FLOOD04806 - Laserfiche WebLink

Home Browse Search

iv 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.BI, ~~~ ;-~y. tv ~~ ~ ~--~~ jj" ...V. ~,-~.Y ;:J ~!l'"''-<--'~ ,,-~~~' ----- ______~~r ----- 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. Mostly in colluvium. Degradation. As applied to streams, synonymous with erosion. In a more general geomorphic sense., the gradual lowering of the landscape by weathering and erosive processes. Detritus. Any material worn or broken from rocks by mechanical means. The composition and dimensions are extremely variable (Stokes and Varnes, 1955, p. 371. Dewpoint (or dew-point temperature). The temperature to which a parcel of air must be cooled at constant pressure and constant water'vapor content in order for saturation to occur. Discharge. The quantity of fluid mixture including dissolved and suspended particles passing a point during a given period of time. Drainage area. The area, measured in a horizontal plane, which is enclosed by a topographic divide. Echoes. In radar terminology, a general term for the appearance on a radar indicator of the electromagnetic energy returned from a target. Entrainment. The mixing of environmental air into a preexisting cloud parcel. Fault. A fracture in the Earth's crust along the sides of which there has been movement par- allel to the fracture plane. Flood. Any abnormally high streamflow that overtops natural or artificial banks of a stream. Flood plain. The nearly flat ground bordering a river and occupied by the river at flood stage, built up from sediment deposited when the river overtops its banks and spreads outside its low-water channel. Floodway. The channel of a river or stream and those parts of the flood plains adjoining the channel, which are reasonably required to carry and discharge the floodwater (Erbe and Flores, 1957, p. 443, quoted in Langbein and Iseri, 1960, p. 11). Usually applied to the part of the flood plain reserved or zoned to accommodate expectable flooding. Front. Boundary separating two different air masses. Gage height. The water-surface elevation referred to some arbitrary gage datum. Gaging station. A particular site on a stream. canal, lake, or reservoir where systematic obser- vations of gage height or discharge are obtained. Gneiss. A foliated metamorphic rock having alternate layers of visibly dissimilar minerals, especially feldspar-rich layers alternating with mica-rich layers. Very common in Big Thomp- son Canyon. Gradient. As applied to streams, the inclination of the bed, usually expressed as a percentage, or feet per mile.