Diary of An Ecological Disaster

 

 One of the very first articles ever published on the topic of erosion control was in the U.S magazine Popular Science in November 1930, and described the loss of an estimated $200,000,000 worth of topsoil into the ocean each year from raindrop erosion. Back in 1930, this was a staggering amount of money, so the U.S Federal Government created a nation wide campaign for the preservation of soils, commencing with a network of experiment stations across the country, to research all stages of rain erosion.

 

However, unfortunately for the American people, the devastating effects of erosion didn't stop there. Between 1930 and 1940, the problem got far worse with one of the largest erosion related disasters to ever occur with what is commonly known as the 'Dust Bowl' or the 'Dirty Thirties'.

During the 1930's the southwestern Great Plains region of the US suffered a severe drought, setting the area up for devastating wind erosion, culminating in 'Black rollers', clouds of dust that reached as far as the east coast.

This disaster was due to a unfortunate series of natural climactic events which were exaccebated by the Homestead Act of 1862 that had allowed thousands of settlers to farm the land or graze cattle. The farmers had ploughed the prarie lands and planted dry land wheat with insufficient understanding of the ecology of the plains. As demand for wheat grew, cattle grazing reduced and millions more acres were ploughed and planted. Dry land farming on the Great Plains led to the systematic destruction of the prarie grasses. Gradually, the land was laid bare, and significant environmental damage began to occur. The area naturally experienced particularly strong winds, and with the onset of drought in 1930, the over-farmed and overgrazed land began to blow away. Winds whipped across the plains, raising billowing clouds of dust. The sky could darken for days, and even well-sealed homes could have a thick layer of dust on the furniture. In some places, the dust drifted like snow. Nineteen states in the heartland of the United States became a vast dust bowl.

After the initial Government response in 1930, Soil conservation was seen as an important issue which needed more attention. President Franklin D. Roosevelt quickly initiated programs to conserve soil and restore ecological balance. The Soil Conservation Service was formed and generated detailed soil maps and aerial photographs.

During this period Australia was not isolated from the 'Dust Bowl' disaster, with anecdotal reports presenting dramatic images of huge dust storms engulfing rural towns and of sand drifts burying fence lines. However it has never been unequivocally established whether these years of the 1940's were due to extreme drought and/or poor land management.

Back in the 1930's and 1940's it was probably a lot more difficult to share experiences and leanings around the globe, possibly resulting in Australians not learning a whole heap from our American cousins. These days Australia has a very active erosion management system with many professionals from the Soil Conservation Service and private industry working hard to improve the way we do things. However the U.S is still ahead of us in terms of erosion and sediment control practices and regulations. But that doesn't mean that we are behind in our awareness or knowledge of the issues. The following case study talks about doing things in slightly different way to how we do them here.

This case study of the U.S Highway 202, discusses how erosion control was managed on this large road project.

In order to capture and treat storm water, mitigation basins were constructed along sections of the upgrade, which incorporate many erosion and sediment control best management practices (BMPs). The basin floor was stabilised with seed and fibre mulch so that the inlets and culverts didn't fill with the typically used straw mulch causing clogs. The side slopes were stabilised with the designated seed and S1 matting, and the spillways were   seeded and installed with P2 matting to ensure permanent, heavy stabilisation.

The ErSed control project was conducted in a staged manner. Stage one saw the protection of the mitigation areas, with silt fence and compost filter socks to control stormwater and remove sediment. These sites are for stormwater control and the plants will act like a rain garden.

Next crews hydraulically applied the bonded fibre matrix with, primarily a hydroseed mix that incorporated hard red fescue (Festuca longifolia), creeping red fescue (Festuca rubra) and annual ryegrass (Lolium multiflorum), seeded at a rate of 11kg per 830m2.

The rye grass gave quick germination so that the site was protected allowing the other seeds time to germinate.

Annual ryegrass such as the type used here is considered ecofriendly. It's non-aggressive, provides vigorous growth, and provides soil stability with its root system. It typically germinates in 5-14 days. It offers some nematode control and will die off after a year cycle in most climates.

In hydroseed mixes used elsewhere along the US 202 corridor, perennial ryegrass (Lolium multiflorum) was used. This type takes longer to germinated but is a more permanent choice because it handles the cold and hot extremes better than the annual. The nature of ryegrasses allows the seeds to germinate with less soil contact, which makes them a good selection for roadways and conservation areas, due to the limited ability to correctly till the soil, however the hydroseed mix usually include one or more fescue grasses, for which soil contact and good seedbed preparation is critical. In order to achieve this a good matting is used which helps hold the soil and achieve better seed contact through germination and plant stand.

To ensure protection of the basin walls, those with a 3:1 slope, required crews to install single net straw blankets. On the side of slopes along the highway with areas of 2:1 slopes,  double-net straw blankets were used. Other matting used included a TRM and jute matting for erosion control.

A product called flexterra was used above the roadside retaining walls due to access restrictions. Flexterra is a high-performance flexible growth medium that bonds immediately to soil, even on severe slopes. Because it gives increased sheet flow resistance, turbidity is reduced, and soil loss minimised, thereby giving seed a good start.

 

With the removal of existing earth and the addition of non pervious road surfaces, more plants were needed to control the additional water runoff on the project. In response to this the storm water management areas were then planted with immature trees from local riparian zones, to help with post-construction storm water and erosion control.

 

Across Australia there are currently a large number of highway projects being completed. Huge projects like the Pacific Highway Upgrade and Adelaide's North-South Corridor usually span several years and several seasons.

 

Storm water and erosion control is an important part of the planning process, and a variety of soil erosion control devices play a role. Challenging topography and limited highway easements sometimes result in steep slopes which present unique challenges for storm water, erosion control and mitigation. At times, overlooked on a construction project is the need to restore the area once construction activities are completed. Temporary methods to trap sediment must be removed, and vegetation must be established to reduce erosion. The best method for controlling sediment is to keep a good vegetative cover on the soil. When sufficient plant cover exists, raindrop or runoff waters cannot dislodge soil particles to cause erosion. Less erosion means less sediment in runoff waters. Therefore, good erosion control results in good sediment control.

 

I would like to encourage ErSed professionals to look outside our own backyard and learn from management practices and innovative approaches across the globe, in order to better manage our precious soil resource.

 

Reference

-Fifield, J.S (2011) Designing and Reviewing Effective Sediment and Erosion Control Plans, 3rd Ed, Santa Barbara: ForesterPress 

-McTainsh, G.H et at, (2011) Wind erosion and land management in Australia during 1940-1949 and 2000-2009, Canberra: Commonwealth of Australia 

-Talman, C.F (1930) 'Saving Runaway Farm Lands', Popular Science Monthly, November p.39. 

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