Juan Pedro Martín-Vide

Fluvial geomorphology and hydrology in the dispersal and fate of pyrite mud particles released by the Aznalcóllar mine tailings spill

The Agrio-Guadiamar River reach affected by the Aznalcollar mine tailings spill can be divided into three main sectors taking account of geomorphic characteristics and human-induced changes. Along the first 15 km, the valley presents the characteristics of a middle fluvial reach of moderate sinuosity and gradient; the floodplain declines gently towards the main channel and is built up mainly by lateral accretion deposits, which are mined by open pits. The second sector is located between 15 and 30 km downstream of the mine, and is characterised by higher channel sinuosity and a lower gradient; the floodplain is built on overbank deposits, separated from the main channel by natural levees, showing some low sinuosity flood channels. Finally, the lowest reach, with a similar low gradient, can be described as the proximal area of the Donana marshlands, whose geomorphology has been modified by agricultural practices for the last 40 years. The spread of mining tailings deposited a mat of pyrite mud (fine milled heavy metal-rich ore) over the main channel and floodplain with a thickness of up to 50 cm in the first sector and then, progressively decreasing downstream. The pyrite mud was deposited over the natural sediments without significant mobilisation of the latter and without relevant geomorphic changes. The subsequent cleaning up of the pyrite mud with the help of heavy machinery resulted in the removal of most of the bushy and herbaceous vegetation as well as the reworking of bed sediments and the destruction of their natural armouring. Relatively small amounts of submerged pyrite mud that could not be removed from the main channel and that mixed with the shallow natural sediments during the cleaning works provides a long-term pollution element in the system. Pyrite mud remnants on floodplains are not being directly removed by fluvial activity in the mid-term because these sedimentation areas are susceptible to erosion only by the wandering of the main channel. However, these materials can be easily oxidised, therefore, plant uptake and underground water pollution should be monitored. On the other hand, the channel bed and banks are source areas of sediments and pyrite materials that can be easily eroded due to the destruction of the natural protection features (vegetation and armouring). Once eroded, the pyrite particles will be transported over longer distances than the coarser host natural alluvium, and deposited on floodplains or areas with low flow velocity.

Thresholds of particle entrainment in a poorly sorted sandy gravel-bed river

Abstract A series of bedload samples and hydraulic measurements were made in the poorly sorted sandy-gravel bed of the Arbucies River to determine the threshold conditions for initiation of motion. Entrainment has been analysed using two different initial-motion methods: the largest-grain method and the reference or fractional transport rate method. Both methods yield different results in terms of the equal mobility hypothesis. The deviation between scaled and non-scaled initial motion relations are clear: (a) the largest-grain critical shear stress varies to the power −0.68 of Di/D50, indicating a tendency towards size-selective entrainment; but (b) computation of the critical shear stress based on the reference transport method indicates only a very weak dependence on grain size (power −0.91). The question is whether such divergent results are caused by real differences or methodological problems. On one hand, estimations of τc from bedload maximum particle size are methodologically problematic. The systematic error associated with determining Di makes it an unreliable estimator of dimensionless critical shear stress. Much of the observed variation between Di and τ may be more shear-stress estimations or bedload sampling effects than a consequence of selective entrainment. Any conclusion following this method cannot be substantiated, at least until more hydraulic and bedload data is available. On the other hand, scaling problems are less important in the reference transport method, because fractional rates incorporate scaling considerations. Results indicate a strong tendency towards equal mobility, a relatively common situation described in many other transport data, although the coefficient still suggests weak particle dependence. Bedload does not become coarser as shear stress increases, but its grain-size distribution remains constant, as expected under prevailing equal mobility conditions.

Apparent friction coefficient in straight compound channels

In compound open channel flow, the strong interaction between the main channel and the shallow floodplains affects considerably the discharge capacity. Since this phenomenon was identified, many authors have estimated experimentally the flow interaction in terms of an apparent shear stress acting at the vertical interface between the main channel and the floodplains. Empirical formulae have been developed to quantify this apparent shear stress, yet without general applicability. Herein, a dimensionally sound expression, depending on the square of the velocity gradient between the main channel and the floodplains, and on the so called “apparent friction coefficient”, is proposed. Its variation with the geometrical and roughness ratios is analysed herein. A generalized formulation to predict the apparent shear stress is presented and validated for a wide range of laboratory data. These include small-scale flumes and the large-scale flood channel facility, with both smooth and rough floodplains.

