Juha Järvelä

Flow resistance of flexible and stiff vegetation: a flume study with natural plants

Flow resistance of natural grasses, sedges and willows was studied in a laboratory flume. The objective was to investigate, how type, density and placement of vegetation, flow depth and velocity influence friction losses. The plants were studied in various combinations under nonsubmerged and submerged conditions in a total of 350 test runs. The results show large variations in the friction factor, f, with depth of flow, velocity, Reynolds number, and vegetative density. The friction factor was dependent mostly on (1) the relative roughness in the case of grasses; (2) the flow velocity in the case of willows and sedges/grasses combined; and (3) the flow depth in the case of leafless willows on bare bottom soil. Leaves on willows seemed to double or even triple the friction factor compared to the leafless case despite the fact that the bottom was growing sedges in both cases. For the leafless willows, f appeared to increase with depth almost linearly and independently of velocity. Unexpectedly, different spacing of the same number of leafless willows with grasses did not have any significant effect on f. Based on the experimental work, a better understanding of flow resistance due to different combinations of natural stiff and flexible vegetation under nonsubmerged and submerged conditions was gained. q 2002 Elsevier Science B.V. All rights reserved.

Determination of flow resistance caused by non‐submerged woody vegetation

Abstract This paper investigates the determination of flow resistance caused by stiff and flexible woody vegetation. A new procedure has been developed which allows the determination of friction factor f or Mannings n using measurable characteristics of vegetation and flow. The procedure is capable of predicting flow resistance due to: (1) leafless bushes or trees and (2) leafy bushes or trees. The application of the procedure is limited to non‐submerged flow (h ≤ H) and relatively low velocity (U < 1 m/s), which are typical conditions in low‐gradient stream valleys, floodplains and wetlands. The procedure is novel in that it uses sound hydraulic principles and methods that are available but incorporates some adjustments based on the knowledge on mechanical design of trees and deformation of foliage in a flow. The procedure is able to account for the natural branched structure in determining area or volume of a woody plant. This makes the prediction of resistance caused by plants more accurate than if they were treated as arbitrary cylinders. The accuracy of the approach to estimate f and U was somewhat better for the leafless condition (mean error of f was -5% to+4%) compared to the leafy condition (mean error of f was -9% to -3%). The presented procedure is intended as a practical tool for estimating the relationship between plant characteristics and flow resistance for flows over floodplains and wetlands growing woody vegetation.

Flow resistance of emergent rigid and flexible floodplain vegetation

This paper summarizes current practices for the estimation of flow resistance caused by floodplain vegetation in emergent flow conditions. The current state-of-the-art for the parameterization of vegetative form drag and associated flow resistance was explored with a view on practical applicability. Specifically, the dissimilar resistance behaviour of simply shaped rigid elements and foliated natural vegetation was emphasized by compiling and reanalysing data published by the authors’ research teams as well as others. It was shown that describing the key hydraulic properties of plants, geometry, and flexibility, with species-specific parameters is superior to the rigid cylinder analogy commonly used in hydraulic engineering practice. The discussion on the limitations of many existing approaches for the determination of vegetative flow resistance is intended to advance the use of modern practices such as the parameterization of vegetation density with the leaf area index, a parameter that can be derived using remote sensing techniques.

Data Processing and Quality Evaluation of a Boat-Based Mobile Laser Scanning System

Mobile mapping systems (MMSs) are used for mapping topographic and urban features which are difficult and time consuming to measure with other instruments. The benefits of MMSs include efficient data collection and versatile usability. This paper investigates the data processing steps and quality of a boat-based mobile mapping system (BoMMS) data for generating terrain and vegetation points in a river environment. Our aim in data processing was to filter noise points, detect shorelines as well as points below water surface and conduct ground point classification. Previous studies of BoMMS have investigated elevation accuracies and usability in detection of fluvial erosion and deposition areas. The new findings concerning BoMMS data are that the improved data processing approach allows for identification of multipath reflections and shoreline delineation. We demonstrate the possibility to measure bathymetry data in shallow (0–1 m) and clear water. Furthermore, we evaluate for the first time the accuracy of the BoMMS ground points classification compared to manually classified data. We also demonstrate the spatial variations of the ground point density and assess elevation and vertical accuracies of the BoMMS data.

Estimation of drag forces caused by natural woody vegetation of different scales

To reliably estimate water levels and velocities in vegetated rivers and floodplains, flow resistance models based on phys-ical plant properties are advantageous. The purpose of this study is (1) to assess the suitable parameterization of woody riparian veg-etation in estimating the drag forces, (2) to address the effect of plant scale on the drag estimates and reconfiguration, and (3) to evaluate the applicability of three recently developed flow resistance models. Experiments on four tree species in a towing tank to-gether with detailed characterization of tree properties were carried out to establish a novel dataset. Despite the variability in the tree height (0.9 m-3.4 m), the stem, leaf and total areas proved to be suitable characteristic dimensions for estimating the flow resistance at different scales. Evaluations with independent data revealed that the tested models produced reasonable results. The performance of the models was controlled by the parameter values used rather than the model structure or the plant scale.

