Since the mid1980s, extensive research has been conducted on overtopping flow over
embankments and on the effectiveness of alternative methods for protecting the
downstream slope of embankments against erosion during overtopping events. A number of
different aspects of overtopping flow have been investigated, including the hydraulic
conditions of flow, the mechanics of overflow erosion, and the prevention of erosion under
varying conditions of soil types, hydraulic conditions, and embankment geometry. The
ASCE Task Committee on the Mechanics of Overflow Erosion on Embankments (Powledge
et al. 1989b) summarizes more than 30 dam, levee and roadway embankments that
experienced overtopping flows during the 1980s. Field studies and laboratory investigations
conducted both at full scale and at model scales during the last two decades have
contributed to the understanding of analysis and design issues associated with overtopping
flow. Many of these research activities are also summarized by the Task Committee
(Powledge 1989a).
6.8.2 Hydraulics of Overtopping Flow
Flow Zones. In the case where there is minimal tailwater during an overtopping event, flow
over an embankment structure transitions through three zones. Zone 1 is characterized by
subcritical flow over the dam crest. In this zone the total energy is essentially equal to the
elevation of the low-velocity pool upstream of the embankment, and erosive energy in this
flow zone is low. In Zone 1, the embankment will experience erosion only when its crest is
comprised of highly erodible material. Zone 2 consists of a transition zone near the
downstream shoulder where the flow transitions from subcritical to supercritical. While the
total energy remains similar to that in Zone 1, an increase in velocity creates an increase in
local shear stress, and the erosive energy begins to build. Zone 3 consists of supercritical
flow on the downstream slope of the embankment. In this zone, the erosive energy
increases significantly as the flow accelerates down the slope. The energy slope increases
as well, causing high erosion potential. Figure 6.25 illustrates these flow regimes (Powledge
et al. 1989b).
Figure 6.25. Hydraulic flow zones on an embankment during overtopping
flow (Powledge et al. 1989b).
Estimating Discharge. Flow over an embankment can exhibit two types of behavior.
Kindsvater (1964) was one of the first to classify these conditions. The first type of flow
condition is known as free flow, and is characterized by tailwater conditions that are low
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