How practical are walls and other tall fixed fortifications when it comes to the defense of a city in a post-apocalyptic setting?

Question

I want to have Washington, DC have a militarized defensive wall (called the Federal Wall) somewhat similar to the Hoover Dam but with artillery, air, and infantry support stationed on its (imagine something like this from Fallout (https://vignette.wikia.nocookie.net/fallout/images/d/db/FNV_concept_art_Hoover_Dam.jpg/revision/latest?cb=20100504192604 (https://images.app.goo.gl/4gApVLP3QVW5N45Y7))). My original idea was to have the Federal Wall basically take up I-495 (https://hobnobblog.com/wp-content/uploads/2013/06/beltwaymap.gif (https://images.app.goo.gl/6B66h5MHvisd24o6A)) in order to make Washington itself a very well defended area but after some thinking, I realized that making the wall go around I-495 was impractical and would be a poor us of resources and manpower. So instead, I’ve decided, I’ve decided to have the Wall protect and encircle Capitol Hill, the Southwest Federal center, the entirety of the National Mall, most of Foggy bottom, West End, DuPont Circle, U Street Corridor, Shaw, Truston Circle, NoMa, and Near Northeast (basically the center of Washington with all of the important buildings). The wall itself won’t be anything super crazy like the three walls from Attack on Titan. Imagine a slightly bigger/wider Great Wall of China that’s able to service aircraft, howitzers and mortars, trucks/support vehicles, infantry, etc.

Although the District of Columbia has greatly expanded its territory to basically directly control the entire DC metro area as well as large swaths of northern Virginia and bits of southern Maryland, the Federal Wall will be used to still act as a last ditch defensive structure in order to defend against both potential marauders and bandits as well as near-peer enemy militaries.

But in a modern (or rather, post-apocalyptic) context, how well do walls work? I know that walls are considered impractical in today’s day and age since modern military hardware and technology like aircraft and artillery can bring down any wall but would they still make sense as a physical protective barrier in a post-apocalyptic setting?

Answer 1

As I understand your question, your main faction controls all of DC, and has built this wall around the governmental 'hub'. They also control some of the surrounding states, so have a fair decent sized buffer around there wall (My knowledge of American geography isnt great so I probably don't fully appreciate the entire lay of the land).

I'm assuming here that this wall is several miles long based on google maps. That would take a lot of building resources and manpower that could be used to kuch greater effect elsewhere (like farming, constructing defences on the frontiers of their territory etc).

While it would be a substantial obstacle for a post-apocalyptic military, provided Virginia has combat engineers (or even just enough artillery), they would be able to make an entry point eventually. The defenders need to protect the entire perimeter, the attackers only need to be successful in a few points.

More than anything else, however, if Virginia manages to push Columbia back to the wall they will have already lost, whether or not Virginia is actually able to breach it. Without any farmland to sustain them and no allied forces that could help them break out they would cease to be any genuine threat, and could just besiege the capital until their surrender. It would make more sense for Columbia to focus on defending further afield.

Answer 2

As I'm sure you're aware, DC, during the Civil War era, was littered with fortifications, including an outer, "last ditch" ring of forts. You're kind of looking for the same thing.

I'm sure that in your scenario, you'll probably want the outer perimeters watched over by sensors & drones & mobile units. You mention the Beltway. That would be a great military highway, a veritable ring fort of its own. Just space out a number of mobile units that will make use of both directions. Anywhere a potential threat is located, at least two units will be close by. You could also create a ring of outer forts at all the Beltway's 40 intersections.

As for the "last ditch" wall around the government sector, I think that's viable, though more useful as a matter of propaganda than anything else. Basically, because once you actually have to rely on that wall for defence, the rest of your territory is most likely lost and you're just hopelessly and vainly staving off the final defeat.

You could make it relatively cheap and effective by simply choosing a large thoroughfare, like K Street, starting at the boundary of Georgetown City, and bulldozing all the buildings for two blocks north of the road. Push all the rubble into the one block area just north of the road. Continue over to 4th St NE or so, then hang a right and do the same on down to the water, in the vicinity of the old Navy Yard.

Close off "non essential" streets with rubble and erect watch towers and gates at intervals. You could make it look more impressive with concrete veneer walls on both sides. Main gates could be made to look nice with brick and fancy guard posts. Turn the cleared block into a long strip of grass.

