Week 9: Floods and Landslides PDF

**[Week 9: floods and landslides]** Uniformitarianism - Principle that geological processes have operated consistently over time. - \"The present is the key to the past\" --- current geological processes can explain past events. - Emphasizes slow, gradual change through erosion, se...

**[Week 9: floods and landslides]** Uniformitarianism - Principle that geological processes have operated consistently over time. - \"The present is the key to the past\" --- current geological processes can explain past events. - Emphasizes slow, gradual change through erosion, sedimentation, and other natural processes. Catastrophism - The theory that Earth\'s landscape has been shaped by large, sudden, and violent events (e.g., volcanic eruptions, asteroid impacts). - Contrasts with uniformitarianism, focusing on rare, catastrophic events that cause rapid changes. - Popular in the 17th-19th centuries but now considered complementary to uniformitarianism. Magnitude-Frequency Concepts & Effective Discharge - Magnitude-Frequency: Relationship between the size (magnitude) of geological events (e.g., floods, earthquakes) and how often they occur (frequency). - Larger events are less frequent, while smaller events occur more regularly. - Effective Discharge: The flow of water (in rivers) that is most effective at shaping the channel over time. - Represents a balance of frequency and flow magnitude that most impacts the river's morphology. Missoula Megaflood - A massive flood event that occurred around 15,000 years ago during the last Ice Age. - The breach of the ice dam holding Lake Missoula in Montana caused the release of vast amounts of water, flooding the Columbia River Gorge. - Resulted in the formation of the Channeled Scablands, leaving behind large-scale erosional features. Megafloods on Earth & Mars - On Earth: Examples include the Missoula Megaflood and other ancient catastrophic flood events (e.g., the Black Sea deluge). - These floods reshape landscapes, carving valleys, depositing sediments, and creating distinct erosional features. - On Mars: Evidence suggests ancient megafloods may have occurred, possibly from the melting of large ice reservoirs. - Features like outflow channels and valley networks suggest that liquid water once flowed in massive volumes, indicating potential past hydrological processes similar to Earth's megafloods. Mountain Basin Cascade - A conceptual model describing the movement of water, sediment, and mass through a mountain landscape. - Describes how sediment is produced, transported, and deposited from the headwaters down to the valley floors. - Characterized by the cascade of processes such as weathering, erosion, landsliding, and sediment transport. - Ultimately influences river systems, sediment storage, and landscape evolution over time. Conditions Leading to Hillslope Failure - Excessive Water: Heavy rainfall, rapid snowmelt, or saturation can reduce soil cohesion, leading to failure. - Steep Slopes: Greater slope angles increase the driving force for movement, making failure more likely. - Weak Materials: Soils or rocks that are poorly consolidated, fractured, or prone to weathering are more susceptible to failure. - Human Activity: Construction, deforestation, or excavation can destabilize slopes. - Seismic Activity: Earthquakes can trigger landslides by shaking the ground and destabilizing weak slopes. - Geological Factors: Faulting, bedrock composition, and existing slope morphology can influence failure risk. Where x = Landslide Size (Area or Volume), C = Normalization Constant, 𝛽 = Power Law Scaling Exponent - Landslide Size (x): The area or volume of the material involved in the landslide. - Normalization Constant (C): A constant used to adjust or scale the landslide size based on empirical data or specific conditions. - Power Law Scaling Exponent (𝛽): Represents the relationship between landslide frequency and size, with larger landslides being less frequent. - The scaling law implies that smaller landslides occur more often, while larger landslides follow a power-law distribution. - The exponent (𝛽) helps quantify this relationship and predict landslide occurrence in different environments. External Controls on Landslides & Sediment Production - Climatic Conditions: Rainfall, temperature, and freeze-thaw cycles control the rate of weathering and mass movement. - Geological Factors: Rock type, structure, and faulting influence slope stability and the type of landslides. - Vegetation: Vegetation can either stabilize slopes by binding soil or contribute to failure through root disruption. - Human Activities: Urbanization, construction, road building, and deforestation can increase landslide frequency and sediment production. - Tectonic Activity: Earthquakes, fault movements, and uplift can trigger landslides and influence sediment supply. Downstream Aggradation - The process by which sediment accumulates in river channels downstream due to an influx of sediment from upstream sources (e.g., landslides, erosion). - Can