Wet weather flow is higher than the dry weather flow due to the contribution from inflow and infiltration in the sewer system.
The rate of wastewater flowing at dry weather combined with stormwater introduced into a combined sewer system, and dry weather flow combined with infiltration/inflow in a separate sewer. Wet weather flow is thereby higher than the dry weather flow due to the contribution from inflow and infiltration in the sewer system. Systems should be designed and constructed such as to minimise flow increase due to wet weather e.g. by incorporating detention tanks or special design of pipes and channels.
However, if the wet weather flow exceeds these arrangements in construction of the sewer system the flow might ultimately lead to overflow of untreated wastewater from the sewer system into the recipient or even flooding.
Figure showing dry and wet weather flow in a combined sewer system.