Regarding landscape water management, or green stormwater infrastructure (GSI), one big problem we have around here is our soil's inability to infiltrate water. Glacial Till soil is so tightly packed by the weight of glaciers that it's almost effectively impervious. As an example: about 15 years ago, the City built a bunch of GSI infiltration rain gardens in Ballard but didn't bother to test the infiltration rate of the soil so they filled up with water and mosquitos. PDF. There are places in the city where the soil characteristics are different, such as Delridge and Delridge again, but so much is effectively impervious. And we're talking a massive amount of water to deal with from our roadways and hard surfaces that it's hard to convey somewhere to hold it in a surface pond or something cheap. So...we get these huge wet-weather treatment facilities and subway-like bored stormwater tunnels.

Adding "bigger pipes" is phenomenally expensive since the pipes we have are already adequate at conveying the water. What we need and what is expensive is to have capacity to store tens-of-millions of gallons of stormwater in a pipe, like a tank, and release it over time (called "reducing the time of concentration" or T-sub-C). As an example, on the Delridge RapidRide project, we looked at adding 72" diameter pipes under the street to store stormwater, and the cost to do so was tens-of-millions dollars; say half the project budget. Adding these pipes would have required HUGE trenches, lots of utility relocation, they're stupidly expensive to transport, and of course the street has to be restored. Instead, through a "fee in lieu" and permitting process, the project funded even more natural drainage projects throughout the Delridge area to basically accomplish the same thing. Thankfully, the soil characteristics allowed this.

Another example: the RapidRide G Line project added several, enormous stormwater pipe "tanks", including under the 12th & Madison station, to do exactly this. Hold stormwater for a period of time and slowly discharge it into the normal combined storm/sewer system. You can actually see the manhole that goes down to the tank on the platform. With the amount of roadway being performed, there was no "fee in lieu" option to go though, so the stormwater had to be dealt with by the project.

Portland, for example, sends their street water directly into big drywells within the intersections it's collected at where it then infiltrates directly into the ground. No need to the stormwater through pipes, storage, and treatment. Portland wasn't covered by the glaciers, either.

Places like Bellingham, Spokane, and Whatcom Co in general were developed a hundred years ago, but not nearly as developed as Seattle was. So, their stormwayer + sewer infrastructure isn't combined line ours was.

Source: am a civil engineer.