Choose the ground
Settle where water lasts the dry season, floods cannot reach the floors, the ground drains and the wind carries smoke away.
Everything for this step
- Field guide
- Planning a settlement → 12 sections Earth sciences → 7 sections
- Builds
- None yet.
- Reference sheets
- None.
- Sciences it rests on
- Earth sciences · Mathematics
- Track
- Settlement and planning, level 2 of 7 · next: The first public good
Why this matters
A settlement stays where it was first built: its streets, wells and boundaries outlast every generation that regrets them. Flooding is the commonest mistake, because the flat, fertile ground beside a river is the easiest to build on and the surest to go under. The flood people remember is not the one to plan for. A flood with a 1 % chance in any year, the "100-year flood", has about a 26 % chance of arriving within 30 years, and few memories reach back 100 years. The engineering standard is to set floors above that flood plus a margin called freeboard: FEMA recommends at least 0.3 m, and many places require 0.6 m.
The shape of it
- Find water that lasts the driest month: at least 15 litres per person per day for everyone who will live there, within 500 m.
- Build above the design flood, not the remembered one: the flood with a 1 % chance each year, plus at least 0.3 m.
- Choose a slope of 2–4 %, which drains itself. Over 10 % erodes; flat ground ponds.
- Test the ground: groundwater at least 3 m down at the end of the wet season, and soil that drains.
- Read the wind: smoke, smell and the noisome trades belong downwind of homes.
Doing it, step by step
The summary above says what has to happen. This says how, with the numbers that decide whether it works.
1 · Find water that survives the driest month
A site is only as good as its water in the worst month of a dry year. Measure the yield in the dry season: time how long a spring or stream takes to fill a bucket of known volume, or bail a well and time its recovery. You need at least 15 litres per person per day for everyone who will live there (find and hold water), within 500 m of every home.
200 people need 3,000 litres a day: a spring that fills a 10-litre bucket in 4½ minutes or less, day and night. Ask the oldest people whether the source has ever failed.
2 · Find the flood line from evidence
Look for silt lines and debris caught in trees and fences, water stains on walls and rocks, flood marks cut or painted on old buildings and bridges, and ask the oldest residents where the highest water reached. Mark the highest point any of them shows. On the flood pole at St George, Queensland, pictured here, the 1890 flood mark was overtopped in 2010, 2011 and again in 2012.
3 · Build above the design flood, not the remembered one
The remembered flood is a minimum. Engineers design for the flood with a 1 % chance in any year. Over 30 years that flood has about a 26 % chance of coming, since 1 − 0.99³⁰ ≈ 0.26, and over 70 years about 50 %; no one alive may have seen it.
Set every floor at least 0.3 m above the highest flood you can find evidence for, which is FEMA's recommended minimum freeboard; many places require 0.6 m. Where you cannot go higher, raise the building on a plinth or posts and keep the ground floor for things water cannot ruin.
4 · Read the slope
Measure the fall with a water level (irrigation) along the line rain would run. 2–4 % is right, a drop of 2 to 4 cm per metre: the ground drains itself without eroding. Under 1 %, water stands and every path is mud. Over 10 %, rain cuts gullies, roads need earthworks and soil can slide. Keep off the foot of steep slopes, ground with fresh cracks, and slopes where trees lean downhill, which are signs of creeping soil.
5 · Dig a test pit
At the end of the wet season, dig a pit at the lowest place you plan to build, as deep as the latrine pits will be, plus 1.5 m. Water seeping in means the groundwater is too shallow for pit latrines there: pits must end at least 1.5 m above the highest groundwater, and UNHCR looks for a water table at least 3 m below the surface of a site. Read the soil as you dig; the soils guide shows how to judge texture and drainage.
6 · Read the wind and put what smells downwind
Watch smoke, grass and cloth for a season and record where the wind comes from most often. Homes upwind; middens, workshops, lime and charcoal burning, tanning and slaughter downwind and downhill, at a planning distance of at least 100 m (plot, street and block). Shelter homes from the strongest winds with a slope or a row of trees, not by putting them in a hollow, where cold air and smoke pool.
7 · Decide once, write it down, and mark it on the ground
Choose with everyone who will live there (decide how you decide) and record the reasons: the water yield, the flood mark, the slope, the test pit and the wind. Stake the flood line and the building line where they can be seen, and write them into the property record so no one builds below them later. Then lay out the plots: plot, street and block.