---
title: "Living with Natural Risks"
book: "Primary & Middle School Earth and Space Sciences"
subject: earth
language: en
chapter: 29
exercises: 12
source: https://one-course.com/books/earth/1/en/chapter/29-living-with-natural-risks
license: CC-BY-NC-SA-4.0
credit: "One Earth and Space Sciences Book, One Course (one-course.com)"
---

# Chapter 29 — Living with Natural Risks

Late in the evening of 13 November 1985, most of the people of Armero, a town of Colombia, were in bed. A [storm](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-storm) hid the snow-capped [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) Nevado del Ruiz, $45\,\mathrm{km}$ away, and nobody in the town saw it erupt. The hot [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) melted part of its ice, and the water, mixed with ash and [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), rushed down the [valleys](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) as mudflows. About two hours later, the mudflows poured out of the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) of the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) Lagunillas, straight into the town: some $23\,000$ people, most of the town, were killed. Two hours was time enough to walk to higher ground. The people of Armero were not told.

![The hazard map of Nevado del Ruiz. A first version, drawn by Colombian geologists, circulated one month before the eruption and already showed the danger of mudflows in the valley of Armero. This version, redrawn after the disaster, adds in bright red the mudflows that really came in November 1985 (the shaded circle around the volcano has a radius of about 20\, km). From T. L. Wright and T. C. Pierson, USGS Circular 1073, 1992 (public domain, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g9-natural-risks/img-0a92f3bf306e.jpg)

*The [hazard map](#def-g9-natural-risks-hazard-map) of Nevado del Ruiz. A first version, drawn by Colombian geologists, circulated one month before the [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) and already showed the danger of mudflows in the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) of Armero. This version, redrawn after the disaster, adds in bright red the mudflows that really came in November 1985 (the shaded circle around the [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) has a radius of about $20\,\mathrm{km}$). From T. L. Wright and T. C. Pierson, USGS Circular 1073, 1992 (public domain, via Wikimedia Commons).*

Six years later, at Mount Pinatubo in the Philippines, the [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) was watched, the warning came in time and the people left ([Chapter 19](https://one-course.com/books/earth/1/en/chapter/19-volcanoes#ch-g7-volcanoes)). Both were explosive [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano), with people living in the path of their mudflows. What differed was what people knew, and what they did with it. This chapter is about that difference.

**You already know.**

A *[natural hazard](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-hazard)* is a natural event that can harm people; it becomes a *[natural disaster](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-disaster)* when it strikes where people live and does great harm ([Chapter 12](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#ch-g5-natural-hazards)). A [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) that overflows covers its [floodplain](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) ([Chapter 10](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#ch-g5-water-shapes-the-land)). A [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) often gives warning signs before a large [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption): small [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake), swelling slopes, gas ([Chapter 19](https://one-course.com/books/earth/1/en/chapter/19-volcanoes#ch-g7-volcanoes)). Scientists cannot predict [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake); [seismometers](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-seismometer) record them, and an [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) under the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) can set off a [tsunami](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-tsunami) ([Chapter 20](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#ch-g7-earthquakes)). A [weather forecast](https://one-course.com/books/earth/1/en/chapter/25-air-masses-fronts-and-weather-maps#def-g8-fronts-and-weather-maps-forecast) says what the [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) will be in the coming days ([Chapter 25](https://one-course.com/books/earth/1/en/chapter/25-air-masses-fronts-and-weather-maps#ch-g8-fronts-and-weather-maps)).

## 29.1 Hazard, vulnerability and risk

The same [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) does not do the same harm everywhere. Under an empty [desert](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-desert) it harms no one. Under a town of strong buildings whose people know what to do, it shakes, frightens and breaks windows. Under a town of heavy, badly built houses, it can bring them down on the people inside. The hazard is the same; what changes is how easily the place can be harmed.

