Planning considerations - Living Roofs in Integrated Urban Water Systems - page 75

Whether using a design storm approach or continuous simulation, the engineer
ultimately designs for assumed scenarios. Engineers design for probable conditions
(e.g., the 95th percentile storm) or several individual storms known to have
environmental, property, public health or infrastructure consequences ( Section 2.2 ) ,
which are ideally underpinned by risk assessments and cost-beneit analysis. In an
ideal scenario, model selection should be based on providing the appropriate level
of accuracy relative to the project's objectives and available resources (including
data and time). On the other hand, most often, the regulatory agency dictates the
method required and the minimum design objective. A designer who chooses to
use more advanced methods may face delays in the review and permitting process
because they have used a “non-standard” approach.

3.5.2 “Curve Number” method (TR-55)
Perhaps one of the most common frameworks endorsed by municipal agencies
for calculating runoff is Technical Release (TR)-55, colloquially known as the
“Curve Number” Method. TR- 55 was developed by the Natural Resources
Conservation Service (NRCS) (formerly the Soil Conservation Service [SCS]) of the
uS Department of Agriculture (uSDA 1986). The methodology is incorporated
into the (USA) National Engineering Handbook , Part 630 Hydrology, which is
periodically updated by the NRCS.
Hawkins et al. (2009) provide a comprehensive, yet concise, analysis of the
Curve Number ( CN ) method, and its continued relevance in water resources plan-
ning. The CN method or a derivative thereof is recommended by stormwater
management design manuals in Alberta (Canada), Iowa, New Jersey, North
Carolina and Auckland (New Zealand), among others (Alberta Environmental
Protection 1999; Auckland Regional Council 1999; Center for Transportation
Research and Education 2008; New Jersey Department of Environmental
Protection 2004; North Carolina Division of Water Quality 2007). The method is
offered as an option for simulating runoff generation in commonly used
stormwater design packages such as HEC- HMS and SWMM.
TR-55's popularity amongst regulatory agencies lies in part in the simplicity of
a method that provides reasonably accurate estimates of storm runoff (speciically
runoff volume) for a given rainfall (Hawkins et al. 2009). The method introduces
a CN , a quantitative representation of the relationship between land use, soil
type and conditions as they relate to water movement, and the resultant
potential for runoff to be generated from a given magnitude of rainfall. In the
method, a set of equations calculates runoff volume as per:

( P - I a ) 2

________

Q =

S + P - I a ;

(3.1)

S =  1000

CN - 10  × 25.4

_____

(3.2)

Where Q is the unit runoff depth per catchment area (mm or inches), P is rainfall
(mm or inches), S is the maximum potential water storage in the catchment