# how fast will water spread id a dam explodes?

I have a post-grad assign mentwith the following scenario: A small town is 18 km away from a big dam and we have to create a system to evacuate the town if something bad happens. For instance, a huge explosion. What I have no idea of how to do is to calculate how much time do the citizens have to run away before they are flushed away… Any help with that?

way too many unknowns. And what do you mean by explodes? Are you postulating that the dam will literally explode? How might that be imagined? I doubt even a nuclear weapon under a dam would “explode” the dam. The dam will fail of course, but not explode in any sense.

A dam exploding would have to be caused by an act of sabotage or the dam itself being struck by a weapon,a meteor impact or large aircraft flying at a high rate of speed into it. It cannot “explode” in any other manner as there no volatile chemicals or hydrocarbons routinely used inside the dam’s structure which could cause it to do so.

Also, if it did “explode” the denotation or the resulting shockwave would likely cause as much damage to the nearby town as the eventually flooding and mudslides created when the water the dam was holding back reached it.

Now if a dam failed, due to sabotage, poor maintenance, a natural disaster,etc, then how long it would take for the water to reach the town would likely depend upon:

[ol]
[li]How much water was released.[/li][li]The terrain between the dam and the town[/li][li]Whether there were one or more natural channels at or in the town to divert the flow[/li][li]The exact location of the town in reference to the dam (if it was on a hillside above the river controlled by the dam or on a plain alongside the river,etc)[/li][/ol]

You would probably get a better answer if you provide the size of reservoir behind the dam, the height/width of the dam,the terrain around the dam and the town,etc.

You might start here:

Johnstown was 23km downstream from a major dam that failed. It took the water 57 minutes to travel that distance. At least that gives a bit of precedent, although there are, as noted, LOTS of variables.

In the 1889 Johnstown, PA flood, which resulted from the catastrophic failure of a dam upriver, water from the breached reservoir traveled the 14 mile (approx. 22.5 km) distance to the city in 57 minutes. Items to consider, obviously, would be the average width of the channel and the slope of the river bed, but this may provide a rough baseline.

ETA: heh, ninja’d as usual.

Here’s another example:

Looks like you’ve got anywhere between 20 minutes and an hour and a half as a reasonable guesstimate.

There’s also this:Teton Dam - Wikipedia

But each dam is different so each scenario would be different.

Is this a theoretical town and dam, or a real-life example? If it’s real, look for the “dam inundation map” associated with the dam in question. If the map is not available online due to terrorism paranoia, try contacting FEMA and/ or the Army Corps of Engineers.

In 1993 a Missouri River levee failed at Chesterfield, Mo, flooding an approximately 4,000 acres.

Considering Highway 40 was not just a major route but the primary evacuation path for the entire valley, they would have kept it open until the very last minute, so 2.5 hours was about the maximum time they could push it.

At the same time NOAA said the Mississippi River at St. Louis was flowing at 1.08 million cubic feet per second, which would fill a bowl the size of Busch Stadium in 69 seconds.

Too late for the edit.

When the Taum Saulk reservoir dam failed in 2005, the lake level dropped by approximately 90 feet in 15 minutes.

Are you supposed to give a best guess answer from analogous cases, or are you supposed to actually calculate it? If you need to calculate it, you’re probably looking at transient flood routing using the Saint Venant equations - check out dynamic wave modeling.

Sorry for the terrible english and lack of details. I had a terrible night of sleep. And a hangover.

It is a theoretical dam and city, and the city needs a system to evacuate its citizens in case of a disaster (a failure on the dam). The data you guys provided is really helpful!

Question: What would you do if you were on a city 18 km away from a dam that just burst open? Is there any chance of survival if one escapes right after the disaster?

Climb the nearest hill at the fastest possible speed ?

Another variable is the amount of pre-planning was done by both the government and the towns citizens.

A town that sits in the path of an upstream dam and reservoir will have a government that has done, or should have done, specific pre-disaster mitigation planning. This would include how to give sufficient warning to all citizens, evacuation routes and alternate routes and sheltering. How do you contact citizens who don’t have phones or internet? How do you evacuate special needs and non-ambulatory citizens? Where will they go when they evacuate? Do you have special shelters that except pets? Most shelters don’t and most people will not go if they can’t take their pets. What transportation will you use to evacuate hospitals, nursing homes and assisted living centers? How will you evacuate the jail or prison? Can you, in an hours time, have enough responders to go door to door and clear every household, school, hospital and business?

Even if you give an hours notice many citizens will struggle unless they too have done some pre-planning. Have they, at some time in the past, collected any non replaceable items and made a evacuation Go Kit? Do they know where they will meet up with other family members if they’re away at work or school at the time of the emergency? Do they have transportation to evacuate? Some citizens will have mobility issues. Some will need medical oxygen, dialysis, special medications or other critical needs to stay alive. Have these citizens done their pre-planning?

As you can see, even if you are given the luxury of a one hour, the evacuation of even a small town is very challenging. If they haven’t done any pre-planning then…Surfs Up!

This again depends upon a number of variables, including:

[ol]
[li]The conditions of the roads - Two lanes vs. four.? State highway vs. interstate? Any bridges or tunnels? Mountainous terrain?[/li][li]What time of day it is - Morning ( better), afternoon ( maybe a lot of traffic, not so good) Evening/early morning before day ( Worst as teh darkness and the people being tired or having to wake up will delay their response time)[/li][li]Planning - If there’s a solid warning and evacuation plan, things may go well. If there isn’t, there will be mass casualties.[/li][li]History - Has this ever occurred before? Johnstown,Pennsylvania for example has experienced two major floods. The one in the 19th century was one of the worst natural disasters in American history. They would expect a dam incident. A town where there has never been a serious natural disaster, night still get caught off guard even with warming.[/li][/ol]

Some free advice: Sit back and really plot out the details on this before you make a decision or a plan. This isn’t going be a simple process.

The most famous incident of catastophic failure that springs to mind is the Vaiont Dam in Italy.

It was a landslide that caused a massive wave of water to overtop the dam and it killed thousands in many of the villages down the valley. It has been extensively studied so I’d image it’d be a good starting point to give you a feel for the sort of “worst-case” timings for evacuations (unfortunately, in this case there was no warning at all)

Move as fast as possible to high ground in a direction about 90 degrees from the direction of flow. If you have, say, 45 minutes warning, you can get two miles away at a brisk, but sustainable walking speed.

In reality though, the warning may come late and you may not be in a position to evacuate immediately (if your kids are at school, for example).

How tall and how sturdy is the tallest building around?
If it’s a steeple in an old wood church I head for the hills.
If it’s a 30 story concrete and steel Hyatt Regency I’m headed for the rooftop bar.

How interesting. I was a young child and yet I remember the news reports of that horrifying event.

Concur - almost. Solid, tall buildings would be the best compromise if the warning comes late, but high ground is always preferable to any built structure, if you can get there in time.