FACTORS AFFECTING LARGE STORM AND FLOOD EVENTS IN THE BLACK HILLS AREA OF WESTERN SOUTH DAKOTA

The Black Hills area has a history of damaging flash floods resulting primarily from exceptionally strong thunderstorms. The best known example is the catastrophic storm of June 9–10, 1972, that caused severe flooding in several major drainages near Rapid City. The potential for storms and flooding in the Black Hills area can be affected by various climatological and physiographic factors.

Thunderstorms that form over and around the Black Hills are largely terrain-driven, especially with respect to their requisite upward motion for initiation. Obstacle effects can also be substantial for the Black Hills in terms of promoting thunderstorm development. A composite of storm total precipitation amounts for 13 recent individual storm events indicates a propensity for heavy precipitation to occur east of the major axis of the Black Hills, from the northern hills (near Spearfish) toward the southeast through the eastern foothills near Hermosa. A proclivity for short-duration but intense convective precipitation events, including supercell thunderstorms, has been observed in this area. Prolonged precipitation from large-scale storm systems becomes progressively more influential with increasing drainage area and tends to be a primary driver for flooding in the main-stem of the Belle Fourche and Cheyenne Rivers.

Topography and geology are two primary physiographic influences on flooding. Peak-flow characteristics for upper Rapid Creek and Spearfish Creek are small (suppressed), relative to other areas, due to generally low topographic relief and high infiltration capacities in the Limestone Plateau area. The reduced potential for development of exceptionally strong thunderstorms in this area, which is located west of the major axis of the Black Hills, may also be a factor. In contrast, the potential for exceptional floods is greatest within and near outcrops of sedimentary formations that encircle the periphery of the lower Black Hills. Narrow canyons with limited potential for attenuation of flood peaks are common, and steep topography within the canyons and associated tributaries also contributes to increased flood potential. Another factor may be increased potential for development of exceptionally strong thunderstorms in these areas.