How much of the area of vertical walls may be included in stormwater runoff calculations, according to some codes?

When calculating stormwater runoff, various codes and regulations allow for a percentage of vertical wall area to be included in the overall calculations, acknowledging the role that these surfaces play in water runoff. The option of including 50 percent of the area of vertical walls reflects an established practice in some jurisdictions, taking into account that while vertical surfaces may contribute to runoff, they typically do not do so to the same extent as horizontal surfaces, such as roofs and pavements.

This 50 percent consideration provides a balanced approach, allowing for an adequate reflection of the contribution from walls without overestimating their impact on runoff volumes. This is particularly important in stormwater management design to ensure that systems are adequately sized to handle expected flows based on realistic contributions from all surfaces involved. It balances efficiency and practicality, addressing the realities of construction and the hydrological characteristics of runoff.

In contrast, other percentages would either undervalue or overvalue the contributions from vertical walls, which might not represent actual field conditions. The selection of 50 percent is thus aligned with best practices in stormwater management effectively.