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Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

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    The ecology of ditches : a modeling perspective
    Gerven, Luuk P.A. - \ 2016
    Wageningen University. Promotor(en): W.M. Mooij, co-promotor(en): Jeroen de Klein; J.H. Janse. - Wageningen : Wageningen University - ISBN 9789462579316 - 137
    ditches - aquatic ecology - ecology - modeling - aquatic plants - aquatic ecosystems - water flow - sloten - aquatische ecologie - ecologie - modelleren - waterplanten - aquatische ecosystemen - waterstroming

    The Netherlands is well-known for its extended networks of drainage ditches, with a total ditch length of about 300.000 km. Their main function is to enable agriculture by draining water. Nonetheless, ditches also have important ecological functions. They serve as ecological corridors and harbor a high biodiversity in which water plants play a crucial role. The last decades, the ecological quality of ditches is at stake. Enhanced nutrient inputs promoted the invasion by dense mats of free-floating plants like duckweed. Underneath these mats the water becomes dark and anoxic, which severely constrains aquatic life.

    In this thesis I developed new concepts to better understand, predict and combat the dominance by free-floating plants in ditches. The following questions are addressed. Are floating plants a self-stabilizing state - an alternative stable state - which would make it more difficult to combat floating-plant dominance (chapter 2)? Does it make sense to fight floating-plant dominance by reducing nitrogen (N) inputs to the ditches or will it lead to an invasion of floating plants that can fix N2 from the atmosphere (chapter 3)? What about spatial aspects, does the vulnerability of a ditch to floating plants depend on the position of a ditch in a polder, like its distance to the polder outlet (chapter 4)? To answer these questions, I used ecological models that predict the abundance of free-floating plants based on the competition for nutrients and light with other plants such as submerged plants, and where possible validated these models with field data. Starting from the ecosystem model PCDitch, I developed and combined models with different complexity to see how theoretical concepts, developed in minimal models, translate to the ecosystem level. Chapter 5 deals with a method that facilitates this up- and downscaling in model complexity.

    Are floating plants an alternative stable state? To answer this question I extended mechanistic resource competition theory with a framework (minimal model) describing the competition of floating and submerged plants for light and nutrients. The model predicts that the competitive advantage of floating plants - they have a primacy for light and shade submerged plants, giving rise to asymmetry in competition for light - makes that floating plants always dominate at high supply of light and nutrients. At intermediate nutrient supply, there can be alternative stable states: either the submerged plants or the floating plants dominate depending on who established first. However, based on the traits of common floating plants (duckweed; Lemna) and submerged plants (waterweed; Elodea) the model predicts, in line with field data, that floating plants are not an alternative stable state. Furthermore, from a theoretical point of view this study shows that the asymmetry in light competition ensures that common rules from standard competition theory do not apply anymore. Like the R* rule, which states that the species that can persist at the lowest resource levels always wins the competition.

    Can duckweed-dominance be combatted by reducing N inputs to the ditches? Or does this promote other floating plants like water fern (Azolla) that can fix N2 from the atmosphere? Important is the question whether such N2-fixers can provide enough N to prevent N-limitation and keep the system P-limited, which would make steering on N inputs ineffective. To investigate this, I considered the competition between Lemna and Azolla for N, P and light. Both a minimal model, an ecosystem model (PCDitch) and field data reveal that N2-fixation is unlikely to lead to P-limitation. This can be explained by N2-fixers typically requiring higher P concentrations to persist, implying that they cannot keep the P concentration low enough for non-N2-fixers to become P-limited. In combination with field data that hint at constraints on N2-fixation that prevent N2-fixers from becoming abundant at low N availability, this suggests that it certainly pays off to combat floating plant-dominance by reducing N inputs.

    Is every ditch in a polder equally vulnerable to floating plants? Each ditch in a polder receives water and nutrients from the adjacent land. This leads to a spatial gradient in water flow and associated nutrient loading, from low in the remote polder sites to high in the direction of the polder outlet where the water leaves the polder. I explored if this spatial gradient affects the vulnerability of a ditch to floating plants, by investigating with a simple nutrient model how this gradient affects the nutrient concentration of the ditches and by subsequently predicting the gradient's effect on the ditch ecology by applying the ecosystem model PCDitch spatially, through coupling PCDitch to the 1-D hydrodynamic model SOBEK. Surprisingly, we found that every ditch is equally vulnerable to floating plants, despite the spatial gradient in water flow and nutrient loading. It turned out that the ecological state of each ditch could already be predicted by regarding only the lateral supply of water and nutrients from the adjacent land, and not the supply from upstream ditches. However, these findings are violated when there is spatial heterogeneity in the water and nutrient supply from the adjacent land or in ditch characteristics like depth and sediment type. Then, the chance on floating-plant dominance differs throughout the network and a spatial modelling approach (PCDitch-SOBEK) is required to predict this chance.

    Developing and combining models of different complexity plays an important role in this thesis. To do so, I used a Database Approach To Modelling (DATM), a recently developed method in which a model is stored in tables in a clear and clean way, which facilitates model development. In addition, with DATM a model can be automatically implemented in a modelling environment of choice. This relieves technical implementation issues and leaves room to focus on ecology rather than technology. I illustrated the use of DATM by implementing and analyzing the ecosystem model PCDitch and its twin model for shallow lakes PCLake in different modelling environments by using DATM. This showed that DATM allows one to use the environment one is familiar with and eases the switch to other environments for complementary analyses, including analysis in a spatial 1-D to 3-D setting.

    The insights provided by this thesis can help us to improve the ecological quality of ditches. A challenging task, given the fast human-driven environmental changes at both local and global level. To predict and to anticipate the effect of these changes on the ecology, it is essential to understand how the ditch ecosystem functions. The developed and applied methods described in this thesis may be helpful in that. For example, using models of different complexity makes it possible to translate fundamental theory to the ecosystem scale, which is essential to better grasp the behavior of an ecosystem. Furthermore, the in this thesis established coupling between PCDitch and SOBEK breaks new grounds for spatial ecosystem modelling. In combination with the growing amount of remote sensing data from satellites and drones, which allow for the continuous and potentially real-time validation and calibration of spatial ecosystem models, such a spatial approach has the potential to greatly increase our ecological understanding of ditches. These advances facilitate the development of successful management strategies that make our ditch ecosystems future-proof.

    Alterra en de kunst van het kronkelen
    Kleis, R. ; Makaske, A. ; Maas, H.A. - \ 2015
    Resource: weekblad voor Wageningen UR 10 (2015)1. - ISSN 1874-3625 - p. 9 - 9.
    waterlopen - herstelbeheer - geomorfologie - natuurlandschap - ecologisch herstel - waterstroming - waterbeheer - streams - restoration management - geomorphology - natural landscape - ecological restoration - water flow - water management
    Een groot deel van onze beken heeft op dit moment een verre van natuurlijk verloop. Om wateroverlast tegen te gaan zijn in de vorige eeuw veel van oudsher kronkelige beken rechtgetrokken. Rechte beken voeren het water sneller af. Maar tegenwoordig is dat niet meer gewenst. Die snelle afvoer leidt tot verdroging en rechtgetrokken beken bieden weinig kansen voor een diversiteit in flora en fauna. Met beekherstel probeert men dat tij te keren. Bart Makaske en Gilbert Maas van Alterra schreven een boek over hoe je beken het beste laat kronkelen.
    Rivers running deep : complex flow and morphology in the Mahakam River, Indonesia
    Vermeulen, B. - \ 2014
    Wageningen University. Promotor(en): Remko Uijlenhoet, co-promotor(en): Ton Hoitink. - Wageningen : Wageningen University - ISBN 9789462572065 - 150
    rivieren - morfologie - waterstroming - hydrologie - indonesië - rivers - morphology - water flow - hydrology - indonesia

    Rivers in tropical regions often challenge our geomorphological understanding of fluvial systems. Hairpin bends, natural scours, bifurcate meander bends, tie channels and embayments in the river bank are a few examples of features ubiquitous in tropical rivers. Existing observation techniques fall short to grasp the complex governing processes of flow and morphology. In this thesis new observational techniques are introduced and applied to study the Mahakam River, East Kalimantan, Indonesia. The observations reveal a new type of morphological regime, characterized by non-harmonic meanders, scour and strong variation of the cross-sectional area. The anomalous geometry induces a complex three-dimensional flow pattern causing longitudinal flow to be concentrated near the bed of the river.

