Robbin B. Sotir & Associates, Inc.

Case Study: Long Leaf

Location: Wilmington, North Carolina
Client: City of Wilmington
Objectives: Improve flood control, stabilize streambanks, enhance aesthetic, riparian and aquatic habitat within an urban watershed. Develop a cost effective low long term maintenance project.

• 2,000 feet of stream length
• Side slopes up to .5V to 1V
• Banks 8' to 18' high
• Highly erosive sandy soils
• Stream had become highly degraded by bed scour and widening
• Poor water quality
• Property loss due to bank failure

Nature of Firm's Resonsibility

• Site analysis and vegetation reconnaissance
• Participation in public meetings
• Soil bioengineering design plans and specifications
• Assistance with COE permit application
• Construction cost estimating
• Pre-bid, pre-construction meetings
• On site construction observation

Long Leaf Creek is located within an urbanized watershed along coastal North Carolina. The specific stream reach addressed in this project is located in a residential neighborhood subdivision. It is a highly sensitive project with a variety of multi-objective goals specific to its location and function in the watershed. Due to the continued development of the watershed,and specifically a new mall and widened road system immediately up stream, the residents saw their creek deepen and widen, a result of increased runoff and high concentration events (hurricanes). Properties were experiencing flooding, bank failures and erosion. Additionally, loss of aesthetic values, riparian corridor vegetation, aquatic and terrestrial habitat and degradation of water quality were issues that needed to be addressed.

Due to the need for increased stormwater capacity and hydraulic efficiency requirements, the City of Wilmington undertook a stream reconstruction, restoration and stabilization project in this reach. Meetings were held with the residents and permitting agencies to ensure all requirements were understood and met.

Robbin B. Sotir & Associates, Inc., serving as the soil bioengineering consultant to the Kimley-Horn's interdisciplinary team, worked closely with the geotechnical design, and the hydrologic/hydraulic efficiencies of a soil bioengineering solution to address the desired goals and critical engineering, environmental and aesthetic issues.

Alternatives were compared with the critical issues, which included such items as: stop erosion and stabilize banks; safer and healthier environment; flood control; timely project completion; environmental and aesthetic improvement; minimize property loss; hydraulic efficiency; and cost feasibility.

After an initial investigation, an alternative analysis was produced in the summer of 1997. This alternative analysis explored numerous approaches to address each of the project goals with cost and risk factors assigned to each alternative. Alternatives such as box culvert, 3:1 grassed slopes, 2:1 riprap rock, 2:1 concrete lining, and soil bioengineering slope systems were considered. The City, with input from the residents and permitting authorities, selected the soil bioengineering approach and commissioned the design team to produce plan and specification documents, including construction cost estimates. Pre-bid, pre-construction and permit application services were provided to support the project. The construction of the project was completed in April of 1999.

The project was designed for the 100 year event. Hurricane Dennis produced an estimated 500 year event coupled with a long duration flow in the channel. While no flooding occurred, some damage was caused in the steepest upper reach of the channel. This was repaired in the winter of 1999 at a cost of 7% of the orignal construction cost. The project has performed well and the City is comfortable with the existing design and performance.