About SPU
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Drainage & Sewer System
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Natural Drainage Systems
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Natural Drainage Overview
Benefits and Challenges
Benefits
Natural drainage systems (NDS) offer a number of benefits to homeowners, municipalities, and the environment:
- • Reduce stormwater volumes, and thereby decrease flooding.
- • Improve water quality because plants and other natural organisms remove or neutralize contaminants.
- • Reduce peak flow volumes and provide more stability for our creek flows.
- • Incorporate more trees and green space into our neighborhoods.
- • Involve neighbors in planning and implementation.
- • Increase pedestrian and traffic safety by reducing average auto speeds.
- • Rather than deteriorating over time like piped systems, NDS become more effective as plants and trees grow.
- • NDS can easily integrated into the landscape and appear more natural than traditional drainage systems.
- • One or more of the swales, cascades, or other components of the system can fail without undermining the integrity of the system as a whole.
- • Controlling runoff at the source eliminates the need for more costly conveyance systems, regional detention facilities, and mitigation for toxic buildups in water bodies.
- • The reduction of impervious surfaces (such as roadways and sidewalks) and use of aboveground swales rather than underground vaults significantly reduce costs of new street and drainage improvements in residential areas of low to medium density.
Challenges
In addition to the benefits associated with NDS, there are also risks and uncertainties:
- • The possible failure of infiltration, resulting in standing water within the swale. Mosquitoes require at least six days of standing water to breed. Therefore, all swales are designed to drain completely within three to five days.
- • The possibility of infiltration causing slope instability. SPU has a policy of not placing infiltration facilities within one block of a steep slope at minimum and further if a geotechnical survey shows higher risk.
- • Uncertainty regarding long-term impact on aquatic species in Seattle’s creeks, lakes, and bays.
- • Safety of car traffic on non-standard design streets.
- • The reduction of available parking.
For municipalities, NDS may be difficult to implement due to institutional barriers:
- • Traditional land use code standards may impede implementation of NDS. Traditional land use codes require wide street widths, curb and gutter, and piped stormwater infrastructure.
- • Street design mangers often prefer to uphold existing national street standards they feel are necessary for car mobility and pedestrian safety.
- • Fire response officials often assume the need for wider streets for emergency vehicle access.The High Point project required a city executive directive to secure street widths that were less than redevelopment code required. Once built, the established rules of standard street design can more easily be questioned as emergency vehicles navigate the new street without difficulty.
Although there can be significant hurdles to overcome, SPU found that the impact on our region’s receiving waters is too high to forgo the study and implementation of natural drainage systems.
Seattle develops at a slow rate (less than one percent per year); therefore new regulatory requirements in the state stormwater manual will take hundreds of years to be fully implemented. Taking a proactive retrofitting approach to improving receiving waters’ health is a priority for the City.
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