How Is Sacramento Delta Levee Seepage Controlled?

Seepage follows the path of least resistance, and in the Delta that path runs through buried sand channels beneath the levee. When the river rises, water enters these permeable layers on the waterside and travels landward under pressure. If the pressure at the landside toe exceeds the weight of the soil blanket holding it down, the ground heaves and water erupts as a sand boil, a small volcano of water and sand that can grow into a pipe, a continuous erosion tunnel working back toward the river. Piping is the failure mechanism behind most levee breaches. The peat foundation complicates everything. Peat compresses under the levee's weight and oxidizes when exposed, so Delta islands have subsided many feet below sea level, increasing the hydraulic head the levee must resist. Cracks from desiccation, animal burrows, and old utility penetrations create preferred seepage paths through the embankment. The geotechnical investigation maps these conditions with borings, cone penetration tests, and geophysics, and seepage analyses model the pressures the control measures must handle.

Stability berms are the most common seepage control on Delta levees. A wide, low berm of compacted fill at the landside toe adds weight that resists heave and lengthens the seepage path, dropping exit gradients below critical values. The berm is designed from seepage analysis, not sized by rule of thumb: its width, thickness, and material are set so the factor of safety against heave and piping meets the required standard at the design flood stage. Drainage within the berm keeps it from trapping water against the levee. Cutoff walls and relief wells handle the reaches where berms alone are not enough. A slurry trench cutoff wall, excavated under bentonite slurry and backfilled with low-permeability material, extends from the levee crown down into an impermeable stratum, physically blocking underflow. Relief wells take the opposite approach, accepting that pressure exists and giving it a safe outlet: screened wells at the toe relieve uplift pressure, and their flow is collected and piped away. The two are often combined, with walls blocking the worst seepage paths and wells managing what remains.