Can we make our land development projects (hydrologically) invisible to downstream properties? Think of Claude Rains, in the 1933 film adaptation of the H. G. Wells novel, The Invisible Man.
“If I work in the rain, the water can be
seen on my head and shoulders.
In a fog, you can see me – like a bubble.
In smoky cities, the soot settles on
me until you can see a dark outline.”
— The Invisible Man, 1933
I believe we can, using a natural drainage approach. To illustrate this we must think through some basic — no actual math required! — hydrology.
Imagine a rectangular area of undeveloped land that has a gradual slope from one corner to another. Imagine that all rain falling on this land drains to the low corner and rain falling outside of this area drains to some other location. Like this:
Imagine that before we develop the site – the “predevelopment” condition – we install a flow measuring device to the low point. This allows us to record the stormwater runoff flow rate leaving the predevelopment site.
If we did this, one hour before a 10 minute rain event, for example, the runoff flow rate would be 0 gallons per minute (gpm). As the first drops of the 10 minute rain hit the ground, the flow would be 0 gpm. Minutes and hours later, the flow would reach its peak and then start to decline back down to 0, like this:
The graph above displays the predevelopment hydrograph. All plots of stormwater runoff flow rate vs. time are known as hydrographs. If we know the history of the flow rate vs. time, we can easily determine the total volume of runoff that left the site as a result of our 10-minute rain. The total runoff volume, if you recall your calculus, is the area under the curve, like this:
This makes sense because if we multiply the dimensions of the x-axis expressed in minutes by the flow rate expressed in gallons per minute we get gallons because the minutes cancel out:
Minutes x Gallons / Minute = Gallons
If we add some kind of development (buildings, roofs, roads, etc.) to the site, thereby increasing the site impervious, the runoff hydrograph changes. The added smooth hard surfaces and concrete storm sewers:
- Reduce resistance to flow;
- Eliminate nooks and crannies for surface storage;
- Accelerate the timing of the runoff;
- Reduce or eliminate water infiltration; and,
- Reduce or eliminate water transpiration (consumption and release to the atmosphere by plants).
These changes to the drainage area change the hydrograph that would be produced if the exact same 10 minute rain event fell on the post-development site. The post-development hydrograph might look something like this:
Note the following characteristics:
- Higher peak flow;
- Earlier peak flow;
- Faster decline back to zero flow; and,
- Larger total volume of runoff.
Well that can’t be good, right?
If we developed this way the bayou receiving this runoff water would see a higher flow rate. This would result in a higher water level, which might cause downstream flooding if that higher water level was higher than the top of the bayou banks.
We mitigate the effect by sizing and constructing detention basins downstream of all new development. The detention volume is generally equal to the “excess volume” produced by the development. The “excess volume” is determined by calculating the difference between the pre- and post-development runoff volumes, like this:
The light blue is the difference between the two ares (volumes). We can place that volume of stormwater runoff anywhere we’d like on the site. Engineers love making them into the shape of nice, regular, rectangles, like this:
Landscape architects have encouraged us to make them into more natural shapes. Regardless of their shape, they are designed to hold the excess volume and to release that water at a rate that does not exceed the predevelopment peak flow rate, like this:
This prevents downstream flooding, but its not perfect. Can you see a few of the problems with this approach?
The main problem is the volume of runoff is not reduced. With detention, the site discharges at the predevelopment peak flow rate for a longer period of time. (Compare the horizontal distance of the blue line to the brown line.) Compare the brown area (predevelopment runoff volume) to the blue area (post-development runoff volume), below:
So how can we deal with this extra volume?
Natural drainage systems (also known as “low impact development”) can help address this. Natural drainage systems are installed to slow the water down, infiltrate water, evaporate water, store water in small or micro scale detention areas, transpirate water, and generally to mimic the predevelopment hydrology. The same rain event falling on the site – mitigated with a natural drainage approach – might produce a hydrograph that looks more like the green hydrograph below:
So how does the runoff volume comparison look using natural drainage?
Pretty good, huh?
The natural drainage approach seeks match the predevelopment hydrology. This means that the downstream folks experience no difference in the timing, rate, or volume of runoff (for a given rainfall event).
Some natural drainage proponents, like me, like to say the development is hydrologically invisible.