**Definition 29.1 (Vulnerability).**

The *vulnerability* of a place is how easily its people, its buildings and the things they need can be harmed by a [natural hazard](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-hazard). Homes on a [floodplain](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood), weak buildings, no warning and no plan make a place vulnerable.

**Definition 29.2 (Risk).**

The *risk* that a place faces from a [natural hazard](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-hazard) is the danger of harm to its people and their goods. It depends both on the hazard, how strong and how frequent it is, and on the [vulnerability](#def-g9-natural-risks-vulnerability) of the place: where there is no hazard, or nothing that can be harmed, there is no risk.

![The risk grid (a sketch). The risk grows with the hazard and with the vulnerability. Moving a town is rarely possible, and a hazard cannot be switched off; but its vulnerability can be lowered, which moves the town down the grid.](https://one-course.com/images/onecourse/chapters/earth-1/g9-natural-risks/fig-760db9ea7446.svg)

*The [risk](#def-g9-natural-risks-risk) grid (a sketch). The [risk](#def-g9-natural-risks-risk) grows with the hazard and with the [vulnerability](#def-g9-natural-risks-vulnerability). Moving a town is rarely possible, and a hazard cannot be switched off; but its [vulnerability](#def-g9-natural-risks-vulnerability) can be lowered, which moves the town down the grid.*

Armero was a place of high hazard: a snow-capped [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) that had woken a year before, and a town built on the deposits of old mudflows, at the [mouth](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-source-mouth) of a [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) that had carried them before. It was also highly vulnerable: the scientists lacked instruments and support, the information given to the authorities was often vague, and no evacuation plan and no quick way to warn the town existed.

## 29.2 Forecasting and prevention

Some hazards can be seen coming. A [storm](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-storm) is forecast days ahead ([Chapter 25](https://one-course.com/books/earth/1/en/chapter/25-air-masses-fronts-and-weather-maps#ch-g8-fronts-and-weather-maps)); a [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) after heavy [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation), hours to days ahead; a large [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) often announces itself days to weeks ahead, though not always. A [tsunami](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-tsunami) crosses an [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) in hours, so a warning can reach distant [coasts](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) before it does. [Earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake), on the other hand, cannot be predicted: scientists can say which [faults](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-fault) will break again one day, never the day.

A forecast, however good, is useful only if something is done before the hazard strikes. Most of that work is done long before any warning.

**Definition 29.3 (Prevention).**

*Prevention* is everything done before a [natural hazard](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-hazard) strikes to lower the [risk](#def-g9-natural-risks-risk): knowing where the hazard can strike, building in safe places and in safe ways, watching the hazard, warning people and preparing them.

[Prevention](#def-g9-natural-risks-prevention) works on both sides of the [risk](#def-g9-natural-risks-risk) grid. Watching a [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) or a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) does not make it less dangerous, but it gives time to act. Building away from the [floodplain](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood), building strong houses, planning the routes to higher ground and practising them all lower the [vulnerability](#def-g9-natural-risks-vulnerability).

![Being prepared: a family checks its emergency kit, kept near the door, ready to take if they have to leave quickly: water and food that keeps for several days, a torch, a radio that works without mains electricity, spare batteries, a first-aid kit and warm blankets (AI-generated illustration of a generic scene).](https://one-course.com/images/onecourse/chapters/earth-1/g9-natural-risks/img-3bf4cbd2d220.jpg)

*Being prepared: a family checks its emergency kit, kept near the door, ready to take if they have to leave quickly: water and food that keeps for several days, a torch, a radio that works without mains electricity, spare batteries, a first-aid kit and warm blankets (AI-generated illustration of a generic scene).*

## 29.3 Watching volcanoes and faults

A [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) observatory watches the warning signs of [Chapter 19](https://one-course.com/books/earth/1/en/chapter/19-volcanoes#ch-g7-volcanoes) with instruments that run day and night and send their readings to the observatory.

