellite remote sensing based overview of average hourly and daily MPB primary production rates at the macro scale. As the proposed model is generic, the model can be applied to other tidal systems to assess spatial variability in MPB primary production at the macro scale after calibration at the site of interest. Model calibration, results and possible applications for regular monitoring of MPB production are discussed.

  • Regioscan Zoetwatermaatregelen : verkennen van het perspectief van kleinschalige zoetwatermaatregelen voor de regionale zoetwateropgave

    Auteurs: Delsman, J.R.; Boekel, E. van; Reinhard, S.; Winkel, T. te; Loon, A.H. van; Bartholomeus, R.P.; Mulder, M.; Massop, H.T.L.; Polman, N.; Schasfoort, F.E. (2018)

    De Regioscan Zoetwatermaatregelen helpt bij de ontwikkeling van een zoetwaterstrategie door het in beeld brengen van de bijdrage van lokale maatregelen afgezet tegen de regionale zoetwatertekorten. Het instrument bestaat uit een kennissysteem Zoetwatermaatregelen en een rekenmodule. Het kennissysteem is een database waarin generieke maatregelinformatie is opgeslagen. Deze is te koppelen aan scenario-informatie over hydrologie en landbouwbedrijven. De rekenmodule berekent de ruimtelijke variërende effecten van maatregelen op droogte- en zoutschade, kosten en baten, zoetwatergebruik en neveneffecten en cumuleert deze voor deel- of beheergebieden. Uit de toepassing op twee proefgebieden blijkt in z’n algemeenheid dat het instrument goed bruikbaar is om snel inzicht te krijgen in de potentie van de maatregelen. Eén van de aanbevelingen is het instrument verder te valideren. Voor concrete investeringsbeslissingen zijn meer detailberekeningen nodig.

  • Regioscan Zoetwatermaatregelen : beperken watervraag landbouw door kleinschalige maatregelen

    Auteurs: Delsman, J.R.; Schasfoort, F.E.; Jeuken, A.B.M. (2018)

    Omdat zoetwatertekorten steeds talrijker worden, zoeken waterbeheerders met landbouwers naar manieren om de vraag te verminderen. Onbekend is in hoeverre kleinschalige maatregelen kunnen bijdragen aan de regionale zoetwateropgave en tegen welke kosten. In de Regioscan Zoetwatermaatregelen worden van maatregelen de kosten en baten voor een landbouwbedrijf doorgerekend. Op basis van geselecteerde maatregelen (hetzij op basis van bedrijfseconomische baten-kostenverhouding, hetzij geselecteerd door de waterbeheerder) worden regionale effecten op de zoetwatervraag ingeschat. Het instrument ondersteunt hiermee de dialoog tussen waterbeheerder en boer. Vooralsnog lijken baten van kleinschalige zoetwatermaatregelen alleen in specifieke gebieden op te wegen tegen de kosten.

  • Evolucao do dano num quebra-mar de taludes em cenarios de alteracoes climaticas

    Auteurs: Lemos, R.; Fortes, C.J.; Mendonca, A.; Rosa-Santos, P.; Taveira-Pinto, F.; Almeida, E. de; Hofland, B. (2018)
    Gepubliceerd in: 14. Congresso da Agua : gestao dos recursos hidricos - novos desafios (7-9 de marco de 2018, Evora) (2018), pagina 1-16

