Natural barriers are rivers, steep slopes, dense woods and swamps that slow people without any building, and each one works only in some seasons and conditions. A 100 m climb up a 30° slope takes a walker roughly 12 minutes where 200 m of flat ground takes about 2.4, but a stream that freezes, a swamp that dries and a wood that burns stop being barriers, so test each one in every season before relying on it.
Part of Perimeter Defense Design
Natural barriers are features of the land, such as watercourses, steep ground, thick undergrowth and wetland, that delay movement and are used as part of a perimeter in place of built defences.
Each Feature Slows A Different Kind Of Movement
The parent article names four: rivers and streams, steep slopes, dense forest and swamps. It says even a stream 5 m wide and 1 m deep is a significant obstacle, a 30° slope is extremely difficult to assault uphill, thick undergrowth is noisy and slow and wetland is impassable to vehicles and exhausting on foot. It recommends placing the perimeter on the far side of water from the likeliest approach, leaving forest intact as an outer barrier with controlled paths, and keeping wetland on the flanks.
None of them is a wall. Each makes one kind of movement slower, and each fails in a way you can predict.
What You Need
- A visit in each season, or local knowledge. Ask people who know the land.
- A way to measure slope and distance: a pace count and a plumb line or a clinometer.
- A map of fords, dry gullies and animal tracks through each feature.
Method: Test The Barrier, Then Use It
Step 1: Rivers and streams. Walk the bank for crossing points: shallows, gravel bars, fallen trees. Moving water is a hazard to your own people: as I recall from flood-safety guidance, about 15 cm of fast-moving water can knock an adult off their feet and 30 cm can float a car; check this for the streams you rely on. Watch for freezing: a stream that freezes hard becomes a road, and the ice is also a danger to your own children.
Step 2: Slopes. Measure the angle and height. Slopes are most useful where the approach must climb under observation. Keep the footpath your household uses off the steepest part and well marked.
Step 3: Dense woods. Walk through the dense growth and notice how noisy it is. Leave a belt intact, and cut one or two controlled paths. Dry woods are a fire risk to the site, so keep a cleared strip between woodland and buildings.
Step 4: Wetland. Mark firm routes through it. Wet ground dries in drought and freezes in winter.
Step 5: Record the fords and gaps as the places for noise devices and lookouts.
Worked Example: What 30° Costs A Walker
I am recalling Naismith’s rule (W. W. Naismith, 1892, a mountaineers’ rule of thumb): allow one hour per 5 km walked, plus one hour per 600 m climbed.
- The climb: a 30° slope climbed through a height of 100 m has a horizontal run of 100 m ÷ tan 30° = 100 ÷ 0.577 = 173 m, and a slope length of 100 m ÷ sin 30° = 100 ÷ 0.5 = 200 m.
- Walking time for the run: 173 m at 5 km/h = 173 ÷ 5,000 h = 0.0346 h = 2.1 min.
- Climbing time: 100 m ÷ 600 m per hour = 0.167 h = 10 min.
- Total: about 12 min.
- For comparison, 200 m on the flat at the same rule is 200 ÷ 5,000 h = 0.04 h = 2.4 min.
The slope is five times slower for the same path length. It is a rule for a hiker with a pack, not for someone scrambling in a hurry, and a fit person with no load can go faster. As an indication it supports the parent article: a slope is the best barrier where the approach must climb it, which is why the farm and the lookout belong on top.
Common Mistakes
- Relying on a barrier that is seasonal. Frozen water and dried swamps are open ground.
- Putting your own exit through the barrier. A river between you and the escape route is a trap.
- Clearing the forest belt for firewood. Cut selectively and leave the outer belt.
Safety
Water, ice and fire
Do not cross moving water above knee depth without a rope and a second person. Do not test ice by walking on it. Dense woods burn and the fire reaches buildings faster than people can leave; keep a cleared strip around the dwelling and plan an exit away from the woodland side.
What’s Next
- Learn next: Channeling Approach Routes — using barriers to steer where people walk
- Combine with: Local Terrain Analysis — the survey that finds them