Flood Forecasting

Surface Runoff

A term used to denote the excess water from precipitation that is available on the land surface after abstractions.

Effective Rainfall

Total incident rainfall minus the abstractions and is sometimes referred to as the rainfall excess.

Abstraction

Term used to describe the amount of rainfall that doesn't turn into runoff.

Saturation Excess

Overland flow is generated at locations where the soil is saturated at the surface.

Infiltration Excess

Overland flow occurs when the rainfall rate exceeds the infiltration capacity of the soil.

Rational Method

Is the most widely used runoff model in small catchments.

Unit Hydrograph Method

Mostly used runoff models in mid size catchments.

Routing Method

Are used where runoff regimes within a catchment vary from overland flow at the smallest scales to river flow at the largest scales.

Dendritic Pattern

Occur where rock and weathered mantle offer uniform resistance to erosion.

Rectangular Pattern

Occur in faulted areas where streams follow a more easily eroded fractured rock in fault lines.

Trellis

Occur when rocks have different levels of resistance, causing streams and tributaries to form and spread more quickly in the less resistant areas.

Perennial streams

Have a continuous flow regime typical of a well defined channel in a humid climate.

Intermittent streams

Generally have flow occurring only during the wet season (SO percent of the time or less).

Ephemeral streams

Generally have flow occurring during and for short periods after storms.

Flood Hydrograph

The stream flow hydrograph or discharge hydrograph is the relationship of flow rate (discharge) and time at a particular location on a stream.

Flood

• Rise, usually brief, in the water level in a stream to a peak from which the water level recedes at a slower rate.

• Relatively high flow as measured by stage height or discharge.

• Rising tide.

Flooding

The effect of a flood that is defined as overflowing of water beyond the normal that it confines whether it is a stream or other body of water, or accumulation of water by drainage over areas that are not normally submerged.

Flash Floods

Occur frequently associated with violent convection storms of a short duration falling over a small area.

Single Event Floods

This is the most common type of flooding, in which widespread heavy rains lasting several hours to a few days over a drainage basin results in severe floods. Mostly due to typhoons.

Multiple Event Floods

These result from heavy rainfall associated with successive weather disturbances following closely after each other.

Seasonal Floods

Floods that occur with general regularity as a result of major seasonal rainfall activity, mostly due to the monsoon.

Coastal Floods

Flooding caused by storm surges and high winds coinciding with high tides.

Estuarine Floods

Flooding caused by the interaction between the seaward flow of river water and landward flow of saline water during high tides.

Urban Floods

Flooding that occurs when intense rainfall within towns and cities creates rapid runoff from paved and built-up areas, exceeding the capacity of storm drainage systems, mostly due to low infiltration.

Prolonged Rainfall (Physical Factor)

Soil becomes saturated after prolonged rainfall which leads to an increase in surface run-off as rainfall can no longer infiltrate the soil.

Heavy Rainfall (Physical Factor)

Water arrives early and too quickly to infiltrate the soil.

Geology (Physical Factor)

Impermeable surfaces such as clay and granite do not allow infiltration, leading to greater surface run-off.

Topography (Physical Factor)

The steeper the slope, the more rapid the flow of water into a river channel.

Size and Nature of River

A bigger, straighter, and smoother river, creek or other channel has a greater capacity to carry water and is less prone to flood.

Vegetation in and around the river

Plants in a river or on its banks slow the speed of the water flowing in it.

Structures

Structures placed in a creek or waterway, such as culverts in an urban drainage system or bridges in a river, reduce the water-carrying capacity of the waterway and may contribute to flooding.

Flood Forecasting

Involves predetermination of flood events necessary to help the implementation of structural measures for the mitigation of flood damages, regulation, and operation of multipurpose reservoirs with a focus on the control of incoming floods and the evacuation of affected people to safer places.

Flood Warning

Issued when an event is occurring or is imminent.

Hydrologic Data

Data that relates to measurement of river flow and level, with monitoring instruments able to accurately record peak values of both.

Meteorological Data

Includes rainfall intensity and duration, precipitation forecasts, and past data for calibration of rainfall runoff models, necessary for developing a successful flood forecasting and warning system.

Topographic Data

Data required for the development of flood forecasting systems, as demands increase for models to produce realistic estimates of spatial flooding.