Is submersion in a canal a good way to shelter from a nuclear strike? I live 1.5 miles from the center of a city in a nuclear-armed country, and an adversarial country has just put its nuclear forces on high alert during a time of extraordinary geopolitical tension. I am thinking about what I could do to shelter from a nuclear strike. A 1Mt airburst over the centre of the city would result in widespread damage to/collapse of structures in my neighborhood with widespread death and near-universal injury according to an online simulator I used, and I have no cellar.
However, I am very near a canal path which radiates outwards from the center of the city. If upon hearing an air raid siren I got on my bike, cycled outwards along the canal for 16-minutes (tripling the distance between me and the city centre to 4.4 miles), jumped off my bike into the canal with ballast as soon as the buildings around me light up due to the explosion, and held my breath for 2 minutes at a depth of 1.5m [edited from 3m after my friend corrected me about the depth of that canal], would the water protect me from the heat and shockwaves? Would sheltering in a body of water at 1.5m depth be better than sustaining injuries in a house?
 A: Apart from assuming that the warning systems indeed sound air raid siren some time before the actual explosion (which should not be taken for granted), the scenario is highly unrealistic.
The primary effects of nuclear explosion are the shock wave, the thermal wave, and the electromagnetic pulse. These are however limited to a radius from a few to a few dozen kilometers from the center of the explosion, depending on whether the bomb hits the ground or explodes in the air, the charge, the presence of mountains, high buildings etc. Note that the resulting shock wave propagates with supersonic speed. Thus, whatever is within radius of a few kilometers from the center will be destroyed instantly - not enough time to jump off a bike. Thermal wave is similarly lethal - in particular, the water in the channel might simply evaporate.
Penetrating radiation propagates much further. Although radiation is nowadays commonly associated with nuclear energy, it was not considered the main factor when the bomb was created, and wasn't the major cause of destruction during the two nuclear bombings. Harmful effects of radiation became known much later - in fact, decades later after numerous open-air tests on Nevada testing grounds. Still, in the case one survives the explosion, one will be left to die more or less slowly from the radiation sickness.
It is naive to hope to survive a nuclear war - a fact that was widely known some 30 years ago, but apparently forgotten (or never learned) in the post-Cold-War era (see Mutually Assured Destruction)
A: Good question from a purely physics point of view.  Here is an answer I found from Karl Lembke, Supervisor, Water Quality Inspectors, Los Angeles Department of Water and Power
In many cases, what you want for absorbing or blocking radiation is a lot of mass. Water, at about one metric ton per cubic meter, is a lot of mass. It’s also useful because you can see through it, and it will assume the shape of whatever container you pour it into, so you don’t have to worry nearly as much about gaps in the shielding.
For gamma rays, you have three major mechanisms for absorbing radiation: the photoelectric effect, Compton scattering, and pair production. At low photon energies, the photoelectric effect matters, and water is less effective than, for example, lead. But if space isn’t an issue, that just means you add more water. At higher photon energies, Compton scatter is pretty much independent of the atomic number of the matter involved, so concrete is often used for shielding. It’s cheaper than lead. Ditto for pair production when photon energies get above about 1.1 MeV.
Water, or anything else that’s rich in hydrogen atoms, is great for stopping neutrons by elastic collision. In a collision between a neutron and a proton (a hydrogen nucleus), most of the kinetic energy can wind up being transferred to the proton, leaving the neutron slowed way down. At this point, a neutron may be absorbed by an atomic nucleus, or failing that, it undergoes a beta decay leaving behind a proton and an electron. (Half-life is 10.8 minutes.)
But in the final analysis, water makes a good radiation shield because it’s relatively heavy and you can put a lot of it around a radiation source for not a lot of money.
For your particular question, I just think you'd need to stay under the water for a long time until the radiation dissipated.
A: Water readily transmits shock waves, so while being under water and in a ditch shields you from infrared and gamma radiation the shock wave may still kill you, dependent on the distance from the blast, the altitude of the detonation and the size of the bomb.
For a 1 Mt bomb at an altitude chosen for a maximum blast radius, 4.4 miles or 7 km is just outside of the area of total blast destruction: Wikipedia estimates a blast radius of 6 km for a 1 MT nuke. As with tsunamis there will be effects of terrain and structures deflecting and diffracting the shock wave, leading to local deviations from the average. While you want to be in a canal running tangentially to the blast, and find a spot on the shore facing the blast for additional shielding from the dirt, it would bee ill-advised to be close to buildings or under bridges which may collapse and bury you under water, which would limit the extension of your life span to a few minutes until oxygen runs out, unless you die from the mechanical impact first.
Water absorbs electromagnetic radiation in the infrared spectrum, so being under water is a a great idea in order to protect against it. 7 km is well within the conflagration radius, according to the same Wikipedia source. Avoiding instant conflagration is a good first step towards survival. I am not sure whether the absorbed energy is sufficient to heat the water significantly; if so, the upper layers would be heated most, so the deeper you immerse yourself the better, in case that wasn't obvious, and don't dawdle when you climb out.
Long term survival depends on so many contingencies that it is impossible to predict. The most important one is that all things considered, nuking your city is likely not a fluke, so you'll find yourself in the middle of a nuclear war zone which somewhat dims your outlook.
But the immediate effect of the nuke may be survivable, and being under water should improve the odds. The general factors determining mid- and long term survival are:

*

*Wind transporting the fallout

*Equipment to protect yourself

*Access to potable water, food and medical supplies

*Availability of transportation to leave contaminated and inhospitable areas

*Depending on your location, the time of year and the weather: Access to shelter

A: As mentioned in mmesser's comment, I see no reason to suspect that a nuclear exchange is likely in the foreseeable future.
For explosions generally, if you live long enough to start making decisions, and aren't trapped in a firestorm, you are concerned about pressure waves, shrapnel, and falling debris.
For pressure waves, being behind something solid, durable, and low to the ground is good. Making yourself small and covering your ears is good. Being underwater might help, I'm not sure. This applies to any explosion - being near to an accidental explosion or a conventional bomb is more likely than getting nuked. If you see an explosion, don't watch it and don't try to run away: immediately get low, get behind something, make yourself small, and cover your ears until well after you hear the sound.
When it comes to shrapnel and falling debris, distance, being behind something solid that can't be turned into shrapnel or blasted into falling rubble, and being low to the ground, are good. Having any kind of barrier between you and falling debris is good. (It's popular to mock "duck and cover" as a useless placebo, but if you've seen an explosion and you aren't already injured, shrapnel and falling debris are your most likely sources of injury and "duck and cover" is your optimal survival strategy. Again, this goes for any explosion.)
Shockwave and shrapnel hazards are reasonably well satisfied by the wall of a canal with sufficient distance to be out of line of sight of the airburst. (Shelter against the wall closer to the blast, make yourself small, and cover your ears.) So would a metro/subway station, a highway overpass, an underground structure without a tall building on top of it, or even being on the far side of a hill and as low to the ground as possible.
The best defense against hazardous materials (including radioactive fallout) is to be upwind. This is probably worth taking half an hour to have a plan for: bookmark a couple of websites with your city's current prevailing wind direction, and know your evacuation route in at least two directions. Again, this applies to non-nuclear threats, in particular forest fires.
Secondary exposure to irradiated objects, especially metals, is a last hazard to consider. Salvaging metal objects (gold, tools, weapons) may inadvertently put you in long-term proximity to radioactive materials.
I don't think being underwater would help against a nuclear weapon's UV pulse (water is mostly transparent to UV at depths of a few meters, and would heat up to lethal temperatures anyway), or against the firestorm. WWII accounts of firestorms inflicted by conventional weapons indicate rivers being no escape for the unfortunate souls who tried. It might protect you from direct gamma radiation, but if you have fatal burns it doesn't matter what your gamma dose is.
There's no guarantee that a weapon will go off in a particular specific place, or a specific time after sirens, so you might end up running in the wrong direction or find yourself in the open at the worst moment. For any major disaster including a nuclear strike, I think it's probably better to seek shelter as near as possible to wherever you are and hope for the best, then work on making your way away from and upwind from the epicenter.
A: By the time you see the buildings around you light up, you have already got the gamma radiation dose. The neutron dose is not far behind. Probably they arrive before your reaction time after the light. The neutron energy depends on the design. But a tritium-deuterium weapon will make a big whack of 14 MeV neutrons that arrive at about 5% the speed of light.
As well, there is sky shine. Both gamma and neutron radiation can scatter off air. So you will be getting dosed off the buildings and the air above you.
The blast wave is coming quite stupendously fast behind that. At 6 km, you might well not have time to react before it reached you. If you leaped into the water after only 0.25 seconds (a not-bad reaction time) you might well still be in mid air when the blast hit.
So the plan of waiting and jumping in is not good. You would not have time to benefit from any protection you might get.
You would be better behind a substantial earthwork. Say a hill that is at least 10 meters tall, taller the better. And with as much of the sky blocked as possible. Earth or stone will block radiation at least as well as water. And it will also block blast wave. If there was a honking-big-and-thick stone wall you could shelter behind, that might give you some chance. A hill full of trees might be a bad choice, since they could be on fire in a second or two.
