Bed load bias:Comparison of measurements obtained using two (76 and 152 mm) Helley-Smith samplers in a gravel bed river

We assess how the size of the Helley‐Smith (HS) bed load sampler nozzle affects the accuracy of bed load sampling. Semitheoretical considerations show that the larger grains resident on the streambed can influence the sample either by blocking the sampler entrance or by causing the sampler to rest in a “perched” position. Probabilities for interference can be derived from the distribution of grain sizes but they do not capture the actual complexity of the influence of the bed on sampler performance. We therefore make an empirical comparison of sediment trapped by HS samplers with 76‐ and 152‐mm intakes during floods in the gravel bed lower Ebro River. Most bed load rates appeared higher when sampled with the HS152. The largest clasts collected by the HS76 also tend to be smaller than those obtained with the HS152 at the same flow. Analyzing paired bed load samples, we find the probability of a bed load sample collected with the HS152 to be biased is around 43% in the conditions of the present study, whereas 65% of samples were biased when obtained with the HS76. The analysis emphasizes the influence of bed material texture over sampler performance and demonstrates that the use of samplers with intake size much larger than bed grain size (i.e., ∼5D) will increase the accuracy of bed load grain size distributions and the precision of annual load estimates in gravel bed rivers.The research was carried out within the framework of the research project REN2001‐0840‐C02‐01/HID (Spanish Ministry of Science and Technology)

Evaluación de fórmulas de transporte de fondo en un río de gravas acorazado

El curso bajo del río Ebro (NE península Ibérica) presenta un cauce con lecho de gravas que discurre aguas abajo de un complejo de embalses. En dicho tramo, el lecho experimenta ciclos de rotura y restablecimiento de la capa superficial acorazada como consecuencia tanto de avenidas naturales como de crecidas de mantenimiento. En el citado contexto, el objetivo de la presente investigación fue doble. En primer lugar, la evaluación de la capacidad predictiva del transporte de fondo de tres fórmulas seleccionadas en condiciones de coraza rota o alterada. En segundo lugar, el análisis de la capacidad de dichas fórmulas para predecir el umbral de rotura o alteración de la coraza. Todo ello se fundamentó en la comparación entre predicciones y mediciones de carga de fondo en el tramo de estudio que abarcaron dos años hidrológicos. Conforme a los condicionantes del tramo de estudio, las fórmulas finalmente seleccionadas fueron las ecuaciones desarrolladas por Parker, Klingeman y McLean (1982) (P-K-M), Bathurst (2007) (B) y Recking (2010) (R). De acuerdo con los resultados, se recomiendan las fórmulas P-K-M y R para la predicción de la carga de fondo cuando se rompa o altere la coraza, y se desaconseja la fórmula B por su marcada tendencia a la infrapredicción. En cambio, para la predicción del umbral de rotura o alteración de la coraza se recomiendan las fórmulas B y R. Sin embargo, cabe advertir que en este caso ambas fórmulas tendieron a predecir umbrales bastante inferiores a los medidos.The lower Ebro River (NE Iberian peninsula) has a gravel bed which flows downstream from a reservoir complex. In this section of the river, the armoured surface of the bed undergoes cycles of breaking and reestablishment as a result of natural and control floods. The objective of the present work is two-fold. First, to evaluate the capacity of three formulas to predict bed load transport under conditions in which the armour is broken or disturbed. Second, to analyze the ability of the formulas to predict the breakage or disturbance threshold of the armour. This was all based on a comparison between predictions and measurements of the bed load transport in the study section for two hydrological years. Based on the characteristics of the study section, the formulas which were finally selected were the equations developed by Parker, Klingeman and McLean (1982) (P‑K‑M), Bathurst (2007) (B) and Recking (2010) (R). Given the results, the P‑K‑M and R formulas are recommended to predict bed load transport when there is breakage or disturbance of the amour, and not formula B because of its considerable tendency to underestimate. Formulas B and R are recommended to predict the breakage or disruption threshold of the armour. Nevertheless, it is worth cautioning that in this case both formulas tended to predict thresholds much lower than those measured.La presente investigación fue llevada a término en el marco de los proyectos REN2001-0840-C02-01/HID, CGL2005-06989-C02-02/HID, HID, CGL2009-09770 CGL2006-11679-C02-01/ (subprograma BTE), SCARCE Consolider Ingenio 2010 CSD2009-00065 y CGL2012- 36394, todos ellos concedidos por el Ministerio Ciencia y Tecnología de España. El segundo autor fue beneficiario de Ramón y Cajal (RYC-2010-06264), ambas otorgadas por el Ministerio de Ciencia e Innovación de España