Alluvial Fans Formed by Channelized Fluvial

Alluvial fans and fan-deltas are of three basic types: those built up primarily by the action of constantly avulsing river and stream channels, those constructed by sheet flows, and those resulting from. ~e successive deposition of debris flows. The present analysis is directed toward the first two types. A mechamstic formulation of flow and sediment transport through river channels is combined with a simple quantification of the overall effect of frequent avulsion to derive relations describing the temporal and spatial evolution of mean (i.e., averaged over many avulsions) bed slope and elevation in an axially symmetric fan. An example of a fan formed predominantly by the deposition of sand is compared to a similar one formed predominantly by the deposition of gravel. In each example the case of channelized flow is compared to the case of sheet flow. The model is applied to the tailings basin of a mine in the companion paper. FIG. 1. View of Alluvial Fan of Kosi River, India, Showing Location of Main Thread of Flow at Various Times [Adapted from Gole and Chitale (1966)] JOURNAL OF HYDRAULIC ENGINEERING / OCTOBER 1998/985 less acts to limit the horizontal growth of fans, as shown by the classic example from Death Valley (Hooke 1968, 1972). As a fan builds outward in a subsiding basin of sufficient extent, it must eventually reach a point at which all of the sediment brought into its head is consumed in providing the deposit just necessary to balance subsidence, so that outward progradation ceases. Under these circumstances the fan reaches a state of equilibrium aggradation, at which the mean aggradation rate due to sediment deposition just balances the subsidence rate, and mean elevation on the fan remains constant in time. This balance was described in a mechanistic sense by Paola (1988, 1989) and Paola et al. (1992). Another mechanism for driving sediment deposition is rising base level, e.g., that of a lake or the ocean. This mechanism has particular relevance to fan-deltas. It is shown here that the equilibrium aggradation associated with a base level rising at a constant rate and vanishing subsidence is identical to that associated with a constant rate of subsidence but invariant base level. Whether actively prograding outward or in a state of equiINTRODUCTION Alluvial fans are fan-shaped zones of sedimentation downstream of an upland sediment source. Fan shape tends to be regular and conical, and can often be described as axially symmetric to a first approximation (Hooke and Rohrer 1979). At least three mechanisms for their formation have been observed; avulsing channelized rivers, sheet flows, and debris flows (Schumm 1977; Blair and McPherson 1994). In the case of a sheet flow, channelized upland flow reaches the fan and spreads out widely with no obvious channel, inundating much of the fan surface and depositing sediment [e.g., Blair and McPherson (1994)]. Debris flow fans are built up as an agglomeration of tongue-shaped deposits of individual debris flows, each one of which is typically much smaller than the fan itself [e.g., Johnson (1984); Suwa and Okuda (1983); Whipple and Dunne (1992)]. Fluvial fans are built up by the successive aggradation, and then avulsion of a river. The river channel may be meandering, split-channel, or fully braided. An example of a large fluvial fan is that of the Kosi River, India, as it emanates from the Himalaya Mountains, shown in Fig. 1 (Gole and Chitale 1966; Wells and Dorr 1987). Fluvial fans may be completely terrestrial, or may have a distal portion with standing water. Fans of the latter type are called fandeltas (Nemec and Steel 1988). Any given fan may be built up by some combination of the previously mentioned mechanisms. It is fair to say, however, that fans dominated by debris flows tend to have higher slopes than those dominated by fluvial processes [e.g., Harvey (1984); Wells and Harvey (1987); Jackson et al. (1987)]. The present paper is devoted to fluvial fans. The analysis also allows for the description of sheet flows as a limiting case. It is not accidental that fans often form in regions that are undergoing subsidence in geologic time. Subsiding regions are natural sinks for sediment; it is by this mechanism that sedimentary basins fill (Allen and Allen 1990). Subsidence, while rarely exceeding rates of a few millimeters per year, neverthe'Prof.• St. Anthony Falls Lab.• Univ. of Minnesota. Minneapolis. MN 55414. 2Assoc. Prof.• Dept. of GeoJ. and Geophys., Univ. of Minnesota. Minneapolis. MN 55455. 3Asst. Prof.. Dept. of Earth. Atmospheric. and Planetary Sci.. Massachusetts Inst. of TechnoJ.. Cambridge. MA 02139. 'Res. Sci.. Exxon Production Research Co.• Houston. TX 77252. Note. Discussion open until March 1. 1999. Separate discussions should be submitted for the individual papers in this symposium. To extend the closing date one month. a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on January 27. 1997. This paper is part of the JourTUZl of Hydraulic Engineering. Vol. 124, No. 10. October. 1998. ©ASCE, ISSN 0733-9429/98/0010-0985-0995/$8.00 + $.50 per page. Paper No. 15069. Nepal 87"10' a: ~ 'f,~I~' nKosi ft su Barahakahetra Chalra li.l..l..l..J 25Km N