## 3 Case Study
### Stormwater Management Model Parameter Sensitivity Analysis
#### 3.4.2 The range of Parameter Value  
The application of the permeable pavement in this study area does not have actual measurement data for verification; therefore, the sensitivity analysis of the pending parameters of the low-impact development technology model should be carried out before the establishment of the SWMM in the case area. The following contents summarize the description of important parameters of permeable pavement in the SWMM simulation. The reference source is the EPA SWMM user manual and LID Design Guide.  
Surface Slope  
The slope of a roof surface, pavement surface, or vegetative swale (percent). Use 0 for other types of LIDs.  
Thickness  
The thickness of the pavement layer (inches or mm). Typical values are 4 to 6 inches (100 to 150 mm).  
Void Ratio  
The volume of void space relative to the volume of solids in the pavement for continuous systems or for the fill material used in modular systems. Typical values for pavements are 0.12 to 0.21. Note that porosity = void ratio/(1 + void ratio).  
Permeability of the concrete or asphalt used in continuous systems or hydraulic conductivity of the fill material (gravel or sand) used in modular systems (in/h or mm/h). The permeability of new porous concrete or asphalt is very high (e.g., hundreds of in/h), but can drop off over time, due to clogging by fine particulates in the runoff (see below).  
Thickness (or Barrel Height)  
This is the thickness of a gravel layer or the height of a rain barrel (inches or mm). Crushed stone and gravel layers are typically 6 to 18 inches (150 to 450 mm) thick, while single-family home rain barrels range in height from 24 to 36 inches (600 to 900 mm).  
The volume of void space relative to the volume of solids in the layer. Typical values range from 0.5 to 0.75 for gravel beds. Note that porosity = void ratio /(1 + void ratio).  
(7)  
Seppage Rate  
The rate at which water seeps into the native soil below the layer (in inches/h or mm/h). This would typically be the Saturated Hydraulic Conductivity of the surrounding sub-catchment if Green-Ampt infiltration is used or the Minimum Infiltration Rate for Horton infiltration. If there is an impermeable floor or liner below the layer, then use a value of 0.  
This study used the modified Morris test method to analyze the sensitivity of the parameters in the stormwater management model. The various hydrological and hydraulic parameters were changed within the 10% adjustment range, and the values were \(-\)50%, \(-\)40%, \(-\)30%, \(-\)20%, \(-\)10%, 10%, 20%, 30%, 40%, and 50%, when one of the parameters was changed, and the other parameters remain fixed. The sensitivity of each parameter was analyzed to the peak flow and outflow volume under the six-hour delay of 350 mm rainfall at the Wugu rainfall station in New Taipei City. For the sensitivity analysis range of permeable pavement with built-in functions in the stormwater management model, the reference range for parameter values were adopted from the stormwater management model user's manual Version 5.1. [7], Low-Impact Development User manual [12], and other manuals and specifications, and the reasonable range of its parameters were selected. The undetermined parameters were changed within a reasonable range, and then the simulation output results were used to analyze the sensitivity of the undetermined parameters. The parameter value range of the stormwater management model del permeable pavement is mentioned in Table 2.