lead to raised riverbeds, changes in flow dynamics, and increased flood risks. - Aggradation can result from an increase in sediment supply (e.g., from landslides) or a decrease in transport capacity (e.g., reduced flow). - Over time, it can impact channel morphology, creating features like braided streams or floodplains. **Lecture 1: Floods** Effective Discharge: Magnitude × Frequency - Definition: The effective discharge of a river is the flow magnitude that, on average, transports the majority of sediment over a given period of time. It represents the flow that most frequently shapes the river channel and floodplain. - Concepts: - Magnitude: The size or volume of water flow. Larger discharges can move more sediment and reshape river channels. - Frequency: The rate at which a particular discharge occurs. It is measured as a return period (e.g., a 100-year flood means there is a 1% chance it will occur in any given year). - Effective Discharge: Typically associated with medium-sized floods that occur often enough to contribute the most to the sediment transport and shaping of the landscape over long time scales. - Applications: Understanding the effective discharge is crucial for river management, designing infrastructure (like bridges and flood defenses), and predicting sediment transport and channel changes. Key Principles of Landscape Formation 1. Uniformitarianism: - Definition: The principle that the geological processes shaping the Earth today, such as erosion and sedimentation, have operated in the same way throughout Earth\'s history. This concept allows scientists to infer past geological events and processes by observing current processes. - Example: The erosion of riverbanks and sediment transport by rivers today provide insights into how similar processes shaped ancient river valleys. 2. Gradualism: - Definition: A theory that the Earth's landscape changes slowly over time due to the continuous action of small-scale processes (e.g., erosion, sediment deposition, weathering) rather than sudden, catastrophic events. - Contrast with Catastrophism: Unlike gradualism, catastrophism posits that major geological changes occur due to sudden, violent events (e.g., volcanic eruptions, massive floods). - Example: The gradual carving of the Grand Canyon by the Colorado River over millions of years versus the sudden effects of a single, large flood. Mega-Floods and Their Impacts - Characteristics: - Rare but Significant: Mega-floods are infrequent events that can have profound effects on the landscape, forming features that may persist for millennia. - Associated Events: Often linked to rapid deglaciation or large-scale glacial outburst floods (e.g., *jökulhlaups*in Iceland). - Example: The Missoula Floods during the last Ice Age, which reshaped the Pacific Northwest, leaving behind features like the Channeled Scablands. - Long-Term Effects: - Persistence: Features produced by mega-floods, such as deep channels or sediment deposition, may remain for thousands of years. The persistence depends on their frequency relative to the time needed for other processes (e.g., fluvial erosion, sedimentation) to obscure or remove them. - Influence on Geomorphology: Mega-floods can change the river's long profile (the elevation change of a river from source to mouth), reshape the landscape dramatically, and deposit large amounts of sediment in floodplains and downstream areas. **Lecture 2: Landslides** Conditions Leading to Hillslope Failure - Driving Forces (Fd): - Definition: Forces that act to move material down a slope. - Factors: - Gravity: The primary force pulling materials downhill. - Shear Stress (τ): The force parallel to the slope, which can be increased by factors like slope steepening or increased load. - Resisting Forces (Fr): - Definition: Forces that act to hold the material in place and prevent it from sliding. - Factors: - Shear Strength (S): The material's ability to resist deformation and movement. - Cohesion and Friction: Increased by factors like vegetation roots binding soil, or compacted rock layers. - Failure Condition: Hillslope failure occurs when the driving forces exceed the resisting forces, expressed mathematically as:Fd \> FrFd \> Fr Driving Forces and How They Change 1. Increasing Shear Stress (τ): - Steepening of Slope: Tectonic activity or erosion can steepen the slope angle, increasing the force parallel to the slope. - Addition of Weight: Factors like heavy rainfall or snowmelt, which add mass to the slope, increasing gravitational force. 