Exercises: Natural hazards

The diagram shows the three components that, when combined, result in a geological risk. Name them (1, 2, and 3), define each one, and apply them to the example of an earthquake.

Classify these natural hazards as internal or external in origin and justify the classification: earthquake, flood, drought, landslide, and volcanic eruption.

State two structural prevention measures (in buildings) and two non-structural prevention measures (concerning the population and planning) against seismic risk.

Distinguish between a natural risk and a natural disaster, and discuss why the same magnitude $6$ earthquake would be minor news in a desert but a catastrophe beneath a city, introducing the role of exposure.

Explain what the vulnerability of a population to a hazard is and give two examples of how it can be reduced without the hazard changing (reinforced buildings, education and drills, insurance, spatial planning).

Name three natural hazards of external origin frequent in Spain (floods, droughts, landslides, heat waves, coastal storms) and associate each one with an especially exposed area.

What is a risk map? Explain which three types of information it combines (hazard, exposure and vulnerability) and what spatial planning and insurance companies use it for.

Explain what risk culture is (the social perception of hazards) and discuss why in areas where disasters are very frequent, or very rare over time, the population tends to undervalue them.

Distinguish between prediction, prevention, and mitigation in volcanic risk management, illustrating each with a real action carried out during the 2021 La Palma eruption.

Explain what the return period of a flood is and how it is used in risk maps, and reason what it means for a flood with a "100-year return period" to potentially occur again the following year.

In eastern Spain, there are housing developments built in dry riverbeds (ramblas) and ravines that have been dry for decades. Explain why building there increases risk even if it does not rain for years and which spatial planning principle is violated.

Wildfire is not just a "natural" risk in the Mediterranean. Analyse which human factors have transformed it (rural abandonment and fuel accumulation, housing developments at the wildland-urban interface, climate change) and which forest and urban management reduces it.

Distinguish between meteorological, hydrological and agricultural drought, and explain why demand management (efficiency, tariffs, priority uses) is as necessary as infrastructure when the drought drags on.

Avalanches are a winter risk in the Spanish mountains. Explain which conditions of the snowpack, slope and trigger provoke them, and which prevention measures exist (mapping of avalanche zones, avalanche galleries, protection forest, forecasting and closure of routes).

Heat waves are the natural hazard that causes the most deaths in Europe. Explain which groups are most vulnerable (the elderly, the chronically ill, outdoor workers), what the urban "heat island" is, and what action plans contain (level-based warnings, air-conditioned centres, workplace adaptation).

Maritime storms periodically damage the Mediterranean and Cantabrian coasts. Explain the risk of coastal erosion and flooding, how the occupation of the beachfront aggravates it (loss of the dune and of the coastal barrier) and what alternatives there are to concrete (dune restoration, strategic retreat).

During the week prior to the Tajogaite eruption (La Palma, 2021), the scientific committee recorded the following monitoring data:

• September 11 to 13: seismic swarm with hypocentres at depths of 10-12 km and a maximum magnitude of 2,8.

• September 15: migration of hypocentres to depths of 2 to 4 km and ground uplift of +10 cm detected by GNSS stations.

• September 18: earthquakes with magnitudes greater than 3,2 at depths of less than 1 km, cumulative deformation of nearly 20 cm, and a sharp increase in diffuse SO2 and CO2 emissions.

1. a) Interpret the evolution of these precursors: deduce what was happening with the magmatic intrusion at each stage and justify why evacuation was not ordered on September 11, but was essential to initiate on the 18th.
2. b) Although the lava destroyed more than 1300 hectares and around 3000 buildings, there were no direct fatalities. Justify which risk factor (R = P · E · V) was successfully reduced to zero for people thanks to this instrumental network, and which one proved impossible to mitigate in the short term.
3. c) If similar seismicity were recorded on another island of volcanic origin but without ground deformation or anomalous gas emissions, could an imminent eruption be ruled out? Justify your answer considering magma viscosity and the presence of pre-existing crustal fractures.