![Watching a volcano (a sketch, not to scale). Seismometers count the small earthquakes; tiltmeters and GPS receivers, fixed to the rock, measure how the ground tilts and moves as magma gathers below; other instruments measure the gases that escape; cameras watch the summit. Rain gauges help to tell the signals of the volcano from the effects of heavy rain.](https://one-course.com/images/onecourse/chapters/earth-1/g9-natural-risks/fig-794133180bc0.svg)

*Watching a [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) (a sketch, not to scale). [Seismometers](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-seismometer) count the small [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake); tiltmeters and GPS receivers, fixed to the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), measure how the ground tilts and moves as [magma](https://one-course.com/books/earth/1/en/chapter/19-volcanoes#def-g7-volcanoes-magma) gathers below; other instruments measure the gases that escape; cameras watch the summit. [Rain gauges](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-rain-gauge) help to tell the signals of the [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) from the effects of heavy [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation).*

None of these signs gives the day and the hour of an [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption). The scientists read them together, as a doctor reads several measurements of a patient, and say how the danger changes; the authorities decide when to close an area and when to move people out. At Pinatubo this chain worked. At Nevado del Ruiz, the [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) had given signs for a year, with swarms of [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake), but the instruments were few, and the chain from the scientists to the people of Armero was broken.

[Faults](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-fault) are watched in the same way, by networks of [seismometers](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-seismometer) and of GPS stations that measure, millimetre by millimetre, how the two sides of a [fault](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-fault) slowly move. This tells which [faults](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-fault) are storing up the strain of the next large [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake), but not when it will come. For [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake), [prevention](#def-g9-natural-risks-prevention) is the main defence.

## 29.4 Hazard maps and flood zones

The first step of [prevention](#def-g9-natural-risks-prevention) is to know where a hazard can strike.

**Definition 29.4 (Hazard map).**

A *hazard map* shows the places that a [natural hazard](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-hazard) can reach, often with several levels of danger. It is drawn from what the hazard has done in the past and from the shape of the ground.

The [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) of Nevado del Ruiz, at the start of this chapter, is a [hazard map](#def-g9-natural-risks-hazard-map): the mudflows of the past had followed the [valleys](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), and the next ones would follow them too. [Flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) [maps](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) follow the same idea. Water spreads over the low, flat ground beside a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water): the higher the [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood), the wider the band of ground it covers. A [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) draws the edge of the water for a chosen [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood), often the [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) that has a chance of 1 in 100 of being reached or passed in any year. It is called the *100-year [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood)*, but it is not a [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) that comes once a century: it can come two years in a row.

![The floodplain of the river of the Varden Hills, which flows east. Each shade is the ground that a flood would cover if it rose 2, 5 or 10\, m above the normal level of the river: each zone contains the smaller ones. The zones stay narrow because the sides of the valley rise quickly from its flat floor. The thin lines join points at the same height, every 10\, m, labelled every 50\, m; such maps are the subject of .](https://one-course.com/images/onecourse/chapters/earth-1/g9-natural-risks/fig-00375e94d436.svg)

![The floodplain of the river of the Varden Hills, which flows east. Each shade is the ground that a flood would cover if it rose 2, 5 or 10\, m above the normal level of the river: each zone contains the smaller ones. The zones stay narrow because the sides of the valley rise quickly from its flat floor. The thin lines join points at the same height, every 10\, m, labelled every 50\, m; such maps are the subject of .](https://one-course.com/images/onecourse/chapters/earth-1/g9-natural-risks/fig-ca3e7dbdd117.svg)

*The [floodplain](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) of the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of the Varden Hills, which flows east. Each shade is the ground that a [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) would cover if it rose 2, 5 or $10\,\mathrm{m}$ above the normal level of the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water): each zone contains the smaller ones. The zones stay narrow because the sides of the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) rise quickly from its flat floor. The thin lines join points at the same height, every $10\,\mathrm{m}$, labelled every $50\,\mathrm{m}$; such [maps](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) are the subject of [Chapter 32](https://one-course.com/books/earth/1/en/chapter/32-maps-of-the-whole-earth#ch-g9-maps-of-the-earth).*

**Method 29.5 (Reading a flood-zone map).**

1. Find which [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) the zone stands for: how high above the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) , or how often it may come.
2. Locate the place on the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) , with the [scale](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-scale) bar or the grid.
3. If the place lies inside the shaded zone, that [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) would reach it; if it lies outside, it would stay dry in that [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) , not in every [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) .
4. To compare places, use their heights above the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) : the lower, the more often flooded.

