Culvert Sizing

Before comparing alternative routing methods in SWMM, it’s essential to determine culvert diameters in the conveyance system. This involves finding the smallest size from Table 2-5 for each culvert, ensuring it handles 100-year, 2-hour storm runoff without flooding. The process follows these steps:

  1. Start with each culvert at max diameter.
  2. Run VOSWMM, gradually reduce C3’s size until flooding, then set C3 larger.
  3. Repeat above for C7 and C11, systematically upstream to downstream.

This systematic approach ensures each culvert manages upstream flow. Not valid if pipe diameters need enlarging or flooding occurs under the baseline. Design scenarios often impact upstream with downstream changes. A slight downstream pipe diameter adjustment can solve upstream flooding.

Keep in mind that VOSWMM offers an Elevation Profile feature. Users have the option to choose a conduit, such as a culvert, and then select Elevation Profile from the simulation tab.

Culvert-sizing uses Example2-Post.inp, KW (Kinematic Wave) routing, 100-year rainfall, and time steps (1 min reporting, 1 hr dry, 1 min wet, 15 sec routing). Routing time is stringent due to system and KW method; Dynamic Wave later uses shorter steps for stability. For just KW routing, 1 min step could suffice. Flooding presence is checked in Node Flooding Summary.

After these sizing simulations, final culvert diameters are: C3 – 2.25 ft, C7 – 3.5 ft, C11 – 4.75 ft. Table 2-6 shows capacity fraction and Manning flow proportion for 100-year peak flow.

SWMM necessitates specifying four time intervals: runoff intervals for both wet and dry conditions, a flow routing interval, and a reporting interval. The most frequent mistake users make is opting for intervals that are excessively lengthy. The wet weather runoff interval should not surpass the precipitation recording span. The flow routing interval must never exceed the wet weather interval and typically ranges from 1 to 5 minutes (or less) for Kinematic Wave routing, and 30 seconds (or less) for Dynamic Wave routing. Dynamic Wave routing can also utilize a Variable Time Interval feature that automatically reduces the interval during periods of rapid flow changes. Substantial continuity errors usually arise from overly large runoff or routing intervals. A high reporting interval might lead to the omission of important output details. While matching the reporting interval to the routing interval mitigates this, it can yield notably large output files. Commencing with smaller intervals, users can experiment with larger ones to identify the most efficient interval that yields adequately accurate outcomes.

Comparison of Routing Methods

To compare the result of different routing methods, duplicate the model for a straightforward comparison. Next, navigate to the Simulation tab and choose Engine Options. Within the General tab, you’ll find the section for selecting Routing Methods. For the duplicated scenarios, opt for Dynamic Wave and Steady Flow, and proceed to execute the model.

After appropriately sizing the culverts, the model is executed using all three routing techniques—Steady Flow, Kinematic Wave, and Dynamic Wave. The objective is to obtain outlet discharges, which are then graphed alongside the discharges from Example 1, based on the design storm. Figures 2-6 display the total inflow to node O1 for the 100-year design storm, utilizing all three hydraulic routing methods.

Regarding flooding, the Steady Flow method identifies potential flooding by computing conduit flow depth using Manning’s equation. If this depth exceeds the channel’s capacity, flow is truncated to the conduit’s full-flow capacity, and flooding is reported.

The other two routing methods, Kinematic and Dynamic Wave, introduce time lags and reduce peak flows, distributing the outlet hydrograph’s volume over time. Dynamic Wave, accounting for backwater and increased storage within the conveyance system, accentuates these effects.

Table 2-7 compares total runoff volumes, coefficients, and peak discharges at the outlet between the post-development model without routing (Example 1) and with Dynamic Wave (DW) routing in this instance. These values are directly sourced from SWMM’s Status Report. Runoff volumes and coefficients remain consistent with Example 1 where no hydraulic routing was involved. Routing’s impact surfaces in peak flow comparisons, which decrease with routing. In this example, Dynamic Wave routing leads to peak reductions of 28.7% for the 2-year storm, 24.8% for the 10-year storm, and 32.4% for the 100-year storm when compared to cases without routing.