An unusual W-weir design reduces erosion and enhances stream ecology.

By Dan Salas, Richard C. Albert, and David M. Williams

Darby Borough is a historic community that has experienced multiple flood events during its long and colorful past. In recent years, this area has suffered a chronic problem of streambank erosion that presented environmental, health, safety, and infrastructure risk for the community. In response, the Delaware Riverkeeper Network, Delaware County Conservation District, Darby Creek Valley Association, and Darby Borough joined forces to craft and implement a restoration project that has addressed these concerns, while at the same time providing a new and natural asset to the community.

Solutions of the Past Cause Problems in the Present
Darby Borough lies 7 miles west of Philadelphia in Delaware County, PA. The town is one of the oldest in Pennsylvania, having been founded in 1682. Almost immediately after its birth, Darby became the site of numerous mills. By the 19th century, Darby was a prominent industrial town surrounded by villages and agriculture. In typical fashion, industry and industrial development occurred on the floodplain of Darby Creek. Property damage and loss of life from flooding was experienced in Darby very early in its history. In 1843, a flood in Darby resulted in the construction of a block of houses outside the floodplain for mill workers.

In the early 20th century, however, trolley lines built through the area connected the borough to Philadelphia; Wilmington, DE; and elsewhere. This transportation network stimulated major suburban development in the Darby Creek watershed. This process continues today, albeit due to highways.

Darby Borough lies in the downstream reach of Darby Creek. Downstream of Darby Borough the creek becomes tidal just before flowing into the Delaware River at the John Heinz National Wildlife Refuge. The refuge preserves the largest freshwater tidal wetland in Pennsylvania. Darby Creek was navigable up to Darby in the 19th century, but urbanization in the watershed has reduced the average stream depths dramatically.

Darby Creek drains a 77-square-mile watershed that includes portions of four counties and 31 municipalities. Almost 500,000 people live in the watershed with an average density of nearly 10 people per acre (20 per square mile in Darby Borough). Populations in the watershed are declining for a variety of demographic and economic reasons. Darby, for example, had a population of 11,140 in 1990 and an estimated population in 2005 of 9,300. More noticeable is the economic decline associated with the population reductions.

In spite of its history as an industrial town, Darby Borough is known for being the home of John and William Bartram, two of America's early and most prominent botanists. In the nearly 300 years since John Bartram's birth in Darby, PA, the area has shifted from agricultural to industrial to urban residential use. As a result, the Darby Creek watershed has become one of the most urbanized watersheds in the state, with well over 50% impervious cover. This shift in land use over the past 200 years has caused this watershed and its streams to make major adjustments to accommodate the changes in water volume, levels of flow, and sediment; the result is major erosion problems. The Bartram Park/Darby Creek Stream Restoration Project provides an opportunity to showcase innovative design concepts and environmentally beneficial structures as alternatives to conventional erosion control.

From Rains to Restoration
Until 2000, the area known today as John Bartram Memorial Park in Darby consisted of row houses. Located directly on the creek, the houses were subject to frequent flooding. In 1999, however, Hurricane Floyd dropped 11 inches of rain, creating significant flooding along the creek within the borough. Homes were flooded with more than 8 feet of water. As a result of the damages sustained, all 38 homes within the 100-year floodplain were subsequently razed as part of a regional Federal Emergency Management Agency buyout, and the land was converted to use as a neighborhood park.

After the storm, the local watershed organization, the Darby Creek Valley Association, and the Delaware County Conservation District restored a small section of streambank in this new park area, creating a naturalized floodplain and riparian corridor. Early erosion control work involved stabilizing 70 linear feet of streambank by regrading banks to a 2:1 slope and installing biodegradable erosion control fabric.

Pre-project erosion and mid-channel bar near a regional sewer line

Unfortunately, a much larger problem was lying just upstream, exacerbated by the Hurricane Floyd flooding. Darby Creek was significantly cutting into its left streambank. The soil lost from the eroding bank had created a large, mid-channel gravel bar that was still growing. The gravel bar, in turn, directed the majority of stream flow toward the right side of the channel, which, as a result, began eroding as well. The width-to-depth ratio of the stream channel was now too low to effectively transport sediment. The problem was predicted to worsen with continued widening.

