Internal Erosion Suite

Blanket Theory Case 3

Case 3 has an impervious top stratum only on the river side of the levee. The pervious substratum is divided into three zones to apply the method of fragments as shown in Figure.

Method Analysis

The method of analysis is the same as in Case 1.

Levee Geometry

Step 2 characterizes the levee geometry. The input includes the levee crest elevation, landside toe elevation, distance from riverside levee toe to the river (L₁), and base width of levee (L₂), as illustrated in Figure.

Figure: Step 2 of BT Case 3 worksheet: Levee geometry.

Pervious Substratum Characterization

The pervious stratum characterization is the same as in Case 1.

Blanket Theory Assumptions

Step 4 checks the BT (method of fragments) assumptions against the input parameters to ensure essentially vertical equipotential lines. For deterministic analysis, the assumptions are checked for the most likely values of the random variables. For probabilistic analysis, the assumptions are checked for the mean values of the random variables. Values outside of the model assumptions have an orange background. Figure illustrates the check of BT assumptions.

Figure: Step 4 of BT Case 3 worksheet: BT assumptions.

Seepage Characterization

Step 5 calculates the net hydraulic head on the levee (H) the same as in Case 1. The flow or seepage per unit length of the levee (Q_s) using Equation.

$$Q_{S} = k_{h,f}H\frac{d}{L_{1}+L_{2}+0.43d}$$

where:

k_h,f = horizontal permeability of pervious substratum
H = net hydraulic head on the levee
L₁ = distance from the riverside levee toe to the river
L₂ = base width of the levee
d = thickness of the pervious substratum

The format for the tabular output is the same as in Case 1.

Likelihood of Heave/Blowout at Landside Toe

Since the excess hydraulic head at the landside levee toe is zero, vertical seepage exit gradients are not computed for this case.

Likelihood of Heave/Blowout at Given Distance from Landside Toe

Since the excess hydraulic head at the landside levee toe and at any distance x from the landside levee toe are zero, vertical seepage exit gradients at any distance x from the landside levee toe are not computed for this case.