Below-Grade Drainage Calculator

No -- the opposite. Water Table Depth is measured below grade, so a larger number means the water table sits farther down and provides less hydraulic head pushing groundwater toward the foundation. Design Flow Rate decreases monotonically as Water Table Depth increases across its full declared range.

Because Soil Type is a coded selection, not a continuous quantity -- it steps between four discrete hydraulic conductivity values (gravel, sand, silt, clay) spanning five orders of magnitude. A small nudge near the default silt value barely moves anything, but switching all the way to gravel or clay can change Design Flow Rate by orders of magnitude, since conductivity itself dominates the groundwater term rather than a gradual slope.

The Pipe Capacity Utilization output flags it, and Pipe Over Capacity switches to 1. The diameter table tops out at 8 inches, so an extremely high design flow still gets an "8 inch" recommendation, but Pipe Capacity Utilization reads well over 100% in that case, signaling that one line at that diameter is undersized. This doesn't require every extreme input at once -- Soil Type set to gravel by itself, with every other input at its default, already pushes utilization to roughly 700%, since gravel's conductivity can dominate the flow calculation on its own. A very long perimeter combined with a high-intensity storm can trigger the same result even with ordinary soil.

Only very slightly, and only near a pipe-size threshold. Design Rainfall feeds the surface-water portion of Design Flow Rate, and Drainage Gravel is driven mainly by trench volume (perimeter x trench width x foundation depth), which has no direct dependence on rainfall at all. The one indirect path: if raising Design Rainfall alone pushes Design Flow Rate across a pipe-capacity threshold (say, from a 4" to a 6" recommendation), the larger pipe subtracts slightly more volume from the gravel-filled trench -- at default geometry, that's about 0.7 tons out of roughly 74, under 1%. Away from such a threshold crossing, Drainage Gravel is effectively unchanged across Design Rainfall's range.

Foundation Depth, near this calculator's default values -- it sets the trench depth directly, so it moves Drainage Gravel more than an equivalent percentage change in Trench Width does at those defaults. Across the full range of possible projects, though, Foundation Perimeter can end up mattering more, since it can vary more than a thousand-fold across its declared range while Foundation Depth's range is comparatively narrow.