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.

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.

Use an artificial system of locks, weirs and dams to control the flow of the river. When the water level is too low, close the downstream gates so the river backs up. When the water level is too high, close the upstream gates so it empties.

In order to be able to deal with droughts or downpour, you will of course need some room to store excess water upstream (a natural lake, artificial reservoir, floodplain or a section of river flowing through a deep valley) and to drain water downstream (like a larger river or ocean).

You see systems like this in many cities which have rivers flowing through them. Let's take the river Alster which flows through the city of Hamburg in northern Germany, for example. The river flows through two artificial lakes in the center of the city. Those lakes have a nearly constant water level all year around. How do they do this? With a system of variable weirs along the 50km upstream which meticulously control the inflow and a set of locks which control the outflow into the larger Elbe river (as well as prevent inflow from the Elbe when it has high tide). The system was built over 400 years ago, so you don't need 20th century technology to achieve that level of water flow control (although modern meteorology and electronic communication do of course help to improve the reliability). There is a German wikipedia article about the Alster lock system with lots of pictures.

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.

So, very much like London then?

Whilst the Thames is an iconic feature of London, virtually all London rivers which feed into it are now culverted and carried in pipes underground. This Wikipedia page lists many of them, but of course there are more. This website and book may also be of interest.

The simple answer is that the culverts start outside the city. If the river overflows, the area around the culvert floods, but the city itself is not affected.

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.

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.

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:)

Nothing easier.

Make the river Very Slow, but able to flow fast

The flow of a river can be measured in cubic meters per second. How fast does it flow? The math is rather straightforward: take a cross section of the river, note its area in square meters, and divide its m3/s flow rate by the area in m2. The result is a speed, in m/s.

Normally, rivers flow at a relatively consistent rate, and have just the cross-sectional area they need. When flow increases, the river rises. Since its banks are sloped, raising the river causes a quadratic or increase in its cross sectional area, but it also causes flooding.

We're going to change that all up. We'll make a river with a very large cross-sectional area. As such, the water moves very, very slowly, e.g. 0.05 metres/sec. When the storm comes, and the river's flow rate increases by a factor of 50, we increase flow to a factor of 50. Now, water is tearing by at 2.5 metres/sec, and canal boaters accustomed to the near stagnant river are like "nope!" We wildly overdesign this thing so even a 1000 year storm won't flood out our town every 5 years (assuming Al Gore exists on your planet).

Level control

The problem is gradient. Every river is on a grade, that's why it's not a lake. At normal / low-in-the-design-range flow rates, the water will want to be on the bottom of the deep channel,like that sad little garden hose trickle known as the Los Angeles River.

We fix that with some sort of weir that can quickly be moved/removed. Something like lock gates, notched to let pleasure traffic by. Or my favorite, inflatable rubber weirs that arch up from the river bottom. This will be a deep channel allowing the passage of deep ships, and if you can't make passable weirs any other way, you just treat the entire river like a flight of locks, opening and closing each weir serially to let the ship through, lowering the weir ahead of the ship and letting water level dip temporarily.

In wartime, all those weirs get blown to hell, and their failure mode is to let the river run free, so it ends up at the bottom, LA style. Wrecks it for navigation, which would be the enemy's aim, but failure mode is "not flood the city".

If you have a waterfall downstream and a relatively straight river bed, limiting the amount of water guarantees your to never have any flooding - all the water that is able to enter the tunnel is able to flow away just as fast.
If managing the amount of water entering the tunnel is not possible, you need to manage the flow speed inside: low friction materials on the tunnel walls and, if necessary, turbines that can use electricity to increase the flow velocity could increase the amount of water you can get rid of indefinitely (the water has to come in through the tunnel entrance, so if you can make it keep its velocity all the way through and the tunnel has a constant size, the amount of water exiting the tunnel always equals the amount entering it).

It's "trivial" (read: rather difficult) to calculate the maximum possible flow of the river:

Take the size of the catchment area.
Multiply it by the maximum rainflow rate over that area that you expect to get.

You will have to figure out what level of that rain, for how long, you wish to handle.

You can put in as many systems of catchment areas and lakes and so on as you like, but once they become waterlogged and saturated, they are no use. So your two options are: either build upstream flood-management systems that can handle every raindrop that can land on the ground, for as long as it might rain; or make a way for the water to flow through the city faster than it can fall on the ground.

Combining both obviously gives you a good backup ability if one fails, and focusing on management and mitigation obviously also saves those upstream from the city.

Management and mitigation is covered well through other answers.

Throughput is best handled through smooth, wide, deep channels through the city, and for some distance beyond.

Another option is to put turbines in there that can be driven by stored power, to drive water through at an accelerated pace, and provide power in non-flood times.

Another possible cause of flooding is by tidal flow and storm surge from the ocean. In this case, a raisable barrier in front of the city would seem to do the job.

But what if both floods happen at once? A day of storm surge, AND a day of maximum flow from upstream? At this point it definitely needs the turbines, since the upstream water under the city will not of its own accord flow into the deeper water of the ocean: it needs to be pushed by the turbines.