This would give you an area of good visibility plus a reasonably lethal and ungothroughsome region that personnel will find extremely hazardous to cross and vehicles won't be able to access and would also find impossible to cross because of all the loose rubble, stones, jagged steel beams and rebar sticking everywhere.

Answer 3

Tall wars are still effective but pointless in modern war: a wall only has to be high enough to stop ground vehicles and/or personnel. Even then, in modern highly mobile warfare it's seen as extremely retro. These days defences are more likely to comprise sensors and suitable covering munitions.

If you're building a higher wall, you must have some specific threat you need to keep out. Low-flying drones might be the problem. Perhaps your wall needs to be high enough to force attackers out of ground clutter and into a zone where they can easily be picked out and destroyed by radar-controlled machineguns.

Answer 4

Sure walls are useful - regardless of the apocalyptic state you find yourself in

Walls surround some complexes and houses today - people build them mainly to keep out opportunistic criminals, or even to 'delay' or 'be aware' of anything undesirable that is approaching. In any societal collapse scenario, these issues become more common. This allows your city to 'exist' more securely even without being actively attacked.

Physically to attack a city with a military force walls only become a minor hindrance though, however once inside the attackers need to contend with 'urban warfare'. Although walls nowadays are too restrictive for our growing economical powerhouse cities, in a post apocalyptic world Urban Warfare may be unpalatable enough to an attacker. Development of the city would be contained, whereas with no wall the city is spread out and easily penetrable.

A wall is also a physical/intelligence holding point - it's more than just useful during battle, it is also useful during 'peacetime' during your apocalypse. A major component of conflict is reconnaissance and intelligence. Your city still needs to trade, get supplies or have immigration. Without a wall, anyone can walk in. With one, you at least have some form of barrier that filters traders/citizens prior to entry giving you an opportunity to screen them.

Answer 5

Lets start with the easy part, a wall would most certainly be effective against raiders and bandits. It doesn't even have to be incredibly substantial, just pose enough of an obstacle that opportunists are going to see significant risk for any reward they might be getting. A well patrolled chain link fence might meet that requirement for all but the most determined.

For a more organised foe that's a different matter, on the tactical level no fixed fortification is insurmountable assuming modern technology, but that does not mean that a wall has no value. The idea is to make it strategically unfeasible to achieve that tactical victory.

Such a wall increases the requirements for an attacker. For something like a chain linked fence it means that you have to provide your forces with wire cutters. For a fortification like what you are proposing the attacker would have to train, equip, and supply a significant cadre of combat engineers as well as a decent sized breakthrough force to take advantage of any holes they make. That's doable for most major powers today but in a post apocalypse where you have likely got far more limited industrial capacity and manpower it might be too much of an outlay of manpower. The enemy may be able to break the fortifications but the more formidable they are the more they risk ruining themselves to to break them, and it takes a lot of fanaticism to get people to commit to a battle where they know the best case scenario is a Pyrrhic victory.

Based on this it sounds like the smartest response for the attacker is just to ignore the walled area and take the rest of the territory, leaving the defender to rot. As a counter to this there needs to be mobile military forces based behind the walls. This forces the attacker to place the city under siege lest the defenders sally out to raid the attacker's supplies and ambush detached parties of men, bleeding the attackers dry. Again we come to the risk of a Pyrrhic victory as a prolonged siege can be as destructive on the attacker as the defender.

But there is a massive issue with all of this. Walls big enough to say to a roughly comparable power, "you may win but you will destroy your economy to do it" are quite likely to destroy your own economy. Which is why I would suggest something different from a monolithic single wall.

My recommendation would be defence in depth. Put in a simple patrolled chain linked fence (other fences are available) where you want the wall, that will stop your opportunistic raiders. For facing a proper military foe build a series of small fortresses around and out from the city (preferably all across the defender's territory but budget constraints will likely limit these to key areas). These fortresses should be in range to support each other with artillery and at least have the capacity to host a small raiding force that can sally out to strike at the attackers. As an addition you can pepper the area in between with pillboxes and bunkers. This means that the attacker is tied down slowly and methodically breaking open these fortifications and clearing them while the defender is free to launch raids and bombardments from any of the fortresses not yet under attack. These strong points can be as temporary or permanent as you want/your nation can afford, trenches, sandbags, and building ruins can work as well here as an armoured concrete bunker network.