Erosion of the left bank created two critical problems. The first was public safety. Undercutting of the bank created a situation in which bank collapse was possible if park visitors stood near the creek. As the eroded bank was now 9 feet high, a danger from falling was a major concern, particularly because a playground and picnic area was located nearby. The second problem was the exposure of a large regional sewer interceptor. In the 1960s, this regional interceptor had been constructed along the path of the creek to deliver sewage to a large regional treatment plant downstream. The elevation of the pipe was below the elevation of the creek bed. However, down-cutting erosion on the left side of the creek had exposed the top of the interceptor below the eroding section of streambank.

The Delaware Riverkeeper Network (DRN) was asked to investigate and help address these growing problems. During the DRN's investigation, we noted that small trees had grown up on the gravel bar created by the creek. By cutting several of the trees and counting their tree rings, we determined that all the trees were post–Hurricane Floyd—that is, less than five years old.

In addition, discussion with long-term residents helped clarify an additional factor. According to one resident, a boulder that sat about one-third of the way across the stream channel from the left side was alongside the streambank 40 years ago (he used to fish from it). The boulder was clearly not of local geology. It is our belief that this boulder was moved into the stream during the construction of the regional sewer line. Stream flow around this large boulder set up the conditions that allowed Darby Creek to create the mid-channel bar and erode its left bank. Ultimately this boulder was moved and actually integrated into the Darby Creek stream restoration project.

Restoring Stream Health
The Bartram Park/Darby Creek Stream Restoration Project was a joint project of the Delaware County Conservation District, the Delaware Riverkeeper Network, the Darby Creek Valley Association, and Darby Borough. The DRN and project partners worked to create a project that effectively protected the exposed sewer pipe and addressed the growing erosion problems while at the same time enhanced the ecological health of Darby Creek and its floodplain. The final result was a project that accomplished these goals and has become a natural gem in the heart of Darby Borough.

The DRN served as project manager and was responsible for overall planning, management, construction, vegetative, and bioengineering work. The Delaware County Conservation District was awarded and administered the project funding through a grant from the National Oceanic and Atmospheric Administration Coastal Zone Management Program via the Pennsylvania Department of Environmental Protection. Biohabitats Inc. was the project consultant, with Water's Edge Excavating serving as the equipment contractor. The Darby Creek Valley Association assisted in public outreach and monitoring. Darby Borough owned and managed the park and project area.

The Delaware County Regional Water Quality Control Authority (DELCORA) and its consultant, Catania Engineering, also provided funding and other assistance to aid in project construction. The timing of construction itself had to be coordinated with a DELCORA sewer-lining project that included funding to correct the exposed sewer line problem. Because the exposed pipe already required protection, a cooperative solution was arranged whereby the DRN designed and installed a W-weir and stone flow deflectors that protected the exposed sewer line and several manholes threatened by the stream widening, while at the same time enhancing stream flows and ecological health.

Before the DRN's involvement, a previous consultant had designed a restoration strategy that consisted of a stream barb, riprap, and root wad installation.However, this design addressed only the erosion along the left bank. The proposal did not properly address the instability in channel morphology. Armoring the banks would have only shifted erosion to the opposing bank or downstream and is inherently counter to the DRN's restoration philosophy, which is to provide long-term relief in addressing the identified problems by restoring healthy stream flows, ecology, and function, thereby providing permanent protection for the community and the creek.

Plan view of the Darby Creek W-weir

Not satisfied with the effectiveness of these project designs, the DRN reviewed alternative design concepts for the streambank stabilization portion of the project. For a more comprehensive erosion control solution, the DRN chose methods developed through the field of natural channel design. Biohabitats Inc., an ecological restoration-consulting firm based in Maryland, designed the in-stream structure needed to restore stability. The selected design called for a W-weir, a structure similar to a cross vane in practicality, but more fitting for wider streams—these structures are carefully designed and installed in order to enhance stream health and function. The width of Darby Creek at the project site was about 90 feet, suggesting that either structure might have been suitable.

