Destructive and killing debris flows developed in the Laguna Beach area during the heavy rains the night of Feb. 23. Frightening examples of the destructive power of these rain-produced debris flows are manifest in a number of places in Laguna Beach. One such example of the destructive power of the debris flows is seen in debris flows which originated in a minor tributary to Laguna Canyon.

On the east side of Laguna Canyon, just south of Castle Rock Road is a small drainage basin, a minor tributary of Laguna Canyon. This tributary, which we will here refer to as Castle Rock drainage basin, is fan-shaped in plan view and covers an area less than 0.25 mi² (Fig. 1). The drainage basin has very steep upper-slopes, not unlike most of the tributary drainages in the lower three miles of Laguna Canyon, and a gentle lower slope where it joins the floor of Laguna Canyon. A rock unit termed the Topanga Formation underlies the basin. The Topanga Formation of middle Miocene age, consists of alternating resistant sandstone and much less resistant mudstone. The very steep upper part of the drainage (the fan part) is covered with a relatively heavy growth of brush. In the upper part of the basin are four minor drainage channels that join downslope to form a single channel, which extends to the floor of Laguna Canyon. The alternating resistant and less resistant layers in the Topanga Formation give rise to a step-like topography along the floor of the main channel of Castle Rock drainage basin.

During the heavy rains on the night of Feb. 23 about a dozen small soil slips developed on the very steep slopes in the headward part of Castle Rock drainage. On the rain-saturated steep slopes, these soil slips were quickly transformed into debris flows that rapidly descended into the main channel of Castle Rock drainage. A short distance down the main channel, the debris flows, as either single and/or as merged debris flows, entered a relatively gentle gradient area, the lower part of which is crossed by a dirt road. It appears there was minor road fill where the dirt road crossed the channel, where apparently the debris flows were temporarily restrained -- there is a considerable amount of debris still ponded in this part of the channel. The road was over-topped and the debris flows continued a short distance to a near vertical "waterfall", about 25 feet in height, where the debris cascaded over and down to the low gradient slope at the side of Laguna Canyon. Rapidly flowing debris splattered high in the air as it cascaded over the "waterfall" and continued with great force through structures in its path before reaching Laguna Canyon channel. The destructive power of the rapidly-moving debris flows is most obvious in the short distance between the "waterfall" and Laguna Canyon channel where the flows wreaked havoc with any structure in their path.