Trying to figure out the planting bed plan, too. Herbs do well in raised beds. On the one hand, a raised bed could help minimize pooled water in the bed during heavy rains - especially if the sidewalk slopes towards the bed. On the other hand, you don't necessarily want to interfere with water draining away from pool. If you decide to raise it, you might consider using something like large rocks to line the bed. That would keep the herbs out of overly damp soil, keep the mulch in, and allow the water to enter bed if it needs to.