Storm and flood of July 31-August 1, 1976, in the Big Thompson River and Cache la Poudre River basins, Larimer and Weld Counties, Colorado

Abstract

The deposition of sediment by a stream of water. Alluvium.Sediment including clay, silt, sand, and gravel in transit and(or) deposited by a stream. Excludes detritus deposited in standing water such as lakes or oceans. Altimeter setting.The pressure required to make an altimeter indicate zero altitude at an ele vation of 10 feet above mean sea level. Cirrus anvil.High clouds which spread outward from the tops of thunderstorms. Colluvium.A deposit of unconsolidated detritus or earthy material that has been carried downslope chiefly by gravity, as opposed to running water. Generally is heterogeneous, poor ly sorted, and poorly bedded. Includes but is not limited to talus, soil creep, landslides, avalanche deposits, and sheetwash. Colorado Piedmont.A section of the Great Plains. The part of the Great Plains lying between the Southern Rocky Mountains and the High Plains. In the Big Thompson region it is the area east of the hogback belt at the foot of the Front Range. The name "High Plains" is sometimes used erroneously for this section. Convection.Vertical motions and mixing resulting when the atmosphere becomes thermodynamically unstable. Cubic feet per second (cfs or ft3/s).A rate of discharge. One cubic foot per second is equal to the discharge of a stream of rectangular cross section 1 foot wide and 1 foot deep flowing at an average velocity of 1 foot per second. dBZ.A measure of the relative power (in decibels) of returned energy to transmitted energy. Debris avalanche.In the Big Thompson Canyon area, term is applied to a very heterogeneous mixture of water-saturated bouldery debris flowing very rapidly down a long, narrow steep channel and leaving behind a conspicuous linear scar on the mountainside. Similar to a debris flow but moving at a higher velocity down a steeper gradient (fig. A). FIGURE A.-Debris avalanche deposit and scar. GLOSSARY Debris fan.A fanlike or conelike subaerial accumulation of sand, gravel, cobbles, boulders, and more or less organic trash deposited where the velocity of a stream is abruptly checked by a change of gradient, as at the mouth of a gulch. Deposit is generally poorly sorted and poorly stratified. Generally, a product of torrential runoff. The fanlike form results from the shifting of the channel as the stream blocks and diverts itself repeatedly with its own debris (fig. B). FIGURE B.-Debris fan. Debris flow.A very heterogeneous mixture of water-saturated debris flowing slowly to very rapidly down a ravine and discharging onto a debris fan. In the Big Thompson Canyon area, many debris flows contained abundant woody trash, such as logs and brush. Some debris flows evidently discharged directly into the Big Thompson River and were swept away by the flood. Similar to a debris avalanche but flowing down a flatter gradient at lesser velocity. Debris slide.The most common type of landslide set off by the Big Thompson storm. In the Big Thompson area, a moist stony heterogeneous landslide that moved downward and out ward without backward rotation. Motion may have been slow to rapid....

Concentration‐discharge relationships during an extreme event: Contrasting behavior of solutes and changes to chemical quality of dissolved organic material in the Boulder Creek Watershed during the September 2013 flood

“…This effect may be associated with other extreme precipitation events in mountainous terrain. For example, substantial debris flow was also generated during the flood on 31 July 1976 in the nearby Big Thompson Canyon, when 300–400 mm of intense rain fell over the span 3–4 h [ McCain et al ., ; Jarrett and Costa , ].…”

Hydrologic conditions controlling runoff generation immediately after wildfire

“…Maximum precipitation is in April or May from cyclonic storms (low intensity, long duration) with a secondary peak in July and August [ Peet , 1981] associated with the North American monsoon convective storms (high intensity, short duration) [ Douglas et al , 2004]. High rainfall intensities (e.g., >25 mm h −1 ) associated with these convective storms are known to produce flash floods and debris flows even in unburned areas [ McCain and Shroba , 1979].…”

Preliminary inventory of debris-flow and flooding effects of the June 27, 1995, storm in Madison County, Virginia showing time sequence of positions of storm-cell center

“…The meteorologic conditions leading to this storm have been compared to those of other stationary thunderstorms that caused rapid onset flooding, i.e. along the Big Thompson River in Colorado in 1976 (McCain andothers, 1979) and at Rapid City, South Dakota in 1972 (Schwarz and others, 1975).…”