Swale Stories

as told by Milt Rhodes, NC Division of Water Quality

This road/swale has slopes that are too steep.  Erosion is likely. Nice use of vegetation in the low spot of the swale.

On this narrow rural road, the swales are wide and shallow.  Runoff enters swale in a diffused manner as it flows over the grass removing nutrients and pollutants deposited by vehicles and atmospheric deposition.  Shallow and wide sloping swales are also easier to maintain, IMO.

In this newer subdivision, the swales alongside the road are very shallow, perhaps to shallow.  Water movement may be a problem hear.  Swales should have at least a 3:1 slope, 5:1 is recommended.  The rock edging material around the planting area prevents water from entering it.  Additionally the planting area is uphill from the depression negating any benefit for filtration from runoff from the road.  These plants must also be artificially irrigated.

This narrow road has a small swale on the left that runs along the side of the road.  The heavy canopy helps reduce rainfall from hitting the road surface. 

This narrow country road also has two small depressions along the sides.   This road has been upgraded a few times, but the canopy has been kept in tact.   Water flows in a diffuse manner along each side of the road.  The road is built up high enough to prevent ponding except in extreme events.  People shouldn't drive then anyway.

This grassed swale has a slope adjacent to the road reflecting the right ratio, but the slope adjacent to the walkway is steep.  Runoff may eventually cut into the bank and cause erosion and a maintenance headache, however, it isn't that bad.

A nice swale in a suburban location.  Density around 3 du/acre.  Trees planted closer to the road which will eventually form a canopy.  Note the drain pipe filling in with grass in the center of the picture.  The pipes mouth should probably be lined with rip-rap to prevent filling, clogging and future flooding problems in extreme events.

A grassed swale in a florida parking lot.  It is a bit steep, but not too bad.  If the depression was a bit more shallow and centered, then the green space could provide an additional community functional area.

A shallow swale in a subdivision at 4 du/acre neighborhood, complete with bioretention/pocket wetland/ planted area of day lilies.  However, because grass is higher than sidewalk, the edge might act as channeling device in storm events.

Very nice, but could more have been done?  If selected curb cuts were introduced along the center median, some water retention and treatment could occur.  Maybe save a little money on infrastructure costs too.

Queens road in Charlotte, a John Nolen layout.  The curb really defines the median, but is it necessary?

Here it is without the curb.  Could bioretention devices be incorporated into the median without destroying Nolens intention.  I think so.   It would complicate the layout, but with careful consideration I think it could occur.  Luckily this part of Myers Park doesn't fare so poorly in storm events, but with greenfields, I think these center medians can incorporate treatment, collection, and filtration devices without destroying integrity of the design.

Vermillion, NC.   A really nice TND in the Charlotte area and DPZ project.  Curious if the curb on the far side of the road was necessary at this point?  If not, water from homes, road and sidewalk could be more diverted into tree'd area on right side of photo in more diffuse way.

BIORETENTION CELLS, POCKET WETLANDS, and RAIN GARDENS

This picture shows a bioretention cell in a new TND development in the Triangle area.  Carpenter Village developers decided to allow NCSU to conduct a demonstration project on an 18 acre drainage area.  Curbs were removed from center median and roads were regraded to direct runoff into bioretention cell in center median.   Vegetation will be added during the year.  PVC pipes are monitoring wells.   The pooled water at the base of the bioretention cell is muddy indicating the sediment in the water entering the device.  Sediment is the number one water pollutant in NC.  Uncontrolled sediment can clog the best designed BMP very quickly.  

A bioretention cell built into a parking area.  The bioretention cell has curb cuts periodically spaced to direct surface water runoff into the device.   Note the collection of larger grained particulates (coarse sand) in the curb.   The curb, in this case helps keep coarse material from entering the center of the cell which increases the functional life of the system and reduces the need for periodic clean out.

A picture of a bioretention garden in a Portland Oregon apartment building.  I don't know anything about how this one works, but it looks nice.   Pocket wetlands should be incorporated into all courtyard buildings!

The end.