2. Decreasing Shear Strength (S): - Loss of Cohesion: Vegetation removal (e.g., deforestation) reduces the soil\'s cohesion, making it more prone to failure. - Water Saturation: Excess water reduces friction between soil particles, leading to a decrease in shear strength. This is common during heavy rain or snowmelt, leading to landslides. How Big Are Landslides? - Size and Frequency: - Small Landslides: Occur frequently but typically do not have a significant impact on the landscape. - Large Landslides: Much rarer, but can contribute substantial amounts of sediment to rivers and are significant for landscape changes. - Power-Law Distribution: - Landslides often follow a power-law scaling relationship, where the probability of a landslide decreases as the size increases:P(x)=Cx−βP(x)=Cx−β - xx: Landslide size (area or volume). - CC: Normalization constant. - ββ: Scaling exponent; typically greater than 1 for landslide size-frequency distributions. - Critical Mass: - Small landslides occur frequently, but once a certain critical mass is reached, it can initiate a larger slide. - Landscape Healing: - Evidence of small landslides can be quickly removed by natural processes, such as surface reworking by rain and vegetation growth, which stabilize the slope. External Controls on Landslides and Sediment Supply Upstream Controls: 1. Climate: - Heavy Precipitation: Increases water saturation, leading to soil slippage. - Freeze-Thaw Cycles: Water expanding and contracting in soil pores can cause soil disintegration. 2. Tectonics (Diastrophism): - Uplift: Steepens slopes, making them more prone to failure. - Earthquakes: Shaking can trigger landslides, especially in mountainous regions. 3. Geology: - Rock Type: Weak rocks like shale are more susceptible to landslides than hard rocks like granite. - Structural Features: Faults, joints, and bedding planes influence landslide potential. 4. Land Use: - Deforestation: Reduces root cohesion, making slopes less stable. - Urban Development: Excavation and construction can destabilize slopes. Downstream Controls: 1. Base Level: - The base level of a river (e.g., sea level or a lake) influences sediment transport and storage. Lowering base levels (e.g., due to sea level drop or river incision) can destabilize the surrounding slopes. 2. Tectonics: - Downstream tectonic movements affect sedimentation rates and the capacity of rivers to carry sediment. Impact of Landslides on Sediment Supply - Post-Landslide Sediment Flux: - Landslides contribute large amounts of sediment to rivers, which may alter channel morphology and sediment transport dynamics. - Quick Depletion: Sediment sources from landslides may be rapidly depleted within a few years as the sediment is transported downstream or reworked by fluvial processes. - Long-Term Impact: - Features from large landslides, like debris fans or terraces, can persist in the landscape for thousands of years. Their longevity is dictated by the frequency of subsequent landslides and the rate of erosion. **Lecture 3: Landscape Responses** W.M. Davis\' Cycle of Erosion - Theoretical Framework: - Describes how landscapes evolve over time through a cycle of erosion that includes stages of youth, maturity, and old age. - Youth: Characterized by steep gradients, rapid erosion, and active river incision. - Maturity: Channels widen, floodplains develop, and gradients decrease. - Old Age: The landscape flattens and reaches a nearly level surface (peneplain). River Terraces - Definition: Step-like landforms found along the sides of river valleys, formed by periods of aggradation (sediment deposition) and degradation (erosion). - Formation: - Result from changes in base level (e.g., tectonic uplift, sea level changes). - May form during glacial and interglacial periods as rivers adjust to changing sediment and water loads. - Significance: - They preserve a record of past river behavior, sediment supply, and climate changes, providing evidence of long-term landscape evolution. Glacial Cycles and Long Profile Change - Glacial Influence: - Glacial Advances: Rivers can become clogged with glacial meltwater and sediment, leading to aggradation (raising of the riverbed). - Deglaciation: Leads to rapid incision, changes in river gradient, and modification of the long profile. - Long Profile: - Describes the gradient of a river from source to mouth. It is affected by factors such as tectonic uplift, erosion, sediment supply, and climate change. - Equifinality: - Different processes can lead to the formation of similar landforms. For example, terraces can be formed by glacial events or tectonic activity. Dynamic Fluvial Systems - System Responses: - Rivers and landscapes adapt to changes in water and sediment supply due to various factors: - Climate Change: Alters precipitation patterns, river discharge, and vegetation cover, affecting erosion and sediment delivery. - Tectonic Activity: Can steepen or flatten the river profile, altering sediment transport and erosion rates. - Landslides and Mass Movements: Sudden influxes of sediment impact channel morphology and sediment transport capacity. - Adjustment Period: - The time it takes for a river or landscape to adjust to these changes depends on the frequency of the driving forces and the resilience of the system. Global and Local Impacts of Climate and Anthropogenic Change - Post-Glacial Adjustments: - Eustatic Changes: Sea level rise or fall can alter coastal river behavior. - Isostatic Rebound: The slow rising of land previously depressed by glacial