Design Example No. 2: Commercial Development - Brown Community Center

This is a sizing example for a commercial site, Brown Community Center. The layout of the Brown Community Center is shown in Figure 1.

In this design example, the NRCS TR-55 method is used as the design methodology.

Step 1. Compute Water Quality Volume WQ_v

Criteria:

• Size for the 90% rainfall event.
• Use a minimum runoff coefficient of 0.2.

Step 1a. Compute Runoff Coefficient

This runoff coefficient is derived from Schueler's Simple Method.

Rv = 0.05 + (I) (0.009)

Where:

I = Impervious Cover (%)
Rv = 0.05+(63.3)(0.009) = 0.62

Step 1b. Compute WQ_v

WQ_v = (P₁) (R_v ) (A)

Where:

P₁ = 90% Rainfall Event (Inches). Assume 1" in this example
A = 38.0 acres

WQ_v = (1.0") (R_v ) (A) / 12
= (1.0") (0.62) (3.0ac) (43560ft²/ac) (1ft/12in)
= 6752 ft³

Check Minimum: (0.2) (3.0 ac) (3630) = 2178 ft³ [okay]

Step 2a. Determine Recharge Equation Based on Hydrologic Soil Group (Table 1).

Step 2b. Compute Recharge Volume

For "B" soils, (0.25 inches) (.62) (3.0 ac) (1/12"/ft) (43,560 ft²/ac) = 1,688 cubic feet; this represents the minimum target volume for recharge purposes.

Step 3. Compute Stream Channel Protection Volume (Cp_v):

In this design example, we assume that channel protection is not required at this site due to the small site size, and relatively flat slopes in the watershed.

Step 4. Compute Peak Discharge Control Volume (Q_p):

Table 2 presents Input Parameters Per attached TR-55 calculations (see Figures 2 and 3).

Step 2b. Compute Recharge Volume

For "B" soils, (0.25 inches) (.62) (3.0 ac) (1/12"/ft) (43,560 ft²/ac) = 1,688 cubic feet; this represents the minimum target volume for recharge purposes.

Step 3. Compute Stream Channel Protection Volume (Cp_v):

In this design example, we assume that channel protection is not required at this site due to the small site size, and relatively flat slopes in the watershed.

Step 4. Compute Peak Discharge Control Volume (Q_p):

Table 2 presents Input Parameters Per attached TR-55 calculations (see Figures 2 and 3).

Assume that 10-year quantity peak control is required (Q_p10).

Per TR-55, Figure 6-1 (Page 6-2 in TR-55), for a Q_in of 13.97 cfs, and an allowable Q_out of 2.91 cfs, the Vs necessary for 10-year control is 0.24 ac-ft or 16,890 ft³, under a developed CN of 83.

Step 5. Analyze for Safe Passage of 100 Year Design Storm (Q_f):

At final design, prove that discharge conveyance channel is adequate to convey the 100 year event and discharge to receiving waters.