Growing Greener Grants have been awarded by Pennsylvania to assess the Choconut Creek watershed and restore a section of the main channel. The Choconut Creek Watershed Association has awarded contracts for this work to Larson Design Group. The stream channel restoration will improve over 3600 feet of unstable stream channel. As part of the recently initiated watershed assessment, stream bank and channel stability were evaluated from St. Josephs downstream to the NY State line.
Over 9,000 feet of unstable stream bank were located through this survey. These eroding banks introduce an estimated 50,000 tons per year of sediment to the system. Much of this sediment is fine-grained soil particles that can severely impact aquatic life in Choconut Creek and further downstream in the Susquehanna River. Poor vegetation cover on the unstable banks allows bank erosion to continue unchecked in many locations.
Gravel bars were located in 29 stream reaches indicating excess sediment loads in the stream system. Gravel bar deposits split the stream channel and cause additional stream channel instability and bank erosion. The bar deposits are indicators of excess sediment that exceeds the stream’s sediment transport capacity.
These conditions upstream from the stream reaches currently targeted for restoration have the potential to reduce the effectiveness of the current restoration project.
Bank Erosion Summary
|
Severity |
Count |
Total Length (Meters) |
|
Extreme |
6 |
311 |
|
Severe |
22 |
688 |
|
Moderate |
55 |
1369 |
|
Mild |
32 |
1370 |
In total over 3,700 meters (2.3 miles) of bank erosion and channel instability threatens property and aquatic life along Choconut Creek.
The stream system was surveyed using high-resolution Global Positioning System (GPS) receivers to locate the stream channel and associated measurements. Features mapped include bank stability, channel dimensions, particle size, embeddedness, woody debris jams, channel encroachments, bridge crossings, dams, and canopy cover.
Many of the features mapped can be used to determine the relative stability or instability of the stream channel. Additionally, this information can be used to determine how far channel changes have progressed since the channel was disturbed.
The maps shown on the following pages summarize bank erosion, mid-channel bars (which are indicators of excess sediment in the stream system), bankfull width & depth, and tree cover near the stream. The maps of bankfull width and depth provide additional insight into stream channel function in the areas of instability.
Natural channel design is used to restore streams to a naturally stable state. Once a stream is reestablished in a stable form it becomes self-sustaining and maintains its form if sediment supply and/or the storm water hydrology of the watershed does not change. It is important to maintain natural hydrology and sediment supplies for natural channel restoration. Upstream erosion in a watershed contributes sediment downstream and will alter the stable form of the stream channel.
Methods used in natural channel design include sediment transport analysis, sediment particle size analysis, hydraulic analysis, surveying of stable stream reaches, and reproduction of stable channel forms. Once the stable geometry of the stream channel has been determined, construction and stabilization of the channel begins. The streambed and banks are graded to the proper form. Large rock structures can be incorporated to control hydraulic function in the stream, maintaining pools and riffles in the channel. Revegetation and streambank grading are important components of natural channel design. Revegetation is used to develop a strong network of roots near the stream channel to strengthen streambanks. Revegetation using natural species in the riparian zone also improves canopy cover over the stream, which reduces stream temperature and can help to remove nutrients from the stream water.
Mapping
Choconut Creek Mid Channel Bars
Proposed Project
There are many instances of bank erosion throughout the reach selected for restoration with the Round 3 Growing Greener Grant. Immediately upstream of the restoration reach is a transport reach that is bounded by high embankments for approximately 0.3 miles to the confluence with Stanley Branch. Upstream from this reach there are numerous gravel bars moving through the stream system. The mean width of the mapped mid-channel bars is 4.9 meters. The mean length is 15 meters. Based on the mean dimension of the bars, an average bar contains 75 cubic meters of material. Unstable banks and poor riparian zones allow rapid channel migration through this reach. Any sediment exported from this reach is transported from the confluence of Stanley Branch (which also contributes significant flow increased the ability of the stream to transport sediment) to the head of the restoration reach.
While natural channel design does in many cases increase the sediment transport capacity (competence) of the stream, it cannot compensate for long-term excess sediment in the system. Natural channel designs count on balanced sediment influx and output. For a short period of time it is likely that the restoration reach will not be impacted by upstream sediment inputs. Over long periods of time, it is critical to address the upstream sediment inputs and bar migration to avoid damage to the restoration reach. Excess sediment from upstream sources can reduce stream gradient further downstream causing channel migration and increased flooding.
Initial estimates of work required to reduce major sediment inputs and migration through the system indicate that approximately 1.5 miles of channel upstream from the confluence with Stanley Branch should be restored. The first phase of this work will be survey, design, and completed permits for the upstream reaches. The second phase will be construction of the natural channel design. Both phases will be submitted as grant applications to the Growing Greener Grant Program through PA Department of Environmental Protection.
Return to Road Runner CCWA Documents