Group 6 (Rainfall-Runoff Analysis)

RAINFALL-RUNOFF RELATIONSHIP

The rainfall-runoff relationship explains how rainfall is transformed into runoff within a watershed.

Rainfall

is the amount of water (precipitation) that falls to the earth's surface.

Runoff

Movement of excess water from the watershed.

reason why rainfall becomes a runoff

• Infiltrate the soil

• Evaporate back to the atmosphere.

• Absorbed by plants.

• Temporary stored in puddles.

• Flow as Surface-Runoff

hydrographs

• Hydrographs are graphs that shows how river flow changes over time.

• Discharge over Time.

• Timeline reaction of rivers on rain

stage 1 before the rain

• Base flow or normal discharge of river.

• The first stage of the graph.

stage 2 rising limb

• Where the rain actually starts.

• The more it rains, the more discharge flows into the river.

stage 3 peak discharge

• The stage where the river produces maximum discharge.

• Highest chance of flooding.

stage 4 recession limb

• After the rain.

• The slow downlone of river discharge.

lag time

• This is the most important metric for flood prediction.

• The time interval between the peak rainfall and the peak discharge.

• The “Warning Window”

• Predictability

• Urbanization Impacts

importance of hydrographs

• Flood Forecasting

• Infrastructure Design

• Ecological Health

unit hydrograph theory

• A graph that shows how a river responds when a small fixed amount of rain falls on the entire watershed.

• One unit depth of effective rainfall occuring uniformly over a watershed during a specified duration.

1cm rainfall a reference amount

• It is small and simple

• Easy to scale later

• Makes calculations easy.

principle of proportionality

• Runoff response assumed to be proportional to the amount of rainfall.

• 1 cm = Q, then;

• 2 cm = 2Q

• 3 cm = 3Q

hydrologic modeling

• is the used of mathematical equations and computer simulations to represent how water moves in watershed.

• Engineers create a digital simulation of watershed.

• Instead of waiting for a real storm to happen, engineers simulate it.

• It is a virtual experiment

importance of hydrologic modeling

• Because the Philippines experiences frequent typhoons, extreme rainfall and flash floods, Hydrologic modeling helps engineers prepare for these events.

• Allowing us to estimate flood levels, river discharge, and drainage systems.

application in civil engineering

These concepts are used in designing bridges, drainage, culverts, dams and reservoirs, flood control channels.