A Discussion Of Urban Groundwater

I would just like to relate some things that I have noticed over the years about groundwater.

Many landscapes have a very subtle slope, which is often barely noticeable. The street on which I live actually has a slope which I never noticed until I became interested in the natural history of the area. It is often impossible to detect a very subtle slope on land without looking along a paved road.

The reason that this is important is water. Water is much more sensitive than the human eye to any slope in the terrain as it follows where gravity takes it. Farmers tend to be especially aware to the slope of the ground, knowing that plowing must be done along the slope and not perpendicular to it.

A typical urban landscape consists of a checkerboard of building with "rough" ground. On built-up areas, including buildings, parking lots, roads, and cared-for lawns, water generally cannot emerge freely. It is on the "rough" areas of empty fields and unused and unpaved ground that water can follow it's natural course.

Years ago, there was an open field where nothing would ever be built because there were electric power lines overhead. The field was much drier then than it is now. There was a little bit of water in low-lying areas in that field, but there is usually considerably more now.

There was a large field not far away from this. At one time, there were two ponds in that field as well as some trees and bushes that would absorb water. But then, that field was developed into a large store and theater. The entire field was paved over.

What happened is simple. Water from rain and melting snow could no longer be absorbed by that field, so it migrated downstream to the next rough area.

Streets block the flow of groundwater close to the surface, but not that which is deeper. Where I live, there is a subtle slope to the southwest. I recall that there was one field with dense bush growth in the southwest corner of the field. There was a nearby field with a pond in the southwest corner.

What this means to you is flooding. If an open area of rough ground is paved over some distance up-slope from you, it can cause your basement to suddenly start flooding when it never has previously. The water will simply migrate down-slope.

Such flooding can be circumvented by digging a ditch or pond between the newly paved over area and the flooded area, or by planting bushes whose roots will intercept shallow groundwater..

Something like a highway will also channel water. There was a large pond in the field described above that eventually was paved over. I later realized that it must have been dug purposely. It was right next to an interstate highway and there was a landfill with high ground not far up-slope. The pond had been dug to absorb water that ran off the high ground of the landfill, and was channeled by the highway, so that there would not be flooding downslope.