>> Dug out to about 2 and 1/2' by the foundation and sloped the 'floor' away from the porch to the sidewalk to about 4'--filled the whole thing back with amended 'soil' including a lot of compost and grit.
So, effectively, I have created a giant bathtub on each side of the porch with well drained soil with no-where to drain to...
That sounds like a great bed. Deep enough for anything!
>> sloped the 'floor' away from the porch to the sidewalk to about 4'
Can't it drain to the sidewalk? If it does, the "runoff" over the sidewalk might be too shallow to notice, even during a heavy rain.
Or maybe there IS a slight amount of percolation down though the muck below 3 feet, and THAT is keeping the bathtubs from filling with water and staying full.
Or being 2.5 feet deep means that top 18 inches drain down and flood the bottom 12", and the roots don’t try to go deeper than 18".
>> When I planted some bulbs in there last fall,
Many bulbs need fairly well-drained soil. If they thrive and come back for a few years, you must have drainage somehow!
>> including a lot of compost and grit.
That is exactly what I would do. Given the budget and carrying capacity.
>> we don't get much annual precipitation around here.
AHHH HAH! Whatever your hardest rainstorm is, say one inch per hour times a few hours, check out how high the water table rose inside that bed, within 10-20 minutes of the rain stopping. Well, really, check how close the water table gets to the soil SURFACE. Then check again a few hours later, and you'll know how fast the whole bed perks (no matter WHERE the water is going TO.)
