The basic principle of LID to use nature as a model and manage rainfall at the source is accomplished through sequenced implementation of runoff prevention strategies, runoff mitigation strategies, and finally, treatment controls to remove pollutants. Although Integrated Management Practices (IMPs) — decentralized, microscale controls that infiltrate, store, evaporate, and detain runoff close to the source — get most of the attention by engineers, it is crucial to understand that LID is more than just implementing a new list of practices and products. It is a strategic design process to create a sustainable site that mimics the undeveloped hydrologic properties of the site. It requires a prescriptive approach that is appropriate for the proposed land use.
Design using LID principles follows four simple steps.
# Determine pre-developed conditions and identify the hydrologic goal (some jurisdictions suggest going to wooded conditions).
# Assess treatment goals, which depend on site use and local keystone pollutants.
# Identify a process that addresses the specific needs of the site.
# Implement a practice that utilizes the chosen process and that fits within the site's constraints.
The basic processes used to manage stormwater include pretreatment, filtration, infiltration, and storage and reuse.