Design Example 1: Residential Development - Swann Center

This section presents a sizing example for a medium residential subdivision, Swann Center. The layout of the Swann Center subdivision is shown in Figure 1.

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

Step 1. Compute WQ_v Volume

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.

R_v = 0.05 + (I) (0.009)

Where:
I = Impervious Cover (%)
R_v = 0.05 + (36.3) (0.009) = 0.38

Step 1b. Compute WQ_v

WQ_v = (P₁) (R_v ) (A)

Where:
P₁ = 90% Rainfall Event (Inches). Assume 0.9" in this example
A = 38.0 acres

WQ_v = (0.9 ) (0.38) (38.0 ac) (1ft/12in)
= 1.08 ac-ft

Check Minimum: (0.2 ) (38.0 ac) (1ft/12 ) = 0.63 ac-ft [okay]

Step 2. Compute Recharge Volume (Re_v)

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

Assume imperviousness is located proportionally in B and C soils.

Step 2b. Compute Recharge Volume

For "B" soils =[(0.25 inches) (.38) (.38 ac)/12"/ft] (0.60) = 0.18 ac-ft
For "C" soils =[(0.13 inches) (.38) (.38 ac)/12"/ft] (0.40) = .06 ac-ft
Add recharge requirement for both soils

Re_v = (0.18 ac-ft) + (0.06 ac-ft)
= 0.24 ac-ft

This requirement can be met either with a structural practice, or with Stormwater Credits.

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

Requirement: Provide 24 hours of extended detention for the 1-year event.

Please note that the length of detention may be smaller based on the stream resource. In cold water trout streams, for example, the detention time may be as small as 6 to 12 hours.

Step 3a. Develop site hydrologic and TR-55 Input Parameters

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

Step 3b. Compute Storage Volume

The following technique is based on the method described in Design Procedures for Stormwater Management Extended Detention Structures, (Maryland Department of the Environment, 1987).

Initial abstraction (Ia) for CN of 78 is 0.564: (TR-55) [Ia = (200/CN - 2)]

Ia/P = (0.564)/ 2.5 inches = 0.226
Tc = 0.19 hours
From TR-55, Exhibit 4-11 (NRCS, 1986):
qu = 740 csm/in
Knowing qu and T (extended detention time), find Qo/Qi from Figure 4, "Detention Time Versus Discharge Ratios."

Peak outflow discharge/peak inflow discharge (Qo/Qi) = 0.024

From TR-55, Volume of storage/Volume of runoff (Vs/Vr)

(Figure 6.1, page 6-2 of TR55)

For a Type II rainfall distribution,

Vs/Vr = 0.683 - 1.43(Qo/Qi)+1.64(Qo/Qi)² - 0.804(Qo/Qi)³ (MDE, 1987)

Note that a different equation would be used to describe hydrographs of Type I or Type III distributions.

Vs/Vr = 0.65

Therefore, Vs = 0.65(0.8 )(1/12)(38 ac) = 1.65 ac-ft

Step 3d. Define the average ED Release Rate

Qi is known (35.0 cfs), therefore,
Qo = (Qo/Qi) Qi = .024 (35.0) = .84 cfs

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

Control the peak flow from 10-year storm event to pre-developed levels.

Step 4a. Model Site Using TR-55 for 10 year storm

Per TR-55, Figure 6-1 (Page 6-2 of TR-55) for a Q_in of 130.0 cfs, and an allowable Q_out of 50.4 cfs, the V_s necessary for control is 2.83 ac-ft, with a developed CN of 78. (See TR-55 Worksheet 6a, Page 6-5 of TR-55). Note that 5.0 inches of rain fall during this event, with 2.7 inches of runoff.

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

Note that this design example doe not assume that control of the 100-year event is required.

At final design, provide safe passage for the 100 year event. Based on field observation, downstream conveyance will require analysis for passing the 100-year event through an existing culvert.