    No âmbito do projeto HYDRALAB+ (H2020-INFRAIA-2014-2015), foi realizado no LNEC um conjunto de ensaios em modelo físico bidimensional (2D) de um perfil de um quebra-mar de talude em enrocamento, construído à escala geométrica 1:30, para diferentes condições de agitação arítima e níveis de maré correspondentes a cenários de alterações climáticas. Estas experiências tinham como objetivo analisar o dano, o espraiamento e o galgamento, nesses cenários. Em paralelo, na FEUP, e em colaboração com Deltares, foram realizados testes com a mesma configuração de quebra-mar, mas utilizando um modelo físico tridimensional (3D), construído à escala geométrica 1:35. Os resultados obtidos permitiram analisar a evolução do dano no manto resistente do quebra-mar (talude de barlamar, talude de sotamar e cabeça), assim como o galgamento, para condições com e sem sobrelevação do nível médio do mar, considerando ou não a reconstrução do talude do quebra-mar entre testes consecutivos da mesma série de testes. Foram também realizados testes com ondas multidirecionais. O presente trabalho incide sobre a análise da evolução do dano para as diferentes sequências de tempestade em cenários de alterações climáticas. O dano é avaliado com base no tradicional método de contagem de blocos removidos e ainda com base no parâmetro adimensional do dano, S recorrendo a técnicas estereofotogramétricas. É também feita uma avaliação do parâmetro E2D. Em termos gerais, concluiu-se que não havia diferenças significativas entre os testes com danos cumulativos e com a reconstrução do quebra-mar após cada teste. Observou-se, também, que o dano era mais elevado na proximidade da superfície livre e que aumentava com a altura de onda significativa. Os danos foram, em geral, menores nos testes com ondas multidirecionais.

  • 14. Congresso da Agua : gestao dos recursos hidricos - novos desafios (7-9 de marco de 2018, Evora)

    Originally published in 2018

  • Measuring damage in physical model tests of rubble mounds

    Auteurs: Hofland, B.; Rosa-Santos, P.; Taveira-Pinto, F.; Almeida, E. de; Lemos, R.; Mendonca, A.; Fortes, C.J. (2017)

    This paper studies novel ways to evaluate armour damage in physical models of coastal structures. High-resolution damage data for reference rubble mound breakwaters obtained under the HYDRALAB+ joint-research project are analysed and discussed. These tests are used to analyse the way to describe damage, the influence of the sequence of testing, and touches on the possible influence of sea level rise. Results of two test programmes were used. Firstly, 3D physical model tests carried out at the University of Porto, in cooperation with Deltares, were used. Here a wide breakwater trunk was used for statistical reasons. Additionally, 2D test results from LNEC were analysed. Tests for a sea level rise scenario resulted in less damage to the seaside slope. In addition, clear differences between “cumulative damage” and “rebuild” test series were noticed. However, significant scatter was also observed in the result of tests carried out under identical conditions. It was also concluded that the damage to the trunk was lower in the tests with short-crested waves. The design values for the damage depth E2D proposed by Hofland et al. (2011) were partly in line with the experimental results presented. Since the relation between S and the depth of damage E does not hold true for nonstandard cases, it seems better to use a parameter based on the local damage depth when testing such a structure. The reliability of a damage number for a test on the stability of a trunk can be improved by either increasing the relative size (width) of the test section or repeating the test.

  • Herstelmaatregel groot zeegras in de Nederlandse Waddenzee : haalbaarheid van de doelstellingen onder de Kaderrichtlijn Water

    Auteurs: Duren, L.A. van; Katwijk, M.M. van (2015)

