leachate flow in leakage collection layers due to defects in ...

GIROUD et al. D Leachate Flow in Leakage Collection Layers Due to Geomembrane Defects
Table 2. Rate of leachate migration through a defect in a geomembrane primary liner as a
function of the defect diameter and the head of leachate on top of the primary liner.
Leachate head on top of the
Geomembrane primary liner defect diameter, d (mm)
primary liner, h prim (mm) 1 2 3 5 10 20 50 100
5 13 51 115 319 1,275 5,101 31,881 127,523
10 18 72 162 451 1,803 7,214 45,086 180,345
50 40 161 363 1,008 4,033 16,131 100,816 403,264
100 57 228 513 1,426 5,703 22,812 142,575 570,301
300 99 395 889 2,469 9.878 39,512 246,948 987,790
Note: The tabulated values of the rate of leachate migration, Q, through a geomembrane defect were
calculated using Bernoulli’s equation (Equation 20) and are expressed in liters per day (lpd).
Table 1 shows that, in the case of a leachate head on top of the secondary liner on the
order of 100 mm, which is possible and acceptable in the case of a large leak, the rates
of leachate flow that can be conveyed by the various leachate collection layers are on
the following order: gravel, 100,000 lpd; geonet, 10,000 lpd; and sand, 1,000 lpd.
Therefore, geonets and, to a greater extent, gravel are suitable for all of the defect scenarios
mentioned above, whereas sand is not. However, it should be noted that, with a
maximum head on the order of 100 mm, a geonet is full in a certain area around the
primary liner defect, whereas a gravel leakage collection layer is not.
If the primary liner is a composite liner (e.g. a geomembrane on a geosynthetic clay
liner) the rate of leachate migration through a geomembrane defect is several orders of
magnitude less than through the same defect in a geomembrane used alone. Therefore,
a geonet leakage collection layer is not likely to be filled with leachate migrating
through a composite primary liner.
4 WETTED ZONE
4.1 Shape of the Wetted Zone
4.1.1 Equations of the Parabola
From Section 2.2, it is already known that the wetted zone has the shape of a parabola.
The equation of the projection of this parabola on a horizontal plane will be provided
in this section, and any subsequent reference to the wetted zone (e.g. equation, surface
area) will be related to the projection on a horizontal plane. However, it should be noted
that the width of the parabola is the same in the actual parabola (on the plane inclined
at angle β ) and its projection on a horizontal plane.
Several points of the parabola (Figure 6) are known from Figure 4. Thus, Figure 4a
and Equation 4 show that the distance between the projection, O, of the flow apex, A,
and the vertex, V, of the parabola is:
228 GEOSYNTHETICS INTERNATIONAL S 1997, VOL. 4, NOS. 3-4