You can realize a large basin upstream, where you can divert the excess water during overflow time.

You can then opt for:

• leaking that excess water on a path avoiding the city, going through expendable areas


• let that water drain through the terrain

First one is preferred, as in case of exceptionally extreme cases, you might prefer flooding low value areas instead of the city. Of course you need to take care that no abusive buildings are built in the area designated for being flooded.

Rivers with stable flow (with data source)

River downstream of a large lake

• The larger the lake, the most stable the flow out of it. Also, it helps that the lake be in a temperate environment, oo water input into the lake doesn't vary too much. The lake can freeze over, as can the outlet river, without really affecting flow much; this happens in the case of the example.




• Example - Saint Mary River (between Lake Superior and Lake Huron in US/Canada) at Sault Ste. Marie. Other good examples are the Rhone downstream of Lake Geneva, and the Neva downstream of Lake Ladoga.




• Pro - Very stable flow rate, year round (assuming the lakes don't freeze). Low ratio and magnitude of seasonal fluctuations. When improved with canals and locks, this makes for a great trading city.




• Con - In many cases, these outlet rivers are steep and un-navigable.

Extreme high flow 3590
Expected annual high 2385
Average flow 2142
Excess over expect high 1205
Excess over average 1448
Ratio over expect high 0.51
Ratio over average 0.68

(Note, all units are in cubic meters per second, except ratio)

Large tropical river

• A large tropical river where half of the river basin is in each hemisphere. As the monsoon rains are pushed by the Intertropical Convergence Zone back and forth between the hemispheres, you will end up with relatively stable rainfall throughout the year.




• Example - Congo River measured at Kinshasa.




• Pros - Very low seasonal flow variance. Also, an enormous basin for river-borne trade with the city.




• Cons - Very high magnitude of flow variance. When the river is this big, even a little bit of flooding is a big deal.

Extreme high flow 80832
Expected annual high 56081
Average flow 39536
Excess over expect high 24751
Excess over average 41296
Ratio over expect high 0.44
Ratio over average 1.04

A temperate river with little snowmelt

• The next best scenario is a river with a small basin with constant, low level rainfall, and little snow accumulation.




• Example - Seine River at Paris.




• Pro - Despite being a big enough river for a truly large city, the magnitude of highest recorded flood to average winter high water is not large.




• Con - Large seasonal fluctuations, can be difficult to navigate in summer time due to low water levels.

Extreme high flow 1284
Expected annual high 560
Average flow 268
Excess over expect high 724
Excess over average 1016
Ratio over expect high 1.29
Ratio over average 3.79

Examples of bad rivers

Continental river with large, dry basin

Example Arkansas River in the US, measured at Little Rock. In the interior US, random thunderstorms lasting for a few days can cause serious flooding, usually in spring or early summer.

Extreme high flow 8220
Expected annual high 2044
Average flow 1066
Excess over expect high 6176
Excess over average 7145
Ratio over expect high 3.02
Ratio over average 6.71

Continental river with enormous spring snowmelt

Example - Tom River measured at Tomsk, in Siberia. While the high river levels with snowmelt is relatively predictable, it is still very large compared to regular river levels.

Extreme high flow 7500
Expected annual high 4622
Average flow 1047
Excess over expect high 2878
Excess over average 6453
Ratio over expect high 0.62
Ratio over average 6.16

River in Monsoon area

Example - Vijayawada river in southern India measured at its mouth. While the timing of the monsoon is predictable, its magnitude is not. An erratic monsoon can produce spectacular flooding.

Extreme high flow 16555 
Expected annual high 6266 
Average flow 1642 
Excess over expect high 10289 
Excess over average 14913 
Ratio over expect high 1.64 
Ratio over average 9.08

1. Build it massively over capacity

This is the expensive option, and would be really hard to get through your funding committee. Though of course it would have to be built above "normal" capacity to handle day to day fluctuations in flow. "Flooding" is basically the point at which your system is over capacity, if the capacity of your system is high enough it will never flood.

2. Beavers

This is one of the flooding prevention options. We've mostly killed off the beavers, but they provide an important service where they still exist. Their dams slow the river flow and hold back floods to smaller areas upstream. This slowed flow means that it never builds up to the point of flooding in downstream areas.

3. More vegetation in your river catchment

Another flood prevention option. The more permanent vegetation, the slower the rainwater reaches the river. Again meaning that even with heavy rains the water filters down to the river at a more measured rate and never builds up to the point of flooding further downriver.

4. Don't build on the floodplain

Leave the river space to flood. This sounds like a very simple and obvious option but you'd be amazed how many cities are built on the main floodplain of their river. Even new build is still going up on the floodplains.

5. Accept that it's going to flood sometimes

I spent a portion of my early childhood living in a house on legs. Under the centre of the building was a staircase leading up to the house, with legs around the outside.

What if it's more about the city's design that the river's ?

Any body of water can overflow.

