Filtration Enhances Suspended Sediment Deposition from Surface Water to Granular Permeable Beds

We present results Irom an experimental study of suspended particle (4.5–36.5 μm silicon carbide powder deposition from surface water to ‘clean’ equi-granular permeable beds in a small 12.5×12.5×15cm box and a re-circulating flume. Enhanced deposition rates of up to 5 times the accepted sediment deposition model (e.g., Einstein,1968 are explained by filtration of particles in the bed. Compared to this model depotiion increases with increasing surface fluid speed, decreasing suspended particle size and increasing bed particle size. These results can be explained by an increased ability of particles to penetrate into the bed with the pore water which increases the effective filter thickness. The predominant deposition mechanism within the bed pores appears to be settling. Enhanced deposition, evident in Einstein's (1968) experimental data, was previously attributed to flocculation but may be explained better by filtration. These enhanced deposition rates drop off to close to the accepted model predicted rate after a certain volume of sediment has entered the bed, which may be due to the stumping of deposits from the top of bed elements. This reduction in deposition rate occurs long before the bed is filled with fine sediment.     

Rundschau

Ohne ZusammenfassungPflanzenschutzorganisation für Europa und die Mittelmeerländer     

Morphological and water-stress characteristics of three Douglas-fir stocktypes in relation to seedling performance under different soil moisture conditions

This one-time greenhouse study examined the phenology, morphology, frost hardiness and response to moisture stress of three Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stocktypes from the same seed lot. The types were mini-plug^TM transplants (MPT), 1+1 bareroot transplants (1+1), and 2+0 bareroot seedlings (2+0). In late summer and fall before lifting, 2+0 seedling set bud before 1+1 seedlings, 1+1 seedlings before MPT seedlings. The 2+0 seedlings appeared slowest to acquire frost hardiness in fall and seemed to deharden most rapidly in spring. Although the 2+0 seedlings were taller than the MPT stocktype, MPT and 2+0 seedlings were relatively similar in other morphological respects, but 1+1 seedlings were much larger. All stocktypes were potted on January 20, 1989, placed in a greenhouse, and subjected to 39%, 18%, 16%, or 6% soil water content (% dry weight) until the end of the experiment in mid-July 1990. The largest decrease in pre-dawn xylem water potential occurred with 16% and 6% soil water content; pre-dawn xylem water potential averaged over the three stocktypes generally declined 219% from low to high soil moisture stress. The 1+1 seedlings used more water than the other two stocktypes, and at maximum soil moisture stress, plant moisture stress increased in the order: MPT <2+0<1+1. During the 6-month greenhouse experiment, the larger 1+1 stocktype showed the most absolute growth, but the smaller stocktypes grew more on a relative scale. Growth of the stocktypes appeared to be related to differences in morphology and water-use patterns as the seedlings competed for available water within each pot. The results show that MPT seedlings, a new stocktype, performed as well as the more traditionally used 2+0 and 1+1 seedlings and that stocktype selection is important in reforestation efforts.