The W-weir, like other vane structures, works to control erosion by redirecting stream flow away from the bank. The structures are constructed using natural materials such as large rocks or logs. They are designed to appear as a natural and integrated part of the stream system. When examined closely, the structures appear "ramp-like" and are designed to dissipate stream energy as the stream flow moves up the slope (similar in practicality to emergency truck ramps along steeply graded highways). A properly designed and constructed vane will effectively reduce erosive forces along the channel edge. The Darby Creek W-weir was designed with outer arm angles set at 15 degrees from the bank edge with a 6% slope. Inner arms of the structure joined outer arms at a distance of approximately one-half of the bankfull channel width.

Vane structures work with the natural stream channel and its flows (rather than armoring against them) to create overall channel stability, rather than simply stabilizing a portion of streambank. Secondary benefits of these structures include enhancing fish habitat by creating flow variation and deep scour pools and creating safe streamside access for pedestrians. The use of rock or logs to construct these structures leads, over time, to a naturalistic structure as opposed to traditional erosion control structures such as riprap, gabions, or concrete walls often considered less aesthetic.

Effort is made to integrate local materials into the constructed structure. Stone from the same parent geology was selected in order to make the structure appear to fit in with its surroundings. In addition, the previous boulder that had been part of the instability problem was integrated into the vane structure to become part of the solution—symbolically, connecting the past and present stream and community.

A W-weir is one of a suite of stream restoration structures employed in the natural channel design school of stream restoration. These types of structures have been used widely throughout the United States in the last 10 years, especially in Colorado and many western states. Only within the past five years has there been growing recognition of this technique's application in the eastern United States, particularly in Pennsylvania, Maryland, and North Carolina. The applicability of natural channel design structures to urban streams, with their flashier and higher peaks, has been questioned. The Darby Creek Stream Restoration Project's W-weir, in its first 10 months, has successfully withstood an unexpectedly large number of high-flow events and is demonstrating the benefits that well-designed and -constructed natural channel design structures can play in our region.

In addition to the in-stream components of the project, restoration of a natural floodplain in the adjacent area was another critical component. The creation of the park following the removal of houses resulted in a floodplain consisting of a parking lot and wide expanse of mowed grass. Under this project, the DRN created a vegetated riparian buffer along 850 linear feet of Darby Creek consisting of trees, shrubs, and herbaceous plants native to southeastern Pennsylvania. Planting design accounted for restoring a healthy riparian ecosystem, passive recreation use by neighborhood residents, and the sewer line's access right-of-way clearance.

In-Stream Construction
With designs and permits in hand, the DRN and the Delaware County Conservation District began coordinating construction with the regional sewer authority, its engineers, the project design engineers, the borough, the Pennsylvania Department of Environmental Protection, and the DRN equipment contractor. This contractor has worked on previous successful projects with DRN staff. In projects of this nature, it is important to hire a consultant and contractor who are sensitive to the goals of the project and the ecology of the creek. Construction of the project involved in-stream use of an excavator with a thumb attachment for easy stone placement. Laser level surveying equipment was used for all survey measurements. Although a rock vane looks primitive, the structure is actually carefully built with tight slope and horizontal angle tolerances.

Prior to construction activities, the structure layout was determined and marked after all the trees and brush were removed from the large gravel bar. A minor deviation in the position of the structure was made to eliminate the need for removal of a large tree. Construction sequencing was dictated by access from one side of the stream, protection of the exposed sewer line, gravel bar removal, flow diversion for erosion and sedimentation control, and the final shape and position of the weir itself.

For erosion and sedimentation control during in-stream construction, flows were diverted around the active work area with 25 large sandbags (approximately 1-cubic-yard capacity). These large sandbags were easily filled and moved using the bucket teeth of an excavator. These bags formed a diversion to keep direct flows away from the work area and contain sediment in the project area through backwater flows. The sediment-laden backwater was then pumped and filtered through a geotextile filter bag before draining overland back into the stream.

To access the stream channel, equipment needed to cross the exposed sewer line. Approximately 6 feet of gravel and riprap were placed over the exposed pipe to protect it during construction. Following this initial protection, the area was covered by gravel and silt excavated from the existing mid-channel bar. This bank was later covered with gravel and soil from the removed gravel bar and vegetated through live brush mattressing and seeded with a native mix of grass and wildflowers.

