In heavy rains, the water flows from my rear yard into the alley.
In preparation (of impending storms!) I cleared the leaves, deposited by my arch-rival the bouganvillea, and trash along the alley side of the run-off, anything that might interrupt or impede my serviceable grading. With regard to fluid dynamics, I was eliminating turbulence, or so the applied science reads.
My alley is ungated, backs a moribund commercial corridor, a pocket park, a library; and, increasingly disused, serves only to access a few garages. Open on both ends, it is prone to dumping. Richard the metalsmith who lives in a twenties warehouse behind me has furnished nearly his entire space in alley cast-offs, even window treatments.
"If I'm willing to wait", he once proclaimed, "it'll show up."
Some houses on my block/alley lack garages altogether. Some have converted their garage (illegally) into an in-law or rental (which makes street parking more scarce). Yet most prefer to park on the street which is less desolate and therefore feels safer. In many places the alleys are gated. By doing so the city abdicates their maintenance responsibilites. The gates are padlocked and only the residents have keys. This tends to discourage their use even further.
One elderly neighbor, exiled by his wife, uses the alley as a place to smoke. He doesn't seem the least bit concerned about fluid dynamics.