    In Chapter 2 a wavelet based technique is introduced to characterize meander shape in a quantitative, objective manner. A scale space forest composed of a set of rooted trees represents the meandering planform. Based on the rooted trees, the locally dominant meander wavelengths are defined along the river. Sub-meander scale spectral density in the wavelet transform is used to determine a set of metrics quantifying bend skewness and fattening. Negative fattening parameterizes the so-called non-harmonic or hairpin bend character of meanders. The super-meander scale tree represents the embedding of meanders into larger-scale fluctuations, spanning from double-headed meander scales until the scale of the valley thalweg. The new approach is used to quantify the anomalous planform geometry of the Mahakam River in a comparison with the Red River and the Purus River.

    The geometry of the Mahakam River is analyzed into more detail in Chapter 3, where the highly curved non-harmonic meanders are related to deep scours in the river bed. A total of 35 scours is identified which exceed three times the average river depth, and four scours exceed the river depth over four times. The maximum scour depth strongly correlates with channel curvature and systematically occurs half a river width upstream of the bend apex. Most scours occur in a freely meandering zone of the river. A systematic reconnaissance of the river banks reveals a switch of erosion-deposition patterns at high curvature. Advancing banks normally observed at the inner side of a bend are mostly found at the outer side of high-curvature reaches, while eroding banks switch from the outer side for mildly curved bends to the inner side for bends with high curvature. The overall lateral migration rate of the river is low. These results indicate a switch of morphological regime at high curvatures, which requires detailed flow measurements to unravel the underlying physical processes.

    Taking flow measurements in the deep scours of the Mahakam River presents a challenge to contemporary methods in hydrography. Acoustic Doppler Current Profilers (ADCPs) are capable of profiling flow velocity over large distances from a research vessel, but the existent data processing techniques assume homogeneity of the flow between the divergent acoustic beams. This assumption fails for complex three dimensional flows as found in the scours. In Chapter 4 a new ADCP data processing technique is developed that strongly reduces the extent over which the flow needs to be assumed homogeneous. The new method is applied to flow measurements collected in a river bend with a scour exceeding 40 m depth. Results based on the new approach reveal secondary flow patterns which remain invisible adopting the conventional method.

    Chapter 5 aims to better understand flow in sharp bends, by combining analyses of the flow measurements from a deep scour with Large Eddy Simulations of the flow. The three-dimensional flow field is strongly dominated by horizontal circulations at both sides of the scour. The dramatic increase in cross-sectional area (from 2200 m2 to 7000 m2 ) plays a crucial role in the generation of the two horizontal recirculation cells. An existing formulation to predict water surface gradients in bends is extended to include the effect of cross-sectional area variations, next to the effect of curvature changes. Variation in the cross-sectional area develops adverse water surface gradients explaining the flow recirculation. The depth increase toward the scour causes a strong downward flow (up to 12 cm s − 1 ) creating a non-hydrostatic pressure distribution, steering the core of the flow toward the bed. The latter aspect is poorly reproduced by the Large Eddy Simulations, which may relate to the representation of turbulent shear stresses.

    In Chapter 6 a novel technique is introduced to better monitor turbulence properties in complex river flows from ADCP measurements, exploiting what is discarded in observations of the mean flow. It extends the so-called variance method, using two ADCPs instead of one. The availability of eight acoustic beams, four from each ADCP, changes an otherwise unsolvable set of equations with six unknowns into an overdetermined system of eight equations with six unknowns. This allows to solve for the complete Reynolds stress tensor, yielding profiles of Reynolds stresses over almost the entire water column. Widely applied assumptions on turbulence anisotropy ratios are shown to be incorrect, which reveals a knowledge gap in open channel turbulence.

    Chapter 7 uses the technique developed in Chapter 6 to investigate the degree in which bed shear stress can be monitored continuously from an ADCP mounted horizontally at the river bank (HADCP). A calibrated boundary layer model is applied to estimate time-series of cross-river bed-shear stress profiles from HADCP velocity measurements. It is concluded the HADCP measurement can represent the regional bed shear stresses, as inferred from a logarithmic velocity profile, reasonably well. These regional bed-shear stresses, in turn, poorly represent the local estimates obtained from coupled ADCP measurements, which are more directly related to processes of sediment transport and complex river morphology. Detailed observations of turbulence properties may be the key to improve our understanding of complex river flow and morphology.

