Module 4: Design Principles of GI
Learning Objective
- Identify the basic design principles that make GI work and how design can inform site locations for GI
Green Infrastructure systems rely on a number of environmental and operational principles to provide the broad array of services. These principles, along with the science, engineering, and materials needed to harness them for effective green infrastructure, are well-established. However, they continue to be updated and refined in response to research studies, municipal experiences, and technical innovations. In this module you will explore the current state of green infrastructure science: what is known, what is still being studied, and what the implications are for GI design and planning.
Stormwater management is a significant motivation for GI implementation. In most urban and suburban areas, rainfall flows quickly along hard surfaces and enters the storm sewer system within five to 10 minutes. This short lag time between the rate of rainfall and the rate of sewer flow means that periods of intense (i.e. peak) rainfall inevitably lead to periods of intense sewer flow that may exceed the capacity of municipal sewers, leading to flooding and combined sewer overflows. The process of slowing water down so it does not quickly enter storm sewers is referred to as peak flow reduction. GI systems that aim to reduce the peak flow into municipal sewers have two primary components: storage capacity and discharge control. Green Infrastructure storage capacity holds rainfall onsite before it can enter the storm sewer system, increasing the time that it takes for rainfall to turn into sewer flow. This storage can be provided in soil, gravel, and may include above-ground and below-ground components. For GI systems to drain properly and retain capacity for future storm events, a discharge path must be designed to drain the system within a time frame deemed acceptable by city planners and policy-makers (usually between 12 and 48 hrs). This time-period is referred to as the maximum drawdown time.
The Water Budget Table is a tool intended as a visual indicator of the distribution of mass outflow among event runoff (Q), evapotranspiration (ET) and percolated or stored drainage (I). You will read about this in the article by Eger et al.
Water Budget Triangle
Source: Page 2 of https://www.researchgate.net/publication/286426965_Water_Budget_Triangle_A_New_Conceptual_Framework_for_Comparison_of_Green_and_Gray_Infrastructure
Water can leave a GI system through several pathways. GI systems may be designed to infiltrate a portion of rainfall into groundwater reserves in order to replenish depleted groundwater reserves and improve regional water security. When the underlying soil cannot infiltrate fast enough to drain the system, a perforated underdrain may be installed at the bottom of a GI system to slowly drain saturated soil layers. The vegetated components of GI turn some rainfall into water vapour through evapo