Why are/were sewers made large enough for somebody to walk down in there?

Question

Today, I have been reading a lot about sewer systems on Wikipedia, in the hope that I would find information or at least photos showing what I think of as sewers, namely somewhere under the streets in a city where you can climb down through manholes with ladders and then walk around on little pavements down there, with rivers of sewer water flowing calmly around and the walls being made of rocks or something.

That's what I think of when I hear "sewer". But I found none of that.

I read about ancient sewers (with very few photos or illustrations of any kind) as well as saw a photo of some modern sewer pipe which definitely doesn't let you walk around in it. Nowhere could I see the "real" sewers, which I associate with Victorian-era England.

Basically, I wonder: why did they have to make them so large-scale? Why do humans need to go down there? And if they had to back in the day, then why don't they now, or in ancient times?

A ton of details are missing in my understanding of sewers. I've also always wondered how they could afford to do this everywhere in a large city. It seems like an insane amount of work just to transfer away some turds. Yes, I realize it's a crucial necessity to prevent diseases and provide for modern/convenient living, but still, it seems so wasteful (no pun intended) to build them so "roomy". Maybe that was only done for certain sections or something?

I especially wonder how in the world they were able to do this when the city was already made. Seems like you'd have to tear down all the houses if you want to make massive sewer systems, and then rebuild all the houses. Did they really do that?

Answer 1

The few important reasons is that the digging was done manually by laborers, who like the miners had to work by getting in and needed cavernous spaces to breath and also needed to move the dirt out on makeshift tracks and wagons.

Modern engineering which strives for efficiency and least damage to echo system, designs the minimal working size pipes which can be placed by just pushing through or drilling or extruding the pipes leading to sanitary treatment centers.

And for maintenance they use digital scopes and routers and sump pumps mostly built onto a sanitation truck as opposed to sending men into the tunnel.

Centralized monitoring stations capable of detecting and rerouting a clogged pipe, etc, with new efficient piping have made it possible to have much more effective system with much less cost.

Answer 2

Some things to consider regarding Victorian era sewers.

How much effluent do humans produce each day and how many people are there and thus how large does a sewerage system need to be to accommodate that quantity of effluent?

The other thing to consider is the technology available at the time and the ease and cost of construction. Allied to this is the need for maintenance and repair of the system and how maintenance will be undertaken.

At the time, brick lined tunnels were an effective and safe means of constructing a sewer, when human labor was abundant and cheap and some tasks were not mechanized.

Also, will the sewer system need to accommodate other things that may intentionally of unintentionally enter the system that could cause blockages or damage the system. Fatbergs have become a recent issue for sewage systems.