Bank Erosion

Erosion Control using Mats of Discarded Christmas Trees

"Grand Canyon of the Sligo"
Eroded channel of a storm drain outfall
on the right bank downstream
of New Hampshire Avenue

When rivers form, the ups and downs of the land direct water into a meandering channel. Erosion inevitably occurs as the force of water affects soft soil and rock differently from hard material, and as unusually bad storms occur. It is normal for erosion to occur slowly, but in Sligo and similar streams where man has disturbed the watershed, water rushes in quickly after storms through storm water pipes, instead of seeping slowly into soil. Few trees are present with roots to retain water. The result is that banks are abraded, the creek bed widens, and trees on the bank cave in.

Erosion is seen as Sligo's greatest environmental problem. During the major restoration of Sligo between 1988 and 1996, the Wheaton holding ponds at Dennis and five more stormwater ponds were built. In the stream, rocks were imbedded on selected banks to form a fairly solid wall of resistance to water. This is known as riprap, and requires bringing machinery into the stream. Some riprap now needs repair. Other techniques to restrain storm water, probably requiring engineering, will have to be studied, especially in the lower end of Sligo where there is little land for ponds.

How does one measure erosion to decide if a new technique is effective? Technology can do so much that it may be surprising to learn there is no esteemed method of measuring general erosion. One technique is to drive a three-foot metal bar into the bank of a stream until the end is flush with the side. After a few storms, the bar may protrude, and a measurement of how much can be made. The finding applies only to that one place in the stream, which may, of course, have undue stress, or unusually little, and so not reflect the average.

Regardless of ability to measure, we will need to add to the trees and shrubs on the banks. Some tree species such as sycamore form a particularly good shield against the rush of waters after a storm.