The role of living roofs in holistic stormwater management systems - Living Roofs in Integrated Urban Water Systems - page 22

The increase in surface runoff and its associated pollutants from developed
catchments triggered regulatory intervention in many Western countries and the
introduction of urban stormwater management techniques known as best man-
agement practices (BMPs). For example, on a widespread basis in the United
States, treatment of runoff-borne pollutants (e.g., sediment, nitrogen, phospho-
rus, zinc and other heavy metals, and pathogens) using BMPs was introduced in
the 1990s with amendments to the Clean Water Act that created the National
Permit Discharge Elimination System. Likewise, Auckland, New Zealand, intro-
duced a stormwater discharge permit system for local development projects in
response to the 1991 Resource Management Act. Similar requirements are not
(yet) consistently found throughout the entire country. The European Union's
2000 Water Framework Directive set goals and implementation plans to achieve
and protect water quality and aquatic system integrity amongst surface, coastal
and ground waters. Few national programs address runoff problems from exist-
ing development; the Total Maximum Daily Load program in the United States is
one such example typically resulting in watershed-scale retroit solutions.
The emerging goal of GSI is to mimic, as much as possible, the hydrological and
water quality processes of natural systems as rain travels from the roof/runoff
source to the receiving water. Land-use planning techniques must be coupled with
engineered interventions (i.e., SCMs) to achieve these goals. Regulations and
permits are often accompanied by technical design protocols (criteria, guidance
and standards) administered at the city, state or regional level. To determine the
dimensions and hydraulic features of any SCM, an engineer calculates the runoff
generated from isolated storm(s) of a speciied magnitude(s) (a “design storm”), or
a sequence of storm events (a “continuous simulation”) according to regulatory
conditions. Quantitative objectives must establish the engineering design point(s).
The fundamental question is: how much rain should be captured by a living roof?
Throughout a wide range of climates, rainfall analysis yields surprisingly simi-
larly shaped spectral frequency curves. For a given location, this type of curve
relates the depth of rainfall to the frequency with which it or larger events has
occurred (measured as a percentile) based on historical records. For example, in
Figure 2.1 , 90 percent of the time, rainfall is 25 mm, or less (i.e., the 90th percen-
tile event is 25 mm). Conversely, only 10 percent of the time do storms occur that
are greater than 25 mm. The larger storms produce more runoff on a storm-by-
storm basis, but occur less frequently. Design to control “everyday” events (such
as those that occur up to 90 percent of the time) means the majority of runoff
from individual storms is controlled.
Rainfall spectral frequency analysis typically shows a sharp curvature (e.g., a
knee or inlection point), normally between the 75th and the 95th percentile
rainfall depth. Quantitatively, if a stormwater solution retains the rainfall from
up to the 75th-95th percentile rainfall depth, a large proportion of the total
annual runoff volume and pollutant loads will also be controlled (NRDC 2011).
The classic economic theory of diminishing returns applies: capturing larger, less
frequent events requires larger SCMs that occupy more space, and/or more