A developer plans a housing development of 30 villas on a slope with an average gradient of 32°. The geological profile reveals permeable sandstone strata resting on impermeable shales and marls, with a dip of 28° in the same direction as the slope of the hillside (concordant strata). At the foot of the slope, a vertical cut excavated for a previous road is observed, and in the middle section several pine trees have curved trunks ("sabre-shaped").

1. a) Identify and classify the type of mass wasting event most likely to be triggered. Justify your prediction by analysing the relationship between the slope angle, the dip direction, and the contrast in lithological permeability.
2. b) Explain the physical mechanism by which an episode of intense and prolonged rainfall could cause the sudden collapse of the slope, making explicit reference to pore pressure and the coefficient of internal friction at the contact surface between strata.
3. c) The developer proposes to resolve the instability by covering the cut with continuous shotcrete without drainage holes. Rebut their proposal technically, showing why this measure would worsen the risk instead of preventing it, and design two viable alternative corrective measures.

A civil protection spokesperson states: "Given that in La Palma the 2021 eruption was successfully predicted days in advance to evacuate the population, we should install a similar sensor network in Lorca or in the Segura river basin to issue an evacuation order 24 hours before the next major earthquake."

1. a) Scientifically rebut this statement: contrast the physics of precursors in a magmatic reactivation with those of brittle failure along a tectonic fault, explaining why short-term temporal prediction is feasible in the first case but unachievable in the second.
2. b) Compare the geodynamic origin of both risks: explain why south-eastern mainland Spain concentrates seismicity associated with a diffuse and compressive plate boundary, whereas the Canary Islands display intraplate volcanism.
3. c) Design the priority preventive strategy for each of the two regions, justifying which specific risk factor (R = P · E · V) public investment should mainly target in each case.

During the October 2024 DANA catastrophe in Valencia, the headwaters of the Poyo ravine basin in Chiva recorded precipitation of nearly 500 mm in just a few hours. In municipalities located 30 km downstream, heavy rainfall barely occurred in the early afternoon, but two hours later they experienced a flash flood with a peak discharge exceeding 2000 m³/s that inundated urban areas.

1. a) Explain hydrologically why the catastrophic downstream flood occurred without it raining heavily there at that moment, defining the concept of "concentration time" and analysing the typical shape of the flood hydrograph of a Mediterranean rambla.
2. b) In recent decades, the floodplain of this basin has experienced intensive occupation by industrial estates, housing, and transversal barriers (motorways and railway lines). Analyse how these anthropogenic actions alter: 1) the surface runoff coefficient, 2) the natural attenuation of the flood, and 3) the rise in water levels due to the damming effect at embankments and bridges.
3. c) Evaluate why warning the population when water was already entering urban areas proved ineffective, and propose how to redesign the early warning chain by integrating real-time stream gauging stations with binding protocols for work shutdown and vertical evacuation.

A Mediterranean coastal municipality suffers from periodic droughts and, at the same time, sudden torrential flash floods from ramblas every 8-12 years. The town council is debating two projects: Plan A proposes channelising with smooth concrete and straightening the rambla's bed down to the sea to evacuate the water as quickly as possible; Plan B proposes restoring the natural channel, creating floodable parks (landscaped retention basins), and replacing asphalt on squares and pavements with permeable sustainable drainage systems (SuDS).

1. a) Compare the impact of both plans on the hydrograph of a torrential flash flood: predict how the lag time (time to peak) and the peak discharge will vary in Plan A compared to Plan B.
2. b) From the hydrogeological perspective of drought mitigation, argue which of the two plans favours the recharge of local alluvial aquifers and reduces municipal water stress, justifying the role of flow velocity and bed permeability.
3. c) If impermeable Triassic clays (Keuper facies) crop out in the rambla's headwaters and there are 15-metre-thick Quaternary fluvial gravels and sands over impermeable basal marls in the middle reach, justify exactly in which reach of the channel infiltration basins should be located to recharge usable water for wells.

In a citizen survey conducted in a municipality in south-eastern mainland Spain and in another in the Canary Islands, more than 80% of respondents stated that "living in their town is completely safe from earthquakes or eruptions because nothing serious has happened in the last 70 years". However, geological records indicate return periods (T) of about 100 years for earthquakes of magnitude M > 6 on that south-eastern fault and about 60 years for eruptions on that Canarian volcanic ridge.

