What is erosion anyway? It is really pretty simple as our headless instructor in the video shows us:
- Weathering causes changes to large solid formations such as rocks. Weathering creates smaller and smaller pieces of the original formation. These smaller pieces are called sediment.
- Erosion occurs when the sediment is scraped away by water, wind, ice and gravity.
- Deposition occurs when the sediment is deposited somewhere other than its original location
As I see it, man has little to do with Step 1. Even if man was not present on earth, the evolution of sediment deposition (steps 1-3) is a fact of nature. Weather and soil composition alone, can wreak havoc causing massive erosion and devastating deposition issues that kill people and destroy habitat in a matter of minutes.
Throw humans into the mix and steps 2 and 3 are exacerbated 1000 fold. (I just plucked that number out of the sky – I don’t really know if it’s 1000 or 1,000,000,000,000 – just understand that the impact of our activities is significant!)
There are too many activities to list that we humans engage in which create situations where erosion leads to destructive deposition. Construction is the activity that Land Rehab and SWP³ built a business around.
Construction activities are essential to our world as we know it, and no one is more appreciative of great architecture, nice roads and accessible facilities than we are. We are even more appreciative of the fact that if left unchecked the world’s construction activities would render our landscapes and water body’s a disaster.
Even with guidelines and controls in place for construction site stormwater runoff, sediment remains the biggest culprit. These are the facts as excerpted from the EPA:
According to the 2000 National Water Quality Inventory, States and Tribes report that sedimentation is one of the most widespread pollutants affecting assessed rivers and streams, second only to pathogens (bacteria). Sedimentation impairs 84,503 river and stream miles (12% of the assessed river and stream miles and 31% of the impaired river and stream miles). Sources of sedimentation include agriculture, urban runoff, construction, and forestry. Sediment runoff rates from construction sites, however, are typically 10 to 20 times greater than those of agricultural lands, and 1,000 to 2,000 times greater than those of forest lands. During a short period of time, construction sites can contribute more sediment to streams than can be deposited naturally during several decades. The resulting siltation, and the contribution of other pollutants from construction sites, can cause physical, chemical, and biological harm to our nation’s waters.
Controlling Erosion
Controls are divided into a couple types – Erosion and Sediment. There is a logical reason for this…
Without erosion there would be no sediment deposition. So, in a perfect world all you would have to do is prevent erosion and then there would be no sediment to control. Yah right! The very minute you move equipment onto a job site you have created the potential for erosion. Oh come on, you say – that’s ridiculous. It’s not ridiculous; just the act of tracking and turning heavy machinery over native grass destroys the grass and leaves exposed soils - that’s how it starts.
Construction project managers have a lot on their plates and there is usually little room left for erosion and sediment issues in the bigger scheme of meeting project schedules – that’s where we come in.
BEST methods to control erosion:
- Disturb as little ground as possible
- Minimize impervious surface area during the design phase
MOST COMMON methods to control erosion:
- Revegetation – more commonly known as “seeding”. Seeding is best done as soon as possible after final grades are achieved. Key concept: In Colorado it takes at least 3 years to achieve a stand of native grasses that are capable of controlling erosion.
- Erosion control blanket and turf reinforcement mat
- Hydraulic Mulching, FGM, BFM
- Surface Roughening
- Scour reducing devices such as rip rap
- Slope and stabilization
- Riparian planting and stream bank stabilization
- Water velocity checking devices such as check dams
- Retaining walls
More often than not there will be sediment issues long before we are able to install erosion control measures.
Controlling Sediment
Sediment, the result of erosion that has not been addressed, is the biggest construction site pollutant and the cause of many a significant fine by the EPA. Sediment deposition can have a damaging affect on natural hydrology, wildlife and plant habitat, and can carry construction pollutants into local waterways and storm sewer systems. If it leaves your construction site it is a big NO NO! In the photo here, this weather event carried so much sediment so fast that all of the upstream protections (hay bales) flowed with the sediment toward the inlet.
There are many of ways and products designed to contain and control sediment. The general idea is to pond or filter stormwater run-off so that the sediment can settle out or be filtered out, allowing clear water to flow off of your construction site. To achieve this, sediment controlling BMPs (Best Management Practices) are ideally installed in a redundant fashion – meaning several devices within the line of flow. In this picture the sediment log has done a great job of containing half of its capacity in sediment. So that this BMP continues to function properly the contained sediment will have to be cleaned out, leaving the log ready for the next weather event.
BEST methods to control sediment:
- Address the erosion potential upstream
- Utilize controlling devices (BMPs) in a redundant fashion
MOST COMMON methods to control sediment:
- Silt Fence
- Straw bales**
- Sediment control logs and wattles**
- Custom pre-fabricated Geotextiles devices
- Rip Rap**
- Constructed basins and ponds
**Many BMPs serve two purposes – one is to allow sediment capture, the other is to slow the velocity of a stormwater flow so that it causes less destruction on its way.
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