InfoSWMM (d/D v. Surcharge d/D)

 

InfoSWMM (d/D v. Surcharge d/D)

1. d/D:

• Calculation: The d/D value is determined by the ratio of the water depth (d) at the midpoint of the link to the maximum depth (D) of the link:
  • Formula: d/D = Link Depth / Link Maximum Depth
  • Range: Since the water depth within the link cannot exceed the maximum depth, d/D always ranges from 0 to 1.
  
  
• Interpretation: This variable shows how full the link is at its midpoint. A d/D of 1 indicates the link is at full capacity at its midpoint, but does not necessarily mean both ends are full or that the link is surcharged.

2. Surcharge d/D:

• Calculation: The Surcharge d/D is calculated using the depths at the nodes at both ends of the link:
  • Formula: Surcharge d/D = (Average Node Depth at Link Ends) / Link Maximum Depth
  • Range: This value can exceed 1 because node depths can go beyond the maximum depth of the link due to surcharging (when water levels rise above the crown of the pipe or conduit).
• Interpretation:
  • Surcharge Indication: A Surcharge d/D greater than 1 indicates that at least one of the nodes at the end of the link is experiencing surcharge conditions.
  • Difference from d/D: While d/D looks at the midpoint, Surcharge d/D considers the ends, which might be under very different hydraulic conditions, especially in complex networks where backwater effects or high inflows occur.

Key Differences and Insights:

• Location of Measurement:
  • d/D reflects conditions at the link's midpoint, which might not represent the entire link's state, especially if there's a slope or varying flow conditions.
  • Surcharge d/D uses an average of node depths at the link ends, giving insight into potential surcharging.
• Implications:
  • A d/D less than 1 does not necessarily mean the link isn't surcharged at one or both ends; this is where Surcharge d/D becomes crucial.
  • If Surcharge d/D is greater than 1, it signals that one or both nodes are surcharged, which could lead to overflows or backflows if not managed.
• Practical Example:
  • As shown in Figure 1 (assumed from your context), plotting both d/D and Surcharge d/D can visually demonstrate how these values might differ in a real simulation, highlighting areas of concern like potential flooding due to surcharges.

Figure 1.  Plot of d/D and Surcharged d/D in InfoSWMM.