A good designer will analyze the nature of your yard, and may “shoot the grades” to establish the exact topography no matter how flat the site may seem. Spot elevations tell the designer where problems lie so that she can solve them through design.
Ground water can also play an important role in drainage, and it is directly related to rainfall patterns. In the low lying areas of the South, the water table can be just inches below the surface. Such conditions create all sorts of problems for construction and limit planting options.
Rainfall is the periodic catalyst that sets drainage problems in gear. Where heavy downpours are common, poorly drained sites can become flooded for a short time if drainage structures are in place, or extended periods if they are not. Add heavy rainfall to a high water table and the potential for damage increases exponentially.