February 2020 was the wettest February in the UK since records began in 1862, according to the Met Office. Storms led to a lot of rivers bursting their banks, with some monitoring stations recording their highest ever river levels.
In Worcestershire (where our Malvern site is located) many cities, towns and villages were badly flooded, damaging properties and closing roads. And once the floodwaters receded, affected places were left covered in a thick layer of squelchy mud, which is time-consuming to clean up.
Storms cause huge amounts of eroded soil and debris to enter the waterways, adding to the sediment already being carried and giving the swollen, fast-flowing rivers their characteristic brown color. In some cases, over half the sediment transported by a river in a year may be moved during one storm period. The larger pebbles and sand particles are usually deposited quickly. However, the smaller silt and clay particles tend to settle out only when the water flow is slow, over flooded land, leaving us with mud.
This fine particle size distribution helps make the mud sticky, as it retains water well. The large specific surface area of the small particles result in increased water adsorption. On top of this, drainage is poor as water can only travel slowly through the densely packed sediment.
And what about when the mud starts to dry? A recent study has attributed the cohesiveness of clay soils to particle size instead of the type or charge of the minerals present. They found that as the water evaporates from clays, thin strands of smaller particles – called solid bridges – form between larger particles. When present, these solid bridges give a 10 to 100-fold increase in stability of the resulting aggregates and improve stability even when re-wetted. These findings could lead to increased material stability across a range of applications, simply through the addition of small particles.
Measuring the Particle Size Distribution of Sediments
While sieves are traditionally used to size soils and sediments, they are unable to reliably measure the clay fraction because the particle size is simply too small. The broad size range covered by laser diffraction makes it an excellent technique for this application. The Mastersizer 3000 can measure particle sizes in the range 10 nm to 3.5 mm in one simple measurement, with the customizable Soils Report allowing the user to really get a feel for the texture, as discussed here.
So while mud can be annoying to deal with, from a particle size perspective it’s fascinating!