TRENCHING INVESTIGATIONS TO ASSESS FLOW MODES AND RECURRENCE

The River Ranch development south of Carbondale, Colorado lies on a latest Pleistocene outwash terrace on the western side of the Crystal River Valley. Three incised, ephemeral tributaries from the west have built alluvial fans onto the terrace in the western part of the development. The largest (Bowles Gulch) fan has the lowest surface slope (4.2°) and the smallest fan (Middle Gulch) has the greatest surface slope (8.1°), while the intermediate-size Holland Gulch fan has an intermediate slope (4.6°).

To assess flood hazards on the 3 fans we excavated trenches to expose fan stratigraphy and sedimentology. The Bowles Gulch trench exposed 8 depositional units and 2 soils, with a typical flood deposit being 3 feet of clast-supported gravel. 90% of deposits have the characteristics of hyperconcentrated flow deposits. This agrees with the low gradient of the fan, and with an observed flood event in 1955. Between 1850 C14 yr BP and 1955 AD there were only 2-3 major depositional events at this location. The estimated return period at-a-point is 617-925 years. However, the 1955 flood covered about 1/6 of the fan, so any trench on a fan probably contains only a partial record of all floods that occurred on a given fan.

Middle Gulch fan exposes a poorly stratified section of matrix-supported angular small gravel derived mainly from the Permian Eagle Valley Evaporite, rather than being recycled pediment gravels as at Bowles Gulch. Matrix-supported deposits comprise 90% of trench deposits, with 10% lenses of well-sorted granules. Vertical distance between lenses (0.3-0.6m) indicates thickness of individual debris flow events.

Holland Gulch trench exposes 11 depositional units; 1/3 are matrix-supported and 2/3 clast-supported. Matrix-supported units are thicker than the clast-supported units, so they comprise about 50% of the exposure. The ratio of matrix: clast supported deposits here is intermediate between that observed on the Bowles Gulch and Middle Gulch fans, indicating both hyperconcentrated and debris flows. There have been 3-4 flood events (including 2 debris flows) since 4790+80 C14 yr. BP yielding an average return period of 1197-1597 years at-a-point.

Trenching provides a “reality check” for flood predictions based on hydrologic models, for determining transport mode, deposit thickness, flow depth, and return period.