Sustainable Development Guide

Section 1: Conservation Design Approach

Introduction

A conservation design approach to industrial site design can have a positive effect on water quality and quantity. By restricting the amount of impervious areas created by buildings, parking lots, roads, and sidewalks, this approach can reduce the runoff peaks and volumes that must be conveyed and controlled on a site. This decreases the size and cost of the drainage and storm-water control infrastructure.

The conceptual site design should lay out the storm-water infrastructure. To accomplish this, follow the steps outlined in the Georgia Stormwater Manual (1).

Gather needed information

Before starting the site concept plan, develop a base map for the project site. Identify natural features and resources such as undisturbed forest areas, stream buffers, wetlands, springs, floodplains, and steep slopes. Conserving these areas will retain some of the original hydrologic function of the site. Delineate areas of different soil types. Detail drainage areas and patterns, topography, and existing storm-water control structures. Determine the depth of the groundwater table.

It’s important to set water quality and quantity goals for the project. These goals must address development and construction activities that may cause water pollution. Each project is unique and requires specific knowledge of the pollution risks associated with the development. Environmental permits, regulations, and watershed plans may contain the information needed to formulate these goals.

To obtain existing permit requirements, regulations, and plans during project planning, make contact with local municipalities and state environmental agencies, watershed associations, and city, county, or local development districts. Where possible, storm-water goals should support already-defined goals in existing regulations and watershed plans. In locales without storm-water goals, consider using the following:

• Provide quality treatment for 90 percent of the expected storms

WQv = water volume to treat for volume

WQv = [(P)(Rv)(A)] /12

Rv = 0.05+0.009(I)

I = impervious cover (percent)

Minimum Rv = 0.2

P = Rainfall Event Number (see NOAA chart of 2-year 24-hour rainfall

A = site area in acres

• Provide 24-hour extended detention in the event of a post-developed one-year, 24-hour storm. Use more detail analysis to show how other approaches adequately protect channels instead.

• To minimize flooding that goes over a stream bank, control the peak discharge from a 10-year storm to 10-year predevelopment rates. Control the peak discharge from the 100-year storm to 100-year predevelopment rates.

• Design best practices to safely withstand a 100-year storm event.

Meeting these criteria provides treatment with the flexibility to choose practices that work best on a particular site. Using runoff volume from a design storm, rather than defined runoff volume, gives the designer credit for reducing impervious surfaces. (2)

Design the system

Follow these steps when designing the system:

Integrate water quality and quantity site features while developing the concept plan.
Design the project so it will infiltrate storm water onsite.
Reduce the overall imperviousness of the site by shortening lengths and reducing widths of roads and sidewalks, and reducing the size of parking lots and building footprints.
Use open space and existing natural features as part of the drainage system.
Use best practices, including swales, vegetative strips, infiltration channels, bioretention areas, and rain gardens.
Consider how all these work together to attain storm-water management goals.
Wherever possible, use native vegetation, since it often grows better, may require less water, and has deeper roots to hold soil in place (see TVA's Riparian Restoration site).

A storm-water management system for a site is made up of best practices. These practices should work together cost-effectively to achieve project storm-water management goals. Practices can be structural, that is, engineered systems such as bioretention areas, swales, or detention ponds that improve the quality and/or control the quantity of runoff. They can also be nonstructural, such as institutional, educational, or pollution prevention techniques designed to limit the amount of storm-water runoff or reduce the pollutants contained in runoff.

No single practice can address all storm-water problems. However, some practices provide more benefit for the investment:
Leaving undisturbed areas around the site perimeter
Fitting the development to existing landscape and topography
Leaving or creating buffers along any streams or drainages that flow through or next to the site
Leaving existing drainage patterns intact.

Factors that influence best-practice choices include drainage area served, available land space, cost, and pollutant removal efficiency, as well as a variety of site-specific factors such as soil types, slopes, and depth of groundwater table. Careful consideration of these factors is necessary to select the appropriate practices for a particular location. (3). Choose practices that are best suited to specific site conditions and that enable the project to meet storm-water management goals in a cost-effective manner.

Best practices are frequently used in combinations of two or more. The capacity of non-structural best practices to remove specific pollutants before those substances contaminate storm water makes them ideal for enhancing the effectiveness of structural best practices (4). Combinations of best practices – sometimes called the treatment train – are picked because they work best with the characteristics of the site and are best suited to achieving the storm-water quality and quantity goals.

The treatment train

A treatment train strategy for storm-water management combines source, onsite treatment, and community-wide controls. Follow these steps when developing a treatment train: (5)

Review the site before and after development. Calculate the amount of runoff likely to be produced pre- and post-development given a particular storm event.
Consider how to control runoff at the source and during construction to achieve water quality and quantity goals for the project.
Consider how swales, filter inlets, baffle boxes, oil-water separators, and other controls will achieve the project goals.
Consider whether any additional treatment through infiltration, inline storage, sediment forebay, or other controls is needed to achieve the goals.
Consider whether any additional inline detention, retention, or other treatment is needed to achieve the goals.
Consider how a system of practices achieves the project’s water quality and quantity goals where storm water discharges into receiving waters.

Some elements of the treatment train are best provided offsite. For example, an evaluation of site characteristics might show that water quality is best managed onsite and flood protection offsite. (6)

At every step of the treatment train, a conservation design approach considers how green space, native landscaping, and natural hydrologic functions reduce runoff from a site. These techniques reduce runoff volume by infiltrating rainfall to groundwater, evaporating it back to the atmosphere after a storm, or finding beneficial uses for it rather than sending it down storm sewers. (7).

Planned open spaces with slopes less than 5 percent may help manage storm-water quality and quantity. (8) Emphasize low-maintenance approaches such as swales, biofiltration practices, and riparian buffers. (9) Landscaped areas can become detention or retention areas. Vegetated areas in the medians of roads or parking lots can become vegetated swales or filters. (10 ) The result may be a landscape functionally equivalent to predevelopment hydrologic conditions.

While most engineering plans pipe water to low spots as quickly as possible, this approach uses micro-scale techniques to manage precipitation as close to where it hits the ground as possible. Strategically placed, linked lot-level controls are customized to address specific pollutant loads, storm-water timing, flow rates, and volume issues. These controls are all elements of a treatment train for storm water on a site. (11).

Some of the most commonly used conservation design practices include:

Rain gardens and bioretention
Rooftop gardens
Sidewalk storage
Vegetated swales, buffers, and strips; tree preservation
Roof leader disconnection
Parking lot storage
Permeable pavers
Soil amendments
Impervious surface reduction and disconnection
Pollution prevention and good housekeeping. (12)

Many of these practices are described in this Guide.

For more information about these and other conservation design practices, review the Center for Watershed Protection’s Better Site Design: A Handbook for Changing the Development Rules in Your Community. The handbook contains 22 model development principles. In 1997, the Center for Watershed Protection convened a roundtable of nationally recognized experts with diverse opinions about land development and its impact on water resources. Through an 18-month consensus-building process, they agreed to the model development principles.(13)

Plan for construction