ice can impact local river flow and sediment transport. - Future Climate Change: - Predicted Effects: - Increased Flooding: More frequent and severe floods due to changing precipitation patterns. - Enhanced Erosion: Warmer temperatures and extreme weather events can accelerate weathering and soil loss. - Reduced Stability: Potential for more landslides and erosion, particularly in mountainous and coastal regions. - Anthropogenic Impacts: - Deforestation: Decreases vegetation cover, reducing slope stability. - Urbanization: Increases impervious surfaces, leading to higher runoff and flash floods. - Dam Construction: Alters sediment transport, river dynamics, and downstream sediment deposition [Friday 29 November: Lecture] River Terraces and Impacts of Glacial Cycles: - River terraces: Formed by alternating periods of erosion and deposition (cut and fill). - Glacial cycles: Influence the formation of terraces through fluctuating water flow and sediment availability. Long Profile Change and Fluvial Knickpoints: - Long profile: The longitudinal profile of a river from source to mouth. - Knickpoints: Sharp changes in the river's gradient due to factors like base level changes or tectonic activity. - Evolution: Aggradation (deposition) and degradation (erosion) shape long profiles, with knickpoints migrating upstream. Climate Controls on Landscape Evolution: - Temperature and rainfall: Control weathering rates, erosion, and sediment transport. - Sea level: Influences river energy and morphology, affecting deltas and river shelves. Global Glacial Isostatic Adjustment: - Isostatic rebound: Post-glacial areas can rise (e.g., Scandinavia), while other regions sink (e.g., parts of the North American coast) due to changes in Earth\'s crust after glaciers retreat River Long Profile Evolution: - Aggradation: Riverbeds accumulate sediment, leading to raised riverbeds. - Degradation: Riverbeds erode, causing lowered riverbeds and reshaping the long profile. Glacial Cycles and Long Profile Change: - Impact of glacial cycles: Glacial advance and retreat can dramatically alter river long profiles, either by eroding riverbeds (degradation) or causing sediment deposition (aggradation). - Braided rivers: Associated with glacial meltwater, they develop in environments with high sediment supply and variable flow, leading to a complex, braided channel network. W.M. Davis\' Cycle of Erosion: - W.M. Davis (1850-1934) proposed a conceptual model to describe the evolution of landscapes over time, known as the \"Cycle of Erosion\". - Phases of the Cycle: 1. Youthful Stage: - Characterised by steep slopes, high energy rivers, and rapid erosion. - Features include V-shaped valleys, waterfalls, and rapids. - Rivers are typically incised into bedrock and may exhibit steep gradients. 2. Mature Stage: - Rivers begin to lower their gradient, forming broad, meandering valleys. - Erosion slows, and sediment begins to accumulate in the valley floor. - Features like meanders and floodplains develop as the river approaches equilibrium. 3. Old Age Stage: - Rivers reach a low gradient, with very little erosion occurring on the surface. - The landscape becomes flat, and extensive floodplains are formed. - The river may form oxbow lakes as it meanders extensively and loses its energy. - Peneplains (nearly flat surfaces) can form as the landscape is almost completely eroded to base level. - Key Concepts in Davis\' Model: 4. The cycle describes a gradual and continuous process of erosion, transport, and deposition. 5. Davis argued that landscapes evolve through successive stages, driven by the interaction between tectonics, climate, and water flow. 6. Base level (the lowest point a river can erode to) plays a critical role in controlling the progression of erosion and landscape development. 7. This model is useful for understanding long-term landscape evolution but has been critiqued for oversimplifying the complex and non-linear nature of geomorphological processes. Fan Entrenchment: - Fan entrenchment refers to the process where a river or stream cuts deeply into a alluvial fan or delta after a period of deposition, leading to the re-exposure of older deposits and sometimes creating a terrace or incised channel. - Alluvial fans: Form at the base of mountains where a river loses its energy, depositing sediment in a fan-like shape. Over time, the sediment builds up. - Entrenchment: If the river\'s energy increases (e.g., due to tectonic uplift, climate change, or changes in sediment supply), it may begin to cut down through these fans, forming incised channels. - This cutting is often vertical and leads to deep, steep-sided channels. - Terraces can form along the sides of these entrenched channels, marking former river levels. - Impact: Entrenchment can significantly alter the landscape, creating more defined valleys and cut-off floodplains. It may also result in a reduction in sediment deposition at the fan's mouth, as the river becomes more erosive and focused on downcutting rather than spreading sediment out.