URBAN HYDROLOGY
During a storm event, a number of processes can occur in a watershed when precipitation falls from the atmosphere toward the earth as rain. Rain can be intercepted by trees and other vegetation where it is stored on leaves, branches, and limbs preventing it from hitting the natural ground or artificial surfaces. Rain can directly fall into surface water features such as creeks, streams, wetlands, ponds, and lakes where it can increase bank storage and recharge aquifers. Rain can fall directly to the ground surface where it infiltrates immediately and is stored in the subsurface. Rain can accumulate on the ground surface and be stored in surface depressions where it can either remain as standing water or infiltrate into the subsurface.
Evaporation of water back into the atmosphere either directly or through plants is generally negligible during and immediately after a storm event.
Depending on the characteristics of both the storm event (duration and intensity) and the physical nature of the watershed where the rain falls, storm water runoff occurs when the storage and infiltration capacities associated with the above processes are exceeded. Storm water then flows over natural and/or artificial surfaces where it eventually may become surface water in creeks, streams, rivers, and lakes, and may eventually end up in a sea or ocean.
In terms of runoff, natural watersheds behave quite differently than urbanized watersheds. Urbanized watershed areas are typically covered with a high percentage of impervious surfaces, including cleared and compacted natural surfaces, pavement, buildings, and other structures.
For a given storm event, an urbanized watershed generates much more storm water runoff faster than a natural watershed because the vegetative interception, surface depression storage, and subsurface infiltration capacities associated with the processes described above are greatly diminished. In many cases, most wetlands, ponds, and creeks in urban watersheds have also been covered and sometimes lined with impervious surfaces further reducing the ability of a watershed to immediately absorb storm water.
The inability of urban watersheds to absorb storm water commonly results in a variety of destructive and costly environmental impacts, including flooding, erosion and sedimentation, surface water degradation, aquifer depletion, property damage, and in some cases, even human injury and death due to car accidents and drownings.
Earthworks, including basins, berms, and swales can serve as storm water control measures to mitigate these environmental impacts by capturing, storing, and infiltrating storm water at or near its source.
On a basin-wide scale, properly designed and located water harvesting earthworks function as enhanced surface depression and infiltration features that capture, accumulate, and percolate water relatively deep into the subsurface where it is stored for plants, animals, and beneficial insects and microbes, to utilize over time as they develop healthy soil and help reclaim damaged environments. Plants, particularly large trees grown in and around earthworks further provide interception capacity to control storm water.
Beyond storm water control, earthworks provide other broad inter-related benefits to urban watersheds, including, but not limited to energy, resource, social, aesthetic, and ecological improvements. Examples include urban heat island mitigation, irrigation reduction (using tap water), urban agriculture, traffic calming, landscaping, and habitat creation.
RESIDENTIAL URBAN STREETS - A PROBLEMATIC SYSTEM
Due to a lack of systematic analysis and integrated design, most residential streets are part of a broken and expensive urban system that actually create more problems than they address for a number of reasons, including:
1) Residential streets are excessively wide - although optimally designed for vehicular traffic, they are not in use for their intended purpose almost all of the time. Direct consequences include:
- Width combined with a lack of traffic calming features (e.g., trees) promotes inattentive/aggressive driving through neighborhoods posing physical hazards to children, pedestrians, bicyclists, pets, & wildlife.
- Extensive areas containing surface soil, trees & other plants are replaced by impervious pavement that accumulate vehicle pollution (e.g., hydrocarbons, metals, dust) that is mobilized & transported by storm water runoff into washes/rivers.
- Poor aesthetics and divisive impact on neighborhoods.
- High economic costs to both initially construct and maintain over their lifetime.
- Decreased urban livability/quality of life (above).
2) Impervious surfaces (pavement) readily shed rather than absorb precipitation, generating storm water runoff that results in flooding and sediment deposition. Direct consequences include:
- Hazardous conditions for motorists, pedestrians & bicyclists.
- Property damage from flooding & vehicle accidents.
- Watershed degradation (water pollution, erosion & sedimentation).
- Environmental and ecosystem degradation (poor infiltration & associated vegetation providing habitat & runoff control).
- High economic costs for flood control, storm water pollution control, sediment removal, endangered species management, and insurance.
- Decreased urban livability/quality of life (above)
3) Excessive pavement creates both surface and atmospheric Urban Heat Islands with extreme daytime surface temperatures and elevated day and night time air temperatures. Problems include:
- Decreased use of pedestrian & bicycle transport modes.
- Increased energy/fossil fuel consumption (home/auto).
- Increased energy-related water consumption (power plants).
- Increased air pollution (particulates & ground-level ozone).
- Increased greenhouse gas emissions (power plants/homes/auto).
- Environmental & ecological degradation (pollution & climate change).
- High economic costs for energy exploration & use, pollution control & cleanup, and health care.
- Increased household energy costs (cooling & transport)
- Decreased urban livability/quality of life (above).
