OVERVIEW of URBAN HYDROLOGY & EARTHWORKS BENEFITS
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, stormwater runoff occurs when the storage and infiltration capacities associated with the above processes are exceeded. Stormwater 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 stormwater 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 stormwater.
The inability of urban watersheds to absorb stormwater 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 stormwater control measures to mitigate these environmental impacts by capturing, storing, and infiltrating stormwater 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 stormwater.
Beyond stormwater 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 STREETS - A BARREN, BROKEN, & COSTLY SYSTEM
Due to ignorant rather than integrated (and economically efficient) design, most residential streets are part of a broken and expensive urban system that create more problems than they address for a number of reasons, including:
1) Streets are excessively wide, optimally designed for vehicular traffic, but not in use for their intended purpose almost all of the time.
Consequences include:
- Lack of calming features (e.g., trees) promotes inattentive & aggressive driving (when in use)
- Unsafe driving poses physical hazard to pedestrians, bicyclists, pets, & wildlife
- Large surface area accumulates vehicle pollution (e.g., hydrocarbons, metals, dust) later mobilized & transported by stormwater runoff
- Quality of life impacts - socially divisive & aesthetically unattractive.
- Problem solving trees & other plants replaced by problem-producing pavement.
- General ecological degradation.
- Associated social & economic costs.
2) Wide streets require excessive impervious surface that readily shed rather than absorb precipitation, generating stormwater runoff that results in downstream flooding.
Consequences include:
- Hazardous conditions for drivers, pedestrians, & bicyclists.
- Property damage from flooding & vehicle accidents.
- Watershed degradation (water pollution, aquifer depletion, erosion & sedimentation).
- Ecosystem degradation (lack of water infiltration & associated vegetation providing habitat).
- Associated economic costs (flood control, stormwater pollution control, endangered species management, insurance).
3) Wide streets are composed of excessive pavement that create both surface and atmospheric Urban Heat Islands with extreme daytime surface temperatures and elevated day and night time air temperatures.
Consequences include:
- Decreased multi-modal (pedestrian & bicycle) transportation
- Decreased outdoor activity & neighborhood social interaction
- Increased energy/fossil fuel consumption.
- Increased energy-related water consumption (e.g., power plants).
- Increased air pollution (e.g., particulates & ground-level ozone).
- Increased greenhouse gas emissions (& climate change).
- Increased household energy costs for cooling & vehicle transport
- Burn hazards to unprotected skin (& paws) from hot surfaces.
- General ecological degradation.
- Associated social & economic costs.
4) The adjacent pedestrian right-of-way (ROW), particularly in Tucson, is often occupied (illegally) by parked cars (despite ample room to park in the street).
Consequences include:
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Decreased pedestrian activity
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Increased hazards by forcing pedestrians into street
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Increased effective street area & related problems
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Limited availability for street tree plantings & other vegetation
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Decreased shade over pavement
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Compacted soil increasing stormwater runoff
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Reduced urban habitat
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Reduced traffic calming (cars parked in street effectively narrow it)
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Socially divisive & aesthetically unattractive
In summary, the design and construction of existing residential streets has created a myriad of social, economic, and environmental/ecological problems that need to be mitigated by an integrated and economically efficient design/retrofit solution if our society wants to reclaim the urban environment by restoring its quality, diversity, and function to the extent practicable.
