New Water Policy and Practice Issue 4, Number 1, Fall 2017 | Page 62

New Water Policy & Practice Journal
ever, none of the sponge city-oriented infrastructures has been considered
for mitigating the flooding of underpasses.
Although there is progress in research studies and government con-
cerns, many problems remain. Current research studies are constrained
within the site scale; therefore, a top-level project for practice is inaccessible.
Cooperation is extremely needed between the bureau, who control most of
the city data, and researchers, and also between different bureaus to avoid
the inefficiency of water projects and the wasting of money.
My Current Progress in Sponge City
GI can play a role in converting the existing city to a sponge city, which
has been developed to mitigate excessive runoff and flooding. Green roof
was one example of this strategy. In most metropolises, the amount of land
available exclusively for GI is limited. So among them, green roof for be-
ing easily accomplished in existing built areas, becomes the most feasible
one. Both urbanization and climate change induce the excessive runoff and
therefore raise the risk of flooding, while green roofs have high potential to
mitigate urban flooding (Brudermann & Sangkakool 2017). Identification of
the spatial repartition of potential green roofs (PGRs) and variation of runoff
reduction effect addresses clear scientific strategies and guidance for urban
administration to prevent the surge runoff under different return periods.
In my study of the hydrological behaviour of large-scale potential
green roofs retrofitting in Beijing, a 19.6% of the potential green roofs within
the study area indicates a strong potential for the future greening.
In the city scale, catchments in the core of the northern city play key
roles in reducing runoff, by more than 10% with the 100% implementation
scenario. A potentially positive hydrological behaviour contributed by roof
greening in the future Beijing at a city scale is promising, and effective runoff
reduction can be achieved only if 50% greening of the potential green roofs
is achieved. Nearly half of the areas in Beijing can tackle the excessive run-
off like 2-year return period or more frequent rainfall, if all the PGR s are
greened.
The study also showed that flooding in 13 of the 20 underpasses would
have been avoided in the 7.21 rainstorm if only 20% of the potential green
roofs had been implemented (Figure 3). It can be carefully inferred that
green roofs can play a very positive role in the flooding mitigation during
a heavy rainstorm, based on the assumption that every piece of the PGRs
are related to the flooding in the corresponding catchment. However, it is
worth noting that green roofs show marginal runoff reduction effect (<10%)
in the southern city where flooding are more serious. That is PGR resources,
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