The term landscape drainage refers to the flow of water through the ground in a particular area. Generally, the two most important factors that influence landscape drainage are the slope, or grade, of the land and the type of soil. The steeper the slope, the more tendency water has to run off and carry the soil with it, which ultimately causes erosion; the flatter the ground, the more likely it is that infiltration — soaking in of water — or puddling will occur. Methods of dealing with landscape drainage include altering the topography of the land, changing the composite of the soil, and installing drainage pipes to remove water from unwanted areas.
A steep slope of ground located above a structure, such a house, can result in the structure being flooded. If designing a landscape on a steep slope cannot be avoided, potential flooding could be avoided by building a berm — a low mound of earth sloping away from the structure to shunt the water around it. Digging a channel or installing drainage pipes are two other landscape drainage methods that could divert water around the structure and down the slope. Terracing — breaking up a slope into successive steps of level areas — will typically slow the runoff and allow water to soak into the soil.
Soil type refers to the size of the soil particles. The three basic sizes are sand, silt, and clay. Water infiltrates sandy or rocky soils much more rapidly because the soil particles are larger. Since these soils do not retain water, plants generally do not grow well in them.
The smallest soil particles are clay, which forms a dense mass that prevents drainage, thereby increasing runoff and creating puddles of standing water. Due to its denseness, clay soils reduce the amount of air around the roots of plants. For this reason, plants typically do not grow well in clay soils.
Silt is an intermediate size of soil particle. Loam is a mixture of all three soil sizes and types. The addition of mulch or humus — decomposing organic material — will generally improve drainage in clay soils and increase nutrient retention in sandy soils. The ideal medium for growing landscape plants typically is a good mixture of humus and loam.