Better still would be a honking-thick-walled enclosure. Say a subway tunnel at least 10 meters under ground, more the better. And not running towards the city center. The blast wave might well find the tunnel. You do have the concern of getting out after, if there is a lot of rubble on the entrance. There is always the chance of tunnel collapse due to the explosion. And every ventilation shaft is going to be dusting down fallout.
Or maybe go stay with your relatives in Outer Nowhere for a few months. One way or the other, this is going to come to a head fairly soon.
A: You have also forgotten that from ground zero out water will evaporate to a point. Once the pressure dissipates water will rush back into the canal from further upstream to take the place of the evaporated water, likely pulling you closer to ground zero, think tsunami as the water flows back.
A: Partial answer re: depth charges, underwater sound and shockwaves
People have commented several places that being underwater may make shockwave effects worse because it is a known phenomenon that water transmits shockwaves somehow "more" than air, and the human body contains hollow air-filled structures (lungs, sinuses) where energy will be deposited.
I'm not confident in my knowledge, so I'm hesitant to say anything, but I think this is a mistake that doesn't need an expert to correct, and which will be clear once anyone thinks about it. This also may be a bit long winded since I'm talking myself through it as I go.
what's going on with depth charges
Water doesn't magically make sounds more energetic. Energy is energy and the inverse square law is the inverse square law. So why can submarines or whales detect sounds many kilometers away with great clarity? Why are underwater explosions so harmful? Why can the clicks of whale communication be harmful to smaller animals like humans?
First let's define harm in a language we can do physics with. Harm is something that has to do with energy deposited into a target much more rapidly than the target can dissipate or radiate energy. Physical structures in the target can be approximated as locally stable energy equilibria. Breaking them means pushing them from that equilibrium, up and over the edge of an energy well.
How you apply the energy affects how fast it can be dissipated or radiated - for instance, shooting a high power laser pulse at somebody might vaporize part of their skin, but steam will carry the energy away as fast as it is put in, so the injury will be superficial. Shooting an equivalent energy bullet at the person will shatter bones and rupture organs, because there's no mechanism for the energy to escape as fast as the bullet is putting it in, except out through the exit wound with what's left of the bullet. Conversely, a high school pitcher throwing a baseball at a person has the same mechanism (physical impact of a mostly rigid object) and the same energy as a handgun bullet (around 500 joules). But it applies the energy much more slowly because of its much lower velocity. Even if it strikes a rigid, small-area structure like the forehead, it will leave only a bruise.
Pressure waves deposit energy into a target. We can therefore infer that, all other things being equal, more energy in less time means more damage.
As I said at the beginning, energy is energy and the inverse square law is the inverse square law. Immersing a bomb in water doesn't make it more energetic. Bombs are solid objects with a well defined minimum radius, so there's no way to cheat the inverse square law. Let's use an old timey spherical bomb: the total energy at the surface of the bomb, is (approximately) the same as the total energy at any larger spherical shell, and the ratio of energy densities is the same as the ratio of squared radii.
So underwater we are doing more "harm" - that is, more energy in less time - with the same energy. It is the speed (literally) with which we are applying that energy that makes the pressure wave more harmful.
The speed of sound about 5 times larger underwater than in air. I have no idea how or if supersonic shockwaves work underwater, but suppose that in any case all pressure waves created by a given explosion propagate much more quickly underwater than in air, and therefore deposit energy in the target much faster than an equivalent energy density pressure wave in air. Same energy flux in less time equals more damage.
so what about in-air explosions underwater