After rebuilding the eroded bank, construction of the W-weir could begin. Construction began by setting the downstream corner to bankfull elevation. This elevation served as a benchmark for the remainder of construction. Elevation readings were taken every 5 feet to ensure that boulders were being set to their proper slope and angle. Stakes were placed upstream to serve as guides for maintaining the proper angle during stone placement. Another critical component of building vane structures is footer rock installation. Often, footers are placed too shallow, which can result in their undermining during high flows. Footers on the W-weir were extended to a minimum depth of 2 feet below the streambed. Upon completion of the project, Darby Creek became home to what might be the single largest stream restoration structures in Pennsylvania: a W-weir measuring 8 feet high, 100 feet wide, and 110 feet long. The completed W-weir consisted of approximately 600 tons of boulders (each boulder weighing 1.5 to 2 tons). Sizing criteria for boulders was determined by the US Geological Survey stream gauge for Darby Creek, which displayed peak flows ranging between 5,000 and 6,000 cubic feet per second.

Weir under high flow during July 12 rain event (above) and the following day (below)

After weir construction was complete, traditional fish habitat structures were installed upstream and downstream of the project. A series of stone deflectors was constructed along the left bank to create backwater pools and cover for small minnow and fry. Stone deflectors were shaped in a 45/45/90 triangle (the 90-degree corner extending out into the channel approximately 15 feet). In addition, boulder clusters were installed upstream and downstream from the weir to provide backwater resting and feeding areas.

In one case, a boulder cluster was installed upstream of the right leg of the W-weir. This proved to be too close to the weir, as it interfered with the stream flow entering the weir. Because it was likely that the structure's long-term integrity was in jeopardy, the boulder cluster was removed. In hindsight, the cluster created a situation potentially similar to the errant upstream boulder that helped cause the problem in the first place.

As most engineers and contractors understand, working on a stream-related project almost ensures interference by heavy rainfall, and this project was no exception. Weir construction took approximately five weeks as a result of numerous rain-related delays. Without such delays, construction was expected to last two weeks.

Project Monitoring and Ecological Effects
Before construction, DRN staff conducted a pre-project survey, including sampling of macroinvertebrate populations downstream of the problem area. The results of this monitoring were compared to similar data collected after the project was completed. Comparison of macroinvertebrate samples showed no observable change in species diversity or richness.

In the post-project period, a local neighbor has joined the DRN's Adopt-A-Buffer program, a volunteer-based initiative that monitors completed restoration projects to ensure their long-term success. Trained volunteers monitor for invasive plants, mowing issues, and other threats to constructed riparian buffers using protocols developed by the DRN. The Adopt-A-Buffer manual is available online at www.delawareriverkeeper.org.

Although permitting agencies and others raise questions during the planning and permitting phases about turbidity and sediment issues during in-stream construction, onsite monitoring during construction showed that, in some cases, these may not be of concern. DRN staff monitored the effects of in-stream construction immediately downstream from the diversion and work area. Turbidity levels were analyzed to review the downstream impacts of construction. Throughout construction, downstream turbidity levels remained at or below the same levels observed in natural high flows. The plume of sediment was contained in the diversion's backwater, dissipated quickly, and was undetectable within sight of construction.

Wildlife was observed utilizing the weir even before it was completed. White-tailed deer tracks were seen on the newly created weir arms, indicating that the deer are using the slope as an easy-access point in and out of the creek. Much to the anxiety of our equipment contractor, a resident water snake used the newly constructed weir arms as a sunning and perch area.

View of W-weir from upstream. Note low-flow deflector (left).

The most interesting of our wildlife observations occurred during construction of the second half of the weir. While we were still building the inner portion of the weir, several white suckers and pairs of ducks were viewed swimming and resting in the newly opened channel of the W-weir. This phenomenon has been noted at our other projects (for example, fish, waterfowl, and other wildlife appearing at the construction site before construction is completed) but was unexpected in such an urban watershed.

Although construction of the weir was an interesting process, it wasn't nearly as interesting as the months following its completion. The project was completed in early May 2004. Between June and September 2004, Darby Creek experienced five bankfull flow events, including several that exceeded the bankfull flow. In fact, the Darby Creek project was subjected to more high-flow events than might be expected in 10 or more years. In April 2005, another significant flow event was experienced, with flooding on the Delaware River being the third worst since 1904.

In spite of the high flows through and over a newly constructed project, both the riparian buffer and in-stream components of the project have held up well against the repeated stress. After the first high flow, the project lost one unstable rock, which was quickly repaired. Since then, damage has been nonexistent.

