Exercises: Hazards of external origin

What is a slope failure and what two factors usually trigger it?

Diagrams a, b, and c represent three slope movements. Assign each diagram its type (rockfall, landslide, or mudflow) and explain how the movement of material differs in each case.

What is the return period of a flood? Explain what it means for a flood to have a return period of $100$ years.

What is the difference between a slow river flood and a flash flood? In what type of basins, in terms of size and gradient, does each usually occur?

Name three factors that increase flood risk in a city, such as soil sealing, occupation of the channel or the silting-up of drains, and briefly explain each one.

What is a torrential flash flood and why are they especially frequent in the ramblas of the Spanish Mediterranean coast?

What is geological hazard mapping and what use does it have for land-use planning? Which public body produces it in Spain?

Write four citizen self-protection rules in anticipation of a flood, such as not crossing water currents, not taking shelter in basements, cutting off supplies and heeding official warnings.

In the Basque Country, flysch slopes suffer landslides after episodes of heavy rainfall. Explain the role of water as a trigger: added weight, pore pressure, and loss of material cohesion.

An ephemeral stream (rambla) floods, on average, once every $25$ years (annual probability of $\frac{1}{25}$). Calculate the probability that it floods at least once in $50$ years, express the result as a percentage, and comment on whether it is advisable to build on its channel.

List three measures for stabilizing unstable slopes (drainage, retaining structures, regrading the head) and explain how each improves stability.

An alluvial plain has a flood return period of $500$ years.

1. a) Calculate its annual probability of flooding;
2. b) explain why that value does not guarantee that "it will not flood for another 500 years";
3. c) discuss which land uses would be admissible in that strip.

Flood-control dams hold back the peak of a flood wave and release it in a controlled way. Explain how they reduce flood risk downstream and what limitations they have (silting-up, exceptional floods, false sense of security).

What are preferential flow zones and flood expansion zones in a floodplain? Why do regulations require them to be kept free of buildings?

Meshes, anchors and shotcrete are installed on a road cutting. Explain which risk each element corrects and what role the drainage of the hillside plays as an essential complement.

Name four precursor signs of instability on an inhabited hillside (cracks in the ground and in buildings, tilted trees and poles, new springs) and explain what each one indicates about the incipient movement.

Following a wildfire that scorched 70% of a Mediterranean basin, two hydrographs corresponding to two identical storms (50 mm in 1 hour) are analyzed: one recorded before the fire and another six months later.

1. a) Qualitatively draw both hydrographs superimposed (time versus discharge), justifying how the time of concentration ($t_c$), peak discharge ($Q_p$), and total surface runoff volume vary.
2. b) Explain the two edaphic mechanisms responsible for this hydrological alteration: the formation of a hydrophobic layer due to the pyrolysis of organic matter and surface sealing by raindrop impact.
3. c) A town council plans to dredge and widen the lower channel to mitigate post-fire flooding. Provide a technical argument showing why this measure transfers and worsens the problem downstream, and defend an alternative hydrological-forestry remediation action in the headwaters.

The general land-use plan of a riparian municipality must rule on four land-use applications on the river floodplain: (1) a nursing home in the $T = 500$-year zone; (2) a chemical storage warehouse in the preferential flow zone ($T = 100$ years); (3) peri-urban community gardens with demountable sheds in the $T = 10$-year zone; (4) a wastewater treatment plant (WWTP) in the $T = 100$-year flood-prone zone protected by a 2-metre earth perimeter levee.

1. a) Give a reasoned ruling on the compatibility or incompatibility of each of the four applications by assessing geological risk (hazard, vulnerability, and exposure) and legal land-use restrictions.
2. b) Explain the phenomenon of "residual risk" and the "trap effect" introduced by the WWTP's defensive levee if an extraordinary flood occurs with a water depth that exceeds the crest level.

During a flash flood in a Mediterranean basin, the hydrograph recorded at the mouth of an ephemeral stream (rambla) showed a time of concentration of barely 45 minutes and an actual peak discharge far higher than estimated by hydrological models based solely on liquid precipitation.

1. a) Analyze the synergistic interaction between meteorology (an isolated depression at high levels [DANA] and moist easterly maritime flow), basin morphology (shape, slope, and lithology), and orographic forcing in the genesis of "flashy" (needle-like) hydrographs.
2. b) Explain how the massive transport of bedload/solid load (boulders, gravels) and floating debris (vegetation, vehicles) modifies the rheological behavior of the flow (hyperconcentrated flows) and exceeds hydraulic predictions based on clear water.
3. c) A bridge collapsed not from hydrostatic overtopping of its deck, but from siltation and blockage of its spans. Justify what engineering design and land-use planning shortcomings this event revealed and what change in construction criteria it demands.

