  Porosity is one of the key properties of dense particle packings like
sediment deposits and is influenced by a multitude of grain characteristics
such as their size distribution and shape. In the present work, we focus on the
form, a specific aspect of the overall shape, of sedimentary grains in order to
investigate and quantify its effect on porosity, ultimately deriving novel
porosity-prediction models. To this end, we develop a robust and accurate
simulation tool based on the discrete element method which we validate against
laboratory experiments. Utilizing digital representations of actual sediment
from the Rhine river, we first study packings that are composed of particles
with a single form. There, the porosity is found to be mainly determined by the
inverse equancy, i.e., the ratio of the longest to the smallest form-defining
axis. Only for small ratios, additional shape-related properties become
relevant, as revealed by a direct comparison to packings of form-equivalent
ellipsoids. Since sediment naturally features form mixtures, we extend our
simulation tool to study sediment packings with normally-distributed forms. In
agreement with our single form studies, the porosity depends primarily on the
inverse of the mean equancy. By supplying additional information about a second
form factor and the standard deviations, we derive an accurate model for
porosity prediction. Due to its simplicity, it can be readily applied to
sediment packings for which some measurements of flatness and elongation, the
two most common form factors, are available.