Lessons Learned
Throughout the course of planning, building, and monitoring the Darby Creek restoration project, a number of important lessons were learned. These lessons are not at all new to this project alone. Some of them are common to many construction projects, all the more underlining their importance:

1. Tight controls must be established on the size and shape of rock purchased for vane construction. Approximately 100 individual boulders were selected and marked at the quarry prior to delivery. However, during installation, stone deliveries gradually yielded smaller sizes and more uneven dimensions than originally specified. Failure to use adequately sized stone will compromise the integrity of the structure. Consequently, we had to clarify our requirements with the quarry during construction to ensure proper specifications were met. Unused stone was then used in construction of the low-flow deflectors.
2. Movement and selection of suitable rock from stockpiles was accomplished by the same trackhoe used in construction. Use of additional equipment (such as a front-end loader) to deliver rock to the construction area would have increased building efficiency.
3. Project teams need to establish quality control procedures throughout construction. The construction team used a series of tables and charts to confirm slope elevations along each arm. However, an error in one set of calculations resulted in the rebuilding of a portion of the structure, setting back construction a day. Having the project designer or a qualified technician oversee installation and check slope and angle calculations in the field is critical for proper construction.
4. Internal quality assurance is needed. In this case, an error in a set of calculations was not checked by those using them the next day, resulting in rebuilding a portion of the W-weir, setting back completion by almost a full day.
5. If possible, projects need to be completed with sufficient time and funds left in the funding period to allow for tests, "tweaking," and repairs. The first bankfull events following construction provide a "settling in" period for these structures. If footers or other rocks are placed insufficiently, they may become dislodged during a high flow. In the Darby project, post-project actions included repairing and re-angling a small section of the inner structure and removing a boulder cluster that interfered with the structure's functions. Allowing time and funds for follow-up inspection and maintenance (if necessary) goes a long way to preventing post-construction problems.
6. Staging the project to accommodate the needs of others (for example, the sewer authority) yielded significant benefits in terms of cooperation, assistance, and funding.
7. The timing of the project (starting on the heels of the sewer pipe re-lining) led to long-term misunderstandings and misconceptions by the public, who assumed there was one large project instead of two separate efforts.
8. The success of the project has created partner enthusiasm for additional work, both within and without the project's partnership itself. The success of this project has led to the pursuit of additional projects of a similar nature in other parts of this watershed.
9. Restoration of Darby Creek, including its in-stream structures, riparian buffer, and the removal of homes inappropriately built on an urban floodplain, represents a modern holistic approach to flooding and urban stormwater management.
10. The Darby project involved several different government agencies and two nonprofit organizations. Having clear sets of responsibilities divided according to expertise and resources allowed a complex project like this one to occur smoothly.
11. Having resources for pre- and post-project monitoring (chemical, biological, and project success) is a concern that needs to be built into the project.

A year after its construction, and several high flows later, the Darby Creek Stream Restoration Project provides an excellent example of natural channel design techniques successfully being applied to a flashy urban stream to control erosion. The project is holistic in that it involves numerous related features such as floodplain management, riparian buffer creation, correction of a threat to a regional sewer line, the correction of a major erosion and public safety problem, and increased aquatic and terrestrial habitat in a highly urbanized area. In addition to stabilizing an area of regional concern, the newly restored Bartram Park provides an opportunity for members of this urban community to reconnect with nature and their local watershed—an effort that literally began from the ground up.

References
Wardell, Lindy Constance, ed. 2003. Darby Borough. Darby Borough Historical and Preservation Society: Arcadia Publishing.

Darby Creek Valley Association. 2004. Darby Creek watershed conservation plan. Prepared by Cahill Associates and Campbell Thomas and Co., November.

Rosgen, Dave. 1996. Applied river morphology. Wildlands Hydrology.

Dan Salas is restoration program manager, Richard C. Albert is restoration program director and staff scientist, and David M. Williams is a restoration program specialist with the Delaware Riverkeeper Network in Washington Crossing, PA. Vince Sortman is a senior fluvial geomorphologist with Biohabitats Inc. in Timonium, MD. William Gothier is a watershed specialist with the Delaware County Conservation District.

EC - September/October 2005