Rainfall erosivity-intensity relationships for normal rainfall events and a tropical cyclone on the US southeast coast

- Raindrops were measured by laser disdrometer on a salt marsh island.
- Power-law equations were best fitted between erosivity indices and intensity.
- Observed kinetic energy was slightly higher than the world average.
- Observed raindrop velocity was lower and more widely distributed than theory.
- Erosivity equations established from normal event were not suitable for cyclone.

SummaryThe work done on the intertidal landscape by low tide rainfall events has been shown to augment the cycling of dissolved and particulate nutrients, but low tide rainfall events are not a well-documented component of coastal ecosystem models. Here we develop the relationships between rainfall intensity (I), and median volume raindrop diameter, and three rainfall erosivity indices (kinetic energy, momentum, and momentum multiplied by the drop diameter) using an optical disdrometer deployed in the intertidal zone during summer and fall of 2010 and 2011. These data include the local effects of Hurricane Irene in 2011. Raindrop data measured for 27 days of late summer were analyzed. The best fit between median volume raindrop diameter and I was a combination of the power-law and logarithm equations, and the best fits of three erosivity indices and I were obtained with power-law equations. Kinetic energy was slightly higher than the world average. Observed raindrop velocity was typically lower and more widely distributed than the theoretical raindrop terminal velocity. Hence, erosivity indices based on observed velocity were lower than those based on terminal velocity. The hurricane provided larger raindrops and more widely distributed raindrop velocity than normal events. Overall, results indicate that it is not suitable to assume that background erosivity-I relationships apply to cyclonic events. We derived new erosivity-I relationships to help characterize soil erosion processes in salt marsh areas for normal events. These results will help predict material and nutrient fluxes between intertidal and subtidal landscapes.