Stormwater, the early years (7/10/2023)

In a previous post I mentioned that I once had the ambition to write a book on stormwater and erosion and sediment control (and I may still do it). This idea was a natural outflow of the job I did pre-retirement. The following post is part of an introductory section that I wrote for the book. Ince I lifted from a chapter, I have changed it a little to make it a narrative for you all to follow and understand. I left the literature citations out, feel free to contact me, if you are interested in them. So here I go:

Very early, North American settlement history may have followed similar patterns as were encountered in Europe of the Middle Ages. The settling population in North America initially concentrated in towns and villages for mutual protection from outlaws and native Indian population. This is evident by some of the older larger urban centers on the east coast. In addition, large tracts of land were deforested very early on in settlement history for the use of timber (building materials) and the growth of cash crops, in particular tobacco. A 17th century document in the historic town of Williamsburg describes that climbers of the church spire in town (or what is now Colonial Williamsburg) were able to see the York River to the north and the James River to the south, a feat that is impossible nowadays because of the regrowth of the forest in the area. The peninsula appeared to have completely deforested, mostly for the growing of tobacco at that time.

The cartographer Lewis Evans observed the same thing when he reported in 1750 about the deforestation and conversion to agriculture land that had already greatly altered the hydrology and clarity of streams and creeks in the Piedmont of Northern Virginia:

"When the country was cover'd with Woods, and the Swamps with brush, the rain that fell was detained by these interruptions, and so had time to insinuate into the Earth, and contribute to the springs and runs. But now the country is clear'd, the rain as fast as it falls is hurried into the rivers, and washes away the earth and soil of our Naked Fields."

 

The reports by Evans were only the beginning. Human caused accelerated erosion increased steadily as is shown in a recent study by Kemp et al (2020) who conclude that European settlement of North America during the past century has resulted in the movement in as much sediment that natural processes can move in 700 to 3000 years. They estimated that geologic sediment production which had been relatively stable for the past 40,000 years increased 10-fold as a result of European settlement and the rapid expansion of agriculture and river modification seen throughout the continent.

So, what are currently the main sources of sediment the waterways in more developed nations? It was reported that while agriculture practices generate considerable amounts of sediment by way of erosion; we now estimate that areas cleared for construction are able to generate the equivalent of many decades of sediment in one year compared to agricultural uses (let me know if you are interested in the source). Research determined that construction can yield 140,000 tons of sediment per square mile per year, compared to 1000 tons from agriculture land and up to 500 tons from forested land in the Piedmont area of Maryland. In other words, construction is a major contributor producing 140 times more sediment than agriculture land per unit area (for example per acre or hectare). However, , there is so much more land being used for agriculture than actively being disturbed by construction activities.

On an interesting note, while sediment production increased at least 10-fold over historical times in North America during the last century, it was also observed that during the same period it seemed that less sediment reached the ocean. This was caused by the retention of the sediment by reservoirs that were constructed by human. Reservoirs served for the storage of drinking water and water for energy generation and irrigation. It has been estimated that a total of 1.4 ± 0.3 metric tons of sediment per year is retained by reservoirs world-wide. Siltation of these reservoirs will eventually greatly reduce the capacity, function, and usefulness of these structures. It appears that Indonesian rivers were a rare exception and that they deliver more sediment to the coastal areas than other rivers that they surveyed.

Here I’ll step away from my book chapter. I saw similar issues in Nepal where the land or better fertile soil was being carried away by runoff. The main reason was deforestation for agriculture. We often joked that a new island was being formed off the coast of Bangladesh. This island was formed from the sediment being carried by the rivers coming out of Nepal. The results in Nepal were the drying out of springs and dropping groundwater table, a phenomenon seen throughout areas that are being deforested.

I’ll end here, there are many posts where I discuss some of these items as well. Just look in the keywords for the words like stormwater, erosion, sediment, erosion and sediment control, runoff reduction and alike. If you like or enjoy what I wrote here, let me know and I’ll pull some additional material out of my writing.

Our daughter was in New Delhi, India these past few weeks and sent me this newspaper shot.  They had 6" (126 mm) of rain in the afternoon and it flooded the town.  Can you imagine all the impervious area in such a large city, it created havoc.