The role of living roofs in holistic stormwater management systems - Living Roofs in Integrated Urban Water Systems - page 21

yards, gardens, reserves and convenient topography alters the pre-developed
condition and integrity of the soil structure, often compacting it, thereby reduc-
ing iniltration capacity (Pitt et al. 1999, 2008; Simcock 2009). The vegetated
canopy is signiicantly reduced, and in many cases, completely eliminated, along
with it the natural interception and ET the canopy previously provided. The
“sponge” of the natural environment is replaced by impervious surfaces such as
roads, rooftops, parking lots and disturbed, compacted soils that readily and
rapidly shed water as surface runoff.
As the hydrologic cycle's components of interception, iniltration, ET and other
surface runoff abstractions decrease, for the same amount of precipitation, the
volume of surface runoff must proportionally increase to maintain the overall
water balance within a catchment. This increased runoff volume travels through
hydraulically eficient urban drainage systems (roofs, gutters, streets, piped storm
sewers and concrete canals), further exacerbating potential receiving water
impacts due to the increased rate and eficiency with which it travels. There is a
possibility of creating even greater peak lows when the post-development lows
from different tributaries converge in the main receiving water or sewer (Fergu-
son 1998; USEPA 2004). Along the way, pollutants are entrained in the runoff.
As the extent of development increases, so too do the potential impacts.
Watershed-speciic conditions inluence ecosystem integrity, but research has
shown that stream quality impacts can occur with as little as 10 percent impervi-
ous area (Schueler et al. 2009).
GSI promotes the components of the hydrologic cycle that are most substan-
tially compromised by traditional forms of Western urban development ( Figure
1.1 ). Design to promote ET, iniltration, stormwater reuse and other abstractions
emerge as critical elements to achieve management goals that more closely repli-
cate pre-developed conditions. Practically, the ability to design for, or implement,
each of these functions is constrained in the urban environment. For example,
hydrologically “tight” in-situ soils (such as clay), areas with signiicant inill, the
presence of underground utilities (including subways), or brownields/redevelop-
ment sites with contaminated soils create challenges for feasible (or sensible)
iniltration. The conined space of downtown areas or central business districts
may limit rainwater harvesting and reuse options. As extensive living roofs are
not typically limited by ground-level GSI constraints, they emerge as essential
contributors to restoring or maintaining the hydrologic cycle.
2.2 an engineer's perspecTive on quanTiTaTive sTormwaTer
design objecTives
Across the board, the majority of regulations apply to only new or redevelop-
ment projects that exceed some minimum level of land disturbance or increase
in impervious area. Across the world, the vast majority of urban stormwater
runoff discharges untreated into receiving systems and ultimately to receiving
waters.