Frequency – Another Keyword to Restore Our Streams


Badly Eroded Stream Bank on the Northwest Branch of the Anacostia.
The height of the bank is about 6 ft.

The words of Volume and Velocity penetrated into environmental and other conservation communities as keywords to restore our streams.  However, here is another keyword that is still not yet well-known. 

The word is Frequency

As the vegetation in a watershed was replaced with impervious surfaces such as roads, roofs, parking lots, etc., stormwater runoff started to flow into a nearby stream directly from the surfaces.  It is concentrated to form a torrential streamflow in a stream – volume and velocity.  Also the frequency of such events has increased dramatically.  A graph shown below illustrates the difference in  frequency of peak streamflow between 1939 and 2000

(Click to enlarge)

A boxing analogy can explain what the increase of frequency of high flow events means to streams: too many jabs in the year 2000 so the stream bank vegetation has little time to recover from the damage; too strong straight punch around 9/25 to 9/28 in the year 2000 so the stream bank vegetation is knocked down after too frequent jabs.  Increasing impervious surfaces are increasing the volume, velocity, and frequency of high flow events.

This is why we want to infiltrate, at minimum, 1” precipitation into the ground.  Ideally, a Channel Protection Volume* of 2.6” – 2.7” precipitation should be infiltrated or at least used for irrigation, toilet flush, etc.  If we infiltrated even 1” precipitation watershed-wide, many small peak streamflows in 2000 seen in the graph would have not appeared. 

*Channel Protection Volume: A design criteria which requires 24 hour detention of the one year post-developed, 24 hour storm event for the control of stream channel erosion.


Frequency – Another Keyword to Restore Our Streams |

Greetings! This is my first visit to your blog! We are a collection of volunteers and starting a new initiative in a community in the same niche. Your blog provided us useful information to work on. You have done a wonderful job!

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I really have learned quite a bit from you.Thanks for sharing. So many people have come to your blog.

Frequency – Another Keyword to Restore Our Streams |

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