Browsing by Author "Box, Walter"

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Development of experimental methods for investigating sediment and nutrient transport in vegetated flows
(2019-10-21) Andelin, Markku
Insinööritieteiden korkeakoulu | Master's thesis
Empirical measurements are needed for improving understanding and modelling of various physical, chemical and biological phenomena present in vegetated flows. Vegetation has a substantial effect for example on flow resistance, velocities and patterns, which in turn affects sediment transport and nutrient deposition. The goal of this work was to develop experimental flume, field and laboratory methods for investigating these processes. The objectives were to develop 1) a downscaled sampler for measuring bed load sediment transport in flume conditions, 2) a drag force sensor for determining vegetative flow resistance in laboratory flows, and 3) a methodology for estimating sedimentary nutrient deposition in vegetated channels. There are knowledge gaps in understanding nutrient deposition in environmentally preferable channel designs, and in using miniature Helley-Smith (HS) bed load samplers. The performance of the bed load sampler and drag force sensor was investigated in different flow conditions. The experiments in vegetated conditions indicated that the downscaled bed load sampler is suitable for determination of bed load fluxes in flume conditions, giving results comparable to an optical backscatter instrument. The measurement variability was found to be relatively low. The sampler was capable of capturing the large differences in bed load transport between unvegetated and vegetated parts of the flume. Experiments with the drag force sensor provided results comparable to other studies, and the variability in results was minor. The soil and sediment cores were collected from an agricultural drainage channel, adjacent to which a vegetated floodplain had been constructed in 2010. The collected samples were subjected to elemental analysis, and the effect of several pre-processing steps was studied. According to the results, in most cases the samples can be analyzed after mechanical grinding without sieving, without significant effect on the measured nutrient and element concentrations. Analysis of soil properties indicated that the bulk density and the ratio of magnesium and aluminum have potential for estimating the amount of post-construction sediment and nutrient deposition in engineered channels. Further experiments are recommended for calibration of the miniature bed load sampler in different flow velocities, if reliable measurements of bed load transport are needed. The extendibility of the proposed soil and sediment properties for determining the rate of net retention is recommended to be investigated in other channels. The developed methodologies have useful applications and provide straightforward and time efficient measurements for example to understand the spatial variations of different processes in vegetated flows.
Floodplain flow resistance in case of a sparse mixture of plants at low relative submergences
(2020-08-27) Box, Walter; Järvelä, Juha; Västilä, Kaisa
A4 Artikkeli konferenssijulkaisussa
Vegetation affects flow hydrodynamics and transport processes across various spatial scales and is increasingly put forward as a powerful tool for nature-based solutions (NBS) in river management. Complex mixtures of different plant types at a wide range of densities are common on real floodplains, but such conditions are rarely properly reproduced in experimental investigations. This paper reports on the first results of an ongoing study, with a view on flow resistance associated with low relative submergences (H/hv = 1-2) and low vegetation densities (LAI = 0.12-0.58). Such experimental investigations are challenging due to the necessity of a high-accuracy sensor system suited for the bulk scale. A mixture of artificial elements was selected to mimic the behaviour of natural floodplain shrubs and herbaceous vegetation in a laboratory flume. Results suggested that also at very low LAI values the vegetative resistance increased linearly with LAI. An increase of H/hv from 1 to 2 caused a 25% to 50% reduction in the friction factor for very low to intermediate LAI, respectively.
Flow resistance of floodplain vegetation mixtures for modelling river flows
(2021-10) Box, Walter; Järvelä, Juha; Västilä, Kaisa
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
River flows are greatly influenced by floodplain vegetation with implications on hydrological and hydraulic conditions from cross-sectional to river reach scales. Flow models need to reliably reflect changes in the riverine environment, such as vegetation growth associated with altered flow regimes, increased sediment loads and eutrophication. Leaf area index (LAI) based approaches are increasingly used as tools to predict the flow resistance caused by natural vegetation. However, current LAI-based modelling involves uncertainty at low and high vegetation densities and flow velocities due to a lack of research and validation at the outer ranges. The aim of this paper is to investigate the flow resistance for a mixture of flexible floodplain vegetation consisting of woody plants and understory grasses of low to high densities (LAI = 1–5) over a wide range of mean flow velocities (0.05–1.2 m/s) at low relative submergences of 1–2. A novel flume setup was designed by using high-accuracy pressure sensors to measure flow resistance and force sensors to measure plant drag forces. Flow resistance decreased by 35–90% when the submergence H/hv increased from 1 to 2, which is a highly relevant range for floodplain flows. The results provided new evidence that LAI-based modelling of vegetative friction factors can be reliably extended from low to high LAI values for non-submerged vegetation. However, adjustments to existing LAI-based approaches are required for water stages higher than the vegetation height. Furthermore, the friction by the understory grasses cannot be neglected as often assumed in literature especially for cases of low plant densities and low relative submergences.
