Key Coastal Hazards

Why This Matters

Coastal hazards aren't random disasters—they're interconnected physical processes that engineers must understand to design effective protection systems. You're being tested on your ability to identify driving mechanisms, cascading effects, and appropriate engineering responses for each hazard type. The key is recognizing how hazards interact: sea level rise amplifies storm surge, subsidence compounds flooding, and climate change accelerates nearly everything.

Don't just memorize a list of hazards. Know what causes each one, how they compound each other, and which engineering solutions address which mechanisms. When an exam question asks you to evaluate coastal vulnerability or propose resilience strategies, you need to connect specific hazards to their underlying physics and appropriate interventions.

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Gradual Onset Hazards

These hazards develop over years to decades, allowing time for adaptive engineering—but their slow progression often leads to delayed action until impacts become severe.

Sea Level Rise

• Driven by thermal expansion and ice sheet melt—global mean sea level has risen approximately 3.4 mm/year since 1993, with acceleration expected
• Amplifies all other coastal hazards by raising the baseline water level from which storm surges, waves, and flooding events occur
• Requires adaptive engineering timelines matching 50-100 year infrastructure lifespans to projected rise scenarios (0.3-2.0 m by 2100 depending on emissions)

Coastal Subsidence

• Land surface sinking occurs through compaction of sediments, groundwater extraction, and tectonic activity—rates can exceed 10 mm/year in deltaic cities
• Creates relative sea level rise that compounds global trends, making some regions experience effective rise rates 2-3x the global average
• Monitoring networks essential for distinguishing subsidence from eustatic rise and targeting groundwater management interventions

Saltwater Intrusion

• Freshwater-saltwater interface migration occurs when hydraulic head in coastal aquifers drops below sea level pressure—often from over-pumping
• Threatens drinking water and agriculture with contamination that can persist for decades even after pumping stops
• Managed through injection barriers and extraction limits—requires integrated groundwater-surface water planning

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Episodic Storm Hazards

These hazards strike rapidly with high-intensity impacts, requiring both predictive warning systems and robust structural design to withstand extreme loads.

Storm Surge

• Wind-driven water pile-up occurs when sustained winds push surface water toward shore faster than it can escape—can exceed 8 m in major hurricanes
• Bathymetry and coastal geometry determine surge height; shallow, funnel-shaped coastlines experience amplified surges
• Design water levels combine surge with astronomical tide and wave setup for stillwater elevation calculations in flood protection

Hurricanes and Tropical Cyclones

• Multi-hazard events combining storm surge, extreme winds, heavy precipitation, and wave action in a single system lasting hours to days
• Wind speed categories (Saffir-Simpson) don't capture flood risk—slow-moving Category 1 storms can cause more flooding than fast-moving Category 4s
• Building codes and freeboard requirements are primary engineering responses, along with evacuation infrastructure and emergency operations planning

Extreme Wave Events

• Significant wave height ($H_s$) represents the average of the highest one-third of waves—design waves typically use $H_{100}$ (100-year return period)
• Wave energy scales with height squared—a 6 m wave carries 4x the energy of a 3 m wave, critical for breakwater and seawall design
• Spectral analysis of wave climate informs structure orientation, armor unit sizing, and overtopping calculations

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Seismic and Rapid-Onset Hazards

These hazards provide minimal warning time, making early detection systems and pre-positioned evacuation plans the primary life-safety strategies.

Tsunamis

• Long-period waves ($T$ = 10-60 minutes) generated by seafloor displacement from earthquakes, submarine landslides, or volcanic activity
• Wave speed in open ocean follows $v = \sqrt{gd}$ where $d$ is water depth—waves travel 700+ km/hr in deep water, slowing dramatically at shore
• Run-up heights can exceed 30 m; vertical evacuation structures and DART buoy warning systems are critical engineering responses

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Chronic Degradation Hazards

These hazards cause cumulative damage through repeated or continuous processes, requiring ongoing maintenance strategies rather than one-time structural solutions.

Coastal Erosion

• Sediment budget imbalance occurs when wave energy removes more material than natural processes or human intervention can supply
• Shoreline retreat rates vary from centimeters to meters per year depending on substrate, wave exposure, and sediment supply
• Hard vs. soft engineering tradeoffs—seawalls protect upland but accelerate adjacent erosion; beach nourishment requires repeated application

Flooding

• Compound flooding combines storm surge, precipitation, and river discharge—coastal urban areas face drainage system overwhelm during concurrent events
• Flood frequency analysis uses statistical methods to estimate return periods; base flood elevation (BFE) sets minimum construction standards
• Green infrastructure (wetlands, permeable surfaces) complements gray infrastructure (levees, pumps) in integrated flood management

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Water Quality and Ecosystem Hazards

These hazards threaten ecological function and human health, requiring monitoring networks and source control rather than traditional structural engineering.

Harmful Algal Blooms

• Nutrient-driven eutrophication triggers rapid algae growth when nitrogen and phosphorus from agricultural runoff and wastewater exceed ecosystem capacity
• Toxin production (e.g., microcystins, brevetoxins) can close fisheries, contaminate shellfish, and cause respiratory irritation in coastal communities
• Watershed management addresses root causes; real-time monitoring and public notification systems manage acute exposure risks

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Quick Reference Table

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Self-Check Questions

1. Which two hazards both raise effective water levels but require fundamentally different engineering responses—and why can't you address one with solutions designed for the other?

2. A coastal city experiences 5 mm/year of relative sea level rise but global rates are only 3.4 mm/year. What hazard explains the difference, and what management intervention could reduce it?

3. Compare and contrast the flood mechanisms of storm surge and tsunamis. How would structural design criteria differ for a coastal building in a hurricane zone versus a tsunami zone?

4. An FRQ asks you to evaluate a seawall proposal for an eroding shoreline. What unintended consequence should you address, and what alternative or complementary approach might mitigate it?

5. Which hazards require watershed-scale management rather than shoreline-focused engineering? Explain why structural solutions at the coast are insufficient for these threats.