Environmentally preferable two-stage drainage channels: considerations for cohesive sediments and conveyance

Design of environmentally preferable agricultural drainage channels calls for an improved understanding of cohesive sediment processes. Flow and cohesive sediments were investigated in a new demonstration and test channel where a floodplain was excavated on one side of the existing channel to improve flood conveyance. In this approach, the existing, naturally recovered channel was mostly left intact to reduce environmental impacts. Continuous monitoring of discharge and suspended sediment concentration (SSC) during 1 year revealed that the construction work of the two-stage channel caused 2% of the annual suspended sediment (SS) load. Agricultural areas covering 13% of the catchment were estimated to contribute over half of the annual SS, predominantly during the rising stages. Seasonal positive hysteresis was found in SSC which was explained by different drainage efficiencies of two distinct sediment sources. The temporally varying shares of the two sources caused scatter in the rating curves between discharge and SSC or SS load. Out-of-channel processes were shown to govern the amount and timing of the SS input into the channel, indicating that environmentally preferable agricultural channel design should consider the cohesive sediment processes and sources at the catchment scale.

Hydrodynamics of Vegetated Channels

Hydrodynamics of vegetated channels and streams is a rapidly developing research area, and this chapter summarizes the current knowledge considering both aquatic and riparian zones. The benefit of an advanced parameterization of plant morphology and biomechanical properties is highlighted. For this purpose, the response of flexible and foliated plants and plant communities to the flow is illustrated, and advanced models for the determination of drag forces of flexible plants are described. Hydrodynamic processes governing flow patterns in vegetated flows are presented for submerged and emergent conditions considering spatial scales ranging from the leaf to the vegetated reach scale.

Flow-Vegetation-Sediment Interaction in a Cohesive Compound Channel

AbstractThe purpose of this study was to quantify how vegetation influences the flow and sediment processes relevant to the design and management of environmental compound channels. Therefore, a two-year field investigation was conducted in a cohesive two-stage channel, focusing on the flow resistance and net deposition in five subreaches with different floodplain vegetation conditions. In the grassy subreaches, the cross-sectional blockage factor was the key vegetation property governing the flow resistance, with a process-based model providing reliable estimates under widely variable hydraulic and vegetative conditions. The net deposition of cohesive sediment was best explained by vegetation height while high stand length and density created supply-limited conditions on the inner floodplain. These results showed that the two-stage approach offers potential for controlling the sediment processes through appropriate vegetation maintenance. The novelty of this research is that straightforward analyses acco...

Vegetative flow resistance: characterization of woody plants for modeling applications

Flow-vegetation interaction is a complex process which causes difficulties in hydraulic modeling. The purpose of this paper is to investigate the characterization of natural woody plants, and further, the determination of flow resistance coefficients for modeling applications. Flume studies with both living and artificial plants were performed to investigate the effect of flow velocity, relative submergence, and vegetation density on flow resistance. Large differences between natural and artificial plants were found. Subsequently, a computational approach for determining flow resistance coefficients for complex woody vegetation was discussed. In the approach, woody vegetation was characterized by leaf area index (LAI), a vegetation parameter =, and a species-specific drag coefficient Cd�. LAI was used to take the vegetation density into account, and the species-specific vegetation parameter = considered the effects of plant deformation in a flow.

Characterizing natural riparian vegetation for modeling of flow and suspended sediment transport

PurposeRiparian vegetation imposes a critical control on the transport and deposition of suspended sediment with important implications for water quality and channel maintenance. This paper contributes (1) to hydraulic and morphological modeling by examining the parameterization of natural riparian vegetation (trees, bushes, and grasses) and (2) to the design and management of environmental channels by determining how the properties of natural floodplain plant stands affect the erosion and deposition of suspended sediment.Materials and methodsLaboratory and field data were employed for enhancing the physical description of flow–plant–sediment interactions with a consideration of practical applicability. A drag force parameterization that takes into account the flexibility-induced reconfiguration, and the complex structure of foliated plants was validated for small natural trees under laboratory conditions, while the data from a small vegetated compound channel demonstrated the approaches at the field scale. Based on the field data, we identified three key vegetative factors influencing the net deposition and erosion on the floodplain. The significance of these factors was evaluated for vegetative conditions ranging from almost bare soil to sparse willows and dense grasses. Overall, the investigated conditions covered flexible and rigid vegetation with seasonal differences represented by foliated and leafless states.Results and discussionThe drag and reconfiguration of woody plants were reliably predicted under leafless and foliated conditions. Subsequently, we present a new easy-to-use methodology for predicting vegetative drag and flow resistance. The methodology is based on a physically solid parameterization for five widely used coefficients or terms (Eqs. (2)–(6)), with the necessary parameter values presented for common riparian species. The methodology was coupled with existing approaches at the field scale, revealing that increasing vegetation density and the associated decreasing flow velocity within vegetation significantly increased net deposition. Further, deposition increased with increasing cross-sectional vegetative blockage and decreasing distance from the suspended sediment replenishment point. Thus, longitudinal advection was the most important mechanism supplying fine sediment to the floodplain, but long continuous plant stands limited deposition.ConclusionsThe proposed parameterization (Eqs. (2)–(6)) can be readily implemented into existing hydraulic and morphological models to improve the description of natural vegetation compared to the conventional rigid cylinder representation. The approach is advantageous for evaluating, for example, the effects of both natural succession and management interventions on floodplains. Finally, guidance is provided on how floodplain vegetation can be maintained to manage the erosion and deposition of suspended sediment in environmental channel designs.