Groundwater, water table 58
Retaining walls 63
Checkpoint summary 71
In addition to all of the items mentioned in the previous chapters, you should inspect the drainage around the house and the landscaping. Any retaining walls, decks, or fences also should be inspected.
As housing developments and shopping centers sprout up in the countryside, they affect the drainage characteristics of the surrounding areas. Normally, in undeveloped areas most of the water falling to the earth soaks into the ground. The remainder flows over the surface into lakes, rivers, and streams, or accumulates in low-level areas, forming ponds. In built-up areas, thousands of acres that had been soaking up rain have been rendered impervious to water because of buildings and paved areas. The surface-water runoff in these areas might be two to ten times more than it was when the land was undeveloped.
In built-up areas, surface water usually flows into storm drains (catch basins) that in turn discharge into rivers and streams. In some cases established housing developments have been inundated with surface water after a heavy rain because their storm-drainage facilities were not adequate for the increased water flow resulting from adjacent new housing developments. In most cases, the increased runoff results in the rivers and streams swelling, although they are usually contained within their banks. However, after a heavy prolonged rain, a river or stream can overflow its banks and flood the surrounding area. Many people do not realize that even a small creek, which might be a trickle when they see it, can become a raging, destructive torrent following an excessively heavy rain.
The area normally flooded when a river or stream overflows its banks is called a flood plain. Between 5 and 10 percent of the land in the United States is in a flood plain. Much of this land is level and from outward appearances seems to be desirable. As land in urban and suburban areas became more and more scarce, builders constructed homes directly on the flood plains of streams and other waterways. These homes are all vulnerable to flooding. (See FIG. 6-1.) In many parts of the country, some owners do not realize that their homes were built in a flood-prone area, and they probably will not realize it until it is too late.
If you have doubts about whether the house is located in a flood plain, you should check with the local town or county engineer. If the engineer is not available, often the local highway superintendent can tell you whether the area periodically floods. In many communities, federal flood insurance is available for those homes located in a flood plain. If you are considering such a home, you should consider purchasing flood insurance.
Surface runoff is of concern to the homeowner because it can result in soil erosion, ponded water, and water in the basement or crawl space. Soil erosion occurs whenever water flows over bare earth. Soil particles are loosened and are carried away by the flow. Water seeks its own level and will therefore flow from a higher elevation to a lower elevation. The paths the water takes when flowing to a lower level are called natural drainageways. Areas particularly vulnerable to erosion are steep banks and drainageways. (See FIG. 6-2.)
The basic principle for preventing or minimizing erosion is to have the ground covered as much as possible with growing vegetation such as grass, trees, bushes, shrubs, and even weeds. If the vegetation does not root and keeps washing out, a substitute cover such as gravel, stones, or mulch can be used. This cover is not as effective, but it does reduce the erosion. Sometimes the banks are too steep for a ground cover of any kind; they must be stabilized by terracing or retaining walls.
In addition to a ground cover, erosion can be reduced by slowing down the water flow. For example, if there is a concentrated surface runoff along a natural drainageway, the water can be diverted to a man-made channel or ridge that follows a level contour. This spreads out the water and slows the flow so that the water does not scour and erode the soil.
The effects of surface runoff can be minimized by reshaping the ground surface. This can be done by terracing and/or regrading the lot into gentle slopes with diversionary ridges and swales. A swale is a depression in the ground that like a ridge will intercept surface runoff and redirect it to an area where the water will not cause damage. When a house is located on a sloping lot, a swale or ridge should be in the portion of the lot that slopes toward the house. This type of a diversion will prevent surface water from accumulating around the house. Otherwise, the surface runoff might seep into the basement or crawl space. (See chapter 11.)
Many building lots have low, level areas that will tend to accumulate water after a rain or from surface runoff. When the soil is slow-draining, as with clay and silt, the water will pond rather than soak into the ground. The ponded areas retain the water until it evaporates or eventually seeps into the ground.
Depending on the location of the pond, the accumulated water might not be a problem. If the pond is over the leaching field of a septic system or is in an area that normally has a lot of foot traffic or is used by children, corrective action is necessary. The problem can often be corrected by bringing in fill and regrading the area. When regrading is not practical, the area can be drained by laying a line of perforated drainpipe through the affected area and directing one end of the pipe to a low spot. The pipe is generally encased in a bed of gravel or broken stones. If conditions warrant it, a concrete-block catch basin with radial spokes of perforated pipe can be installed at the low point. (See FIG. 6-3.) Water collected in the pipes and catch basin can then be directed to another area. If there are no other low areas to which the ponded water can be redirected using a pipe with gravity flow, the water can be directed to a sump pit and pumped to the desired location.