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Silt, Sand, and Storm Water

What is silt?
Where does silt come from?
How do we deal with silt and sand?
Why don't we do more street sweeping?
Who is responsible for maintaining retention ponds?
What more should we do in Fairfax County?


Silt has often appeared in local news in 2007:

Beaver dam in the middle ground, some water behind it. Huntley Meadows from below the beaver dam:  much less of the wetlands is under water and there are almost no deep pools. This has produced major changes in the plant and animal life in the Park.
Canadian geese feeding on open ground From the boardwalk toward the tower:  Canadian geese feed on ground that used to be under water.

What is silt?

Silt is generally defined as fine particles of earth, ranging in size from slightly larger than clay to slightly smaller than sand. Silt is deposited as sediment at the bottom of bodies of water. While sand is technically not silt, it is often included under silt in common usage.

Where does silt come from?

In nature, rain water falling on the ground is absorbed by the earth and is filtered as it sinks down into the water table. When land is covered by development, the houses, roads, sidewalks, driveways, and parking lots become impervious to such infiltration of rain water (or melting snow) and it is carried off by storm water drainage. Water from roads flows into the concrete or metal pipe of the storm sewers, but these soon empty into local creeks or streams, and these eventually empty into rivers, bays or oceans. Anything on impervious surface is therefore washed into streams — trash, chemicals, sand, and silt.

The more intense the growth and development, the more impervious surface will be created, and the faster the greater amount of storm water will flow into local streams.

Silt mainly comes from four sources:

Pike Branch streambed scoureded much wider than normal flow. Pike Branch between Telegraph Road and Old Telegraph Road. Note how stormwater has scoured the banks. Normal flow is shown by the wet area on the stream bottom, about 18 inches wide and 1 inch deep.

How do we deal with silt and sand?

Good storm water management practices slow and/or reduce water runoff, protecting stream banks and allowing silt and sand to settle out early on.

Riparian buffer and stream restoration is also important.

Street sweeping sounds like a great idea! And we'd have clean streets! Why don't we do more of it?

Sand on the parking lot Sand from March snow storms still on the Franconia Governmental Center parking lot in May.

It is a great idea, but its effectiveness depends on many factors.

Who is responsible for maintaining wet and dry storm water retention ponds?

In theory, the property owner/association maintains wet ponds, and the County maintains dry ponds. However, in the development application process, the County has often had the developer proffer the maintenance of dry ponds as well. This means that the burden of maintenance falls on the homeowners' association. Cutting grass isn't expensive. Dredging is.

What more should we do in Fairfax County?

The point of this paper is that, while it may be cheaper in the short run not to do anything about storm water, in the long run not doing anything may be very costly indeed. Witness the three items in the news at the top of this page.

Here's what I think needs to be done:


This article can be found at
http://www.dougboulter.com/policy/silt.html


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