A city that could float, though, wouldn't necessarily suffer from this - it could just rise with the waters.

This is not as complicated as it seems and a wholee lot more work then it seems

Your Tunnel

First off you need to design the tunnel to meet your requirements whatever they may be, Rivers want to find the easiest and shortest route to lower ground every turn in the river slows it down fractionally. so make your tunnel nice and straight. but as few blockages like grates and covers in the tunnel as possible this will allow the water to flow freely without impediment and also stop anything that gets washed down the tunnel from creating a blockage.

Overflow Pipes

Have these staggered along the the length of the tunnel, should water reach high enough then it will fall into these overflow pipes and out of the tunnel. have these every so often so that if 1 somehow gets overwehlemed for some reason the water will then drop into the next one and the next one etc etc.

These pipes should lead to either a second much large tunnel that would spend a lot of time mostly empty, or out of the city and into another river somewhere else.

Flow Control

This is one of the most important parts have something like a Dam or series of Dams upriver from your city, this will ensure enough water can enter your tunnel but stop or at least limit flash flooding. obviously build your Dam how they try to build them in real life, with overflow piping and the ability to divert water elsewhere if required.

So long as you build it along these lines then your city should be fine. although there is always the old proverb about "best laid Plans"

Have a restricted entry for the river - for example, it passes through a hole in the bottom of a wall.

If the river starts to flood, then the hole limits the amount of water that can come through into the city portion of the river, while the wall redirects the flood waters to tributaries or around the city (into a moat?)

The river then exits through a matching hole at the other end of the city, and (optionally) rejoins the overflow.

(The flow rate of water through the hole when it is submerged should be slightly under 14 * Size_of_Hole * Height_of_Water_above_Hole m³s^-1. Build your wall and the flood-plains alongside it accordingly.)

Similarly - if you have a deep /wide ditch or gorge around your city, you have bring the water in via an aqueduct, which will overflow into the gorge in case of a flood, instead of into your city.

Build a top water limit for the river, like a roof.

When the initial river overflows, at the river will contain always the same amount of water, while the overflow water will go somewhere else.

You can even make the roof out of glas, so you have a modern view of the river

Hope this helps:)

This actually pretty hard to do. Here's why: Water flows downstream. You need a difference in water level between upstream and downstream of your city. Flow through a pipe is essentially a rather complicated function - typically you use the Darcy Weissbach equation in combination with a Moody chart - of this level difference. Approximately, the head loss, or required level difference between upstream and downstream, quadruples when you double the flowrate.

In open flows like rivers the relationship is far more complicated because with higher flow rate the river bed or channel is usually filled more, which means less then quadruple head loss for double flow rate. Open channel flow is not trivial but please read around a bit

What does this mean for your city?

Let's take one of the rivers from Kingledions answer, the Seine with low flow around 100 m³/s, average flow of 280 m³/s and extreme flow of 1280 m³/s

It's often a good idea to look at flow systems starting downstream. Let's say at average flow conditions you have a downstream water level of 50m above sea level. You city is 5 km across, really small. This calculator tells me, with an 8m diameter, 5km long pipe, my pressure loss is 72.445 Pa wich is equivalent to a head loss of 7.2 m. So my upstream reservoir level will be 57.2 m at average conditions.

Now, let's take the extreme flow, now I have 1513.962 Pa - 15m! We need a dike between our upstream reservoir that's 65m above sea level, and 7 m above the normal level of the lake or what have you. Actually more, because at extreme flows the downstream level will be higher too, by a few m!

I suggest a second channel or even third that's only opened at high flow events, also play around with the sizing of the pipes.

On the other hand, what happens at low flow? At normal flow, we have a flow velocity of 3.6 m/s . At low flow - 100 m³/s - the flow velocity is 1.3 m/s. The DWA-M 275 (An industrial code, Germany, for designing piping systems in wastewater treatemtn plants that I happebn to have open at the moment) advises a flow velocity of at least 2m/s for raw sewage. Why? Sedimentation! at lower flow velocities, sand etc. will sediment and remain in the pipe. In actuality I don't see this problem, because in all likelyhood your pipe will be the fastest streaming part of the river system.

Environmental impact assessment

The pipe will stop migrating fish, at least most of the time, from swimming upstream. This could seriously impact aquatic ecosystems along your river.

The easiest solution is to build your city where existing geography supports your goals. The most obvious (and extreme) example of an overflow-proof river on Earth would be the Grand Canyon in the Southwestern United States. With the walls of the canyon rising an average of 670 meters above the river bed, there’s no chance of it overflowing a hypothetical city built above it.

You’d want something similar, though probably not as extreme - a river cutting through (and eroding) hard rocks to create high, nearly vertical canyon walls that prevent overflow. The specific rocks and age of the river would determine how high the canyon walls are, so adjust as desired. It took the Grand Canyon anywhere between 6 and 70 million years (depending on who you ask) to develop into its current state, so adjust your time scale as necessary and you’ll get what you want.