Final answer:
Passive flood mitigation strategies, such as preserving natural forests and wetlands, are often preferred for their cost-effectiveness, environmental sustainability, and long-term effectiveness. However, active mitigation measures like seawalls may be necessary in high-risk areas.
Step-by-step explanation:
When considering flood mitigation strategies, the preference between passive and active approaches depends on a variety of factors including cost, environmental impact, and effectiveness. Passive flood mitigation, such as the preservation of mangrove forests and wetlands, is often more cost-effective and environmentally sustainable compared to the construction of artificial barriers.
An example of the effectiveness of passive mitigation is the observation that thirty "waru" trees planted along a 100-meter band can reduce the impact of a tsunami by 90%. Moreover, the maintenance of natural forests has been shown to significantly decrease both flood risk and severity, as afforestation programs may not provide the same benefits and can even exacerbate flood risks due to factors like soil compaction and altered drainage regimes.
On the other hand, active mitigation strategies like seawalls and levees might be necessary in certain high-risk urban areas, especially those facing sea level rise. However, the creation and conservation of wetlands can serve as a natural protection measure against storm surges and flooding, demonstrating the potential for combined active and passive approaches in certain contexts. Ultimately, while both strategies have their uses, passive strategies are often preferred for their sustainability and long-term effectiveness in reducing flood impact.