**Example 29.6 (Three houses beside the river).**

On a [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) like the one above, a house stands on ground $3\,\mathrm{m}$ above the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). A [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) $2\,\mathrm{m}$ above the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) does not reach it; [floods](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) of 5 and $10\,\mathrm{m}$ do. A second house, $8\,\mathrm{m}$ above the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), is reached only by the $10\,\mathrm{m}$ [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood), and a third, on a slope $30\,\mathrm{m}$ above the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), by none of the three. The first house is the most vulnerable: smaller [floods](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood), which come more often, are enough to reach it.

## 29.5 Building to resist earthquakes

In an [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake), the ground moves back and forth under a building, and the building is shaken from below; in a strong [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake), the danger comes above all from buildings that crack and fall. Engineers cannot stop the shaking, but they can build so that the structure carries the forces of the shaking without falling.

![Four frames shaken by the same ground (a sketch). A frame of posts and beams alone leans over and can collapse (a). Diagonal braces (b) and stiff shear walls (c) stop it from leaning; strong joints between the beams and the posts hold it together. Isolators placed under the building (d) give way as the ground moves, so that much less of the motion reaches the building.](https://one-course.com/images/onecourse/chapters/earth-1/g9-natural-risks/fig-79796bcb0e80.svg)

*Four frames shaken by the same ground (a sketch). A frame of posts and beams alone leans over and can collapse (a). Diagonal braces (b) and stiff shear walls (c) stop it from leaning; strong joints between the beams and the posts hold it together. Isolators placed under the building (d) give way as the ground moves, so that much less of the motion reaches the building.*

The same principles serve a house, a school or a bridge. They are easiest to apply when the building is designed, rather than added to an old building afterwards.

**In the lab — A tower on a shaking tray.**

Indoors, build two small towers of the same height from drinking straws or wooden sticks and modelling clay, as two-storey frames. Give the second one diagonal braces on every side. Stand both on a tray, then slide the tray quickly back and forth, gently at first, then harder. Which tower leans first? Which one keeps its shape? Then put the bare tower on four marbles under a light card, and shake again: what changes?

## 29.6 Warnings in time

The last part of [prevention](#def-g9-natural-risks-prevention) is the warning. For [tsunamis](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-tsunami), it starts on the floor of the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). At sites near where destructive [tsunamis](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-tsunami) have started before, a pressure recorder lies on the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor and reads the weight of the water above it every $15\,\mathrm{s}$, which gives the height of the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) surface. It sends its readings by sound through the water to a buoy moored at the surface, and the buoy relays them by satellite to the [tsunami](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-tsunami) warning centres. A rise of the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) far from any [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) shows a [tsunami](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-tsunami) on its way, hours before it reaches distant shores.

![A crew member of a United States Coast Guard ship inspects a tsunami warning buoy in the Pacific Ocean, in November 2021. The buoy floats above a pressure recorder on the sea floor, thousands of metres below. U.S. Coast Guard (public domain, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g9-natural-risks/img-ed4a9b226c57.jpg)

*A crew member of a United States Coast Guard ship inspects a [tsunami](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-tsunami) warning buoy in the Pacific [Ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), in November 2021. The buoy floats above a pressure recorder on the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor, thousands of metres below. U.S. Coast Guard (public domain, via Wikimedia Commons).*

**History — A network of buoys.**

In 2001 the first array of six such stations was complete, in the Pacific [Ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). After the [tsunami](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-tsunami) of December 2004, which reached the [coasts](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of the Indian [Ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) with no warning system in place ([Chapter 20](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#ch-g7-earthquakes)), the network grew: by March 2008, there were 39 stations.