    Heteroaggregation and sedimentation rates for nanomaterials in natural waters
    Quik, J.T.K. ; Velzeboer, I. ; Wouterse, M. ; Koelmans, A.A. ; Meent, D. van de - \ 2014
    Water Research 48 (2014)1. - ISSN 0043-1354 - p. 269 - 279.
    sedimentatie - zwevende deeltjes - aggregatie - nanotechnologie - emissie - schatting - colloïden - waterstroming - zeewater - oppervlaktewaterkwaliteit - sedimentation - suspended solids - aggregation - nanotechnology - emission - estimation - colloids - water flow - sea water - surface water quality - engineered nanomaterials - silver nanoparticles - carbon nanotubes - manufactured nanoparticles - aggregation kinetics - aquatic environments - ceo2 nanoparticles - organic-matter - fate - exposure
    Exposure modeling of engineered nanomaterials requires input parameters such as sedimentation rates and heteroaggregation rates. Here, we estimate these rates using quiescent settling experiments under environmentally relevant conditions. We investigated 4 different nanomaterials (C60, CeO2, SiO2-Ag and PVP-Ag) in 6 different water types ranging from a small stream to seawater. In the presence of natural colloids, sedimentation rates ranged from 0.0001md-1 for SiO2-Ag to 0.14md-1 for C60. The apparent rates of heteroaggregation between nanomaterials and natural colloids were estimated using a novel method that separates heteroaggregation from homoaggregation using a simplified Smoluchowski-based aggregation-settling equation applied to data from unfiltered and filtered waters. The heteroaggregation rates ranged between 0.007 and 0.6Lmg-1 day-1, with the highest values observed in seawater. We argue that such system specific parameters are key to the development of dedicated water quality models for ENMs.
    Basisafvoer van de Baakse beek : onderzoek naar perspectieven voor aquatische natuur in een laaglandbeek
    Jansen, P.C. ; Massop, H.T.L. ; Houten, G.J. van den; Klutman, W.A.J. ; Bakx, W. - \ 2013
    Wageningen : Alterra, Wageningen-UR (Alterra rapport 2383) - 101
    waterlopen - hydrologie van stroomgebieden - droogte - waterstroming - historische ecologie - hydrologie - achterhoek - streams - catchment hydrology - drought - water flow - historical ecology - hydrology
    Een groot aantal van de oppervlaktewateren binnen het beheergebied van waterschap Rijn en IJssel heeft binnen de Kader Richtlijn Water de typering langzaam stromende midden- en benedenloop op zand (R5) gekregen. Vanuit ecologisch oogpunt houdt dat in dat er ook in droge perioden stroming in de waterloop moet plaatsvinden. Het waterschap wil weten hoe groot deze zogenaamde basisafvoer vroeger was en wat de invloed van latere antropogene ingrepen op de basisafvoer is geweest. Het moet antwoord geven op de vraag of een herstel van de basisafvoer een reële optie is. Het stroomgebied van de Baakse beek is door het waterschap aangewezen om daar onderzoek naar te doen.
    Solute transport in soil
    Zee, S.E.A.T.M. van der; Leijnse, A. - \ 2013
    In: Environmental sciences: soil processes and current trends in quality assessment / Hernandez Soriano, M.C., Rijeka, Croatia : InTech - ISBN 9789535110293 - p. 33 - 86.
    bodemchemie - waterstroming - bodemoplossing - modellen - soil chemistry - water flow - soil solution - models
    Solute transport is of importance in view of the movement of nutrient elements, e.g. towards the plant root system, and because of a broad range of pollutants. Pollution is not necessarily man induced, but may be due to geological or geohydrological causes, e.g. in the cases of pollution with arsenic, and salt. For the polluting species, a distinction can be made between dissolved and immiscible, and between conservative and reactive. Dissolved pollutants (aqueous phase pollutants) will spread with the groundwater due to groundwater flow, diffusion and dispersion.
    Ontwikkeling emissiemanagementsysteem grondgebonden teelt; de lysimeter en drainmeter
    Voogt, W. ; Zwinkels, F. ; Balendonck, J. ; Dorland, H. van; Winkel, A. van; Heinen, M. - \ 2012
    Bleiswijk : Wageningen UR Glastuinbouw (Rapporten GTB 1190) - 52
    cultuurmethoden - glastuinbouw - lysimeters - instrumenten (meters) - waterstroming - emissie - drainage - evapotranspiratie - drainagesystemen - cultural methods - greenhouse horticulture - lysimeters - instruments - water flow - emission - drainage - evapotranspiration - drainage systems
    Het hoofddoel van het project ‘Glastuinbouw Waterproof, grondgebonden’ was het ontwikkelen van een aantal middelen voor telers van grondgebonden teelten, waarmee zij emissiedoelstellingen kunnen halen. De leidende gedachte hierbij is dat een gesloten waterkringloop zoals toegepast bij substraatteelten onhaalbaar is. Emissiereductie zal vooral via het waterspoor behaald moeten worden en daarom is een brongerichte aanpak, de irrigatie afgestemd op de evapotranspiratie, het meest effectief. Het project omvatte in de eerste plaats het ontwikkelen en combineren van een aantal technische hulpmiddelen en in de tweede plaats het installeren en testen in de praktijk. In dit rapport worden de lysimeter en de bijbehorende drainmeter besproken.
    Onzekere debietschattingen
    Mulder, H.M. ; Schipper, P.N.M. ; Jongman, E. ; Heijkers, J. - \ 2011
    Stromingen : vakblad voor hydrologen 16 (2011)3. - ISSN 1382-6069 - p. 33 - 43.
    hydrodynamica - afvoer - waterstroming - schatting - hydrodynamics - discharge - water flow - estimation
    De meest fundamentele en daarmee belangrijkste activiteit van hydrologen is het opstellen van waterbalansen. Immers, waterbalansen vormen het hart van elke watersysteemanalyse, inclusief het ontwikkelen, kalibreren en valideren van modellen. Naast de neerslag en verdamping vormen de aan- en afvoer van oppervlaktewater de belangrijkste componenten van de Nederlandse waterbalans. Die worden indirect afgeleid uit metingen bij klepstuwen, afsluiters, pompgemalen e.d. In dit artikel gaan wij in op de vraag hoe accuraat met zulke meetmethodes de debieten aan- en afvoeren in de praktijk kunnen worden geschat en hoe dit verbeterd kan worden. Wij concluderen dat de onzekerheden groot zijn en dat er veel te verbeteren is over het gehele traject: het uitvoeren van de metingen, het ijken van de formules van de meetopstellingen met alternatieve meetmethodes, de opslag en verwerking van de ruwe meetdata naar debieten en het kwantificeren van onzekerheden daarin.
    Beekherstel vergt integrale aanpak
    Lototskaya, A.A. ; Verdonschot, P.F.M. - \ 2011
    Land + Water (2011). - ISSN 0926-8456 - p. 73 - 75.
    waterlopen - herstelbeheer - waterstroming - beekdalen - morfologie - ecologisch herstel - streams - restoration management - water flow - brook valleys - morphology - ecological restoration
    Bij beekherstel gaat het niet alleen om de waterstroom zelf, maar om het hele beekdal. Ook is gebleken dat morfologie en hydrologie van de beek sterk samenhangen. Een goed herstel is alleen mogelijk als het beekwater door de seizoenen heen een vrij constante stroming heeft. Dit is bijvoorbeeld te bereiken door dood hout in de beek te plaatsen.
    Stationary flow solution for water levels in open channels
    Opheusden, J.H.J. van; Molenaar, J. ; Beltman, W.H.J. ; Adriaanse, P.I. - \ 2010
    Wageningen : Alterra (Alterra-report 2084) - 37
    waterstroming - kanalen, klein - afvoer - waterstand - constante stroming - modellen - water flow - channels - discharge - water level - steady flow - models
    We study stationary flow in open discharge channels. A model is derived from basic principles, which is solved numerically for the water level and discharge as a function of position along the channel. The model describes the effect of external inflow from fields adjacent to the channel. Several scenarios are calculated, both for very slowly, and more rapidly flowing water courses.
    Role of sediment in the design and management of irrigation canals : Sunsari Morang Irrigation Scheme, Nepal
    Paudel, K. - \ 2010
    Wageningen University. Promotor(en): E. Schultz, co-promotor(en): N.M. Shakya. - [S.l. : S.n. - ISBN 9789085858515 - 271
    irrigatie - irrigatiesystemen - irrigatiekanalen - waterbouwkunde - ontwerp - bedrijfsvoering - sediment - waterstroming - nepal - irrigation - irrigation systems - irrigation channels - hydraulic engineering - design - management - sediment - water flow - nepal
    Sediment transport in irrigation canals
    The sediment transport aspect is a major factor in irrigation development as it determines to a large extent the sustainability of an irrigation scheme, particularly in case of unlined canals in alluvial soils. Investigations in this respect started since Kennedy published his channel-forming discharge theory in 1895. Subsequently different theories have been developed and are used around the world. All of them assume uniform and steady flow conditions and try to find the canal dimensions that are stable for a given discharge and sediment load. In the past irrigation schemes were designed for protective purposes with very little flow control, hence steady and uniform flow conditions could be realised to some extent.
    Modern irrigation schemes are increasingly demand based, which means that the water flow in a canal is determined by the crop water requirements. Accordingly the flow in the canal network is not constant as the crop water requirement changes with the climate and the growing stages of the crops. Also the inflow of the sediment is not constant throughout the irrigation season in most schemes. The situation is even worse for run-of-the-river schemes where fluctuations in the river discharge have a direct effect on the inflow of water and sediment.
    The conventional design methods are not able to predict accurately the sediment transport behaviour in a canal, firstly due to the unsteady and non-uniform water flow conditions and secondly due to the changing nature of the sediment inflow. Hence, the actual behaviour of a canal widely diverges from the design assumptions and in many cases immense maintenance costs have to be met with to tackle the sediment problems.
    An irrigation scheme should not only be able to deliver water in the required amount, time and level to the crops on the field, but also should recover at least its operation and maintenance cost. Cost recovery is, to some extent, related to the level of service provided by the irrigation organization and the expenditure for operation and maintenance of the scheme. Past experiences in Nepal have shown that modernization of existing irrigation schemes to improve the level of service has also increased the operation and maintenance costs. These costs are, in some cases, high compared to the generally low level of ability of the water users and farmers to pay these costs. The search of making schemes more equitable, reliable and flexible has resulted in the introduction of new flow control systems and water delivery schedules that may, if not carefully designed, adversely affect the sediment transport behaviour of a canal. In quite some schemes unpredicted deposition and/or erosion in canals have not only increased the operation and maintenance costs but also reduced the reliability of the services delivered.
    Irrigation development in Nepal and the study area
    Nepal is a landlocked country in South Asia lying between China and India. It is situated between 26º22' N to 30º27' N latitude and 80º4' E to 88º12' E longitude of the prime meridian. Roughly rectangular in shape, the country has an area of 147,181 km2. It is 885 km in length but its width is uneven and increases towards the West. The mean North-South width is 193 km. Nepal is a predominantly mountainous country, with elevations ranging from 64 m+MSL (Mean Sea Level) at Kechana, Jhapa to 8,848 m+MSL at the peak of the world highest mountain, Everest, within a span of 200 km. Nepal has a cultivated area of 2.64 million ha, of which two third (1.77 million ha) is potentially irrigable. At present 42% of the cultivated area has some sort of irrigation, out of which only 41% is receiving year round irrigation water. The existing irrigation schemes contribute approximately 65% of the country’s current agriculture production.
    Nepal has a long history of irrigated agriculture. Most of the existing large-scale irrigation schemes are located in the southern alluvial plain (Terai). The canals are unlined and the sediment load forms an integral part of the supplied irrigation water. The schemes are predominantly supply based and have a very low duty for intensive cropping. In view of the increased competition among the different water using sectors and low performance of these schemes, many of them are undergoing modernization. For example, the Sunsari Morang Irrigation Scheme (SMIS) is one of the schemes under modernization, and it has been taken as a study case for this research. A better understanding of the sediment transport process under changing flow and sediment load conditions, a shifting management environment and different maintenance scenarios will be very useful in pulling out the schemes from the present vicious cycle of construction-deterioration-rehabilitation.
    The Sunsari Morang Irrigation Scheme (SMIS) is located in the eastern Terai. The Koshi River is the source of water. A side intake for the water diversion, an around 50 km long main canal of capacity 45.3 m3/s for water conveyance and 10 secondary canals and other minor canals of various capacities for water distribution were constructed to irrigate a command area of 68,000 ha. The system was put into the operation in 1975, but faced a serious problem of water diversion and sediment deposition in the canal network. Hence from 1978, after 3 years of operation, rehabilitation and modernization work of the scheme has been started. During modernization the intake has been relocated to increase the water diversion and reduce the sediment entry. Besides, a settling basin with dredgers for continuous removal of sediment has been provided near the head of the main canal. Apart from that the command area development and modernization of existing canal network is in progress and till third phase (1997-2002), around 41,000 ha area has been developed.
    Sediment transport research
    The aim of this research is to understand the relevant aspects of sediment transport in irrigation canals and to formulate a design and management approach for irrigation schemes in Nepal in view of sediment transport. In the process, the design methods used in the design of irrigation schemes in Nepal and their effectiveness on sediment transport have been studied. The impact of operation and maintenance on sediment movement has been analysed taking the case study of SMIS. An improved design approach for sediment transport in irrigation canals has been proposed. A mathematical model SETRIC has been used to study the interrelationship of sediment movement with the design and management and to evaluate the proposed design approach for irrigation canal based on the data of the SMIS.
    The mathematical formulation of sediment transport process in an irrigation canal is based on the previous works in this field, most notably the work of Mendez on the formulation of the mathematical model SETRIC. Subsequent analysis, improvement and verification works by Paudel, Ghimire, Orellana V., Via Giglio and Sherpa have been used. The model SETRIC has been verified and improved where found necessary and has been used to analyse the irrigation scheme and to propose an improvement in the design and management from sediment transport point of view.
    Assessment of design parameters
    The methods of selecting the design discharge and sizing of canals for modern irrigation schemes based upon the present concept of crop based irrigation demand, water delivery schedules and water allocation to the tertiary units have been analysed. The selection of a crop depends upon the soil type, water availability, socio-economic setting and climatic conditions. The type of crop together with the soil type determines the irrigation method and irrigation schedules, while the type of crop and climatic condition determines the irrigation water requirement. The required flow in a canal is then derived based on the water delivery schedule from that canal to the lower order canals or to the field to meet the water requirement.
    The factors that influence the roughness of an irrigation canal have been analysed and a proposal for a more rational roughness determination process has been formulated based on the available knowledge. The roughness in the sides depends upon the shape and size of material, vegetation and surface irregularities, while the roughness in the bed is a function of shape and size of material and the surface irregularities (bed form in case of alluvial canals). For the prediction of roughness in the bed mostly two approaches are in use – methods based on hydraulic parameters (water depth, flow velocity and bed material size) and the methods based on bed forms and the grain related parameters. In this research, the method based on the bed form and grain related parameters, as suggested by van Rijn, has been used. Similarly, for the determination of roughness in the sides, the influence of surface irregularities have been included by dividing the maintenance condition as ideal, good, fair and poor and accordingly applying the correction to the standard roughness value for the type of material. The influence of vegetation has been accounted based on the concept of V.T. Chow. The various methods of computing the equivalent roughness have been compared and the method proposed by Mendez has been found to be better when tested with the Kruger data.
    Most of the sediment transport predictors consider the canal with an infinite width without taking into account the effects of the side walls on the water flow and the sediment transport. The effect of the side wall on the velocity distribution in lateral direction is neglected and therefore the velocity distribution and the sediment transport are considered to be constant in any point of the cross section. Under that assumption a uniformly distributed shear stress on the bottom and an identical velocity distribution and sediment transport is considered. Majority of the irrigation canals are non-wide and trapezoidal in shape with the exception of small and lined canals that may be rectangular. In a trapezoidal section the water depth changes from point to point in the section and hence the shear stress. The effect would be more pronounced if the bed width to water depth ratio (B-h ratio) is small. The change in velocity distribution in a canal in view of the change in boundary shear and water depth along the cross section has been analysed and evaluated with the field measurements. The change in velocity and shear stress in a canal section has been used to evaluate the influence of B-h ratio and side slope in the prediction of sediment transport capacity by selected predictors (Brownlie, Engelund-Hansen and Ackers-White). The evaluation with the available data set showed that the proposed correction improved the predictability for non-wide irrigation canals.
    Canal design approaches for sediment transport in Nepal
    For the design of canals having erodible boundary and carrying sediment loads two approaches are in practice, namely the regime method and the rational method. The regime design methods are sets of empirical equations based on observations of canals and rivers that have achieved dynamic stability. The rational methods are more analytical in which three equations, an alluvial resistance relation, a sediment transport equation and a width-depth relationship, are used to determine the slope, depth and width of an alluvial canal when the water and sediment discharges as well as the bed material size are specified.
    In Nepal, the design manuals of the Department of Irrigation recommend Lacey’s regime equations and White-Bettess-Paris tables with the tractive force equations for the design of earthen canals carrying sediment. But in practice, there is no consistency in the design approaches that has been found to vary from canal to canal even within the same irrigation scheme. The use of Lacey’s equation for computing the B-h ratio has generally resulted in wider canals. This is so, because flatter side slopes than predicted by the Lacey’s equations are used from soil stability considerations.
    The White-Bettess-Paris tables are derived from alluvial friction equations of White, Bettess and Paris (1980) and sediment transport equations of Ackers and White (1973). No records regarding the use of this method for the design of canals was found and hence its performance in terms of sediment transport could not be verified. However, the Ackers and White sediment transport equations over-predicted the sediment transport capacity of a canal when tested with the SMIS data. The sediment load entering into the canals of SMIS is mostly fine (d50 < 0.2 mm) and most of the large scale irrigation schemes in Nepal have similar geo-morphological settings. That means that the White-Bettess-Paris tables will result in a canal with a flatter slope than actually required to carry the type of sediment prevailing in SMIS and other similar irrigation schemes of Nepal. Analysis showed that the Brownlie and Engelund and Hansen equations are more suitable for the type of sediment that has been found in SMIS.
    During the modernization, the secondary canals (S9 and S14) of SMIS have been designed by two different approaches. Secondary Canal S9 has been designed using Lacey’s regime concept while Secondary Canal S14 has been designed using an energy approach. In the energy approach the erosion is controlled by limiting the tractive force and the deposition is controlled by ensuring equal or non-decreasing energy of the flow in the downstream direction. Both the canals have been evaluated for their sediment transport capacity for the prevailing sediment characteristics. The carrying capacities of both canals (~ 230 ppm) have been found to be less than the expected sediment load (~ 300 – 500 ppm) in the canal. The energy concept assumes that the sediment transport is proportional to the product of velocity and bed slope. The carrying capacity of the canal designed by this principle has been found to be variable along its length. It means that the sediment transport capacity is not only a function of bed slope and water depth as assumed in the energy concept.

