Our last field trip for Land Use Planning was a squlchy walk through the headwaters of Piney Branch in southwestern Montgomery County, as we looked at stormwater management structures there. Piney Branch is within one of several Special Protection Areas in the County. About ten years ago, Human Genome Sciences built a campus on land near Travillah and Darnestown Roads under conditions meant to ensure best practices for stormwater quality and quantity control.
Current thinking encourages more, smaller retention chambers, like this series of three. In the image at left, you’re looking at the last chamber, where (behind you) the outfall structures drain into the stream. The two upstream chambers are the depressions you see in the middle ground, this side of the road and lie of bare trees. In the image at right, you’re looking in the opposite direction, at the first of the chambers. The dark gunk is sediment and petroleum washed from the various impervious surfaces of the campus and settled into the sand at the bottom of the chamber. The white PVC tubes at upper right at test wells for checking groundwater levels.
A little farther along Shady Grove Road Extended is this chamber. The primary outfall is partially obscured by the dead Typha stalks, and it carries water to the stream in a small pipe (about 10 cm diameter). In the event of a major rain event, the large outfall structure at center left carries water away in a big pipe (30 cm or more). Most of these large outfall structures are notched so that a medium-sized inundation can be slowed down by the chamber. Also notice the retaining wall at right, which is holding up the graded fill so that offices and parking could be built on level ground, out of frame at right. The retaining wall is already showing some cracks and streaks.
Old-fashioned stormwater practices depended on in-stream dams that formed artificial ponds, like this one in a different development, part of the Universities at Shady Grove. At any rate, the three Ring-necked Ducks (Aythya collaris) that we saw were enjoying the water.
Despite management efforts, Piney Branch is not in the prime of health. Scouring of the banks is apparent in the image.