SWMM5 roadway.c Summary

 The roadway.c module in the EPA SWMM5 code is specifically designed to compute flow overtopping a roadway using the FHWA HDS-5 methodology. It calculates flow over a ROADWAY_WEIR object typically used in scenarios involving a culvert crossing where the conduit is placed at zero offset and the roadway weir has an offset equal to the height of the roadway.

Here’s a breakdown of the functions in this module:

Constants and Parameters:

1. Discharge Coefficients:
   • These coefficients are used to compute the flow rate based on the water depth over the roadway weir. There are two sets:
     • Low Head: For (head / road width) <= 0.15
     • High Head: For (head / road width) > 0.15
2. Submergence Factors:
   • These factors adjust the discharge coefficients based on the submergence (depth of water over the roadway weir). Different factors are provided for paved and gravel road surfaces.

Functions:

1. roadway_getInflow:
   • Purpose: Computes the flow across a roadway weir based on upstream and downstream water levels, the road width, and the type of road surface.
   • It calculates the discharge (q) using the formula for weir flow: $$q = cD \times \text{length} \times h^{1.5}$$
   • It adjusts the discharge coefficient based on the water depth over the road (hWr) and the road surface type (paved or gravel). It also calculates the derivative of flow (dqdh) for further computations.
2. getCd:
   • Purpose: Returns the appropriate discharge coefficient (cD) based on the water depth (hWr), the difference in water elevation between the upstream and downstream sides of the road, the road width, and the road surface type (paved or gravel).
   • The function checks if the head-to-road width ratio (hWr / roadWidth) is low or high, and selects the appropriate coefficient tables accordingly.
3. getY:
   • Purpose: Interpolates the value of a coefficient (cD or kT) based on the provided x value and a given lookup table (table). The function searches through the table for the appropriate value using linear interpolation between two points.

Key Concepts:

• Discharge Coefficients: These are critical in determining the flow rate over a roadway. They depend on the height of water (head) over the roadway and the road width.
• Submergence Factor: A factor that adjusts the discharge coefficient based on how submerged the roadway is.
• Flow Calculation: The flow is calculated based on the relationship between the head over the roadway and the road width. If the head-to-width ratio is small (low head), a different formula is used compared to when the ratio is large (high head).

Summary:

This module calculates the flow overtopping a roadway during rainfall or other events using the FHWA HDS-5 methodology, and integrates those calculations into the SWMM 5 model to analyze the hydrological behavior of roadways within the drainage network. The calculation depends on the water depth, road width, and road surface type, and includes adjustments for submergence and flow conditions.