Flume investigations using natural-like vegetation with a view on fine sediment processes
(2019-09-06) Box, Walter; Västilä, Kaisa; Järvelä, Juha
A4 Artikkeli konferenssijulkaisussa
Quantifying vegetative effects on the flow, suspended sediment concentrations, and sediment transport is complicated by uncertainties associated with the correct conceptualization of flow-vegetation-sediment interactions. The aim of this paper is to investigate the vegetative effects on the flow, the spatially varying sediment transport mechanisms, and sediment fluxes for both the unvegetated and vegetated areas of the channel. Experiments were conducted in a laboratory flume at medium to high bulk flow velocities. Care was taken to reproduce vegetated conditions typical of floodplain flows, where the unvegetated main channel and vegetated floodplain are clearly separated, inducing a strong shear flow. Vegetation was represented by a combination of artificial understory grasses and flexible woody plants. Instantaneous flow velocities were measured by acoustic Doppler velocimetry and suspended sediment concentrations by optical turbidity sensors. The suspended sediment concentration increased for positions closer to the bed in the unvegetated part of the channel. However, in and adjacent to the vegetative areas, the vertical profiles of concentration showed more complex distributions. Based on the paired measurements of the flow and concentration the streamwise sediment fluxes were estimated. In the investigated partly vegetated channel the unit sediment discharge was two to four times higher in the unvegetated part compared to the vegetated part of the channel. Data and findings in the present study provide insight on the vertical and lateral variability of suspended sediment fluxes and are useful for predicting sediment transport in partly vegetated channels.
The impact of natural floodplain vegetation on flow resistance and fine sediment transport
(2022) Box, Walter
School of Engineering | Doctoral dissertation (article-based)
Suitable implementation of sustainable river management strategies and Nature-Based Solutions requires detailed understanding of the impacts of riverine vegetation on flow characteristics, sediment transport, water quality, river health and ecology. Modelling of the flow resistance of floodplain vegetation has received substantial attention, as many research and engineering purposes require reliable estimates. Existing drag force and flow resistance models are based on highly simplified representation of vegetation. In particular, there remains a lack of adequate description of the effects of reconfiguration and relative submergence. Subsequently, many models are used outside their developed domain, resulting in poor predictions. This thesis extends current knowledge on flow resistance of flexible flood-plain vegetation mixtures as well as on sediment transport processes under vegetated settings. The specific objectives were to 1) quantify vegetative drag and measure flow resistance of vegetation mixtures of woody plants and a herbaceous understory to evaluate existing resistance equations; 2) develop leaf area index (LAI)-based flow resistance models suitable for both emergent and submerged conditions, and 3) advance knowledge on transport and net deposition of fine sediments under the effect of seasonal variability in plant properties. The LAI-based models developed in this thesis provide for accurate predictions of vegetative flow resistance of mixtures of flexible floodplain vegetation also at low relative submergences. Most accurate predictions are obtained when a reliable estimate of the deflected vegetation height hd is available and the newly introduced von Kármán scaling factor α is used. A workflow for implementation of the proposed LAI-based approaches was developed for modelers and practitioners. The newly introduced LAI-based drag ratio approach can be used for estimating the influence of seasonal variation in foliage on sediment transport. The presence of foliage increased the net deposition in the vegetated areas and enhanced deposition near the main channel-floodplain interface by¬ two to five times. The new insights allow for improvements in future predictions of suspended sediment transport and deposition in partly vegetated, main channel-floodplain or bank/riparian vegetation settings. The potential applications range from flood inundation studies to river restoration, floodplain reforestation, channel design and management of riverine environments.
