25 October 2004
reduce energy costs, create an attractive environment, and have reduced replacement
and maintenance, and longer life cycle costs. There are many types of vegetated roofs
that can be developed including pre-made grids, or cells, or whole systems.
LID MANAGEMENT AND DESIGN STRATEGIES. LID design is an iterative
process that requires a thorough understanding of the management objectives, a
detailed understanding of the physical and natural resources of the site, a conceptual
site design that can be refined to achieve the goal of a hydrologically functional
landscape, and a long-term maintenance plan.
LID Site Planning Components. This section presents the aims of LID site
planning and, in light of existing site development requirements, describes how LID site
design can be best approached to manage runoff.
5-4.1.1 Hydrologic and Hydraulic Objectives. The purpose of LID site planning is
to significantly maintain the predevelopment runoff volume and flow rate. Ideally, and
where site conditions allow, this will be achieved in a way that replicates the site's
predevelopment hydrologic functions. Sites that are characterized before development
by porous soils, substantial vegetative ground cover, and ungraded topography
naturally perform several important hydrologic functions:
Facilitate infiltration, evapotranspiration, retention and detention of runoff
Limit runoff flow rates because of ground surface roughness
Help control water quality through surface and subsurface filtering of
pollutants and sediments
On a developed site, these hydrologic functions can continue to be provided
by the preservation of natural features or construction of a variety of man-made features
(as described in Chapter 9). Taken together, the utilization of these features comprises
a distributed source control strategy that is designed to not only meet regulatory
requirements but also to provide superior natural resource protection.
Maintaining areas with high soil porosity, vegetative ground cover, and
shallow ponding will help meet the following objectives:
Flood control. Facilitating the infiltration of runoff and decreasing overland
flow rates reduces the risk of flooding in receiving waters. To meet design
objectives and regulatory requirements completely, supplemental controls
may still be required.
Volume Control. The overall volume of runoff that leaves a site is kept as
close as possible to predevelopment levels.
Peak Control. The peak runoff rate does not increase above
predevelopment levels, and the entire runoff hydrograph emulates the