Hollywood Branch: A neighborhood stream restored

Hollywood Branch
Located on the east side of Colesville, Hollywood Branch is a tributary in the greater Paint Branch Watershed. From 2008 to 2015, more than 4,400 feet of stream were restored by the Montgomery County Department of Environmental Protection (DEP). The cooperative restoration process included input from landowners, Montgomery County Parks (M-NCPPC), and other stakeholders.  

What caused Hollywood Branch’s poor water quality?

Much of the development in the Hollywood Branch watershed occurred before today’s stormwater management regulations were in place. In older neighborhoods, stormwater runs off roofs, driveways, and roads into storm drains and directly into streams, sometimes carrying trash, oils or pollutants. The runoff also moves rapidly over paved surfaces, causing streams to have higher flow during storms. These high flows can alter the natural stability of the stream, causing erosion and instability, poor water quality, and damage to valuable habitat for fish and other organisms.
Erosion on Hollywood Branch

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.

  Hollywood Branch Stream Restoration was identified as a priority project in the 2006 Lower Paint Branch Watershed Study. Watershed Planners are assigned to restoration projects like Hollywood Branch to ensure that resident input is incorporated during the design and construction process.
Pre-Restoration photo

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 bridge

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 Stream

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 of stream

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 of the Hollywood Branch

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.

 

Restoration: A Natural Approach

DEP used a natural systems approach to the restoration at Hollywood Branch with the goal of using the stream’s own natural hydrological processes to establish a stable and self-maintaining channel. The project stabilized bank erosion, improved the stream’s ability to access its floodplain during storms, and enhanced habitat for aquatic organisms. The design emphasized the use of natural materials, including logs, boulders, and live plantings. Hollywood Branch runs through private backyards and Montgomery Parks property. DEP Watershed Planners ensured landowner input was incorporated throughout the design process and easements were obtained for private property prior to construction.  
During construction of the stream

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 of the Hollywood Branch

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 image of the Hollywood Branch

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.

 
Map from public meeting

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.

 
Mid-construction image of the Hollywood Branch

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.

 
Hollywood Branch community walk

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 along Hollywood Branch

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.

 

Hollywood Branch: Before and After Restoration

Pre-restoration – September 2011

Pre-restoration – September 2011

Post-restoration picture in October 2016

Post restoration – October 2016
The stream channel has been widened, boulder habitat installed, and banks graded to allow the stream to access its floodplain.


Pre Restoration – August 2011

Pre Restoration – August 2011

Post restoration – March 2018

Post restoration – March 2018

The curve of the meander here has been made less severe. Banks have been graded to reconnect the stream to its floodplain and establish a “floodplain bench.” The floodplain bench is the lower area between the immediate stream bank and the higher banks on the far sides of the photo above. When the stream overtops its banks during a storm, water spreads out over the floodplain bench, slowing it down and minimizing erosion. The high banks keep high flows from entering yards.
Pre-restoration – August 2011

Pre-restoration – August 2011

Post restoration – December 2014

Post restoration – December 2014
The eroding meander was graded back to allow the stream to access its floodplain. Boulders were added to provide habitat and bank protection. Native plants were added to further stabilize the banks.


Pre-restoration – November 2006

Pre-restoration – November 2006

Post Restoration – December 2014

Post Restoration – December 2014

This side channel to Hollywood Branch was rerouted away from the bridge approach. The bank was graded and vegetation added for stability. The twigs sticking up along the bank are “live stakes.” Planted during the dormant season, these cuttings begin to grow and take root in spring. Black Willow and Silky Dogwood were two species planted at Hollywood Branch.
Pre-restoration – April 2008

Pre-restoration – April 2008

Post Restoration – March 2018

Post Restoration – March 2018
The severe meander here has been lessened at the upstream approach to the walking path bridge. J-hook in stream structures provide stability and habitat


Pre-restoration – April 2008

Pre-restoration – April 2008

Post Restoration – March 2018

Post Restoration – March 2018
Pool habitat looking downstream from the walking path bridge. Boulder protection and native plantings provide stability to the previously eroded outside bend of this meander.


Pre-restoration – April 2008

Pre-restoration – April 2008

Post Restoration – March 2018

Post Restoration – March 2018
The pre-restoration stream rerouted itself around these fallen trees causing bank erosion and instability. The original channel shape was restored. In stream j-hook structures provide channel stability and habitat. Vegetation provided added bank protection.

2 comments

I love this article! Thank you so much for sharing this level of detail on the design elements of the restoration.

This kind of project always makes me wonder whether intervening with the natural geologic and hydrologic processes is a good idea. I suppose it’s necessary when we want to preserve the stream habitat while also maintaining the integrity of the built environment (eg, neighborhoods) surrounding it? Is there any habitat lost due to this restoration? I know new vegetation have gained habitat by grading the floodplain and encouraging their root systems, and some older trees will be saved from the erosion processes. I’d love to hear more on Montgomery Parks’ perspectives on non-intervention vs. restoration. Was the intervention done primarily to protect the properties of homeowners whose yards back up to the stream?

The results are beautiful. Job well done! Thank you for sharing in such depth!

Many thanks for posting this along with the explanations of why the various structural changes were made. I understand, however, that currently 25 already designed projects to reduce stormwater runoff have been canceled. Given that it is the force of that storm water runoff from our many hard surfaces that causes the super erosion this project aimed to address, and given there are many other streams in distress like this one and getting worse all the time, does it make sense to cancel projects intended to reduce run off?

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