Effect of flow resistance in open rectangular channel

The determination of flow resistance for open channels remains a challenge in practices. In this research, an experimental study was carried out to investigate the hydraulic roughness characteristics in an open channel. The experimental flume (10 m length, 0.30 m width and 0.46 height) was carried out with two conditions ; contain gravel bed and without gravel bed (as controller). Blocks of concrete foam were fabricated and laid on the bed surface for providing uniformly roughened along the open channel. A velocity flow meter was used to quantify the average velocity, and other parameters such as cross-section, hydraulic radius, wetted perimeter, and channel slopes were also calculated. Finally, the flow resistance, n obtained using the flume with gravel bed surface is higher than the flume without gravel bed surface. The flow conditions were declared as subcritical as the Froude number is less than 1 for both conditions in the flume. As a conclusion, it was identified that the hydraulic roughness n, was influenced by the type bed roughness, flow rate and channel slope

Unscreened water-diversion pipes pose an entrainment risk to the threatened green sturgeon, Acipenser medirostris.

Over 3,300 unscreened agricultural water diversion pipes line the levees and riverbanks of the Sacramento River (California) watershed, where the threatened Southern Distinct Population Segment of green sturgeon, Acipenser medirostris, spawn. The number of sturgeon drawn into (entrained) and killed by these pipes is greatly unknown. We examined avoidance behaviors and entrainment susceptibility of juvenile green sturgeon (35±0.6 cm mean fork length) to entrainment in a large (>500-kl) outdoor flume with a 0.46-m-diameter water-diversion pipe. Fish entrainment was generally high (range: 26-61%), likely due to a lack of avoidance behavior prior to entering inescapable inflow conditions. We estimated that up to 52% of green sturgeon could be entrained after passing within 1.5 m of an active water-diversion pipe three times. These data suggest that green sturgeon are vulnerable to unscreened water-diversion pipes, and that additional research is needed to determine the potential impacts of entrainment mortality on declining sturgeon populations. Data under various hydraulic conditions also suggest that entrainment-related mortality could be decreased by extracting water at lower diversion rates over longer periods of time, balancing agricultural needs with green sturgeon conservation

Design, commissioning and performance of a device to vary the turbulence in a recirculating flume

Ambient turbulent flow structures are one of the key drivers that will determine the rate of wake recovery downstream of tidal turbines. For second and third generation arrays or farms such a parameter is critical for the determination of inter-device spacing and the optimisation of energy extraction per unit surface area. At present offshore flow characterisation is dominated by seabed or surface-mounted diverging-beam acoustic Doppler profilers that whilst having a good spatial capture cannot characterise turbulent flow structures to the same accuracy as single point converging laboratory-scale velocimeters. So a paradox presently exists: We can measure the (mean) flow characteristics at real tidal energy sites but lack the ability to accuracy ascertain high-frequency flow characteristic at discrete spatial locations. This is possible at laboratory-scale with convergent-beam devices but as we do not know the real site conditions replication at small-scale can only be approximated. To date there has been few laboratory studies where the ambient flow turbulence has been varied. The standard method is to generate turbulence from a static structure such as a grid. Here we have developed an articulated rig that has the ability to oscillate cylindrical members along two axes in the flow upstream of tidal turbine models. Initial results presented in this paper show the effect upon the ambient flow that the turbulence-generating rig can impose and the effects upon wake dissipation for varying levels of turbulent length and time scales. Also the formation and insistence of turbulent structures shed from the device are reported. As expected increasing ambient turbulence intensity serves to dissipate the turbine wake more rapidly and whilst we cannot directly relate these laboratory flow characteristics to full-scale tidal energy sites at present it is hoped that offshore measurement technology and that of laboratory replication can converge so that device performance prediction can be performed at smaller-scale and at a corresponding lower cost to the technology

