A few basic things are found to be responsible for most residential flooding. Poor design and offsite factors do play a role but generally, most residential flooding is caused by erosion and trees.
When a home is first completed, even 50 years ago, building codes require that the foundation be 6 inches minimum above the soil in the yard and that this soil is sloped (graded) away from the house to provide positive drainage for at least six feet. In other words, as one moves away from the foundation, the soil is even lower than adjacent to the foundation.
Very few (if hardly any) homes escape this building code requirement—upon final inspection. This “freeboard,” or the elevation of the foundation above the surround yard, leaves plenty of room for runoff to accumulate and flow out to the street or alley. Over time however, many things can contribute to a reduction of this “freeboard.”
Homeowners like to landscape. They can unknowingly reduce the freeboard by installing flower beds that block water flow out of the yard. Fences are installed and these too block the flow out of the yard. But there are other things that contribute to residential flooding more than any others. These things take place over time and get worse over time.
One is that those beautiful trees that we plant in our yards to provide shade and aesthetics grow up. They often turn into huge and valuable assets. But as they do so, not only does their increasing trunk size block water flow, these tree roots push soil up. It is not uncommon for a tree to raise the level of the soil a foot or more over 30 to 50 years.
So when a tree is planted or grows up between houses, along fence lines, etc., or in a front or back yard and off to the side of the lot, flow paths can become blocked, ponding occurs and water sometimes enters homes. Trees growing up on other lots uphill, even several lots away, can block critical flow paths too and send large quantities of stormwater runoff in a completely different direction from where it was originally intended to go.
Another primary reason that home flooding increases over time is directly related to those trees. Over time, most neighborhoods become heavily treed forests. The yards beneath these trees, especially the backyards, become heavily shaded which makes it difficult for turf to grow. Less grass creates more exposed soil. This soil erodes during heavy rain events, sometimes easily, and is swept downhill. When trees block flow paths between houses, ponding occurs on the uphill sides of these houses and the eroded sediment settles out.
This sediment builds up on the uphill sides of houses over time and reduces freeboard allowing ever smaller rain events to pond and threaten to come into the house.
There are dozens of other reasons why homes flood, but these are the biggest. Often, the trees that have caused this flooding are too valuable to remove. Backyard drains are then installed and the third most common reason for residential flooding then occurs. This is the 4-inch drainage pipe used in almost all backyard drainage solutions.
Four-inch pipes are fine for very small flows, but how often do we get very small flows, especially these days when climate change (man-caused or natural) is actually increasing the intensity of current rainfall events?
A 6-nch pipe can carry almost four times the amount of water as a 4-inch pipe and an 8-inch pipe can carry almost eight times the capacity of a 4-inch pipe. Most landscape drainage systems are simply undersized, regardless of climate change enhanced rainfall intensity. The 4-inch pipes installed to fix the problem actually do very little to alleviate flooding in most cases.