    An improved approach for the design and management of irrigation canals
    In general the reliability of sediment transport predictors is low and at best they can provide only estimates. As per Vito A Vanoni (1975) a probable error in the range of 50-100% can be expected even under the most favourable circumstances. There is no universally accepted formula for the prediction of sediment transport. Most of them are based upon laboratory data of limited sediment and water flow ranges. Hence they should be adjusted to make them compatible to specific purposes, otherwise the predicted results will be unrealistic. An improved rational approach has been proposed for the design of alluvial canals carrying sediment loads. To find the bed width, bed slope and water depth of a canal for a given discharge and sediment characteristics three equations, namely a sediment transport predictor (total load), resistance equation (Chézy) and a B-h ratio predictor are used.
    A canal design program DOCSET (Design Of Canal for SEdiment Transport) has been prepared for the improved approach including the above mentioned improvements. The program can also be used to evaluate the existing design for a given water flow and sediment characteristics. Basic features of the new approach are:
     concept of dominant concentration. Instead of using the maximum concentration, the approach suggests to look for a concentration that results in net minimum erosion/deposition in one crop calendar year;
     determination of roughness. The proposed method makes use of the elaborated and more realistically determined roughness value in the design process. The roughness of the cross section is adjusted as per the hydraulic condition and sediment characteristics. Moreover the influences of the side slopes and the B-h ratio are included while computing the equivalent roughness of the section. This should result in a more accurate prediction of hydraulic and sediment transport characteristics of the canal and hence, a better design;
     explicit use of sediment parameters. The sediment concentration and representative size (dm) is explicitly used in the design. That will make the design process more flexible as different canals might have to divert and convey sediment loads of different sizes (dm) and amounts;
     Use of an adjustment parameter. An adjustment parameter has been used that includes the influence of non-wide canals, sloping side walls and exponent of velocity in the sediment transport predictor. This adjustment should increase the accuracy of the predictors when they are used in irrigation canals, an environment for which they were not derived;
     holistic design concept. This approach uses one canal system as a single unit. The canal system may have different canals of different levels, but the water and sediment management plans are prepared for the whole system. Then the hydraulic design of the individual canal can be made to meet the design management plan for that canal;
     Selection of B-h ratio. A B-h ratio selection criterion has been proposed considering the side slope selection practices in Nepal as well as the sediment transport aspects.