An industrial estate was built on the debris cone of a torrential gully, channeling its flow under an avenue through an underground concrete culvert. During an extraordinary flood, the water failed to enter the culvert and swept through the estate's facilities.

1. a) Explain geomorphologically and hydraulically why the culvert inlet collapsed, considering the depositional dynamics typical of an alluvial fan and the transport of sediment and floating debris.
2. b) Justify how the waterproofing of 40 hectares of roofs and roads in the industrial estate altered the runoff coefficient ($C$) and the time of concentration ($t_c$), worsening the flood peak.
3. c) Design an alternative technical solution to underground culverting that combines peak attenuation in the headwaters or via floodable parks with sustainable drainage systems (SuDS), explaining its effect on the outlet hydrograph.

A sandy-clay slope with a 32° inclination remained stable for decades under mixed forest. Following clear-cutting for timber harvesting, shallow landslides and rotational failures began to occur after moderate-intensity rainfall events.

1. a) Justify the instability of the slope using the concept of safety factor ($Fs = \text{fuerzas resistentes} / \text{fuerzas desestabilizadoras}$): detail the mechanical role of the loss of apparent root cohesion ($\Delta c$) and the increase in pore water pressure ($u$) due to the cessation of evapotranspiration.
2. b) A technical proposal suggests immediately reforesting with large, fast-growing tree species, intensively watering the slope during the first year. Refute this proposal by pointing out two adverse effects on mechanical stability and the short-term water balance.
3. c) Design a three-phase slope bioengineering plan that combines surface drainage techniques and temporary mechanical support with progressive revegetation using native species with complementary root systems.

Two plots in the same municipality lie, respectively, in the $10$-year return period strip and in the $100$-year one.

1. a) Calculate the annual probability of flooding of each plot;
2. b) calculate the probability of each one flooding at least once in $40$ years;
3. c) decide which land use you would assign to each plot and justify it.

A mountain road crosses a well-stratified limestone formation through a trench cut with two artificial slopes with a 65° inclination: slope A (facing north) and slope B (facing south). The rock strata dip 40° towards the north and exhibit a main family of vertical joints orthogonal to the road alignment.

1. a) Reasonably determine on which of the two slopes there is a predominant risk of planar sliding along the bedding planes and on which the risk of block toppling or free fall predominates.
2. b) Records show that 85 % of rockfalls occur in early spring, on days with nighttime frost followed by daytime temperatures above 10 °C. Explain the geomechanical mechanism that triggers failure during daytime thaw rather than in the middle of winter with permanently sub-zero temperatures.
3. c) For the slope at risk of individual block falls onto the roadway, select and justify two stabilization or protection measures: one of an active nature (preventive) and another of a passive nature (corrective).

The Clausius-Clapeyron relation establishes that the saturation capacity of water vapor in the atmosphere increases by approximately 7 % for every 1 °C of thermal increase. In the western Mediterranean, the sea surface temperature has increased by 1,5 °C in recent decades.

1. a) Justify physically and thermodynamically how the warming of surface seawater intensifies convective available potential energy (CAPE) and the torrential nature of rainfall events in coastal basins.
2. b) Traditional hydrological calculations in public works assume the hypothesis of "stationarity" of climatic series. Reason how the breakdown of this principle affects the calculation of the return period ($T$) and the actual risk of failure of a dam spillway or channelization designed with 20th-century data.
3. c) A local government proposes to concrete and deepen the bed of an urban river to accelerate drainage towards the sea. Rebut this action technically by assessing its downstream impact and defend a nature-based solution (NbS) for flood risk management.

Unit challenge. Act as an environmental consultant to evaluate the geomorphological risk of your municipality (or a nearby one). Prepare a brief technical report with a mitigation proposal following these phases:

1. Diagnosis of landforms: Enter the Iberpix viewer of the National Geographic Institute (IGN). Identify the morphoclimatic system and two dominant landforms, deducing the external geological processes (weathering, erosion, transport, or sedimentation) that shaped them.

2. Risk assessment with real data: Access the official cartography of the SNCZI (Ministry for the Ecological Transition) or the torrential rainfall records from AEMET. Locate urbanized areas or infrastructure exposed to flooding (return period $T = 100$ years) or to slope movements on gradients steeper than $20,5\,\%$.

3. Intervention proposal: Design a table that synthesizes the relationship between landforms, dynamic process, and potential risk. Add a reasoned proposal with two feasible preventive measures: one structural (such as slope bioengineering) and one non-structural (such as land-use planning).

Assessment criteria:

• Accurate and coherent identification of geological processes and landform shapes from official cartography.

• Rigorous integration of real external hazard data (SNCZI or AEMET) into the territorial diagnosis.

• Technical and environmental feasibility of the proposed mitigation measures.