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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:

• The Army Corps of Engineers determined that the June 2006 flooding of the Huntington Neighborhood was caused by accumulation of large amounts of silt that raised the stream bed in Cameron Run. The shallower channel could not accommodate the large and rapid storm water runoff and produced the flooding.
  
  
• In 2006, Kingstowne residents received an additional assessment of $300,000 that "was added to the capital reserve program to offset the projected needs of removing silt from storm water management ponds." (see the article in the May 2007 Kingstonian.)

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.

• Huntley Meadows will undertake a multiphase renovation of the park, due in part to silt accumulation in the wetlands, especially near the beaver dam. Shallower wetlands have changed the plant and animal life in the park. The roughly $2 million renovation will add a sediment pond to allow silt to precipitate out of incoming stream water, provide some dredging, and rebuild the dam to allow deeper water overall at various points in the park

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:

• Large amounts of water flowing fast in streams scours earth off the stream banks and moves along sediment that has settled on the stream bottoms. Erosion of stream banks is a major cause of silt.

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.

• Construction projects, especially large ones where earth is left bare, produce runoff containing large amounts of the earth, especially in heavy rains.
  
  
• When the earth is very dry because of the lack of rain, wind will move exposed small particles of earth and deposit it on impervious surface. In suburban environments, exposed earth is a result of large construction and homeowner landscaping or construction. In rural, environments, plowed fields are major contributors.
  
  
• Highway departments deposit sand (technically not silt) on roads and parking lots during snow storms to increase traction and help with snow melting. While sand is a much friendlier to the environment than salt, if not swept up sand will be washed by the melting snow or subsequent rains into the storm sewers. Because sand is comparatively heavier, it isn't carried as far before it settles out onto the stream bottoms. But in violent storms the speed and swirl of the moving water can move sand a long way, or pick up sand already deposited on the bottom of the stream bed and carry it further downstream.

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.

• filtering devices can be built under parking lots and other paved areas to allow storm water to infiltrate into the ground instead of running off. When practicable (because some soils, mostly clays, won't allow much water to infiltrate), this is the best solution.
  
  
• storm water retention ponds (wet or dry) delay the flow of rain water into streams and allow suspended solids such as silt and sand to settle out at the bottom of the pond. However, as time passes, these ponds must have the silt dredged out or the bottom of the pond will rise and it will hold less water. Dredged silt, depending on the metals it contains, may require disposal as hazardous waste.
  
  
• sand laid down on roads can be swept up and re-used, or it can be used as fill or cover or layers of a sanitary land fill. Street sweeping will also get some of the wind-blown dust deposited on the roads.
  
  
• construction should be done with silt barriers (or lately, "super silt barriers") to prevent runoff from the construction site. These barriers should be inspected at regular intervals and after every heavy rainfall.

Riparian buffer and stream restoration is also important.

• concrete stream beds greatly increase the speed of the water. Concrete or pipe should be removed where possible and natural, meandering stream beds should be constructed.
  
  
• ideally streams should have a buffer zone full of plants, shrubs, and trees. First, these plant buffers protect the stream banks from erosion as the roots help hold the soil. Second, the buffers act as natural filters of water flowing off the land to the streams. Finally, not directly related to storm water, the buffers keep the streams cooler, protecting the natural aquatic life and therefore helping the overall health of the stream.

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

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.

• you have to have the right street sweeping equipment, and it's not inexpensive to buy and maintain — and maintenance is key to how well it works.
  
  
• you must have a well-thought-out street sweeping plan, and you must sweep as soon as you can after the snow has melted.
  
  
• street sweeping isn't very effective where streets don't have curb and gutter. A local study has shown that where curb and gutter don't contain the sand, sweeping may only get about 10% of what was laid down.
  
  
• on-street parking detracts from sweeping effectiveness.

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:

• have commercial property owners upgrade storm water management using best management practices (BMP) as they re-develop their properties.
  
  
• insist that new residential development also incorporate BMP. The goal should be no new net runoff, and, where possible, reduction.
  
  
• do more restoration of streams and riparian buffers.
  
  
• in older neighborhoods, review the adequacy of storm sewers in light of subsequent development. Replace and upgrade as necessary
  
  
• spend the money to implement a street sweeping plan to get sand and silt off the roads.

Copyright Doug Boulter, 2007

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