Channel evolution after dam removal in a poorly sorted sediment mixture: Experiments and numerical model

Dam removal is commonly used for river restoration. However, there are still some uncertainties associated with dam removal, mainly related to the sediment transport rates released downstream from the deposit that had previously filled the impoundment. This research studies the physical response to dam removal in the antecedent deposit by answering the following questions: (a) how does an initial channel excavated into the deposit evolve, and (b) what is the time distribution of the material released during the early stages of the process. These goals are achieved by an experimental campaign using a poorly sorted mixture of sediment in the antecedent deposit. The research shows that for the given conditions of our experiments, the rate at which the sediment is released depends on the height of the removed dam, the water discharge and the maximum potential volume of sediment to be eroded. This investigation provides new insights of the width evolution when the sediment is composed of a poorly sorted mixture. This evolution is linked to the bed degradation rates: channel narrows during a rapid incisional phase, and subsequently widens when bed degradation rates decrease. Channel width changes propagate upstream as a convection-like perturbation associated with a kinematic wave starting at the location of the antecedent dam. These features are modeled through a new numerical model accounting for mixtures. More specifically, a set of equations has been derived for the variation of bed elevation, channel bottom width, and bed grain-size distribution, which when solved numerically, describe the observed channel processes.

Improved 1-D modelling in compound meandering channels with vegetated floodplains

This paper shows that common open channel flow one-dimensional (1-D) numerical models behave poorly in terms of flow distribution across a section in a naturalized meandering channel with vegetated floodplains.We compared numerical models with new experiments carried out in a physical model on a reach of the Besòs river close to Barcelona that has been restored. The case examined has floodplains with thick vegetation, a low-flow meandering channel and high water depths over the main channel and floodplains. 1-D models overestimate velocities in the main channel but underestimate them on the floodplains. The paper gives a detailed description of an improved 1-D computation method based on the consideration of the total shear stress at the main channel-floodplain interface (the sum of turbulent friction plus lateral momentum transfer) on the one hand, and the influence of upstream velocity distribution with the help of two-dimensional numerical models on the other hand.

Backwater of arch bridges under free and submerged conditions

Experimental research on backwater effects in semicircular arch bridges is reported. Both pressurized and free-surface flows at the bridge were investigated. Flows on a mobile bed in clear-water conditions were compared to those with a rigid bed. Methodologies for backwater computation by Yarnell, the US Geological Survey, the US Department of Transportation and the HEC-RAS model were compared with the experimental data. A simple expression for the head loss coefficient as a function of the obstructed bridge area is derived.

Riprap stability: transverse and longitudinal versus continuous protections

An experimental study was conducted to determine the influence of length, width, and protrusion of noncontinuous riprap protections on shear failure conditions. The incipient motion of particles as a failure criterion and the reference transport method as the threshold of motion were used. In each test, riprap transport rates were measured at different time intervals using a sediment trap placed immediately downstream from the test reach so that time dependence could be well described. Results reveal that incipient motion conditions of transverse (cross-sectional) protections are strongly influenced by both the protrusion and length of bed protection, which indicates that stability significantly increases as protection length increases and decreases as protrusion increases. In the case of longitudinal protections, almost the same failure conditions were found as in the case of continuous protection. Furthermore, these conditions are unrelated to the width of the protection. A coefficient to correct design formulas obtained by other authors is proposed to take into account the effect of the geometry of transverse protections on their stability.

Local scour in a protruding wall on a river bank

Vertical banks with changes in alignment are called protruding walls. Local scour at the wall nose was investigated in almost 40 clear-water tests lasting up to 2 weeks. The main results were expressions for local scour development as a function of relative depth, flow intensity, wall angle and contraction rate. An expression for the flow intensity needed for scour inception was also derived from the tests.

A Sand-Gravel Gilbert Delta Subject to Base Level Change: GILBERT DELTA UNDER BASE LEVEL CHANGE

Laboratory experiments were conducted on a sand-gravel Gilbert delta to gain insight on its dynamics under varying base level. Base level rise results in intensified aggradation over the topset, as well as a decrease in topset slope and topset surface coarsening, the signals of which migrate in an upstream direction. Preferential deposition of coarse sediment in the topset results in a finer load at the topset-foreset break, which creates a fine signature in the foreset deposit. Base level fall has the opposite effects. Entrainment of the topset mobile armor causes a coarsening of the load at the topset-foreset break and so a coarse signature in the foreset deposit. The entrainment of the topset substrate and fine top part of the foreset may follow, which causes a fining of the load and a fine signature in the foreset deposit. The fact that the upstream sediment supply requires a certain slope and bed surface texture to be transported downstream under quasi-equilibrium conditions counteracts the effects of base level change. This information travels in the downstream direction. In nature base level change is likely so slow that the upstream sediment load maintains the topset slope and bed surface texture and so keeps the topset in a quasi-equilibrium state. Base level change is therefore not expected to leave a clear signal in a mixed-sediment Gilbert delta other than a change in elevation of the topset-foreset interface.