The Least NRCS NRCS Dam: Using HEC-RAS to Evaluate a Unique NRCS Dam.

Sophia Hassan, EIT, Engineer I, Freese and Nichols, Inc.

Kyle Jacobs, PE, Engineer IV, Freese and Nichols, Inc.

Crabtree Creek Dam No. 5A (CC5A) is a Natural Resource Conservation Service (NRCS) dam located in Wake County, North Carolina. The dam was constructed in 1973 along Interstate 40 (I-40). I-40 is a major interstate highway along the southeastern and southwestern portions of the United States. I-40 was constructed prior to the construction of CC5A, and originally contained three 10’x10’ box culverts that drained to the south side of the highway. To build CC5A, compacted earth fill was placed along the north side of I-40 to serve as the dam embankment, a concrete intake riser was constructed as the principal spillway, and a separate concrete riser serving as the auxiliary spillway was constructed upstream of the existing three box culverts. The original reservoir area consisted of one contiguous reservoir until Interstate-540 (540) was constructed in 1993. Interstate-540 was constructed north of I-40 bisecting the CC5A reservoir. The two sections of the reservoir and a single 10’x10’ box culvert was constructed through the new road embankment with the purpose of allowing water to flow from the upper portion of the reservoir into the lower. This created an in-series effect of inflow to CC5A. The watershed was divided into two sub-basins, a larger basin upstream of 540, and a much smaller basin between 540 and I-40 upstream of the actual CC5A dam. A hydrologic analysis of the watershed was performed, and the inflow hydrographs were developed according to Technical Release No. 60 (TR-60) criteria. NRCS’s SITES computer program was originally used for the hydraulic capacity assessment of the dam, however modeling the design storms within SITES suggested that a 2D modeling approach would produce a more realistic model. While TR-60 provides guidance on how to assess dams in series, the road embankment for 540 does not classify as a dam, and therefore does not comply with TR-60. To assess the unique flow conditions at CC5A, the project team collaborated with the National Design, Construction, and Soil Mechanics Center to develop a plan on how to assess the dam. This presentation will highlight the following:
  • The design inflow hydrograph scenarios developed alongside the National Design Center to model CC5A;
  • How NRCS’s SITES and USACE’s HEC-RAS were used to model the complex discharges from the upper drainage area;
  • The results from these models, and lastly how these techniques can be used to model dams in series that TR-60 cannot fully account for.

Author Bio

Sophia Hassan graduated from North Carolina A&T State University with a degree in Biological and Agricultural Engineering. She has a focus on hydrology and hydraulics and has been working at Freese and Nichols for one year. Kyle Jacobs is a graduate of Virginia Tech with BS and MS degrees in Biological Systems Engineering. He is a water resources design engineer with Freese and Nichols in the Raleigh, NC office. His primary responsibilities are in hydrologic and hydraulic modeling and analysis, Emergency Action Planning, NRCS assessments and spillway design for dam projects.