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Effects of Stormwater Pollutants - Rhode Island's New Stormwater Manual

Dry swales are open vegetated channels or depressions explicitly designed to detain and promote filtration of stormwater runoff into an underlying fabricated soil matrix. Dry swales should have topsoil composed of loamy sands or silt loam. Silty clay loam or sandy clay are poorly suited for significant infiltration rates. Source: Rhode Island Stormwater Design and Installation Standards Manual

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Rhode Island has approximately 1,498 miles of rivers, 20,917 acres of lakes and ponds and approximately 15,500 acres of shrub swamps and marshes, plus 72,000 acres of forested wetlands. Narragansett Bay and coastal ponds cover 156 square miles. Underlying the state are 22 major stratified sand and gravel aquifers, plus bedrock aquifers.

The Rhode Island Smart Development for a Cleaner Bay Act of 2007 required the state's Department of Environmental Management and the Coastal Resources Management Council to amend the state's 1993 Stormwater Manual.

The new Rhode Island Stormwater Design and Installation Standards Manual is the product of a two-year technical analysis and revision. The new manual was written, compiled, designed and illustrated by Horsley Witten Group (HW), with technical services from the University of New Hampshire Stormwater Center, Durham, N.H., and Loon Environmental, LLC, Riverside, R.I. HW is a consulting group specializing in low-impact development (LID) techniques, stormwater management, land use regulation, site design, coastal/watershed planning and protection, civil/environmental engineering, hydrology and hydrogeology.

The updated stormwater manual provides enhanced practice performance criteria and mandates LID strategies for stormwater management in site planning and design, including: 1) maintaining pre-development groundwater recharge and infiltration on site to the maximum extent practicable; 2) Demonstrating that post-construction stormwater runoff is controlled, and that post-development peak discharge rates do not exceed pre-development peak discharge rates; and 3) Using low impact-design techniques as the primary method of stormwater control to the maximum extent practicable.''

Infiltration basins or shallow vegetated permanent pools store water in a surface depression before it is infiltrated into the underlying soils or substratum. They are generally constructed by excavating a pit into permeable soils with acceptable infiltration capabilities. Infiltration basins are appropriate for relatively small drainage areas (5-50 acres. Development sites larger than 50 acres are best served by wet ponds or extended detention basins. Source: Rhode Island Stormwater Design and Installation Standards Manual

The Problem with Conventional Stormwater Management

The new stormwater manual notes: ''Traditionally, stormwater has been managed using large, structural practices installed at the downstream end of development sites, often as an afterthought, on land segments leftover after developing property. Stormwater is typically conveyed from rooftop to driveway to street, where it is then quickly conveyed via a drainage system to a downstream structural practice such as a dry detention pond. This approach, sometimes referred to as end-of-pipe management, yields the apparent advantages of centralizing control and limiting expenditure of land. These structural drainage systems are designed to be hydraulically efficient for removing stormwater from a site as fast as possible. However, in doing so, these systems limit groundwater recharge, can degrade water quality of receiving waters, and increase runoff volumes, peak discharges, and flow velocities.

'''As research, technology, and information transfer have improved over recent years, alternative approaches are being sought ... to reduce the environmental impacts from new development and redevelopment. Developers and designers are also seeking alternatives to expedite permitting processes, reduce construction costs, reduce long-term operation and maintenance costs, and increase property values. LID has emerged as an effective way to address these issues by combining a site planning and design process with runoff reduction and treatment practices, resulting in benefits that far surpass the end-of-pipe approach.''

From a water quality standpoint filter strips are similar to grassed swales, except that they are designed to receive runoff as overland sheet flow. Channelization of runoff within a filter strip significantly reduces the amount of infiltration and subsequent pollutant removal. Level spreaders are used to capture and evenly distribute runoff to the filter strip while reducing the potential for channelization and maximizing treatment efficiency. Source: Rhode Island Stormwater Design and Installation Standards Manual

Low-Impact Development (LID) Site Planning and Design Criteria - LID site planning and design criteria require proposed projects meet the following measures:

  1. Protect as much undisturbed open space as possible to maintain pre-development hydrology and allow precipitation to naturally infiltrate into the ground.
  2. Maximize the protection of natural drainage areas, streams, surface waters, wetlands, and other regulated areas.
  3. Minimize land disturbance, including clearing and grading, and avoid areas susceptible to erosion and sediment loss.
  4. Minimize soil compaction and restore soils compacted as a result of construction activities or prior development.
  5. Provide low-maintenance, native vegetation that encourages retention and minimizes the use of lawns, fertilizers, and pesticides.
  6. Minimize impervious surfaces.
  7. Minimize the decrease in the "time of concentration" from preconstruction to postconstruction, where "time of concentration" means the time it takes for runoff to travel from the hydraulically most distant point of the drainage area to the point of interest within a watershed.
  8. Infiltrate precipitation as close as possible to the point it reaches the ground using vegetated conveyance and treatment systems.
  9. Break up or disconnect the flow of runoff over impervious surfaces.
  10. Provide source controls to prevent or minimize the use or exposure of pollutants into stormwater runoff at the site in order to prevent or minimize the release of those pollutants into stormwater runoff.

Role of the Landscape Architect

How does the new manual affect landscape architects? LID often utilizes vegetative practices for enhanced stormwater treatment and infiltration. This presents opportunities for designers to play a significant role in the implementation of these techniques for residential, commercial and municipal clients. By choosing appropriate native plants for varying hydrologic conditions, landscape architects can help create LID projects that are not only in compliance with the new stormwater standards, but are beautiful functioning landscapes.

Wet ponds or wetlands are permanent pools of water that decrease the velocity of runoff as it enters the basin, allowing settling of sediments and suspended matter. Microorganisms and plants in the pond or wetland bottom sediments assist in biological uptake and degradation of many pollutants. Source: Rhode Island Stormwater Design and Installation Standards Manual

Horsley Witten will be assisting the Rhode Island Department of Environmental Management and the Coastal Resources Management Council with a series of LID workshops in the Fall/Winter of 2010. Visit for more information.

The 498-page final draft of the new manual is at

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May 26, 2019, 3:17 pm PDT

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