1. a) Rebut the respondents' logic using the concept of return period and the stress-accumulation recurrence model: why does the fact that no destructive event has occurred in the last 70 years not reduce the geological risk and, according to that model, indicate a higher probability of occurrence in the short to medium term?
2. b) Analyse the geological mechanisms underpinning this hazard accumulation: contrast the progressive accumulation of elastic strain along the plane of a locked fault with the progressive pressurisation of a magma chamber due to deep replenishments.
3. c) Design three non-structural land-use management measures that increase resilience and risk perception among the population of south-eastern mainland Spain without inducing unfounded alarm or paralysing economic activity.

The official seismic hazard map of Spain sets a basic seismic acceleration of a_b = 0,19 g (for a return period of 475 years) for Lorca (Murcia), whereas for Valladolid it sets a_b < 0,04 g.

1. a) Interpret these data: explain what basic acceleration measures exactly with respect to the acceleration of gravity (g = 9,8 m/s²) and justify why this mapping constitutes a probabilistic forecast useful for engineering rather than a deterministic prediction of date and time.
2. b) During the 2011 Lorca earthquake (Mw = 5,1), two buildings with the same structure located at a similar distance from the hypocenter suffered very disparate damage: one built on sound limestone rock suffered minor cracks, while another on soft floodplain sediments suffered severe collapses. Explain this contrast based on the phenomenon of soil seismic amplification and how the seismic-resistant building code (NCSE) accounts for it.
3. c) A developer argues that it is preferable to invest public funds in researching earthquake early-warning systems that provide minutes of warning rather than making construction more expensive by applying the NCSE code. Justify why this stance is unfeasible for shallow intraplate earthquakes, considering the velocity of P and S waves and the distance to the epicenter.

A tourist campsite located on the floodplain of a rambla (preferential flow zone with a return period T = 10 years) has been flooded and destroyed by flash floods on three occasions during the last 25 years. After each disaster, the Insurance Compensation Consortium (Consorcio de Compensación de Seguros) has fully compensated for the material losses, allowing the owner to rebuild the permanent facilities at the same elevation in the channel.

1. a) Analyze this case using the risk formula (R = P · E · V): deduce which factors are kept artificially high and explain what the "moral hazard" resulting from economically compensating disasters in recurrent risk zones without requiring mitigation measures consists of.
2. b) According to fluvial dynamics, preferential flow zones fulfill the geomorphological function of evacuating flood discharges and transporting bedload. Predict the hydraulic and geomorphological consequences on downstream land of erecting a perimeter concrete wall to shield said campsite against T = 10-year floods.
3. c) As a geological hazard specialist in public administration, draft three binding technical criteria based on the National Flood Zone Mapping System (SNCZI) to condition the Consortium's compensation and mandate the setback or rezoning of activities to areas outside the T = 100-year flood zone.

Unit Challenge. Imagine you are advising your local town council on disaster prevention. Your mission is to draft a brief technical report on the geomorphological vulnerability of a slope or watercourse in your area.

1. Landform and slope: Open the Iberpix (IGN) viewer, locate your municipality, and select a slope or riverbank. Calculate the approximate slope by applying $P = (\Delta h / d) \times 100$, where $\Delta h$ is the elevation difference and $d$ is the horizontal distance in meters.

2. Pedogenesis and permeability: Inspect the soil of a nearby garden or vacant lot. Estimate by feel its texture and the presence of organic matter in the surface horizon, and deduce whether the profile development favors infiltration or runoff.

3. Risk and mitigation: Check AEMET for the maximum $24\text{ h}$ precipitation recorded in your region (e.g., $> 50,0\text{ mm}$). With these data, write a one-page reasoned proposal assessing the risk of landslides or flash floods and including two bioengineering measures to mitigate erosion.

Assessment criteria:

• Rigor in topographic calculations and landform characterization.

• Coherent justification of the soil's influence on erosive dynamics.

• Technical quality and feasibility of the risk prevention solutions.