    Since, the sediment transport process is influenced by the management of the irrigation scheme, the design should focus to have a canal that is flexible enough to meet the demand and still have a minimum deposition/erosion. The provision of sufficient carrying capacity up to the desired location (conveyance), providing controlled deposition options if the water delivery plans limit the transport capacity (provisions of settling pockets) and preparation of maintenance plans (desilting works) are some of the aspects that would have to be analysed and included in the design to reduce the sediment transport problems.
    The canal design methods can give the best possible canal geometry for a given water flow and sediment concentration only. For water flows and sediment concentrations other than the design values, there may be either erosion or deposition. The aim of the design would have to be to balance the total erosion and deposition in one crop calendar year. So, a design may not be based on the maximum sediment concentration expected during the irrigation season, but on a value that results in the minimum net erosion/deposition. The best way to evaluate a canal under such scenario is to use a suitable sediment transport model. Besides, the roughness of the canal depends upon the hydraulic conditions, sediment characteristics and the maintenance plans that are constantly changing throughout the irrigation season. The canals are designed assuming a uniform flow and sediment transport under equilibrium condition. However, such conditions are seldom found in irrigation canals due to the control in flow to meet the variation in water demand. Hence, the design of a canal would have to be evaluated using a sediment transport model for the selection of proper design parameters and to evaluate the design for the proposed water operation plans.
    The mathematical model SETRIC
    The mathematical model SETRIC is a one-dimensional model, where the water flow in the canal has been schematised as a quasi-steady and gradually varied flow. This one dimensional flow equation is solved by the predictor-corrector method. Gallappatti’s depth integrated model for sediment transport has been used to predict the actual sediment concentration at any point under non-equilibrium conditions. Galappatti’s model is based on the 2-D convection-diffusion equation. The mass balance equation for the total sediment transport is solved using the modified Lax’s method, assuming a steady condition of the sediment concentration. For the prediction of the equilibrium concentration one of the three total load predictors: Brownlie, Engelund and Hansen or Ackers and White methods can be used.
    The model SETRIC was evaluated using other hydrodynamic and sediment transport models (DUFLOW and SOBEK-RIVER) and was validated by the field data of SMIS. Predictability of different predictors has been compared. The Brownlie and Engulund and Hansen methods predicted reasonably for the sediment size of 0.1 mm (d50), while predictability of Ackers and White for the sediment size was found to be poor. The sensitivity of Brownlie’s method was more uniform than the other two methods for a sediment size range of 0.05 to 0.5 mm.
    Field data collection
    For the field measurements of the sediment transport process, one of the secondary canals of SMIS (S9) was selected. Since, the objective of field data was to test the design approach for sediment transport; preference was given for a canal that was recently designed and constructed. The field measurement of water and sediment flow was carried out in 2004 and 2005. During field measurements the water inflow rate into Secondary Canal S9 system was measured. A broad crested weir immediately downstream of the intake for Secondary Canal S9 was calibrated and used for discharge measurement. For sediment concentration measurements, dip samples just downstream of the hydraulic jump were taken on a daily basis. The samples were then analysed in the laboratory and the sediment concentration was determined. Point sampling across the section using pump samplers were also taken and the calculation results showed that the dip samples underestimated pump samples by around 8% in case of the total load and by around 35% for the sediment of size > 63 μm. At the end of the irrigation season, the deposited sediment samples along the canal were taken to determine the representative sediment size and other properties.
    The irrigating water delivered to the sub-secondary canals, delivery schedules and the set-points upstream of water level regulators were also measured. For morphological change, the pre-season and post-season canal geometry was measured. The velocity distribution in the trapezoidal earthen canal section was measured. Besides, field measurement roughness (indirect measurement) was also made in the beginning, mid and end of the seasons to determine the change in roughness in time.
    Modelling results
    The model SETRIC was used to study the effect on the sediment transport process due to system management activities namely, change in water demand and supply, water delivery modes based on the available water and the change in sediment load due to the variation in sediment inflow from the river or problems in proper operation of the settling basin. For the design water inflow into Secondary Canal S9, a water delivery schedule has been designed and has been evaluated for sediment transport efficiency under changing sediment inflow conditions. The improved canal design approach was evaluated comparing the results with the existing design of Secondary Canal S9. Some of the findings of the modelling results are:
     water delivery schedules can be designed and implemented to reduce the erosion/deposition problems of a certain reach even after the system is constructed and put into operation;
     the design operation plans and assumptions have not been followed in Secondary Canal S9 of SMIS. From sediment transport perspective, the existing water management practice results more sedimentation in the sub-secondary and tertiary canals than Secondary Canal S9;
     the periodic change in the demand and the corresponding change in sediment transport capacity of the canal can be manipulated to arrive at the seasonal balance in the sediment deposition. In one period, there may be deposition but that can be eroded in the next period;
     the proposed water delivery plan is based on the existing canal and its control structures and covers discharge fluctuation from around 46% to 114%. Hence, it can be implemented with the present infrastructure and can handle all possible flow situations in the canal;
     the proposed delivery schedule ensures either full supply or no supply to the sub-secondary canals which have been designed for the same principle. This could reduce the existing deposition problem faced by these canals;
     proper operation of the settling basin is crucial for the sustainability of the SMIS;
     the secondary canals need to be operated in rotation when there is less demand or less available water in the main canal. This will ensure design flow in the secondary canals and reduce the sedimentation problem. The main canal would have to be analysed for the best mode of rotation from sediment transport and water delivery perspective.
    Major contributions of this research
    Apart from the recommendations made in the design, management and operation of Secondary Canal S9 from sediment transport perspective, the following contributions are made by this research:
     an elaborate analysis of the velocity and shear stress distribution across the trapezoidal canal is made to derive the correction factor for the sediment transport predictor. This will help to increase the predictability when the predictors are used for the analysis of sediment transport in irrigation canals;
     an explicit method of including roughness parameters in the calculation of the equivalent roughness for the mathematical model has been proposed;
     the sediment transport model SETRIC has been updated and its functionality has been improved. The model can now be used as a design as well as research tool for analyzing the sediment transport process for different water delivery schedules and control systems;
     an improved approach for the design and management of irrigation canals has been proposed. A computer program DOCSET has also been prepared based on the approach. The program is interactive, easy to use and can be used by designers with limited modelling know-how;
     a water delivery plan has been designed and tested for the changing water and sediment inflow condition that can be implemented with the existing canal infrastructure;
     the causes of sedimentation in the sub-secondary canals of Secondary Canal S9 have been identified.
    Conclusions and outlook for the future
    Canal design is an iterative process where the starting point is the preparation of management plans. Then the design parameters need to be selected and the preliminary hydraulic design of the canal can be made. The design results can then be used in the model to simulate and evaluate the proposed management plans and the sediment transport process in the system. Necessary adjustment can be made either in the design parameters or in the management plans, if needed. Then the canal need to be redesigned and the process would have to be continued till a satisfactory condition is reached.
    The coarser fraction of the sediment is mostly controlled at the headwork and settling basin of an irrigation scheme. The sediment that is encountered in main and secondary canals is generally fine sand. Most of the silt fraction (sediment < 63 μm) is transported to the lower order canals and fields where it gets deposited. In the sub-secondary and tertiary canals, it has been observed that the fine sediment does not roll down to the bed as normally assumed in the case of sand and gets deposited on the slope also. Thus the canal section becomes narrower and the side slope becomes steeper. This phenomenon can not be analysed with the present sediment transport assumptions and an investigation in this aspect to address the transport process of fine sediment would be beneficial for improving the design and management of irrigation canals.
    Flexibility of operation and sediment transport aspects restrict each other. A canal without any control can be designed and operated with higher degree of reliability in terms of sediment transport. Once the flow is controlled the sediment transport pattern of the canal is changed and the designed canal will behave differently. Hence, both flexibility and efficient sediment management are difficult to achieve at the same time. A compromise has to be made and this needs to be reflected in the design.
    All the methods to transport, exclude or extract the sediment are temporary measures and just transfer the problem from one place to the other. They are not the complete solutions of the sediment problem. A better understanding on sediment movement helps to identify the problems beforehand and look for the best possible solutions.

