Alluvial Fans

Citation

Hooke, Roger LeBaron (1965) Alluvial Fans. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/4KN6-1E95. https://resolver.caltech.edu/CaltechETD:etd-04162003-095459

Abstract

NOTE: Text or symbols on renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Alluvial fans were studied in the field, largely in the desert regions of California, and in the laboratory. Field study consisted of detailed mapping of ages and sizes of debris, channel patterns, and deposits of different types on parts of four fans, and reconnaissance work on over 100 additional fans. Reconnaissance generally consisted of outlining the fan, noting material size and channel patterns, and measuring a few slopes. In the laboratory small alluvial fans were built of mud and sand transported through a channel into a five-foot square box under controlled conditions. Material is transported to fans by debris flows or water flows which follow the main channel. This channel is generally incised at the fanhead, because there water is able to transport on a lower slope the material deposited earlier by debris flows. Since the main channel at the fanhead has a lower slope than the adjacent fan surface, it emerges onto the surface near a midfan point herein called the intersection point. On the laboratory fans most deposition above the intersection point is by debris flows that exceed the depth of the incised channel. Fluvial deposition dominates below the intersection point. This is also inferred to be true on natural fans. Fans deficient in fine material may have so high an infiltration rate that even moderately large discharges are completely absorbed before reaching the toe of the fan. Under these conditions the coarse debris in transport is deposited as lobate masses on the fan. In many respects these deposits resemble and may, in the past, have been mistaken for debris-flow deposits. The empirical relationship between fan area, A[subscript f], and drainage-basin area, A[subscript d] = cA[subscript d][superscript n] has been recognized previously (Bull, 1964; Denny, 1965). The present study suggests that this relationship results from a tendency toward a quasi steady-state between coalescing fans in the same lithologic, tectonic, and geographic environment. The quasi steady-state exists when all fans are increasing in thickness at the same rate. If rates differ, the areas of the fans will change to approach a quasi steady-state. The rate of deposition is determined by the influx of debris, which is a function of drainage basin area. The exponent [...] is less than unity because a storm of a given recurrence interval is less likely to envelop a large drainage basin than a small one. The coefficient [...] is a function of the lithologic, tectonic, and geographic environment. Rates of deposition on fans may be estimated from this relationship using Langbein and Schumm's (1958) data on sediment yield as a function of precipitation. A typical average rate is on the order of one foot per 1,000 years. If a long-term tectonic process is superimposed upon the quasi steady-state relationship between fans in the same lithologic and geographic environment, the rate of deposition may be used to estimate the rate and nature of the tectonic process. As an example, the difference in depositional rates on opposite sides of Death Valley suggests a present rate of eastward tilting of 0.018 degrees/1000 years. The slope of an alluvial fan is determined primarily by debris size and water discharge. Large fans have larger drainage basins and hence larger discharges than small fans. Consequently fan slope generally decreases with increasing fan area. Photographic materials on pages 16, 31, 33, 55, 63, 64, and 81 are essential and will not reproduce clearly on Xerox copies. Photographic copies should be ordered.

Thesis Files