The Incredible Drop of Rain

Our blue planet’s hydrologic cycle has three main components (evaporation, condensation, and precipitation). Precipitation can be either liquid (drizzle, rain) or solid (snow, hail). The universally accepted definition of rain is that it is a form of liquid precipitation that comprises drops greater than 0.5 mm in diameter. Liquid precipitation less than 0.5 mm diameter is classified as drizzle.


During a given storm event, rainfall comprises an array of drop sizes. Storms of high intensity can carry the large rain drops (5 mm or larger). Large drops of rain (greater than 6 mm diameter) loose stability and disintegrate while travelling through the atmosphere. It is interesting to note that terminal velocities of raindrops are approximately 10 meters per second (36 km/hr) for the largest drop size and around 2 meters per second (7 km/hr) for the smallest drop of rain.


The interaction between the particles within the rain drop and the surrounding environment (surface tension) keeps the rain drops spherical; however, rain drops greater than 2 mm diameter have greater forces acting on them which result in them assuming a flatter shape. For rain drops with a diameter greater than 6 mm, the excessive aerodynamic forces result in their undersurface becoming concave. Eventually, the large raindrop assumes a bubble shape and bursts into numerous micro-droplets.


As mentioned above, the terminal velocity of large raindrops can approach 36 km/hr. Therefore, it is not difficult to imagine the resultant impact of a raindrop as it exerts significant erosive and compaction forces on bare soil. The damaging impact of raindrops can be mitigated through vegetation or other appropriate erosion protection measures and techniques. Typically, varying percentages of the total rain volume infiltrate into the soil (unless the soil is sealed by a synthetic erosion control blanket or an impermeable liner), evaporate and transform into surface runoff. The surface runoff causes the secondary damage to unprotected soil by dislodging and washing away nutrient rich topsoil. Excessive runoff volume can also cause drainage problems by flooding of the low-lying areas.


The two basic flow patterns for runoff are sheet flow and concentrated flow. Sheet flow occurs when runoff flows overland in a uniform sheet pattern whereas concentrated flow occurs when runoff flows through defined pathways (rills, catch drains, drainage channels, waterways etc.). Both sheet and concentrated flow present a significant risk of erosion and scour to unprotected surfaces.


In case of sheet flow, a layer of organic matter and nutrient rich topsoil (dislodged by rain drop impact) is carried downstream in an even manner resulting in significant soil loss. During a storm event, several tons of soil can be lost due to sheet erosion. Sheet erosion is common in areas which have poor soil structure and / or are not managed as per the established principles of erosion and sediment control.


Runoff on uneven surfaces or areas where soils are dispersive can result in rill erosion. Rill erosion occurs when channelized runoff flows in an uncontrolled manner. These small channels can transform into larger gullies. Rill erosion also has a tremendous potential to move soil material in a given storm event as it exposes the underlying subsoil which are often significantly more unstable than the topsoil.


It is not uncommon to observe sheet and rill erosion on the same site. Together they can transport several hundred tons of material in a single storm event and result in breach of license conditions, penalty infringement notices, significant clean-up costs and reputation damage. The cumulative harm resulting from rain drop impact, sheet erosion and rill erosion can be mitigated through effective source control measures and techniques designed by an appropriately qualified professional in erosion and sediment control.