Correctly disconnecting your downspouts from the sewer system helps reduce some of the stress heavy rainfall has on the combined system. A combined system is where pipes carry both wastewater (water from our toilets and house / business drains) and stormwater (rain or snow that washes off streets / roofs) to the wastewater treatment plant.
Disconnecting your downspout can be accomplished fairly easily with an elbow, pipe extension, and splash block. If done correctly, it helps direct water away from your house to a suitable area where the runoff can spread and soak into the surrounding landscape or another pre-designated area.
Besides having a suitable area for water runoff, other factors determining the success of downspout disconnection include having adequate distance from adjacent property lines, the amount of slope your property has, and assurances that excess water will not affect neighboring properties. If your property does not meet these minimum requirements, downspout disconnection is not recommended.