**3. Excess rainfall**

Flooding is commonly brought about and intensified by tropical cyclones along with other weather systems that generate storms. The presence of abundant moisture in the air mass carried by the different weather systems, when precipitated, make our cities prone to excessive surface runoff from streams and through floodplains where most of these cities are located. Flooding is a natural reaction of a watershed when the excess rainfall which is the amount of rainfall that is not absorbed by infiltration to the ground, flows as surface runoff. However, urbanization tends to alter the permeability of the soil and reduces the infiltration rates of the surface by replacing the permeable ground with impermeable concrete

pavements and building structures. Natural waterways and floodplains that are the natural drainage systems and detention basins of floods are also blocked or replaced with built up environment. This makes flooding a major problem in areas where people and anthropogenic activities are thriving.

In many parts of the world, unprecedented experiences of flooding are documented. The distribution of rainfall is disrupted usually attributed to climate change. Climate change increases the frequency of heavy precipitation events (or the proportion of total rainfall from heavy falls), an observation reported in the IPCC Technical Paper VI on Climate Change and Water [1]. The increase will in turn increase the annual average runoff making previous efforts of flood control and drainage system insufficient to accommodate the runoff.

The amount of excess rainfall is also dictated by the residence time the rain water will remain on the surface such as by the interception of vegetation and the ponding on soil surfaces to be infiltrated. Since rainfall events are observed and even projected by the IPCC [1] to become more intense, i.e. higher rainfall depth on a shorter period of time due to climate change in the wet tropical regions, risk of flooding is projected to rise. Higher intensity rainfall results to lesser residence time for the rain water to be intercepted by the plant's foliage and to be infiltrated to the ground, thereby, causing the hydrograph to peak earlier resulting to flash floods.

## **4. Flooding consequences**

In tropical watersheds, the natural environment is naturally designed to assimilate in its system the excess rainfall that flows as direct runoff through the hillslope, accumulating and flowing as channel flow in the river network. From the steep slopes of the headwater source, it reaches the flatter terrain of the floodplain that functions as natural detention basin before it finally exits to the sea or ocean.

The floodplain serves as a natural barrier to make the flow of water more efficient to in maintaining equilibrium in the watershed system. This equilibrium defines the natural environment of the watershed that hosts different flora and fauna unique to the environment.

Human intervention to promote development to support the demands of mankind and in modern civilization to support the requirements of the economy of a country has put the natural balance of the watershed environment in danger. The modification of our environment leads to the alteration of the hydrological processes. From the watersheds headwaters, the forests are cleared at a rate that leaves the mountains denuded reducing the capacity of the watershed to capture the rainfall for infiltration and recharge of the groundwater. The effect is an increase in surface runoff that endangers the downstream. Moreover, clearing of the forests also reduces the surface roughness of the terrain that tends to increase the velocity flow. Climate change, on the other hand, also modifies the hydrologic cycle and its effects is cascading. An increase in temperature will affect the atmospheric water and hence, the water balance. With increase flowrate and velocity due to anthropogenic alterations coupled with climate change, more floodwaters flow downstream.

Development often favors the flat terrain of the floodplain for accessibility and ease. It is also advantageous for growing crops as it is fertile enriched with nutrients coming from the mountains. Because of this, our floodplains became the usual sites of urbanization. The replacement of the natural environment with the built environment robbed the floodwaters of its own space. Every rainy season or typhoon season people's habitation are inundated with flood. It even encroaches now areas that we thought of not flood prone but are actually are. We just occupied the natural course and detention area of the floodwaters. During typhoons that amplify the

*Flooding and Flood Modeling in a Typhoon Belt Environment: The Case of the Philippines DOI: http://dx.doi.org/10.5772/intechopen.98738*

**Figure 2.**

*Effects of flooding in Metro Manila, Philippines in 2012.*

effects of the monsoon season, low-lying populated areas in the floodplains are a common scene in countries like the Philippines.

In 2009, the Tropical Cyclone Ketsana (Tropical Depression Ondoy which did not reach the typhoon category) wreak havoc to the capital and its suburbs that dumped a month's rainfall in just more than 3 hours. It was an unprecedented flooding event with the scale of its damage. Just three (3) years after in 2012, the same areas were affected even just by the Habagat or Southwest Monsoon rains enhanced by a tropical storm. **Figure 2** shows a swollen river and flooding in low-lying areas.

In countries frequented by tropical cyclones, riverine flooding is even exacerbated by sea level rise brought about by climate change and amplified by land subsidence, making worse the net sea level rise. Moreover, during typhoons, the effects of storm surge brought about by the gusts of winds, even worsens the flooding problem. In the Philippines, when the Tropical Cyclone Haiyan (Typhoon Yolanda) hit the central islands in 2013, it was estimated that more than 6000 people died and left the coastal towns and cities devastated.
