Planning considerations - Living Roofs in Integrated Urban Water Systems - page 80

available for manipulation to a full catchment runoff hydrograph; however, these
were derived from studies of runoff from a wide variety of natural land uses
(Bedient et al. 2013; NRCS 2007). As the composition of the living roof system
and low path of rainfall through the system vary signiicantly from rain falling on
a ground-level surface, existing generalized unit hydrograph models are not con-
sidered valid transformations. Only a single study has been identiied to date that
attempted to derive a living-roof-speciic unit hydrograph (Villareal and Bengts-
son 2005).
Altogether, application of the CN methodology to living roof systems should
be approached with caution. The CN method should only be used where regula-
tory agencies insist on it. Again, the suggestions here are offered as a starting
point to enable implementation. Somewhat more lenient CN values are sug-
gested for planning purposes to acknowledge the variability in results, and the
wide range of beneits provided by living roofs beyond stormwater control. Criti-
cal to the living roof CN herein is the assertion that runoff is assumed to be equal
to zero (i.e., CN ≤ 1) for storms less than the laboratory-measured water holding
capacity of the growing media ( Section 4.1.3 ).

3.5.3 Rational Formula
The Rational Method is widely employed in predicting peak runoff rates in the
design of urban drainage systems. The method is most commonly attributed to
Mulvaney (1851) in Ireland. It is a simple empirical formula relating the peak low
rate to the drainage area, the rainfall intensity and a runoff coeficient. In metric
units, the Rational Formula is:

Q p = 0.0028 C i A Equation 3.3

Where Q p is peak runoff rate (m 3 /s), C is the runoff coeficient, i is the constant
rainfall intensity (mm/hr) occurring for a storm with duration equal to the
catchment's time of concentration, and A is the effective (connected) area of the
drainage basin (km 2 ). In uS Customary units, the coeficient of 0.0028 is
dropped, and units are Q p (ft 3 /s), i (in/hr) and A (acres).
The term “runoff coeficient” is deined as ratio of runoff to rainfall for a given
time period. In most hydrology text books, the runoff coeficient is the ratio of total
runoff volume over total rainfall ( C v ), on either annual, seasonal or event basis. In
some local design manuals and academic literature, the runoff coeficient is instead
considered the ratio of the peak runoff to rainfall intensity ( C p ), on speciic event
basis. In either case, the value of the runoff coeficient may vary signiicantly from
event to event in practice, but this is rarely accommodated in design procedures.
Similarly to the CN method, the value of the runoff coeficient is assumed to inte-
grate effects of land use, soil, slope, etc. Some jurisdictions endorse a ixed value of
the runoff coeficient. Other design manuals, such as Colorado's urban Drainage
Criteria Manual (uDFCD online) and textbooks, such as McCuen (2004), suggest
increasing values of the runoff coeficient depending on either storm depth or soil
type (speciically in reference to the iniltration potential of the soil), or both.