## An important difference between Capacity d/D and Surcharge d/D in InfoSWMM, Capacity is based on Area and Surcharge d/D is based on Depth

InfoSWMM is a comprehensive tool for the design and management of wastewater and stormwater systems. In InfoSWMM, the terms 'Capacity' and 'Surcharge' are used to describe different aspects of the system's performance under various flow conditions.

Capacity d/D:

The term 'Capacity d/D' in InfoSWMM refers to the ratio of the flow depth (d) to the diameter (D) of the pipe. However, when we talk about 'Capacity', it's crucial to understand that this is derived based on the area of flow within the pipe, not the depth. The 'd/D' ratio here gives us an understanding of how full the pipe is under normal flow conditions. The 'Area' here refers to the cross-sectional area of the flow within the pipe.

This ratio gives an idea of how much of the pipe's total potential flow area is being used. When the ratio is low, the pipe has plenty of additional capacity. When the ratio is close to or at 1, the pipe is at or near its total flow capacity, and any additional flow could result in surcharging.

Surcharge d/D:

'Surcharge d/D', on the other hand, refers to the situation where the flow depth (d) exceeds the pipe's diameter (D) - in other words when water rises above the crown of the pipe. Unlike 'Capacity', which is based on the area of flow, 'Surcharge' is directly concerned with the actual depth of the water above the pipe crown.

In this situation, the pipe is essentially functioning as a pressurized conduit, as it is completely filled with water and the water level in upstream manholes (or other structures) is higher than the crown of the pipe. This is a situation that typically needs to be avoided in gravity-fed systems, as it can lead to backups and overflows.

In conclusion, 'Capacity d/D' and 'Surcharge d/D' represent different operational conditions in a drainage system modeled by InfoSWMM. 'Capacity d/D' is an indicator of how much of the pipe's flow area is being used under normal conditions, while 'Surcharge d/D' is an indicator of conditions when the water level exceeds the top of the pipe, potentially leading to problematic conditions in the system.