A warning saves lives only if it reaches people who know what to do with it: the routes to higher ground are signposted and practised, the emergency kit is ready, schools hold drills. At Armero, the hazard was known and the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) was drawn; the warning never reached the town.

## 29.7 Exercises

**Exercise 29.1 ★.**

Hazard or [vulnerability](#def-g9-natural-risks-vulnerability)? (a) A [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) that [floods](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) almost every spring. (b) A village with no warning system. (c) Heavy houses of brick with no frame. (d) A strong [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) on a nearby [fault](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-fault). (e) A town built at the [mouth](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-source-mouth) of a [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) where mudflows have run before.

**Solution of Exercise 29.1.**

(a) Hazard. (b) [Vulnerability](#def-g9-natural-risks-vulnerability). (c) [Vulnerability](#def-g9-natural-risks-vulnerability). (d) Hazard. (e) Both: the mudflows are the hazard; building the town in their path makes it vulnerable.

**Exercise 29.2 ★.**

True or false? Correct the false sentences. (a) A [natural hazard](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-hazard) in a place where no one lives is a high [risk](#def-g9-natural-risks-risk). (b) [Prevention](#def-g9-natural-risks-prevention) can lower the [vulnerability](#def-g9-natural-risks-vulnerability) of a town. (c) A [hazard map](#def-g9-natural-risks-hazard-map) shows where a hazard can strike. (d) A good warning system makes a [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) less dangerous.

**Solution of Exercise 29.2.**

(a) False: where no one lives, nothing can be harmed, so the [risk](#def-g9-natural-risks-risk) is low. (b) True. (c) True. (d) False: a warning does not change the [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano); it lowers the [vulnerability](#def-g9-natural-risks-vulnerability) of the people, who can leave in time.

**Exercise 29.3 ★.**

For each hazard, say whether it can be forecast days or hours ahead: (a) a [storm](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-storm); (b) a [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) after heavy [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation); (c) an [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake); (d) the arrival of a [tsunami](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-tsunami) on a [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) far from where it started.

**Solution of Exercise 29.3.**

(a) Yes, days ahead. (b) Yes, hours to days ahead. (c) No: [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) cannot be predicted. (d) Yes: once the [tsunami](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-tsunami) has started, it takes hours to cross the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), and the warning travels faster.

**Exercise 29.4 ★.**

Use the drawing of a watched [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano). What does each instrument measure, and what warning sign of an [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) does it watch for? (a) A [seismometer](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-seismometer). (b) A tiltmeter. (c) A GPS receiver. (d) A gas instrument.

**Solution of Exercise 29.4.**

(a) The shaking of the ground: swarms of small [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) as [magma](https://one-course.com/books/earth/1/en/chapter/19-volcanoes#def-g7-volcanoes-magma) rises. (b) The tilt of the slope: the [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) swelling. (c) How the ground moves, by millimetres: the swelling as [magma](https://one-course.com/books/earth/1/en/chapter/19-volcanoes#def-g7-volcanoes-magma) gathers. (d) The gases of the plume: more gas escaping as [magma](https://one-course.com/books/earth/1/en/chapter/19-volcanoes#def-g7-volcanoes-magma) comes up.

**Exercise 29.5 ★★.**

In 1985, the mudflows of Nevado del Ruiz reached Armero, $45\,\mathrm{km}$ away, about two hours after the [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption). (a) What was their average speed, in kilometres per hour? (b) A person walks at about $4\,\mathrm{km}/\mathrm{h}$. How far could the people have walked in two hours? (c) Why did they not leave? (d) Give two things that could have saved them.

