When rain falls on the ground it will at first soak in but when the soil cannot take any more water the water runs across the ground. It will flow down hillsides finding the path of least resistance until it reaches a creek or stream. When normally dry depressions on hillsides gather water and begin to flow it is known as overland flow.
Urban development in our towns and cities introduces hard surfaces such as roofs, roads, driveways and paths which stop rain soaking into the ground. This means more water runs off than would naturally occur. Although gutters, pipes and drains have been built to direct the water where we want it to go, the biggest storms have more water than pipes or gutters can carry. The extra water runs across the ground as an overland flow, finding its own path of least resistance. This can take it through private properties and even through buildings.
While the total amount of floodwaters from overland flows will generally be less than from riverine flooding, overland flows can cause just as much damage to an individual property as riverine flooding. It is the depth and velocity of water at each property which determines how destructive the flood waters are.