Water Flow and Geography
In urban settings, a big pipe system diverts water directly to streams or lakes, causing low water loss and rapid discharge increase during storms.
In rural areas, natural elements like trees and soil absorb and hold rainwater, filtering it gradually to rivers.
Example: Water can take days to reach rivers due to soil retention.
Flood Hydrographs
Hydrographs showing rapid increases in discharge are termed "flashy" indicating a quick storm response.
Example of flash flooding due to prior rainfall (antecedent rainfall) affecting soil saturation.
Ground conditions: Frozen ground acts like pavement, increasing runoff and flash flooding.
Urban vs. Rural Floods
Comparison of flood heights between urban and rural areas in Nova Scotia illustrates how urban areas experience higher flooding due to less absorption.
Urban flood hydrographs can show increased peak heights and rapid responses to precipitation.
Causes of Flooding
Factors: Weather patterns, climate change, urbanization, permafrost, wildfires, and sea level rise.
Flooding events may be exacerbated by human-made alterations to the landscape and water systems.
Base Level Concept
Sea level represents the ultimate base level. Proximity to sea level increases vulnerability to flooding due to sea-level rise.
Base level influences how much a river can erode its surroundings and impacts flood risk.
Variability Factors
Different conditions such as storm surges, heavy rains, and rapid snowmelt can lead to flooding.
Example: The effects of multi-variable flooding combining rain, snowmelt, and high tides can significantly increase flood risk.
Recurrence Interval of Floods
Recurrence intervals help predict the probability of flood events based on historical data.
Method 1: Analyze complete historical records of floods for a river.
Method 2: Utilize numerical models for predictions, which may not be fully accurate.
Method 3: Examine paleohydrological records for long-term flood trends, enriching the understanding of flood risks.
Example: If the average of a specific flood height occurs once every ten years, this would result in a ten-year return period based on ranked historical data.
Ranking and Probability
Floods are ranked based on observed heights or discharge levels.
The formula for predicting recurrence interval is given by:
Recurrence Interval = (Number of Years Observed) / (Rank of the flood height/discharge)
Understanding probabilities: A 1 in 20 chance of flooding indicates a 5% chance in any given year.
Data Interpretation Challenges
Predicting based on limited data poses challenges; the more years of data, the better the prediction reliability.
Data extrapolation beyond observed records must be done with caution to avoid inaccuracies.