Runoff and Land Use Change

Burlington, VT has changed from being completely forested to an urban area. This change has increased the percentage of impermeable spaces throughout the city, leading to more runoff into the Winooski River and Lake Champlain and less infiltration into the soil.

To calculate the total runoff for these areas, I first had to calculate the average weighted curve numbers. This was done by multiplying the curve numbers for each type of surface by the percentage of the properties that was covered by each land type (seen above). These numbers were multiplied by the amount of rain for each type of storm in order to determine the amount of rain that will run off (seen below). The total runoff volume was calculated by multiplying the amount of rain that was generated as runoff by the area. All of the calculations were done using inches. The volume of runoff generated by largest storm is equal to nearly 275 cubic meters, which is a lot of rain from a small area.

Over the past 19 years, the land uses for these properties have not changed significantly, but they have still changed. The amount of green space decreased from 60% to 52%, while the amount of impervious or nearly impervious space increased from 40% to 48%. Overall, these are fairly small changes for an area and this does not produce significantly more runoff. However, if you assume that the entire city of Burlington, or at least multiple neighborhoods, changed as much as this one area did, there would be a much greater amount of runoff generated.

Compared to the pre-European settlement numbers, the runoff between the 1600s and 1999/2018 has nearly doubled in many cases, or at least increased significantly. Forests have very low curve numbers, arguably lower than 30 in many cases. In an entirely forested area, tress will take up a great deal of precipitation through their roots. Evapotranspiration also helps pull this water into the roots and release some of it back into the atmosphere. Assuming Burlington had mostly sandy, well draining soils, the water would drain relatively well. Today, there are far fewer trees in Burlington, which have many negative impacts including less stable soils. Deforestation, especially on a slope, will increase landslides, erosion, and sediment fluxes. The water table will rise because there are fewer trees to intake water from it. When this is combined with an increase of impervious surfaces, water is often forced into channels or on to the sides of roads, which increases the risk of a significant geomorphic event occurring.

Let's not forget that Burlington is situated on a hill facing Lake Champlain. Water moves downslope, so it would make sense that most of the water that falls in Burlington eventually reaches the lake. With an increase in impervious surfaces, water can easily run down the roads, pretty much straight into the lake, unless it goes into a drain which will also reach the lake. The water will pick up any dirt, debris, oils, salts, trash, gasoline, or heavy metals. The addition of any of these substances into the lake negatively impacts the water quality, which in turn impacts the entire lake ecosystem and overall health. While the lake is quite large, continuously adding runoff especially in one area will increase the concentrations of harmful substances. Neighborhoods near UVM have been adding more and more impervious surfaces, which increases the amount of water that will enter the lake as runoff opposed to infiltrating into the soil. More students create the need for more impervious surfaces, which produces more runoff that has the ability to negatively impact the Lake Champlain ecosystem.