SCOUR BELOW AN OVERFALL: PART I. INVESTIGATION

Abstract

Scour below an overfall contributes to headcut instability and gully advance. Thirteen largescale scour tests wereconducted by preforming an overfall in a compacted cohesive soil bed. Horizontally compacted soil layers were placed usingtypical embankment construction methods. Water flowing over a horizontal approach was allowed to plunge over an overfalland impinge on the compacted soil bed. Different combinations of soil moisture and soil density were examined for two similartailwater or backwater conditions. For a constant flow rate, the vertical scour was measured downstream of the gully overfall,and typical erosion behavior is discussed. The rate of vertical scour was observed as the plunge pool deepened.<br><br>The vertical scour rate increased as the moisture content at the time of compaction decreased. The scour rate was alsoobserved to decrease as the dry density of the fill material increased. For the soil conditions tested, the vertical scour rateincreased as the unconfined compressive strength increased. The stressstrain modulus and the vertical scour rate alsodisplayed a direct relationship. The layered soil placement methods, typical of embankment construction, can have aninfluence on the observed vertical scour rates. These fieldscale erosion tests were conducted to enhance our understandingof scour processes. This scour investigation focuses on the test description, observed results, and fundamental relationships,while a companion article addresses prediction methods.