🌊 Table 3-6: Sensitivity of Runoff Volume and Peak Flow to Surface Runoff Parameters 🌧️ from the SWMM5 Hydrology Manual

 

🌊 Table 3-6: Sensitivity of Runoff Volume and Peak Flow to Surface Runoff Parameters 🌧️

Parameter	Typical Effect on Hydrograph	Effect of Increase on Runoff Volume	Effect of Increase on Runoff Peak	Comments
🌳 Area	Significant	📈 Increase	📈 Increase	Less effect for a highly porous catchment
🏙️ Imperviousness	Significant	📈 Increase	📈 Increase	Less effect when pervious areas have low infiltration capacity
📏 Width	Affects shape	🔽 Decrease	📈 Increase	Increasing width tends to produce higher and earlier hydrograph peaks, especially for storms of varying intensity. Affects volume only when reduced width on pervious areas allows more time for infiltration
⛰️ Slope	Affects shape	🔽 Decrease	📈 Increase	Similar to width but less sensitive, as flow is proportional to square root of slope
🌾 Roughness	Affects shape	📈 Increase	🔽 Decrease	Inverse effect as compared to width
💧 Depression Storage	Moderate	🔽 Decrease	🔽 Decrease	Significant only for low-depth storms. Losses like evaporation, depression storage, and infiltration become less important as storm depth increases

🔍 Additional Notes:

• In flooding scenarios, the land surface behaves increasingly like an impervious surface, hence urbanization has less impact on high-return period events than on common events.
• Ground saturation consideration may invoke groundwater routines to allow water table rise to the surface.
• For small storms, depression storage becomes crucial, although it is difficult to estimate and depends on initial conditions.