    In de Nederlandse Waddenzee is groot zeegras (Zostera marina) bijna verdwenen en daarmee scoort de Waddenzee "slecht" voor deze ecologische parameter onder de Kaderrichtlijn Water (KRW). Volgens modelvoorspellingen is er voldoende geschikt habitat voor deze soort, maar zeer waarschijnlijk is een tekort aan zaadaanvoer een belangrijke factor waardoor zeegras in de Nederlandse Waddenzee zich niet spontaan kan herstellen. In 2011 is begonnen met een herstelproject op basis van zaad uit de Duitse Waddenzee. De doelstelling was om op drie locaties in Nederland een dusdanig grote hoeveelheid zeegras uit te zaaien dat hier robuuste zeegraspopulaties zouden ontstaan met voldoende kritische massa om natuurlijke fluctuaties op te vangen. Het project moest tevens leiden tot een advies m.b.t. de haalbaarheid van de huidige KRW-doelstellingen. Uitzaai heeft plaatsgevonden in 2011 en 2012 en de ontwikkeling van zeegras in opgevolgd tot 2015. In 2012 en 2013 is op alle drie de locaties zeegras opgekomen. De totale verspreiding van zeegras in 2013 bedroeg 300 ha, zij het met een gemiddeld zeer lage bedekkingsgraad (<1 %). In dat jaar is, in tegenstelling tot andere jaren, nauwelijks zaadzetting opgetreden. In 2014 is een zeer sterke teruggang opgetreden van de zeegrasbedekking op alle locaties. Recent is gebleken dat er, naast de reeds bekende ziekteverwekker Labyrinthula, nog een andere ziekteverwekker (Phytophthora) in de Waddenzee zit die mogelijk de reproductie en kiemkracht van zeegraszaad aantast. Dit was bij aanvang van het project niet bekend. Dit heeft er mogelijk toe geleid dat er veel minder kiemkrachtig zaad in Nederland is uitgezaaid dan oorspronkelijk berekend was. Ondanks het feit dat de doelstelling van het project (een robuuste populatie zeegras) niet is gehaald, wordt op dit moment geadviseerd om de KWR doelstellingen voor zeegras te handhaven. De nieuwste kansenkaart voor groot zeegras bevestigt dat er waarschijnlijk voldoende geschikt habitat in de Waddenzee is. Geadviseerd wordt om een onderzoekstraject in te zetten naar methoden om de kieming van ingevoerd zeegras te verbeteren en te zoeken naar voldoende grote geschikte bronnen van zeegraszaad. Het is niet zeker dat dit inderdaad haalbaar zal zijn, gezien het feit dat groot zeegras ook in buurlanden een beschermde status heeft. Mogelijk zal daar onvoldoende draagvlak zijn om grote hoeveelheden zaadmateriaal te leveren voor herstel van deze soort in Nederland. Onderzoek in het buitenland heeft aangewezen dat herstel van deze soort een zaak kan zijn van lange adem.

  • The relation between land use and subsidence in the Vietnamese Mekong delta

    Auteurs: Minderhoud, P.S.J.; Coumou, L.; Erban. L.E.; Middelkoop, H.; Stouthamer, E.; Addink, E.A. (2018)
    Gepubliceerd in: Science of the total environment, volume 634 (2018), pagina 715-726

    The Vietnamese Mekong delta is subsiding due to a combination of natural and human-induced causes. Over the past several decades, large-scale anthropogenic land-use changes have taken place as a result of increased agricultural production, population growth and urbanization in the delta. Land-use changes can alter the hydrological system or increase loading of the delta surface, amplifying natural subsidence processes or creating new anthropogenic subsidence. The relationships between land use histories and current rates of land subsidence have so far not been studied in the Mekong delta. We quantified InSAR-derived subsidence rates for the various land-use classes and past land-use changes using a new, optical remote sensing-based, 20-year time series of land use. Lowest mean subsidence rates were found for undeveloped land-use classes, like marshland and wetland forest (~6–7 mm yr−1), and highest rates for areas with mixed-crop agriculture and cities (~18–20 mm yr−1). We assessed the relationship strength between current land use, land-use history and subsidence by predicting subsidence rates during the measurement period solely based on land-use history. After initial training of all land-use sequences with InSAR-derived subsidence rates, the land-use-based approach predicted 65–92% of the spatially varying subsidence rates within the measurement error range of the InSAR observations (RMSE = 5.8 mm). As a result, the spatial patterns visible in the observed subsidence can largely be explained by land use. We discuss in detail the dominant land-use change pathways and their indirect, causal relationships with subsidence. Our spatially explicit evaluation of these pathways provides valuable insights for policymakers concerned with land-use planning in both subsiding and currently stable areas of the Mekong delta and similar systems.

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