FLOOD05589 - Laserfiche WebLink

Home Browse Search

6 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS Collapse of gully walls is greatly facilitated by piping or tunnels which develop in the gully walls and lead waters from the adjoining surface to the channel by an underground route (see fig. 5). It was Our observation FIGURE 5.-W'ater pouring into open pipe or tunnel leading downward to arroyo channel. The upper opening of this piping hole is at least 15 feet from waU of gully. Rio Pescado near Ramsh, N. Mex., August 17, 1946. that only a small proportion of total flow in a gully reaches tho. gully channel by direct overpour of the vertical banks, Piping tunnels and tributary gullies and rills deliver the bulk of the discharge, The manner in which relatively large pebbles or cobblcs move during flash flows is particularly worthy of comment, The bed material in the ephemeral streams studied in the Santa Fe area characteristically is composed of a matrix of moderately well sorted coarse sand, but it includes a certain number of cobbles, rocks, and even some small boulders. The eross sec- tion of flowing water during flash floods is wide and shallow, but the velocity of tho. water is high. Despite the small depth of flow, the large cobbles are effectively moved by rolling. ;'vludballs move in a similar fashion, rotating about the longest axis. Even cobbles which are irregular in shape and subangular roll along the stream bed for long distances without stopping. Cob- bles were observed t<l roll spasmodically but rapidly even when the water was no deeper than halj the dmmeter oj the rolling object. At this depth the water seems to splash up on the upstream side of the cobble and plunge over its top, so providing a torque. It is indeed com- mon to see particles, small and large, sticking well out of the general water surface and rolling rapidly down- stream with only temporary interruptions, PROBLEMS OF MEASUREMENT Attempts to obtain precise measurements of arroyo floods are fraught with many difficulties and inherent dangers, It was necessary to adopt unorthodox methods that yield data which are admittedly crude. K evertheless, the data themselves are unique, and they appear to be adequate for the kinds of analyses undertaken, Three factors militate against good measurements of the rising stage of arroyo floods. First, the stage rises to peak so quickly that one can seldom be present during the few minutes of rise even when he is trying to. Second, peak stage is dangerous for a person wading in the flood because of high velocity and the Occurrence of surges or bores. Finally, peak flow of consequence Oecul's generally near the storm center where lightning is a deterrent to wading operations. Hence, most of the hydraulic data presented here were obtained during the falling stage of the individual floods. All measurements were made by wading; velocities ,vere measured with a. Price cunent meter. When rocks as much as half a foot in diameter batter one's feet and meter in a current flowing 6 feet per second, and when the sand is constantly undermined from under one's heels, short-cut methods inevitably are adopted. For reasons which follow, our discharge measurements of arroyo flow must be considered rough approximations. Instead of using 20 to 30 measuring points or "verticals" a.cross the channel~ only 10 to 15 were used. The duration of eurrent-meter observa- tion at each point was reduced from 40 to 20 seconds, and the meter was set at 0.6 depth in most cascs, It is standard procedure to make adjustments for varia- tion in water level during the measurement, but stage could be measured only crudely, We traversed back and forth along the tag-line 'Without interruption during the falling flood. Each traverse of the 10 or 15 sections across the channel ,vas considered a measurement of discharge, and the mean stage during the traverse was assumed to apply to that measurement. Suspended-sediment samples were collected with the DH 48 hand-sampler or, in a few cases, by dipping a bottle without the aid of a hand-sampler. The samples were collected near tho. midpoint in time of discharge measurements. Usually two samples, each depth- intcgrated, were collected at two points in the channcl cross section. The eoncentration of sediment was de- termined separately in the laboratory. The average of the two concentrations' was considered to be re.pre- sentative of the flow during the discharge measurement.