Now let's say we detonate the same bomb above the water surface. Suppose for the sake of argument that no energy is reflected at the water-air interface. (This is not realistic - in reality most energy is reflected at the interface.)
The water can't transmit energy to a submerged target any faster than the surface of the water receives energy. That would violate energy conservation or at least require the water to have some mechanism for storing the energy for delayed release. In terms of power flux, then, the power flux through a target's cross-section at some distance $l$ from a surface which is itself a distance $r$ from the source of an in-air pressure wave is somewhere between the power flux at a distance $r$ from the source in air and the power flux at a distance $r+l$ from the source in air.
If we're 1.5 meters under water, ignoring other factors, our power flux is some value less than what it would be at the surface, and more than what it would be another 1.5m away from the weapon in atmosphere.
That would be a big range if you're talking about a bomb 3 meters away, and experiment or calculation would be needed to determine whether you'd be better off in air or underwater. But we're talking about a bomb at least 3 kilometers away - otherwise we're already dead no matter what we do. Our maximum increased power flux from diving 1.5m instead of walking 1.5m is $1-(3000m)^2/(3001.5m)^2 \approx 0.001 $ times more power flux.$^1$
So, in terms of power flux, water needs only to reflect less than 0.1% of incoming energy in order for you to receive less "harm" than someone standing at the same radius in air.
what about the human body interfaces?
Okay, but maybe there's something about getting hit in water that makes a given pressure wave power flux worse in water than in air. Pressure waves deposit most of their energy when they reflect at interface changes, so in water, your arms, legs, and guts will mostly transmit the energy unperturbed, while your lungs and sinuses will mostly reflect the pressure wave, and therefore the pressure wave will deposit lots of energy in those places. One imagines that one's lungs and sinuses might be more fragile than arms and legs, even accounting for how the articulation of the body will transmit most of that energy to vulnerable joints.
Again, this is a mistake that is easily seen and I don't need to know anything about the relative structural strengths of lungs and joints to know it. All I need to know is that in both cases, there are the same number of interface changes made of more or less the same thing.
That is: suppose a human is a bag of water (flesh, bone, etc) around a bag of air (lungs, sinuses)
If you are hit by a pressure wave in atmosphere, there is an interface change at your skin and a second interface change at your lungs. At the first interface (air to body) a certain amount of energy is reflected, a certain amount is deposited at the interface, and a certain amount is transmitted. The transmitted amount reaches the second interface (body to lungs). A certain amount is reflected, a certain amount is deposited, and so on.
If you are hit by a pressure wave in water that originated in atmosphere, there is an interface change at the surface of the water and a second interface change at your lungs. At the first interface (air to water) a certain amount of energy is reflected, a certain amount is deposited at the interface, and a certain amount is transmitted. The transmitted amount reaches the second interface (body to lungs). A certain amount is reflected, a certain amount is deposited, and so on.
Which would you rather absorb whatever percent of the energy is deposited by the wave on the air-to-water phase transition? The surface of the water, or your body?
Now, it may be that the energy deposited at the surface of the water is, in practical assessment, as bad for you at a depth of 1.5m as if it was deposited to you directly. Water deforms and a cataclysmically violent event like a large bomb might blast all the water away with a fraction of the energy deposited to the surface, making there be no practical benefit. I don't know. But surely, it's not significantly worse for the energy to be deposited, at least at first, a distance 1.5m away from you than inside your own body.

1: It's more complicated than this because of ground effect (the ground reflects most of the energy, so shockwaves propagate along the ground with somewhere between $1/r$ and $1/r^2$ dependency). And because of barriers (as noted in my other answer, taking cover behind something heavy and solid will cause a drastic reduction in the amount of energy you're exposed to, and the surface of the planet is a very heavy and solid thing to hide behind). But for a general idea of why 1.5m of distance doesn't matter much, I think it's true enough.
