FLOOD05589 - Laserfiche WebLink

Home Browse Search

32 PHYSIOGRAPHIC AND m-DRAULIC STUDIES OF RIVERS This line of reasoning which follows the principles derived from interrelation of width, depth, velocity, and suspended load at constant discharge in natural rivers does provide an explanation for the small gradient of the channel bed in the early and narrow stage of the discontinuous gully. It indicates why an increase in hed slope could be expected to be associated with the channel widening. The principles outlined apply to the changing rela- tions at constant discharge, suspended load, and rough- ness. Before the individual discontinuous gullies coal- esce to form a continuous, fiumelike channel, there is still sufficient channel storage in the ungnllied reaches of the valley to attenuate flood peaks originating far upstream. Thus, the most effective flows in a given reach probably are derived from storm rainfall in the immediate vicinity of the reach in question. The suspended load in the reach is derived from the bank caving and headcutting of the individual discontinuous gully. In the early stages of gully development these conditions justify the use of the assumption of constant discharge and suspended load. Constant load and discharge imply constant sediment concentration which is one of the important determinants of roughness. So, the assumption of constant roughness is logical. In developing an argument to explain the low gradient associated with the narrow and deep channel of the early stages of a discontinuous gully, it is inappro- priate to reason from the downstream relation of slope to width shown in figure 18, for there is implicit in that diagram an increase in discharge associated with increasing width downstream. The stages in develop- ment of the discontinuous gully must be explained in terms of changes in channel characteristics whicb can take place at constant discharge, In summary, it is reasoned that mutual accommoda- tion causes the small width of the initial discontinnous gully to be associated with the development of a small slope, in fact so much flatter than that of the original valley floor that the two gradients intersect (diagram 2, fig. 26), The depth rapidly decreases downstream to the point of extinction, and below that point a small fan develops where the sediment is spread on the orig- inal valley floor. These same forces operate to expand the gully head- ward and to deepen it, the combination of which makes the gnlly increase in total length. Because there are usually several gullies along the valley floor, the toe of one gully tends to extend itself downstream while the head-cut of the succeeding one is extending itself up-valley, decreasing the total length of unchanneled valley floor between them. This change is indicated by diagram 3 of figure 26. As the process continues through time, a stage wij] be reached when the head-cut of the downstream gully meets the toe of the one upstream and the two discon- tinuous gullies coalesce, This stage is indicated by diagram 4. The lengthening and deepening of the gully is accompanied, however, by a gradual increase in width. As both width and depth continue to become greater, the channel reaches a size sufficient to contain without overflow on the valley floor the largest discharge which the basin upstream can produce. At this stage the effect of the large amount of channel storage which characterized the flat-floored alluvial valley in its ungullied condition has been lost and the discharge in a given reach increases greatly. Finally a condition is reached in which the i"tal width between the arroyo walls is sufficient to allow the stream to wandel' back and forth between the walls and the development of local patches of flood plain can begin. At that stage the gradual increase in width has required such an increase in slope that the gully bed becomes almost parallel to the original valley floor (diagram 5). This is characteristic of most of the large arrovos throughout the West. There may still remai; a difference, however, between the total width of even a large gully and the width of flowing water character- istic of the condition when the ungullied valley floor carried the same discharge, Probably it is for this reason that through long reaches in the down-valley direction the depths of some arroyos gradually decrease. The hypothesis implies that gullies in the process of development could generally show the features indi- cated diagrammatically on figure 26, To test this hypothesis several gullies in the Santa Fe area were mapped. The profile of figure 27 is a typical example. This gully is discontinuous, as can be seen by the fact that its depth becomes progres- sively shallower downstream to point of extinction at a distance of 2,750 feet. Immediately upstream from this point the gradient of the gully floor was .014, a much smaller figure than the original gradient of the valley floor, ,028, Some deposition in the form of a fan can be seen between 2,750 and 3,200 feet on the distance scale, but only a small proportion of the total evacuated materials is ever deposited in the fan below a discontinuous gully; most of it proceeds downstream. The situation downstream from point 2,300 feet indeed bears a marked similarity to that shown in diagram 2 of figure 26. At 2,240 feet it can be seen that the gully bed drops more than 4 feet, and this is certaiuly the point of coalescense of two formerly separated discontinuous gullies, exactly as shown in diagram 4 of figure 26.