**Solution of Exercise 29.5.**

(a) $45 \div 2 = 22.5\,\mathrm{km}/\mathrm{h}$. (b) $4 \times 2 = 8\,\mathrm{km}$, far enough to reach higher ground. (c) Nobody saw the [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption), hidden by the [storm](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-storm), and nobody warned them: there was no quick way to send the information to the town, and no evacuation plan. (d) For example: an evacuation plan with routes to higher ground, practised in advance; a chain of warning from the scientists to the town; more instruments on the [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano).

**Exercise 29.6 ★★.**

Use the flood-zone [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) of the Varden Hills. (a) Which [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) covers the widest band? (b) With the grid of the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map), measure the width of each zone across the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) at $5\,\mathrm{km}$ east. (c) A barn stands on ground $6\,\mathrm{m}$ above the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). Which of the three [floods](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) reach it? (d) Why is the [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) $2\,\mathrm{m}$ above the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) the one that comes most often?

**Solution of Exercise 29.6.**

(a) The [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) $10\,\mathrm{m}$ above the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). (b) About $0.47\,\mathrm{km}$ for the $2\,\mathrm{m}$ [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood), $0.56\,\mathrm{km}$ for the $5\,\mathrm{m}$ [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) and $0.67\,\mathrm{km}$ for the $10\,\mathrm{m}$ [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) (readings within about $0.05\,\mathrm{km}$ are fine). (c) Only the $10\,\mathrm{m}$ [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood): the ground is higher than 2 and $5\,\mathrm{m}$ above the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). (d) A small rise of the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) is needed to reach it, and small [floods](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) come more often than large ones.

**Exercise 29.7 ★★.**

Place each situation on the [risk](#def-g9-natural-risks-risk) grid (low or high hazard, low or high [vulnerability](#def-g9-natural-risks-vulnerability)) and say whether the [risk](#def-g9-natural-risks-risk) is low, moderate or high. (a) Strong buildings and drills in a town on an active [fault](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-fault). (b) A camp site on the [floodplain](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) of a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) that rarely [floods](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood). (c) A village of weak houses at the foot of an active [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano), with no observatory. (d) A town on a hilltop far from any [fault](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-fault), [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) or [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano).

**Solution of Exercise 29.7.**

(a) High hazard, low [vulnerability](#def-g9-natural-risks-vulnerability): moderate [risk](#def-g9-natural-risks-risk). (b) Low hazard, high [vulnerability](#def-g9-natural-risks-vulnerability): moderate [risk](#def-g9-natural-risks-risk). (c) High hazard, high [vulnerability](#def-g9-natural-risks-vulnerability): high [risk](#def-g9-natural-risks-risk). (d) Low hazard, low [vulnerability](#def-g9-natural-risks-vulnerability): low [risk](#def-g9-natural-risks-risk).

**Exercise 29.8 ★★.**

Three schools are built on the same ground. School A is a frame of posts and beams with brick walls between them; school B has the same frame with diagonal braces and two shear walls; school C has the frame of B, standing on isolators. (a) Rank them from the safest to the least safe in an [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake). (b) Explain what the braces do. (c) Explain what the isolators do.

**Solution of Exercise 29.8.**

(a) C, then B, then A. (b) The braces form triangles, which cannot lean without stretching or squashing a brace: the frame keeps its shape instead of leaning over. (c) The isolators give way as the ground moves back and forth beneath them, so much less of the motion reaches the school.

**Exercise 29.9 ★★.**

A [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) shows the 100-year [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood), which has a chance of 1 in 100 of being reached or passed in any year. (a) Write this chance as a percentage. (b) A neighbour says: “The 100-year [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) came last year, so we are safe for 99 years.” What is wrong? (c) On average, how many such [floods](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) would come in 300 years?

**Solution of Exercise 29.9.**

(a) $1 \div 100 = 1~\%$ per year. (b) The chance is the same every year, whatever happened the year before: the [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) can come again next year. (c) $300 \div 100 = 3$ [floods](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood), on average.

