stream

Erosion Complaint: DEP Watershed Planners are routinely called out on erosion complaints such as this one on Hollywood Branch in 2005. The County considers residents’ concerns during both project selection and design.

Pre-Restoration: Downstream scour, or erosion, on M-NCPPC walking path bridge. Following increased development in the watershed, the opening under the original bridge was no longer large enough to carry stream flow during heavy rain storms. Water would overtop the banks and flow around the bridge, causing erosion and damage to the asphalt pathway and threatening the bridge structure.

Post-Restoration: A new bridge was installed by Montgomery Parks in coordination with the restoration project. The new, longer span allows Hollywood Branch to stay within its banks, even when running high.

Pre-Restoration: Water moves fastest around the outside bend of a meander (at left, above). The faster the water, the more erosion. An unstable stream, like pre-restoration Hollywood Branch, will continue to erode into and under the root zone on the outside bends of meanders, exposing and undermining tree roots and eventually causing otherwise healthy trees to fall into the stream. The erosive condition seen here on the outside of the meander is known as a “cut bank.”

Pre-Restoration: Water moves slowest on the inside bend of a meander, which allows sediment to settle out and change the shape of the stream channel. This can be seen in the sandy area at center, above. The lack of vegetation here indicates that this was a relatively recent change to the stream. This feature is known as a “point bar.” Increases over a short period of time in both point bar deposition and cut bank erosion can indicate an unstable stream.

Pre-Restoration: The stream undermined the root zone of this tree, causing it to fall into the bed of the stream and damage a homeowner’s existing fence.

During Construction: An in-stream “J-hook” structure. J-hooks provide valuable pool habitat while helping to direct stream flow towards the center of the channel, causing less erosion to stream banks. The hose at the top of the photo carries stream flow around the work area during construction, minimizing the erosion of bare sediment in the work area.

Post-Restoration: J-hook in stream structures help create riffle and pool habitats for fish and other creatures. A mixture of flow and depth provide a variety of habitat. Pools provide a calm refuge for fish and mollusks. The rocky bottom of the riffle habitat supports a variety of macroinvertebrates (bugs) that provide food for fish and larger organisms. The agitation of the water over the rocks provides the stream much needed dissolved oxygen to support life.

Post Restoration: Pool habitat with “Rock Toe Protection” at right. Rock toe protection helps protect the streambank from the erosive forces found on the outside bends of meanders.

Public Meeting: DEP holds public meetings to present proposed engineering designs to the public for input during various stages in the design process. This poster depicts the Hollywood Branch restoration design near the M-NCPPC walking path bridge crossing. The J-hook structures mentioned above can be seen on the design.

During Construction: Banks are graded to reconnect the stream to its floodplain, helping to dissipate erosive energy and create channel stability. Erosion control matting is installed to cover the bare soil until vegetation can take root. Note the hose running through the center of the stream valley pumping stream flow around the active construction site.

Stream Walk: DEP hosts stream walks to discuss the proposed designs in the field. The walks allow residents to better visualize the design. Stream walks were also held during the construction phase of the Hollywood Branch project.

Tree plantings: After construction is complete, landscapers plant native trees and shrubs along the stream banks and valley. The deep roots of these native plants help hold together the soil along the stream banks, preventing erosion and sediment transfer downstream.