    Upward groundwater flow in boils as the dominant mechanism of salinization in deep polders, the Netherlands
    Louw, P.G.B. de; Oude Essink, G.H.P. ; Stuyfzand, P.J. ; Zee, S.E.A.T.M. van der - \ 2010
    Journal of Hydrology 394 (2010)3-4. - ISSN 0022-1694 - p. 494 - 506.
    kwel - grondwater - zout water - waterstroming - polders - chloride - holoceen - grondwaterkwaliteit - zuid-holland - seepage - groundwater - saline water - water flow - polders - chloride - holocene - groundwater quality - zuid-holland - rhine-meuse delta - saltwater intrusion - surface-water - sand boils - discharge - aquifer - temperature - soil - hydrology - quality
    As upward seepage of saline groundwater from the upper aquifer is leading to surface water salinization of deep polders in the Netherlands, we monitored the processes involved in the Noordplas Polder, a typical deep polder. Our results show three types of seepage: (1) diffuse seepage through the Holocene confining layer, (2) seepage through paleochannel belts in the Holocene layer, and (3) intense seepage via localized boils. They differ with regard to seepage flux, chloride concentration, and their location in the polder; thus, their contributions to surface water salinization also differ. Permeable, sandy paleochannel belts cut through the lower part of the Holocene layer, resulting in higher seepage fluxes than the diffuse seepage through the Holocene layer where there are no paleochannels. The average chloride concentration of paleochannel seepage is about 600 mg/l, which is sixfold higher than the average concentration of diffuse seepage. The highest seepage fluxes and chloride concentrations are found at boils, which are small vents in the Holocene layer through which groundwater preferentially discharges at high velocities. This results in upconing of deeper and more saline groundwater, which produces an average chloride concentration of 1100 mg/l. Despite the fact that seepage fluxes are difficult to measure, we were able to calculate that boils contribute more than 50% of the total chloride load entering the Noordplas Polder and they therefore form the dominant salinization pathway
    The influence of light and water flow on the growth and physiology of the scleractinian coral Galaxea fascicularis
    Schutter, M. - \ 2010
    Wageningen University. Promotor(en): Johan Verreth; Rene Wijffels, co-promotor(en): Ronald Osinga. - [S.l. : S.n. - ISBN 9789085855354 - 230
    koralen - cnidaria - lichtsterkte - waterstroming - groei - metabolisme - dierfysiologie - aquariums - aquacultuur - corals - cnidaria - light intensity - water flow - growth - metabolism - animal physiology - aquaria - aquaculture
    Background
    Zooxanthellate scleractinian corals are sessile colonial animals that live in symbiosis with photosynthetic algae, the zooxanthellae. They can feed both phototrophically and heterotrophically and produce an external skeleton of calcium carbonate, which process is enhanced by light. They are the key organisms of tropical coral reefs and responsible for building the large carbonate structures. Tropical coral reefs are increasingly threatened by both natural and anthropogenic stresses. Concurrently with the gradual decline of coral reefs, a growing interest in keeping this delicate ecosystem in aquaria has emerged. To reduce harvest from the wild, increasing effort is put in developing cost-effective coral aquaculture culture. The objective of this thesis was to study the influence of light (irradiance and photoperiod) and flow on coral growth and physiology. Furthermore, the interaction between light and water flow was studied.

    Methods
    The effect of flow (Chapter 2), light (Chapter 3), photoperiod (Chapter 4) and the interaction between light and flow (Chapter 5) on coral growth were studied in long-term experiments monitoring several growth parameters such as buoyant weight (i.e. skeletal mass), surface area and polyp number. Physiological parameters such as photosynthesis and respiration were measured in respirometric flowcells to provide an explanation for the observed differences in growth. Moreover, an overview was given of different factors controlling coral growth and how such knowledge can be translated to aquaculture practice (Chapter 6).

    Results
    In the absence of water flow, coral growth was significantly lower and corals appeared unhealthy. In the presence of water flow (10, 20 and 25 cm s-1, at 90 µE m-2 s-1), growth rates were significantly increased. However, growth was not significantly different between 10 cm s-1 and 20 cm s-1, but again significantly increased at 25 cm s-1. Differences in growth could not be explained by net photosynthetic rate and Scope for Growth based on phototrophic carbon, since these parameters decreased with increasing water flow (Chapter 2). Increasing irradiance significantly increased the specific exponential growth rate of Galaxea fascicularis. The relation between skeletal growth and net photosynthesis was not directly proportional, but distorted at high irradiance levels (Chapter 3). Increasing photoperiod did not increase the specific exponential growth rate of Galaxea fascicularis. However, since growth neither increased with increasing irradiance, it is suggested that growth was limited by another factor and light was therefore saturating. The corals in the 24 hour light treatment were not able to adapt to prolonged light duration. However, the corals in the 16 hour light treatment probably photo-acclimated to prolonged photoperiod under light-saturating conditions by reducing their hourly photosynthetic rates (16 hour vs. 8 hour light) As a result, daily net photosynthetic was not significantly different, just as their growth rates (Chapter 4). The interaction between light and water flow for coral growth was significant. Water flow stimulated coral growth more at 600 µE m-2 s-1 than at 300 µE m-2 s-1 and the highest growth rates were attained at high irradiance (600 µE m-2 s-1) in combination with high flow rates (15-25 cm s-1). Nevertheless, enhancement of coral growth with either increasing irradiance or increasing water flow could not be explained by net photosynthetic rates (Chapter 5). There are many factors (both environmental and genetical) that can potentially limit or inhibit coral growth. Optimization of coral aquaculture therefore requires close fine-tuning of factors. Growth models can be used as a tool to determine the best culture strategy (Chapter 6).

    Conclusion
    Increasing water flow has a positive effect on coral growth at a wide range of irradiance levels (90, 300 and 600 µE/m2/s). Increasing irradiance also has a positive effect on coral growth. However, the positive relation between irradiance and coral growth is disturbed when other factors are limiting. Corals were able to retain their growth rates upon photoperiod extension under light-saturating conditions, presumably by means of reducing their hourly photosynthetic rate. At high irradiance levels, the enhancement of coral growth was not proportionally related to net photosynthesis, suggesting that other factors become limiting. Since the interaction between irradiance and water flow is significant, this indicates that water flow can remove some of those limitations at high irradiance levels. The mechanism of enhancement did not seem related to differences in net photosynthetic rate, but is possibly related to reduced energy allocation toward costly photo-protective mechanisms at high irradiance and high flow.
    The information in this thesis should not be used as a blueprint for coral aquaculture, however, its value lies in providing a blueprint for targeted optimization studies of coral aquaculture. Since the magnitude of effect of one factor often depends on the other, it is of importance for the future to perform multi-factorial experiments to provide insight in the interactions between factors.




    Under light-saturating conditions, further increases in irradiance or extension of photoperiod do not result in more growth. [


    Increasing irradiance has a positive relation with both coral growth and net photosynthesis. At high irradiance levels, the enhancement of coral growth was not proportionally related to net photosynthesis, suggesting that other factors become limiting. Since the interaction between irradiance and water flow is significant, this indicates that water flow can remove some of those limitations at high irradiance levels. The mechanism did not seem related to differences in net photosynthetic rate, but is possibly related to reduced energy allocation toward costly photo-protective mechanisms at high irradiance and high flow.
    Under light-saturating conditions, further increases in irradiance or extension of photoperiod do not result in more growth. [


    Both increasing irradiance and water flow enhance coral growth. Enhancement of coral growth by light was not proportionally related to net photosynthesis at higher irradiance levels, suggesting that other factors become limiting. Since the interaction between irradiance and water flow is significant, this indicates that water flow can remove some of those limitations at high irradiance levels. The mechanism did not seem related to differences in net photosynthetic rate, but is possibly related to reduced energy allocation toward costly photo-protective mechanisms at high irradiance and high flow.
    Under light-saturating conditions, further increases in irradiance or extension of photoperiod do not result in more growth. [


    Increasing water flow has a positive effect on coral growth at a wide range of irradiance levels (90, 300 and 600 µE/m2/s). In our studies, differences in growth could not be explained with differences in net photosynthetic rate. Rather differences in growth could be related to algal competition and sedimentation or other physiological parameters such as (in)organic nutrient uptake, relief of photo-oxidative stress and dark respiration that are influenced by water flow.
    Increasing irradiance also has a positive effect on coral growth. Enhancement of coral growth by light was not proportionally related to net photosynthesis at higher irradiance levels, suggesting that other factors become limiting. The significant interaction between light and water flow indicates that water flow can remove some limitations and that the presence of water flow is very important for optimal light use for coral growth. The mechanism did not seem related to differences in net photosynthetic rate, but is possibly related to reduced energy allocation toward costly photo-protective mechanisms at high irradiance and high flow.
    The effect of increasing photoperiod under light-limiting conditions still needs to be established.



