Decommissioning of Silted-in Reservoir – What Constitutes “Flowable” Material?

Information request from Phil Gregory, Cal Engineering & Geology, Inc.:

I am curious if anyone has recent experience with determining what would reasonably be considered as "flowable material" under a dam safety evaluation. We are working with a dam and reservoir that has been completed silted-in (40+ feet thick) and that no longer impounds water. The original spillway was
lowered so that a lowered channel cuts through the sediment as a stream. No other water is impounded and the lowered stream channel and spillway can safely pass the design event without impounding floodwater. The goal is to eventually remove the facility from being considered as a jurisdictional dam. My logic is that since the water content of much of the sediment is now below the liquid limit of the soil (or will be soon due to ongoing self weight consolidation), I am leaning toward concluding that the sediment behind the dam should not be considered as flowable as if it were water. If the moisture content of the sediment is below the liquid limit, the material by definition acts as a solid and not as a liquid and therefore the sediment should not be considered as part of an impoundment. If anyone has any perspective on this issue (even if it is to tell me how wrong I am) please comment or provide any references that you may know of regarding this specific issue.

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