Hillside Greenhill Stream Restoration Update: Part Two

This is the second in a two part series. ran on Dec. 27.

All of the stream and stream banks in the first section of the Hillside Greenhill stream project have been covered in four inches of Leafgro compost, except for the two plunge pools at either end.

That section, the project’s phase 1, runs from the Hillside underpass to the first pedestrian bridge, along a pedestrian path. McDonnell Landscaping, Inc. Foreman Robert Thayer said that section of the stream would be planted with grass, willow trees and other vegetation suited to stream areas, which will further hide the stream in this section of the project.

More than three dozen trees and shrubs, including tulip poplar trees, have already been planted on the stream banks and near the new storm drain on Hillside Road.

The stream will only be visible with 100-year floods, Thayer said.

In January, McDonnell Landscaping plans to have one crew work on building the new bridge, while another fashions the second set of stairs at the project’s endpoint where the stream drops down to a concrete flume, at the bridge to Fayette Place. The bridge is likely to be the last piece built, according to Thayer.

The entire half million dollar project, from Hillside Road to Fayette Place is designed to slow down the stream and filter out soil and attached nutrients and other pollutants before they reach Greenbelt Lake, Thayer stated.

Originally from Brunswick, Md., Thayer has been doing this kind of work for 24 years. He said the “coastal plain outfall” design has the intermittent stream pour from a large diameter concrete pipe under Hillside Road into a large depression filled with large rocks, called a plunge pool.

From the pool, McDonnell Landscaping plans for the stream to flow invisibly through, rather than over, a thick wall of rocks to flow through a smaller depression lined with smaller rocks and then through another thick wall of rocks to flow through a second depression.

The stream flows through five such depressions or pools, designed to collect rainwater — each separated by a thick wall of rocks—before flowing into the second plunge pool, where a smaller diameter drainage pipe pours water from a tributary stream, just before the first pedestrian bridge.

Since the stream will flow freely under the new bridge after flowing out of that plunge pool, it will be more visible than ever there, according to Thayer.

During the heavy rains of December 7 upon observation, the main 36-inch diameter pipe water was pouring fairly gently into the first plunge pool and the water disappeared as it joined the stream flowing through the rocks under the Leafgro.

The only places water was visible late that evening were in the pools designed to collect water from just such a rain. And the only thing resembling a stream was a narrow wisp of water flowing out of the filled ponds.

The smaller diameter pipe that pours into the last plunge pool had much more force than the larger pipe at the first plunge pool. But the rocks immediately under the smaller pipe broke its force. And the weakened flow was further dissipated as it spread throughout the large rock-lined plunge pool near the bridge.

The heavy rains washed away much of the Leafgro that hid the stream bed rocks. But Thayer said that was expected, it's part of the natural process, he said, with the Leafgro carried to the rain collection pools.

The Greenhill/Hillside Road stream renovation project is expected to be completed by the end of January 2012, except for the possibility of more plantings in March, according to Thayer.