Urban Hydrology 2

Summary

This video delves into the impact of urbanization on hydrology, focusing on impervious surfaces like asphalt and concrete that limit water infiltration into the ground. It explains how urban development increases runoff and flash flooding due to the reduction in natural drainage networks. The video covers the complex infrastructure of stormwater management, including gutters, man-made channels, and sewage systems. Additionally, it explores the environmental consequences, such as sediment flow and water pollution from construction, oil leaks, and sewage. Ultimately, urbanization alters natural water flow, with significant implications for flood risk and water quality.

Takeaways

• 😀 Impervious surfaces, such as asphalt and concrete, prevent water from infiltrating the ground, reducing groundwater replenishment in urban areas.
• 😀 Urbanization increases surface runoff due to the lack of natural infiltration, leading to higher overland flow in cities.
• 😀 Smaller residential lot sizes tend to have more impervious surface area, while larger lot sizes allow for more porous, water-absorbing ground.
• 😀 Urban drainage networks are man-made systems designed to manage water flow and prevent flooding, especially in areas with altered topography.
• 😀 Man-made drainage networks, like gutters and crowning of roads, direct water into specific channels, unlike natural streams that change course over time.
• 😀 City infrastructure, including drainage pipes, sewers, and wastewater treatment plants, is highly water-dependent and complex, often hidden beneath urban areas.
• 😀 Channelizing rivers, such as the LA and Santa Ana Rivers, prevents natural meandering that could damage infrastructure, reducing the risk of flooding.
• 😀 Floods in urbanized areas can occur more frequently, especially during extreme rainfall, as drainage systems may not be able to handle the volume of water.
• 😀 Flash floods are more common in urban environments due to concentrated runoff and a high amount of impervious surfaces, which lead to water accumulation in specific areas.
• 😀 Urban development increases sediment flow into water channels, often carrying pollutants from construction sites, vehicles, and chemicals into rivers and streams.
• 😀 Pollution in urban runoff can include oil, pesticides, herbicides, fertilizers, and other chemicals, all of which eventually flow into groundwater or the ocean.
• 😀 Leaking sewage pipes and polluted landfills contribute to water pollution, as harmful substances seep into groundwater or are carried by rain into the water system.

Q & A

What are impervious surfaces, and why are they a problem in urban environments?

Impervious surfaces are materials like asphalt and concrete that prevent water from infiltrating into the ground. In urban environments, these surfaces reduce groundwater recharge and increase surface runoff, leading to potential flooding and less water absorption by the soil.

How does urbanization affect the infiltration of water into the ground?

Urbanization reduces the natural infiltration of water into the soil by covering land with impermeable surfaces like concrete and asphalt. This prevents water from seeping into the ground, forcing it to flow over the surface instead.

What is the relationship between lot size and the amount of impervious surface in urban areas?

Smaller lot sizes typically have more impervious surfaces, such as concrete and parking lots, leaving less space for natural vegetation. Larger lot sizes usually have more space for gardens and porous surfaces that allow water to infiltrate.

What are the consequences of reduced water infiltration in urbanized areas?

When water infiltration is reduced, runoff increases, and water must be collected in storm sewers and drainage systems. This can lead to faster water flow, higher peak discharge, and a higher likelihood of flash floods during heavy rainfall.

What is the concept of 'crowning' in urban drainage systems?

Crowning refers to the design of roads where the center of the road is higher than the edges, which directs water away from the middle towards the gutters, helping to manage surface water drainage.

How do man-made drainage networks differ from natural drainage systems?

Man-made drainage networks are designed to direct water flow through specific paths to prevent flooding, whereas natural drainage systems are flexible and can change course over time due to erosion and water flow patterns.

Why were rivers like the Santa Ana and Los Angeles rivers channelized?

Rivers were channelized to prevent them from meandering and damaging infrastructure, such as roads and railroads. The process became more common after the catastrophic 1938 floods that caused widespread destruction.

What is the impact of urbanization on the likelihood of flash floods?

Urbanization increases the chances of flash floods because it reduces water infiltration, increases surface runoff, and can overwhelm drainage systems during heavy rain, causing rapid flooding in urban areas.

What role do construction activities play in water pollution in urban areas?

Construction activities expose sediment and increase the likelihood of pollutants like oil, pesticides, and fertilizers being washed into nearby water systems. This leads to contamination of rivers and groundwater.

What kinds of pollutants are commonly carried by urban runoff into water systems?

Urban runoff can carry pollutants such as oil from vehicles, herbicides, pesticides, fertilizers, sewage leaks, and chemicals from construction sites, all of which contaminate rivers, streams, and groundwater.