Thick or Thin, Buffers (Tainted Waters ?)

By Don Flora  PhD

Thick or Thin, Buffers May Not Stop Tainted Waters

Last month narrow buffers were mentioned that capture very well the effluents of critters including people.   They are on gently sloping ground with porous soils, covered by grass. 

But what about places in the Puget Sound country where those conditions aren’t present?  This is important because of concerns about pollutants that may be carried by overland stormwater and because vigorous buffering precludes beneficial land uses. 

First, how steep the slope.  If your side yard slopes so that the front of your house is three feet below the back that’s a 10 percent slope.[1] During significant storms surface rainwater flows past at about 25 feet per minute.[2]  If the buffer out front has that same slope, stormwater travels four times that fast there, assuming a shrubby environment that divides the incoming water into rills .[3] And that’s on a gentle slope.  Yep, the Department of Ecology acknowledges that buffers can do less for water quality than your yard, on account of stormwater’s speedy buffer transit.

Does speed matter?  Yes, because quadrupling the velocity of water increases its erosive power 16-fold, and the amount of material that can be transported is increased 1,024 times![4]  And, yes, if it is important to hold the water long enough to soak in. 

Now, what if it can’t soak in? Puget Sound’s glacial history has left us an abundance of hardpan (till) soils, close to the surface, that resist infiltration.  It is estimated that hardpan accepts rainfall at only about 0.03 rainfall inches per hour.[5] During significant storms rainwater accumulates above the hardpan until, soon, it can seep no more. The flood of surface water goes swiftly to the Sound.  In general it reaches aquifers only sparingly.  That may be good if the water is tainted.  But the bad water goes into good waterways the buffers are supposed to protect.  Buffering does not solve the hardpan problem. 

Doesn’t buffer vegetation appreciate and ingest the arriving water?  Yes, in parts of the country where rains are significant during the growing season.   Our storms come in a time of dormancy; our heaviest rains are in months when most vegetation has shut down its intake manifolds.  Its valves are closed to stormwater and whatever nutrients and pollutants are riding along.  Vegetated buffers (are there any unvegetated buffers?), with their conduits to the sky, don’t work when we want them.  In every summer week each buffer tree ships as much as a thousand gallons away, and we can’t stop that despite its drain on moisture reservoirs.  In this respect buffers not only don’t work, they may do harm, proportional to their sizes. 

Last month I pointed to some narrow buffers that have corralled impressive volumes of bodily discharges.  Those narrow buffers are working. 

This month it’s steeper slopes, impermeable soils, vegetation and seasonality problems that raise large and serious questions for the Puget Sound country.  Above our hardpan soils in particular we should consider that buffers may not work.  Perhaps they have never worked but we chose not to notice.  Or perhaps the really bad pollutants are not generated or are not reaching the buffers along our residential shorelines.

[1] Assuming the house is 30 feet ‘thick’.

[2] From equations in the state Department of Ecology’s 1991 Stormwater Management Manual for western Washington, Volume III, Chapter 2.

[3] Based on sheet flow (an even covering of wetness) in the yard, with concentrated flow in the shrub/tree  zone, in line with DOE’s assumptions.

[4] These effects vary with the second and fifth powers of velocity respectively.  Forbes, Reginald D. 1955.  Forestry Handbook, Society of American Forester and Ronald Press.

[5] Per the state’s stormwater manual.


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