No Where Else for the Rain to Go
While the primary driver of the Great Chapel Hill Flood of 2013 was a record rainfall of over five inches in 24 hours, there was another important factor at work, impervious surfaces. When rain falls, some is absorbed into the ground, some evaporates, and some runs off. The more impervious (or less permeable, if you like) a surface is, the higher the percentage of the rain that runs off. When the rate of run off exceeds the carrying capacity of the trenches, drains, and ditches which make up our storm water systems, we end up with flooded streets, parking lots and buildings like we saw last week.
Both the heavy rains of last week and Tropical Storm Andrea earlier in the month prompted numerous observations that flooding seems worse now than in the past. This is not a trick of memory but the direct result of how we have altered the landscape in our beloved town.
The amount of run off from an area depends strongly on the characteristics of the surface. A mature forest with branches and leaves to slow down and break up rain drops that has a thick layer of leaf mulch below will allow only 10% of the rain to run off. Run off from a parking lot is in the range of 90%, with the other 10% of the rain water settling in low lying areas and eventually evaporating. While we tend not to think of it as impervious surface, housing developments and other areas with graded landscapes are more like parking lots than forests and result in run offs in the range of 50 to 60 percent. The blue bars in Figure 1 below show the percentage of run off for different land uses.
I live in the Parkside development off of MLK. As my home was being constructed all vegetation and top soil were removed, leaving a hard-packed clay surface which was subsequently covered with a thin layer of sod. The sod clung to life for a little while before dying. While I have since managed to coax a reasonable covering of grass to exist on the lawn, shortly after any reasonably strong rain shower starts you can see the water running off my yard, on to the street, and into the storm drains. Each time I see this I experience pangs of guilt at having paid someone to create this problem.
The reason people are observing greater flooding now than in years past is that we are allocating more and more area to less permeable land uses. I do not have the precise data to quantify this for you, but let me walk you through an estimation exercise which helps to illustrate the point.
The purple bars in Figure 1 represent my estimated area percentages for current land use in Chapel Hill. I estimated 55% housing development and similar landscapes, 20% paved areas, and split the remaining 25% evenly between meadow and forest. (If anyone has the actual data please email it to firstname.lastname@example.org.) With these land use estimates and the percent run off data from Figure 1, I calculate that 55% of the rain the falls on Chapel Hill will run off into the storm drains. I found some data for heavily urbanized areas showing 70% run off, so my estimate of 55% for modestly urbanized Chapel Hill seems reasonable to me.
Next I looked at the population growth in Chapel Hill from 1970 to 2010. (Shown below in Figure 2.)
In order to estimate land use in the past, I made the assumption that the percentage of land devoted to housing and pavement was proportional to population. For example, my estimate that 55% of the land in Chapel Hill is devoted to housing developments and similar landscapes in 2010 scales back to 33% in 1970 by using the population data.
While some of the estimates I made are likely a bit off, the general trends should hold. Taking all of the data together, I can now generate Figure 3, which shows the percentage of rain run off into Chapel Hill as a function of time.
My model predicts that the percentage of run off from rain in Chapel Hill has almost doubled since 1970, from 33% to 55%. The engineers who did the calculations to install the storm water system back in the 1970s likely included some excess capacity in the design, to accommodate future urban growth, but certainly not enough to allow for a doubling of the run off. The cost of retrofitting the system to the extent necessary to accommodate all of this additional water would no doubt be prohibitive.
So my recommendation would be to steer clear of the intersection of Franklin and Mallette streets on stormy days.
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