**Exercise 29.10 ★★.**

A [tsunami](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-tsunami) warning station reads the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor pressure every $15\,\mathrm{s}$. (a) How many readings does it make in one hour? (b) In one year of 365 days? Write the answer in scientific notation. (c) A [tsunami](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-tsunami) crosses the deep [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) at about $800\,\mathrm{km}/\mathrm{h}$. How long does it take to reach a [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) $2400\,\mathrm{km}$ away? (d) Why does that time matter?

**Solution of Exercise 29.10.**

(a) $3600 \div 15 = 240$ readings. (b) $240 \times 24 \times 365 =
2\,102\,400 \approx 2.1 \times 10^6$ readings. (c) $2400 \div 800 =
3\,\mathrm{h}$. (d) In these three hours, the warning centres can warn the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) and people can move to higher ground.

**Exercise 29.11 ★★★.**

A family of four prepares an emergency kit with water for 3 days, at about $3.8\,\mathrm{L}$ per person per day, for drinking and washing. (a) How many litres of water do they need? (b) How many bottles of $1.5\,\mathrm{L}$ is that? (c) Name four other things the kit holds, and say why each one is useful. (d) Why is the kit kept near the door?

**Solution of Exercise 29.11.**

(a) $4 \times 3 \times 3.8 = 45.6\,\mathrm{L}$. (b) $45.6 \div 1.5 = 30.4$, so 31 bottles. (c) For example: food that keeps, to eat when the shops are shut; a torch, if the electricity is cut; a radio working on batteries or by hand, to hear the warnings; a first-aid kit, to treat small injuries; warm blankets. (d) So that it can be taken at once if the family has to leave quickly.

**Exercise 29.12 ★★★.**

A town lies beside a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), at the [mouth](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-source-mouth) of a [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) that comes down from a snow-capped [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano). Old mudflow deposits lie under its streets. The [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) has begun to shake with small [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake). Write a prevention plan of five points for the town. For each point, say whether it acts on the hazard side (knowing and watching the hazard) or on the [vulnerability](#def-g9-natural-risks-vulnerability) side of the [risk](#def-g9-natural-risks-risk).

**Solution of Exercise 29.12.**

For example: (1) draw a [hazard map](#def-g9-natural-risks-hazard-map) of the mudflows, [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) by [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) (hazard side); (2) install [seismometers](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-seismometer), tiltmeters, GPS receivers and gas instruments on the [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) (hazard side); (3) set up a fast chain of warning from the observatory to the town, with sirens ([vulnerability](#def-g9-natural-risks-vulnerability) side); (4) plan and signpost routes to higher ground, and practise them with drills ([vulnerability](#def-g9-natural-risks-vulnerability) side); (5) build no new homes or schools in the path of the mudflows ([vulnerability](#def-g9-natural-risks-vulnerability) side).

## 29.8 Problem: Where should the new school go?

**Problem 29.1.**

Weekend problem — a new school is placed on the map of the Varden Hills, away from floods, steep slopes and the fault; the final answer is the number of houses inside the flood zone

The village of the Varden Hills needs a new school, and three sites are offered: A, B and C, on the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) below. The [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) zone shaded on the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) is all the ground less than $5\,\mathrm{m}$ above the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), which a large [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) would cover. The table gives, for each site, the elevation of the ground, the level of the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) beside it, and how much the ground rises over $200\,\mathrm{m}$ measured flat across the slope.

| site | ground elevation | [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) level beside it | rise of the ground over $200\,\mathrm{m}$ |
| --- | --- | --- | --- |
| A | $137\,\mathrm{m}$ | $137\,\mathrm{m}$ | $1.4\,\mathrm{m}$ |
| B | $276\,\mathrm{m}$ | $126\,\mathrm{m}$ | $83\,\mathrm{m}$ |
| C | $236\,\mathrm{m}$ | $143\,\mathrm{m}$ | $20\,\mathrm{m}$ |

