Internal Erosion Suite

Background

Breach is the final phase of internal erosion and is defined as a catastrophic failure characterized by the sudden, rapid, and uncontrolled release of impounded water or liquid-borne solids (Federal Emergency Management Agency P-1025, 2015 [?]). Fell et al. (2008) [?] lists four breach mechanisms caused by internal erosion and typically considered for earthen embankments.

• Gross enlargement of a pipe or concentrated leak: When an erosion pathway or pipe connects to the impounded water, the earthen sidewalls rapidly erode. This continues until the embankment collapses unless the impounded water level drops below the pipe entrance or the impounded water level drops enough so that the hydraulic shear stress on the pipe sidewalls becomes less than the critical shear stress of the soil. If the amount of crest drop is greater than the available freeboard, embankment overtopping could quickly lead to full breach formation. If overtopping does not occur, the embankment could be severely damaged, and breach may still occur by concentrated leak erosion through cracks.

• Downstream face sloughing or unraveling: Increased seepage into the downstream or landside zone of an embankment can over-steepen the exit face, leading to progressive sloughing or unraveling. Sloughing requires a cohesionless material and progresses more rapidly with steep slopes. In some cases, as soil particles erode, a void may grow near the exit face until a roof can no longer be supported, and the void collapses (mass wasting), like sapping involving seepage erosion undercutting. Unraveling refers to progressive removal of individual rocks by large seepage flows through a downstream rockfill zone. The sloughing and unraveling repeats and progresses until freeboard is lost.

• Overtopping due to sinkhole development: Internal migration can create a sinkhole or depression in the embankment that must be large enough to lead to overtopping. If the sinkhole is on the downstream slope away from the crest, progressive instability leads to breach. Internal erosion can also lead to excessive crest settlement and overtopping due to embankment loss or foundation materials.

• Downstream slope instability: Internal erosion may cause high pore pressures in the foundation or embankment, resulting in reduced shear strength and slope failure. Breach could occur if the failure surface intersects the impounded water level, or if the slope deformations are significant enough that the remnant embankment cannot resist the water load.

All four breach mechanisms ultimately lead to crest settlement and embankment overtopping. The most likely breach mechanism depends on the internal erosion mechanism, embankment zonation, and the specific failure mode evaluated. Table, adapted from Fell et al. (2008) [?], lists the breach mechanisms based on the embankment zoning. Although one or more of the mechanisms may occur during the breach, risk assessment usually only considers the critical breach mechanism. The breach mechanism also informs time for intervention, warning issuance, and evacuation, and breach parameters.

This toolbox informs the likelihood of breach due to gross enlargement of a concentrated leak pipe using the excess shear stress equation, downstream slope unraveling due to flows through the rockfill, and sinkhole development and provides general guidance for assessing breach due to slope instability.

Table: Breach mechanism screening by zoning type (adapted from Fell et al. 2008) [?].

Dam Zoning Type	Breach Mechanisms
	Gross Enlargement	Slope Instability	Sloughing or Unraveling	Sinkhole Development
Homogeneous earthfill	$\checkmark^*$	$\checkmark$	Exclude, except if downstream fill is cohesionless	$\checkmark$
Earthfill with filters	$\checkmark^*$	$\checkmark$	Exclude, except if downstream fill is cohesionless	$\checkmark$
Earthfill with rockfill toe	$\checkmark^*$	$\checkmark$	Exclude, except if downstream fill is cohesionless	$\checkmark$
Zoned earthfill	Exclude, except if downstream fill can support a roof	$\checkmark$	Exclude, except if downstream fill is cohesionless	$\checkmark$
Zoned earthfill and rockfill	Exclude, except if downstream fill can support a roof	$\checkmark$	$\checkmark^*$	$\checkmark$
Central core earth and rockfill (or gravel shells)	Exclude, except if downstream fill can support a roof	Exclude, except if existing dam has marginal stability	$\checkmark^*$	$\checkmark$
Concrete face earthfill	$\checkmark$	$\checkmark^*$	Exclude, except if downstream fill is cohesionless	$\checkmark$
Concrete face rockfill (including gravel fill)	Exclude	Exclude, except if dam is gravel or low permeability	$\checkmark^*$	Exclude
Puddle core earthfill	$\checkmark^*$	$\checkmark$	Exclude, except if downstream fill is cohesionless	$\checkmark$
Earthfill with core wall	Exclude	$\checkmark^*$	Exclude, except if downstream fill is cohesionless	$\checkmark$
Rockfill with core wall	Exclude	Exclude, except if existing dam has marginal stability	$\checkmark^*$	$\checkmark$
Hydraulic fill	Exclude, except if downstream fill can support a roof	$\checkmark$	$\checkmark^*$	$\checkmark$

1. ✓ Breach mechanism can occur.
2. ✓* Breach mechanism can occur and is usually the more critical mechanism.