The emerging use of magnetic resonance imaging to study river bed dynamics

The characterization of surface and sub-surface sedimentology has long been of interest to gravel-bed river researchers. The determination of surface structure is important as it exerts control over bed roughness, near-bed hydraulics and particle entrainment for transport1. Similarly, interpretation of the sub-surface structure and flow is critical in the analysis of bed permeability, the fate of pollutants and maintaining healthy hyporheic ecology 2.For example, many invertebrates (e.g. mayfly, caddis) and fish (e.g. salmon) lay their eggs below the river bed surface, and rely on sub-surface flows to supply the necessary oxygen and nutrients. Whilst turbulent surface flows drive these small sub-surface flows, they can also convey sand and silts that clogs the surface and sub-surface pore spaces. Reduction in sub-surface flows can starve eggs of oxygen such that larvae or juveniles do not emerge. This is particularly critical in Scottish gravel-bed rivers as the rising supply and deposition of fine sediment (silts and sands) is contributing to the dramatic decline in wild salmon. In order to gain a better understanding of such flow-sediment-ecology interactions in river systems, laboratory experiments are conducted using long rectangular flow tanks called “flumes”, see figure 1a,1b. Here, traditional techniques for analysing sediment structure are typically constrained to 1D or 2D approaches, such as coring, photography etc. Even where more advanced techniques are available (e.g. laser displacement scanning), these tend to be restricted to imaging the surface of the sediment bed. Using Magnetic Resonance Imaging (MRI) overcomes these limitations, providing researchers with a non-invasive technique with which to provide novel 3D spatio-temporal data on the internal pore structure. In addition the important sub-surface flows can be investigated by adding MRI contrast agents to the flowing surface water

Anatomy of extraordinary rainfall and flash flood in a Dutch lowland catchment

On 26 August 2010 the eastern part of The Netherlands and the bordering part of Germany were struck by a series of rainfall events lasting for more than a day. Over an area of 740 km2 more than 120 mm of rainfall were observed in 24 h. This extreme event resulted in local flooding of city centres, highways and agricultural fields, and considerable financial loss. In this paper we report on the unprecedented flash flood triggered by this exceptionally heavy rainfall event in the 6.5 km2 Hupsel Brook catchment, which has been the experimental watershed employed by Wageningen University since the 1960s. This study aims to improve our understanding of the dynamics of such lowland flash floods. We present a detailed hydrometeorological analysis of this extreme event, focusing on its synoptic meteorological characteristics, its space-time rainfall dynamics as observed with rain gauges, weather radar and a microwave link, as well as the measured soil moisture, groundwater and discharge response of the catchment. At the Hupsel Brook catchment 160 mm of rainfall was observed in 24 h, corresponding to an estimated return period of well over 1000 years. As a result, discharge at the catchment outlet increased from 4.4 × 10-3 to nearly 5 m3 s-1. Within 7 h discharge rose from 5 × 10-2 to 4.5 m3 s-1. The catchment response can be divided into four phases: (1) soil moisture reservoir filling, (2) groundwater response, (3) surface depression filling and surface runoff and (4) backwater feedback. The first 35 mm of rainfall were stored in the soil without a significant increase in discharge. Relatively dry initial conditions (in comparison to those for past discharge extremes) prevented an even faster and more extreme hydrological response

Investigating periphyton biofilm response to changing phosphorus concentrations in UK rivers using within-river flumes

The excessive growth of benthic algal biofilms in UK rivers is a widespread problem, resulting in loss of plant communities and wider ecological damage. Elevated nutrient concentrations (particularly phosphorus) are often implicated, as P is usually considered the limiting nutrient in most rivers. Phosphorus loadings to rivers in the UK have rapidly decreased in the last decade,due to improvements in sewage treatment and changes to agricultural practises. However, in many cases, these improvements in water quality have not resulted in a reduction in nuisance algal growth. It is therefore vital that catchment managers know what phosphorus concentrations need to be achieved, in order to meet the UK’s obligations to attain good ecological status, under the EU’s Water Framework Directive. This study has developed a novel methodology, using within river mesocosms, which allows P concentrations of river water to be either increased or decreased, and the effect on biofilm accrual rate is quantified. These experiments identify the phosphorus concentrations at which algae becomes P-limited, which can be used to determine knowledge-based P targets for rivers. The ability to reduce P concentrations in river water enables algae–nutrient limitation to be studied in nutrient-enriched rivers for the first time