A Case Study in 3D Stormwater Modeling

Garrett Johnston

A typical detention pond design will include an overflow weir for safely passing high inflows downstream. In many cases, an engineer will design the weir using a one-dimensional (1D) hydraulic model with “textbook” weir coefficients.

These commonly-accepted values are based on lab tests and measurements, and are limited by certain assumptions (for example, flow is assumed to be uniformly distributed across the weir). The following case study illustrates how 3D modeling software can provide valuable insights into complex flow patterns and provide confidence in 1D modeling results.

This urban park project included a large regional detention pond with an overflow weir located at the upper limits of the pond’s grading (pictured above). Flow over the weir is captured by an existing downstream sewer system. Design criteria required the following:

  1. The pond’s overflow weir had to reduce flooding of the surrounding area during the 100-year event.
  2. The pond had to fully detain the 25-year storm within the pond to limit flooding conditions downstream of the pond.
  3. Site constraints required a minimal difference between the allowable peak 25-year and 100-year pond levels.
  4. For park aesthetics, the weir needed to be placed as close as possible to a retaining wall, where the overflow would then enter the downstream sewer system through two pipe openings.

Taken together, these criteria required a long, low weir with a sudden transition from an 80-foot width of shallow flow across the weir to two full 42-inch arch pipes. In this situation, standard 1D weir flow assumptions do not apply. If the 1D weir coefficient were set to a conservatively high value, the peak water level would exceed the highest allowable level; if it were set to a conservatively low value, the peak outflow would exceed the maximum allowable flow.

For such an unusual design, one might try to design the weir using 2D hydraulic modeling software, e.g. HEC-RAS 2D or InfoWorks ICM. These 2D modeling packages can account for complex flow patterns, but still require simplifying assumptions regarding the vertical component of flow over the weir and the sudden transition into the closed system.

Three-dimensional hydraulic modeling software, also known as computational fluid dynamics (CFD) software, can solve both the Navier-Stokes fluid equations and a separate set of free-surface equations to determine the free flow of a fluid in three dimensions. Various 3D/CFD software packages have been commercially available for decades, but the long computation times have typically restricted its use to projects much larger than a regional detention pond.

Freese and Nichols’ Flow-3D software license proved to be the perfect solution for this unique design problem. FNI had previously used Flow-3D for complex dam analysis and design, but this overflow weir presented the perfect opportunity for a stormwater/park application. Within days, our project team was able to build a 3D hydraulic model from Civil 3D design drawings, test multiple weir configurations, and calibrate the weir parameters in the 1D hydraulic model. Although 3D hydraulic modeling software has typically only been used to support the design of large, heavy-civil projects, recent advances in computing power have lowered the barrier to entry. Now projects of any size with challenging design constraints can benefit from 3D hydraulic analysis.