    Since enhancement of coral growth did not seem to be d… by photosynthesis at higher irradiance levels [WEL, maar niet proportional], it suggested limited.. One of such factors that can remove limitations is water flow. A significant interaction between light and water flow is detected, …
    but this effect is dependent on water flow.
    The interaction between light and water flow indicates that water flow is very important for optimal light use for coral growth. The mechanism is still unclear, but is possibly related to reduced energy allocation toward costly photo-protective mechanisms at high irradiance and high flow.


    Both increasing irradiance and water flow have a positive effect on coral growth, and a significant interaction is found between light and water flow.

    The positive effect of increasing water flow on coral growth was found to be significant both at an irradiance of 90 µE/m2/s (Chapter 2), 300 µE/m2/s and 600 µE/m2/s (Chapter 5). Neither of these differences in growth were supported by a significant increase in net photosynthetic rate, in contrast to our expectations. The positive effect of increasing water flow on coral growth is probably a consequence of both external (algal competition and sedimentation) and internal mechanisms( (in)organic nutrient uptake, relief of photo-oxidative stress, respiration). Different at different irradiance levels. External at 90, internal at 300 and 600
    The positive effect of irradiance on skeletal growth was demonstrated in both Chapter 3 and 5. The relationship with photosynthesis irradiance curve not proportional.

    - mediation by P not clear.
    - The relation between skeletal growth and net photosynthesis was not proportional,

    In contrast, no positive effect of irradiance on skeletal growth was found in Chapter 4.
    Both specific growth rate and net photosynthesis increased with irradiance, however, this relationship was not proportional.
    It is suggested that .at high irradiance levels, skeletal growth (i.e. calcification and organic matrix synthesis) is not limited by light or photosynthesis. At high irradiance, other factor (e.g. availability of bicarbonate (i.e. aragonite saturation state), heterotrophic feeding and/or water flow) may become limiting.

    Water flow and light…
    Photoperiod… still needs to be established. .. offset limitations . optimize balance between factors






    At an irradiance of 90 µE m-2 s-1, water flow enhanced coral growth
    Increased water flow enhanced coral growth at a wide range of irradiances: 90 µE m-2 s-1 (Chapter 2), 300 and 600 µE m-2 s-1 (Chapter 5). AT 90 µE m-2 s-1, no The mechanism of enhancement is however not clear.

    Flow enhanced growth.. absence of flow detrimental.. did… however.. P, algae..
    Light… relation growth and photosynthesis distorted.. light enhanced calcificcaiotn.
    Photoperiod… no effect .. light not limiting.. photoacclimation.. 24 not able to adapt, but 16 hours well, probably by reducing hourly P.

    Light x flow…

    Interaction between light and water flow (chapter 5)..
    … no diff chapter 4… Review factors (chapter 6)
    .. examined… chapter.. Furthermore… In addition..

    different mechanisms at different irradiance levels. No relation with P. At 90… reduce disturbance of coral growth by competing algae. at 300 and 600 …. Reduce photo-protective mechanisms

    Evaluatie van de effectschatting waterkwaliteit Terra Nova Zuid na aanbrengen damwand
    Kosten, S. ; Faassen, E.J. ; Lürling, M.F.L.L.W. - \ 2009
    Wageningen : Wageningen UR - 6
    watersystemen - waterkwaliteit - waterstroming - dammen - aquatische ecologie - water systems - water quality - water flow - dams - aquatic ecology
    Waternet is van plan om in het gebied Terra Nova een damwand aan te brengen tussen de delen Terra Nova Noord en Terra Nova Zuid om daarmee de overlast van blauwalgen in Terra Nova Noord tegen te gaan. Een groep bewoners van Terra Nova Zuid vreest echter voor negatieve gevolgen van deze maatregel op de waterkwaliteit van Terra Nova Zuid. Deze bewonersgroep heeft Wageningen UR gevraagd te beoordelen of de effecten van het aanbrengen van een dam op het watersysteem van Terra Nova Zuid goed zijn onderbouwd.
    Ruimtelijke verdeling van gewasbeschermingsmiddelen in de kas: Hoe komt een middel via een gewasbehandeling in het recirculatiewater terecht?
    Os, E.A. van; Staaij, M. van der; Michielsen, J.M. ; Slooten, M.A. van; Velde, P. van; Beerling, E.A.M. ; Zande, J.C. van de - \ 2009
    Wageningen : Wageningen UR Glastuinbouw (Rapporten GTB 1006) - 15
    pesticiden - waterkwaliteit - kastechniek - ecologische risicoschatting - oppervlaktewaterkwaliteit - waterstroming - glastuinbouw - pesticides - water quality - greenhouse technology - ecological risk assessment - surface water quality - water flow - greenhouse horticulture
    Gewasbeschermingsmiddelen vormen een probleem in het oppervlaktewater om een goede ecologische kwaliteit te bereiken (Teunissen, 2005; Kruger, 2008; Van der Staaij, 2009). In 2015 moet het oppervlaktewater een goede kwaliteit hebben en lozen van middelen is dan niet meer toegestaan. Het is daarom belangrijk te weten hoe middelen in het oppervlaktewater terecht komen. In deze notitie wordt verslag gedaan van een inventariserend onderzoek naar de route die het spuitmiddel in de kas aflegt. Komt er via de standaard spuitmethode spuitmiddel in het recirculatiewater en is dit te voorkomen of te verminderen? Telen met toekomst heeft de taak vragen en knelpunten bij geïntegreerde teelt en emissie van gewasbeschermingsmiddelen naar het onderzoek terug te koppelen. Dit onderzoek is dan ook op verzoek van Telen met toekomst uitgevoerd in het project ‘Knelpunten bij de implementatie van Best Practices’, onderdeel van het LNV programma Plantgezondheid
    Process description of SWQN : A simplified hydraulic model
    Smit, A.A.M.F.R. ; Siderius, C. ; Gerven, L.P.A. van - \ 2009
    Wageningen : Alterra (Alterra-rapport 1226.1) - 54
    hydraulica - modellen - peilbeheer - stuwen - duikers - pompen - waterlopen - waterstroming - hydraulics - models - water level management - weirs - culverts - pumps - streams - water flow
    SWQN is a simplified hydraulic model for surface water systems which computes water levels and flows in a network of nodes labelled as ‘volumes’ and segments labelled as ‘connectors’. The user can specify a variety of connectors like open water courses or structures such as weirs, gates, culverts or pumps. Water levels are calculated in the ‘volumes’ driving the one dimensional flows through the ‘connectors’ linking up the ‘volumes’. The assumption is that the flow between two nodes with an open connection in between is linearly dependent on the difference in water level, if necessary augmented with the difference in velocity head, the wetted profile, and a given resistance.
    Functional magnetic resonance microscopy of long- and short-distance water transport in trees
    Homan, N. - \ 2009
    Wageningen University. Promotor(en): Herbert van Amerongen, co-promotor(en): Henk van As. - [S.l.] : S.n. - ISBN 9789085855071 - 125
    kernmagnetische resonantiespectroscopie - waterstroming - xyleemwaterpotentiaal - bomen - sapstroom - spinthout - nuclear magnetic resonance spectroscopy - water flow - xylem water potential - trees - sap flow - sapwood
    Due to their long life span, changing climatic conditions are of particular importance for trees. Climate changes will affect the water balance, which can become an important limiting factor for photosynthesis and growth. Long-distance water transport in trees is directly related to the transpiration stream and very sensitive to changes in the soil-plant-atmosphere water continuum. Therefore the study of long distance transport gives information about tree response to changing climatic conditions. Here the dynamic behaviour of water transport processes in trees has been studied by the MRI method, which is a direct and non-invasive tool.. MRI flow imaging has been applied to diffuse- and ring-porous trees to study drought stress and the occurrence of xylem vessel cavitations.
    World-wide unique dedicated MRI hardware is described that allows imaging of sap flow in intact trees with a maximal trunk diameter of 4 cm and height of several meters. This setup is used to investigate xylem and phloem flow in an intact tree quantitatively. Flow is quantified in terms of (averaged) velocity, volume flow (flux) and flow conducting area, either in imaging mode or resolved on the level of annual rings.
    Results obtained for the same tree, imaged at two different field strengths (0.7 and 3 T), are compared. An overall shortening of observed T2 values is manifest going from 0.7 to 3 T. Although susceptibility artefacts may be present at 3 T, the results are still reliable and the gain in sensitivity due to the higher magnetic field strength results in shorter measurement time (or a better spatial resolution or a higher signal to noise ratio) with respect to the 0.7 T system. By use of such dedicated hardware xylem and phloem flow, and its mutual interaction, can be studied in intact trees in relation to the water balance and in response to environmental (stress) conditions (Chapter 2).
    To further investigate the effect of susceptibility artefacts on MRI flow imaging by PFG-STE MRI on 3 T, water flow was studied in a number of model porous media with or without surface relaxation, internal magnetic field inhomogeneities (susceptibility artefacts) and exchange with stagnant water pools, mimicking the tree situation (Chapter 3). In such situations a clear dependence of the flow characteristics on the observation time is demonstrated. The most reliable results are obtained at relatively short observation times. This limits the observation of low flow velocities and the discrimination between flowing and non-flowing water. It is shown that correlated displacement-T2 measurements are available to improve the discrimination of flowing and non-flowing water and can be of help to decide about the functional activity of xylem conduits (Chapter 4). A method that reveals exchange between the flowing and stagnant fractions in the system is presented. Further it is demonstrated how this exchange can be quantified (Chapter 3).
    Xylem flow, flow conducting area and water content in the storage pools of sapwood and cambial zone were investigated simultaneously and non-destructively by MRI in diffuse-porous laurel (Laurus nobilis) and viburnum (Viburnum tinus) trees during a drydown period and recovery after watering (Chapter 4). The development of the drought stress was detected by the decrease in average velocity, volume flow and flow conducting area as observed by MRI flow imaging. A decrease in flow conducting area was observed with a delay of one day in comparison to the observed reduction in average velocity and volume flow. The re-watering of the plants resulted in the fast restore of the flow conducting area to the value observed under well watered conditions, demonstrating that if cavitations had been induced they refilled quite fast. In addition, a significant increase in the average velocity and volume flow was observed, but still lower than the original values. Imaging water content in the cambial zone indicated a gradual decrease of the water content, which speeded up during the drought stress. The rate of decrease was dependent of day/night conditions. Watering resulted in the partial restore of water content in this zone. Water content in sapwood showed a clear diurnal variation. The water storage pool in sapwood depleted quickly upon switching on the light, gradually restoring in the afternoon. Drought stress did not change the character of diurnal variation of water content significantly, but it increased the amplitude of the diurnal variation. Re-watering of the tree resulted in a 10% water loss in sapwood. Thus, for the first time the coupling between water floe in xylem vessels and water content in storage pools was demonstrated. The oldest annual ring was rather inactive in long distance water transport. We found that the transport activity of this ring was not sensitive to any environmental change and that the variation of water content in sapwood was uniform in all annual rings
    Non-destructive measurements of cavitation were made with MRI to test whether large earlywood vessels of ring-porous xylem are as vulnerable as some standard methods have suggested (Chapter 5). Potted, 3-4 year old Quercus robur L. trees were droughted to water potentials measured with temperature-corrected stem psychrometers. Imaging of (vessel) water content indicated that earlywood cavitation in trunks was not detectible until water potentials dropped below -3 MPa. Most earlywood vessels were cavitated below -4 MPa. Dye perfusions through excised branch segments gave comparable results. Imaging of flow conducting area (FCA) indicated a gradual decline in trunk water conduction that was not solely associated with cavitation, but probably resulted from stomatal closure and too low velocities to be discriminated from non-flowing water. Dye perfusion and FCA indicated a significant portion of earlywood vessels were non-conducting even at the most favorable water potentials. No refilling of embolized vessels was detected in rewatering experiments. Contradictory to the MRI results, standard centrifuge and air-injection methods on Q. robur stem segments indicated complete cavitation at xylem pressures at or below -1 MPa. An artifact in these destructive methods was revealed by experiments on the related species Q. gambelii Nutt. When earlywood vessels became air-filled during collection prior to being refilled in the lab, they became much more vulnerable to cavitation. Residual bubbles left behind in the refilled vessels may be responsible. These results suggest revised protocols for measuring vulnerability curves by destructive methods.
    An about linear correlation between water potential and decrease of water content in cambial zone of oak (Quercus robur L.) was observed (Chapter 5).


