Chapter 3 Decision Support Model (IUWS-DSM) - Tubdok

38 New Conception and Decision Support Model for IUWS
2.2.5 Rainwater utilisation
Rainwater itself is very clean and already distributed over the area, but its temporal
distribution is mostly very uneven. Also, due to the environmental pollution and the city
complexities, many contaminants enter the rainwater in urban area. Therefore, despite being
a very good water source, rainwater is barely extensively utilised in cities. To the contrary,
due to its flooding risks, rainwater attempts to be eliminated from urban area as soon as
possible, whereby the huge storm drains are often constructed. Today, the universal water
scarcity together with the better understanding of the nature brings rainwater as water source
back to our sight.
2.2.5.1 options
Storm water elimination. The rainfall strength in one event has the bell-shape. In order
to avoid the flooding risks, the sizes of storm drain are designed based on the peak flow rate
of one rainfall event with considering its recurrence possibilities. The design of storm drains
is a well developed subject, and the national or local design standards and handbooks are
generally available.
Urban storm water detention started in the 1960s when it became known that development
tends to be followed by increasing the rainfall peak rate and aggravated flood damage
(Ferguson 1998). Open ponds are probably the most common type of detention used in
storm water management (Urbonas and Stahre 1993). Other methods to delay and reduce
the peak flow are the increase of the permeable area and the construction of green area.
Such measures do not only diminish the sizes of strom drains, but also potentially improve
the urban environmental conditions.
Instead of being discharged out of the city with huge drains, the storm water can be
retained inside and possibly converted to urban water resources, not only in water-scarce
areas, but also in water-rich regions (Handia et al. 2003, Herrmann and Schmida 1999).
Surface runoff management. Impoundment is one good option for utilising surface
runoff, which acts as the buffer storage during the rainfall events and as the irrigation water
source for the vicinal green area during sunny days. Moreover, impoundments can improve
the local ecological conditions and even be regarded as recreational sites. Constructed
wetland can also be used to retain and utilise the rainwater, as well as to enhance the urban
ecological system (e.g. Lawrence & Breen, 1998, Wong et al. 1999). For supplementing the
local water source, the storm water is recharged into the groundwater usually by two
measures: gravity filtration and direct injection. In this conception, groundwater recharge of
rainwater is considered as the indirect rainwater utilisation.
As an example, an Australian program called Water Sensitive Urban Design is developed
nationwide, which is particularly for designing and managing urban storm water in the
sustainable way (e.g. Brisbane City 2005, Melbourne Water 2005). Other cities like Hamburg
manage rainwater in the decentralised nature-closed system (HBSU 2006). All can be the
good references for managing storm water in modern cities.
Roof-water utilisation. As barely touched area, roofs of buildings are the proper catchment
area for collecting the clean rainwater that is called roof-water. After diverting the first