Water, Water Everywhere, Flooding and Why
by Ruth S. Foster
Did you ever wonder why we have so many water problems? And flooding problems? It's more than just lots of rain. To illustrate, consider the geology of a small town, Belmont, which lies on the edge of the Boston Basin.
Water runs downhill from Belmont Hill to the Charles River (past Harvard and MIT) or Mystic River (where the Pilgrim's fished for alewives or shad, who swim up stream to spawn each spring). The water from Belmont runs 6 miles to Boston harbor and then out to sea, like the whaling ships.
We all know that water runs downhill. But understanding it, is not so simple. Follows a simple semi-scientific discussion. First, comes prehistoric geology. The Boston Basin was formed by an ancient semicircle of mountains. During the last ice age, glaciers shaved off the tops leaving cracked granite thinly covered with sand, gravel and soil sediments. There is even a geologic fault line running across Belmont that goes right under the Police Station.
The Boston Basin is the low land inside the semicircle of hills. And even though it is not under water, it's underground aquifers are subject to the daily tides with their high and low water tables. Most of Belmont is actually in this Boston Basin even though it is quite a few feet above sea level. The elevations fall off rapidly from the high of Belmont Hill, several hundred feet actually, to the low lying areas from where one could row a boat into the harbor a century ago, Especially low and vulnerable is the Winn Brook area, where the water comes shooting way up out of the manhole covers several times a year after heavy rains.
In the Winn Brook area, Clay Pit Pond is supposedly only 8 feet above mean high tide in Boston Harbor. I wonder what the pond's under ground water table is at flood tide ( with a full moon) and a northeaster. Also adjacent Little Pond (where the alewives still run) is similarly impacted.
Everyone knows that water runs downhill, but it's not so simple. There are underground aquifers, springs, feeder creeks, streams, perched water tables, and glacial outwash deposits.
The UNDERGROUND AQUIFER is sub-surface water that hopefully never comes up in the basement. It is fed by numerous underground and above ground SPRINGS which are forced up through the cracks in the shaved off and cracked granite that is Belmont Hill. When the underground aquifers from Lexington (the next town) hit the Belmont granite mountain base, the water has no place to go but up through those cracks.
These SPRINGS come up all over the place, and change location frequently. After a heavy rain, you can dig a hole on the hill, and it will often spontaneously fill with water. Belmont Springs a Victorian source of bottled water that's still used today.
These springs rise up above ground into FEEDER CREEKS which often run for two or three days after the rain stops. After a heavy rain, driving downhill on steep Old Concord Avenue (built 1840) there are often streams of water running down the sides of the road. Sometimes you can actually hear feeder creeks gurgling in some of the old stone walls. Unfortunately these creeks also appear in people's backyards and basements too.
Historically the water and flooding was controlled by digging drainage channels and by keeping the STREAMS open and running. Not only to keep buildings and roads dry but, for health reasons. It was necessary for control of malaria, yellow fever, encephalitis, black flies and mosquitoes, which breed in swampy bogs.
When the channels and brooks get clogged up, we get swamps, bogs, flooded backyards, underwater roads, soggy athletic fields where the water doesn't drain. Technically all these are caused by a PERCHED WATER TABLE. where water stands and doesn't drain. And it doesn't percolate down into the soil.
Belmont's soil is mostly GLACIAL OUTWASH which was left as the glaciers melted. It consists of sand, gravel and topsoil and tends to have a high water table. It doesn't drain well and is more susceptible to flooding, particularly basements dug below grade.
So next time it rains, if you need to, dig a little trench to keep the water flowing downhill, around your basement. Or if necessary, dig a drywell (which is a big hole filled with stone and concrete type rubble, then covered with soil proof fabric to keep the dirt out, then grass or groundcover).
Or even dig a big area, sunken about 1 foot below grade, to use as a sunken terrace or play area or rain garden. Plant grass, flowers, or use crushed stone over a weed barrier. The water will go into the drywell hole or sunken terrace, and slowly seep back into that underground aquifer. Better there than the basement! (Thanks to W, Crosby's "The Boston Basin", 1898.)