How does urban development alter hydrograph characteristics?

Urban development alters hydrograph characteristics by increasing runoff, reducing infiltration, and shortening lag time.

Urban development significantly impacts the hydrological cycle, which in turn, alters the characteristics of a hydrograph. A hydrograph is a graph showing the rate of flow (discharge) versus time past a specific point in a river, or other channel or conduit carrying flow. It is used in hydrology to study the relationship between rainfall and the water level in a river.

One of the main ways urban development alters hydrograph characteristics is by increasing surface runoff. This is due to the construction of impermeable surfaces such as roads, pavements, and buildings, which prevent water from infiltrating into the ground. Instead, the water is directed into storm drains and then into rivers, leading to a higher and faster peak in river discharge following rainfall. This is reflected on the hydrograph as a steeper rising limb and a higher peak discharge.

Another way urban development impacts hydrograph characteristics is by reducing infiltration. Infiltration is the process by which water on the ground surface enters the soil. In urban areas, the natural ground surface is often replaced by impermeable materials, reducing the amount of water that can infiltrate into the ground. This means that more water is available for surface runoff, which can lead to a more rapid rise in river levels following rainfall. On a hydrograph, this is shown as a shorter lag time, which is the period between peak rainfall and peak discharge.

Furthermore, urban development often involves the straightening and deepening of rivers to allow for better navigation and flood control. This can speed up the flow of water, reducing the time it takes for water to travel from the point of rainfall to the river. This can further shorten the lag time on a hydrograph.

In conclusion, urban development can significantly alter the characteristics of a hydrograph, primarily by increasing surface runoff, reducing infiltration, and shortening lag time. These changes can have significant implications for flood risk in urban areas.