The interplay between flow field, suspended sediment concentration, and net deposition in a channel with flexible bank vegetation
(2019-11-01) Box, Walter; Västilä, Kaisa; Järvelä, Juha
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
This paper investigates the interplay between the flow, suspended sediment concentration (SSC), and net deposition at the lateral interface between a main channel and riverbank/floodplain vegetation consisting of emergent flexible woody plants with understory grasses. In a new set of flume experiments, data were collected concurrently on the flow field, SSC, and net deposition using acoustic Doppler velocimeters, optical turbidity sensors, and weight-based sampling. Vegetation largely affected the vertical SSC distributions, both within and near the vegetated areas. The seasonal variation of vegetation properties was important, as the foliage strongly increased lateral mixing of suspended sediments between the unvegetated and vegetated parts of the channel. Foliage increased the reach-scale net deposition and enhanced deposition in the understory grasses at the main channel–vegetation interface. To estimate the seasonal differences caused by foliation, we introduced a new drag ratio approach for describing the SSC difference between the vegetated and unvegetated channel parts. Findings in this study suggest that future research and engineering applications will benefit from a more realistic description of natural plant features, including the reconfiguration of plants and drag by the foliage, to complement and replace existing rigid cylinder approaches.
New formulas addressing flow resistance of floodplain vegetation from emergent to submerged conditions
(2024) Box, Walter; Järvelä, Juha; Västilä, Kaisa
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Hydraulic modelling of natural floodplain vegetation using leaf area index (LAI) has been applied successfully for non-submerged conditions whereas its suitability for submerged conditions requires further development. This study investigates the vegetative flow resistance at low relative submergences and extends existing LAI-based approaches building upon new flume data and prior experiences from field-scale applications. We provide advanced LAI-based formulas for modelling the flow resistance from emergent to submerged conditions, with water depth up to three times higher than the vegetation height. Such low relative submergences are highly relevant in hydraulic analyses of riverbank and floodplain flows but not adequately represented in existing formulas. The use of the deflected vegetation height as the characteristic height provided the most accurate modelling results, whereas the use of undeflected height resulted in significant errors. As a new development for submerged conditions, we proposed von Kármán scaling factor for improved model predictions. Overall, the results proved that LAI-based modelling is suitable also at low relative submergences for a wide range of vegetation densities (LAI = 1–5) and mean flow velocities (0.05–1.2 m s−1). For both emergent and slightly overtopped vegetation the JAR and VAS approaches outperformed the BAPmod-LAI approach that does not account for reconfiguration. For modellers, we provide a workflow and guidance on the use of the newly developed LAI-based formulas in 1D/2D hydrodynamic models for both emergent and submerged conditions.
Transport and deposition of fine sediment in a channel partly covered by flexible vegetation
(2018-09-05) Box, Walter; Västilä, Kaisa; Järvelä, Juha
A4 Artikkeli konferenssijulkaisussa
Riparian plants exert flow resistance and largely influence the flow structure, which affects erosion, deposition and transport processes of fine sediments. Predicting these vegetative effects is important for flood, sediment and nutrient management. However, predictions on the fate of sediments are complicated by uncertainties associated with the suitable parameterization of natural plants and the associated effects on the turbulent flow field and on the variables in the transport equations. The aim of this study is to quantify deposition and transport of fine sandy sediment in a partly vegetated channel under laboratory conditions. Care was taken to reproduce conditions typical of vegetated floodplain flows including dense flexible grassy understory as a starting point. The experiments were conducted in a flume that is specifically designed to recirculate fine sediment. We measured suspended sediment concentrations with optical turbidity sensors and determined patterns of net deposition over the vegetated parts of the cross section. The flow field was determined with acoustic Doppler velocimetry. Our investigations are intended to improve future predictions of fine sediment storage and transport in natural or constructed vegetated channels, and the first results reported herein were useful in designing further, on-going experiments with complex combinations of vegetation and channel geometry. Key words: sediment transport, suspended sediment, deposition, riparian vegetation, flow field.