Terhi Helmiö

Unsteady 1D flow model of compound channel with vegetated floodplains

Abstract In areas that are occasionally inundated by floods, it is essential to know the quantity and density of vegetation that can be allowed while avoiding flooding beyond the intended floodplains. An unsteady flow model was developed for a channel with vegetated floodplains, to take into account the retention effects of the vegetated areas on flood wave conveyance. A one-dimensional (1D) unsteady flow model was combined with Nudings method, which takes into account the friction caused by vegetation and the additional resistance caused by the interaction between the main channel and vegetated areas. Nudings method was used to compute the total Darcy–Weisbach friction factor for each cross section and each water level as input data for each time step of the flow model. The model was applied to a double-trapezoidal channel, and the results were compared with a traditional model, in which floodplains were considered only as storage. Differences in the conveyance capacity of the channel were found between the models.

Unsteady 1D flow model of a river with partly vegetated floodplains—application to the Rhine River

Abstract A relatively simple unsteady flow model was proposed which estimates velocities, friction factors and the components of discharge in the main channel and on the floodplains simultaneously. Bottom roughness of the main channel and the floodplains, the flow resistance of vegetation on the floodplains and the flow resistance caused by the momentum transfer between the main channel and the floodplains were included in the model using Nudings method. A pre-processing program was developed to convert topographic field data that can be derived from a digital terrain model into cross-sectional input data of the flow model. The model was applied to a 28-km reach on the Upper Rhine for three steady flow cases and for two unsteady flood event cases to investigate resistance effects of partly vegetated floodplains. Computed discharges and water levels correlated well with the measured data. It was found that a significant component of discharge was transported by the floodplains in some cross-sections during high flows.

Hydraulic Considerations in Restoring Boreal Streams

The physical habitat that controls ecosystem functioning is determined by local hydraulics and channel morphology. Hydraulic field studies were conducted in a boreal stream (1) to test the hypothesis that the local hydraulic conditions are determined by cross-sectional geometry and flow resistance in boreal conditions by analysing the relationship between flow velocities, cross-sectional geometry and flow resistance, and (2) to suggest success criteria for the restoration of local hydraulic conditions. Results suggest that, in the case of small channels, cross-sectional geometry and flow resistance are weakly interconnected and influenced by factors such as local roughness elements and channel forms. The study showed that both flow resistance and cross-sectional geometry are vital factors in determining local hydraulics. In stream restoration, a design based on consideration of only one of these two factors is inadequate and may result in a failure to replicate natural hydraulic conditions. Simple success criteria for the restoration of local hydraulics are developed.