![The village, the three school sites and the flood zone (shaded), on the map of the Varden Hills (thin lines of equal height every 10\, m). Each small black square is a house. The thick black line is a fault.](https://one-course.com/images/onecourse/chapters/earth-1/g9-natural-risks/fig-a5a42dd3d3fd.svg)

*The village, the three school sites and the [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) zone (shaded), on the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) of the Varden Hills (thin lines of equal height every $10\,\mathrm{m}$). Each small black square is a house. The thick black line is a [fault](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-fault).*

**Part I — The flood zone.**

1. Using the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) levels of the table, say which way the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) flows.
2. Compute the height of each site above the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) beside it.
3. Which of the three sites lies in the [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) zone?
4. Use the [scale](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-scale) bar to measure the width of the [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) zone, across the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) , at site A.

**Part II — The slopes and the fault.**

5. A slope in percent is the rise of the ground for every $100\,\mathrm{m}$ measured flat. Compute the slope at each site.
6. Which site would be exposed to a [landslide](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-landslide) ? Explain.
7. Use the [scale](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-scale) bar to estimate the distance from each site to the [fault](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-fault) .
8. Suppose that the [fault](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-fault) could still move. Why is the ground right on top of it a bad place for a school?

**Part III — Choosing.**

9. Draw a table with one row per site and three columns: in the [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) zone? steep slope? on the [fault](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-fault) ? Fill it with yes or no.
10. Which site would you choose? Justify your choice with the table.
11. The chosen site still lies in a region where the ground can shake. Name two ways of building the school, and one habit of its pupils, that lower its [vulnerability](#def-g9-natural-risks-vulnerability) .
12. The school will also serve as a shelter during [floods](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) . Count the houses of the village that stand inside the [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) zone. What fraction of all the houses is that?

**Solution of Problem 29.1.**

**1.** The [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) is at $143\,\mathrm{m}$ beside C, $137\,\mathrm{m}$ beside A and $126\,\mathrm{m}$ beside B; on the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) C is to the west and B to the east, so the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) flows east.

**2.** A: $137 - 137 = 0\,\mathrm{m}$; B: $276 - 126 = 150\,\mathrm{m}$; C: $236 - 143 = 93\,\mathrm{m}$.

**3.** Only A, which is less than $5\,\mathrm{m}$ above the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water).

**4.** About $0.57\,\mathrm{km}$ (between 0.5 and $0.65\,\mathrm{km}$).

**5.** The rise over $100\,\mathrm{m}$ is half the rise over $200\,\mathrm{m}$: A $1.4 \div 2 = 0.7~\%$; B $83 \div 2 = 41.5~\%$; C $20 \div 2 = 10~\%$.

**6.** B: its slope is very steep, and loose [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) and soil can slide down a steep slope, especially after heavy [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation).

**7.** A: about $1\,\mathrm{km}$; B: almost on the [fault](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-fault) (less than $0.05\,\mathrm{km}$); C: about $1.3\,\mathrm{km}$.

**8.** If the [fault](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-fault) moves, the ground breaks and shifts along it, which can tear a building apart; and an [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) on the [fault](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-fault) would shake the ground there very hard.

**9.**

| site | in the [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) zone? | steep slope? | on the [fault](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-fault)? |
| --- | --- | --- | --- |
| A | yes | no | no |
| B | no | yes | yes |
| C | no | no | no |

**10.** Site C: it is the only site with three “no”: $93\,\mathrm{m}$ above the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), a gentle slope of $10\,\%$, more than $1\,\mathrm{km}$ from the [fault](https://one-course.com/books/earth/1/en/chapter/20-earthquakes#def-g7-earthquakes-fault).

**11.** For example: diagonal braces and shear walls in the frame, strong joints, or isolators under the building; the pupils practise [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) drills (drop, cover, hold on).

**12.** **17 houses** out of 28 stand inside the [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) zone: $17 \div 28 \approx 0.61$, about 3 houses in 5.