**4. Results**

#### **4.1. Site design**

**Figures 3**–**6** present the basic configuration the site design as completed by the team. **Fig‐ ure 3** is a general site plan for the study area, while **Figure 4** illustrates the layers of fea‐ tures, surfaces comprising the site plan. **Figure 5** illustrates the location of stormwater surfaces for the site. Finally, **Figure 6** presents four elevations of site details related to stormwater.

**Figure 3.** The resulting plan for the study area.

Metrics in Master Planning Low Impact Development for Grand Rapids Michigan http://dx.doi.org/10.5772/63708 73

**Figure 4.** Various layers of the design.

**Figure 3.** The resulting plan for the study area.

72 Sustainable Urbanization

**Figure 5.** Stormwater surfaces of the proposed design.

**Figure 6.** Elevations for the proposed design.

#### **4.2. Site metrics**

#### *4.2.1. Stormwater*

The study computes the stormwater for 25-year 24-h storm events by comparing predevelop‐ ment land use to postdevelopment without LID and with LID treatments. The SWC results indicate that the LID controls in the study improve stormwater management efficiency. When predevelopment scenario and postdevelopment scenario generate 53 and 43% runoff, respec‐ tively; the design with LID treatments decreases runoff to 15% (**Figure 7**).

**Figure 7.** SWC scenario differences.

**Figure 5.** Stormwater surfaces of the proposed design.

**Figure 6.** Elevations for the proposed design.

The study computes the stormwater for 25-year 24-h storm events by comparing predevelop‐ ment land use to postdevelopment without LID and with LID treatments. The SWC results

**4.2. Site metrics**

74 Sustainable Urbanization

*4.2.1. Stormwater*

The study site is 98.5 acres, with B Hydrologic Soil Group (HSG) derived from the U.S. Department of Agriculture's Natural Resources Conservation Service (NRCS) SSURGO database, which represents moderately low soil type. The hydraulic conductivity of the site is 0.3 inch/h. The general surface slope is approximately 5%. The precipitation and evaporation data are from Grand Rapids Gerald R. Ford International Airport weather station. Wet day threshold is 0.1 in. The climate change scenario is warm and wet and is of long term.


**Table 2.** SWC statistic analysis.

There is neither a meadow area nor a pervious pavement within the current boundary, although the city of Grand Rapids is proposing and promoting the use of permeable pavements for the city center renovation. The LID treatments used in the postdevelopment area are substituted by normal grasses and shrubs in the traditional design. The team added a skywalk area covered with trees, LID treatments, and pervious pavement to predict the scenario which employs the LID controls, ending up with a total site area of 107.04 acres. **Table 1** shows other different parameters that the team used in the SWC tool.

According to **Table 2**, the number of days with runoff of the existing condition, traditional design, and the design employing LID controls are 46.93, 39.29, and 12.03 days, respectively. The percentage of water infiltration into the site is 47.42, 47.68, and 83.98%, respectively. The greatest rainfall without runoff increases by the design deploying LID treatments from 0.3 in. of predevelopment to 0.94 in. The SWC model indicates that the LID treatments can retain much more water on site and reduce the burden of stormwater infrastructure and their costs.

**Figure 8.** SWC analysis results.

**Figure 8** shows how effective the design with LID controls contributes to reduce the runoff intensities and the frequency of large-volume rainfall/runoff treatment. The rainfall retention frequency of the scenario with LID controls does not decrease dramatically like the existing condition and the traditional design solution. It is able to retain the water for larger rain events.

The runoff contribution by rainfall of pre-design and traditional design scenarios have a peak at 0.52 in., while the design employing LID controls eliminates the peak and increases visually at 0.8 in. of the daily rainfall depth. LID treatments can also calm rainfall/runoff events and reduce rainfall or runoff exceedance frequency, compared to predevelopment and traditional development scenarios.