    Contaminant release from sediments: a mass flux approach
    Smit, M.P.J. - \ 2009
    Wageningen University. Promotor(en): Wim Rulkens; Tim Grotenhuis. - [S.l. : S.n. - ISBN 9789085854586 - 120
    sediment - overstromingen - dieldrin - verontreinigende stoffen - stroming - waterstroming - deeltjes - verdunning - waterverontreiniging - organische verontreinigende stoffen - verontreinigde sedimenten - sediment - floods - dieldrin - pollutants - flow - water flow - particles - dilution - water pollution - organic pollutants - contaminated sediments
    With the predicted climate change it is expected that the chances of flooding increase. During flood events sediments will suspend and if sediments are polluted, contaminants can be released to water. Also under gentle flow regimes, when sediments are settled and form a sediment bed, transfer of contaminants to water is possible. The release of contaminants from the sediment –particles and bed– to the aqueous phase is the first step in a sequence of processes. When this step is the rate limiting process we speak of mass transfer limitation. In this thesis the release of (mainly) dieldrin from field aged sediment was studied using the SPEED reactor we developed for this purpose. In this reactor parameters like mixing intensity and water flow –or dilution– rate can be controlled and low aqueous concentrations of contaminants can be quantified. Dieldrin release from the sediments was assessed for different situations: simulating a flood event and simulating more gentle flow conditions. In line with the variable discharge rates in rivers, we performed experiments with different flow rates and assessed the release of dieldrin in time.
    Development of model for macropore flow of plant protection products in soil
    Boesten, J.J.T.I. ; Tiktak, A. ; Hendriks, R.F.A. - \ 2009
    drainage door leidingen - stroming door macroporiën - grondwater - bodem - simulatiemodellen - waterstroming - pesticiden - risicoschatting - vollegrondsteelt - pipe drainage - macropore flow - groundwater - soil - simulation models - water flow - pesticides - risk assessment - outdoor cropping
    Poster presenting a model for preferential flow of water that was developed (i.e. part of the water flow model SWAP). Main assumption is that the macropores in the soil can be divided into a bypass domain and an internal catchment domain
    Redistribution of velocity and bed-shear stress in straight and curved open channels by means of a bubble screen: laboratory experiments
    Blanckaert, K. ; Buschman, F.A. ; Schielen, R. ; Wijbenga, J.H.A. - \ 2008
    Journal of Hydraulic Engineering 134 (2008)2. - ISSN 0733-9429 - p. 184 - 195.
    waterbouwkunde - kanalen, klein - hydrodynamica - waterstroming - buigen - herverdeling - snelheid - turbulentie - laboratoriumproeven - hydraulic engineering - channels - hydrodynamics - water flow - bending - redistribution - velocity - turbulence - laboratory tests - submerged vanes - secondary flows - alternate bars - topography - bends - rivers
    Open-channel beds show variations in the transverse direction due to the interaction between downstream flow, cross-stream flow, and bed topography, which may reduce the navigable width or endanger the foundations of structures. The reported preliminary laboratory study shows that a bubble screen can generate cross-stream circulation that redistributes velocities and hence, would modify the topography. In straight flow, the bubble-generated cross-stream circulation cell covers a spanwise extent of about four times the water depth and has maximum transverse velocities of about 0.2 ms¿1. In sharply curved flow, it is slightly weaker and narrower with a spanwise extent of about three times the flow depth. It shifts the counter-rotating curvature-induced cross-stream circulation cell in the inwards direction. Maximum bubble-generated cross-stream circulation velocities are of a similar order of magnitude to typical curvature-induced cross-stream circulation velocities in natural open-channel bends. The bubble screen technique is adjustable, reversible, and ecologically favorable. Detailed data on the 3D flow field in open-channel bends is provided, which can be useful for validation of numerical models
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