Vegetated Filter Strip System Design Manual
Design Guidelines of VFS System Components

| 3.1 Calculate Runoff Quantity | | 3.2 Design of Storage/Settling Basin |
| 3.3 Determination of Discharge Rates from Runoff Storage/Settling Basin |
| 3.4 Runoff Discharge System | 3.5 Conveyance System |
| 3.6 Design of Distribution System | 3.7 Design of Infiltration Area | 3.8 Preparation of Design Package |
| 3.9 Operation and Maintenance |

3.3 Determination of Discharge Rates from Runoff Storage/Settling Basin

• 3.3.1 Discharge from an Integrated Storage/Settling Basin
• 3.3.2 Discharge from an External Storage/Settling Basin
  

The discharge rate from the runoff storage/settling basin depends upon the level of storage provided and its configuration as an integrated or external storage facility. The discharge rate from an integrated storage/settling basin can be established using one of the two following methods.

3.3.1 Discharge from an Integrated Storage/Settling Basin

Discharge Rate of Conservative Maximum Storage Volume

In situations where sufficient storage volume can be integrated into the outdoor livestock yard/confinement area or permanent solid manure storage facility, serving to accommodate the conservative maximum storage volume (total rainfall amount generated by a 25-year/24-hour storm event), then the discharge rate will correspond to the length of time required to drain the total runoff storage/settling volume over a given length of time. The length of time for the discharge of the stored volume will typically extend over a 4 to 10-hour period, depending on the operator's preference. An orifice will be used to establish the desired rate of release. The orifice will be sized to allow the stored volume of runoff to be drained within the minimum period of four hours. Equation 3.5 outlines the variables, units, and mathematical relationship to establish the discharge rate for the various release periods. The discharge rate is calculated by dividing the total storage volume (v_max) by the period within which the volume is to be drained from the storage/settling basin (h_tm).

Equation 3.5 Calculate Discharge Rate of Conservative Maximum Storage Volume

qmax = v_max/h_tm

Where:
q_max = discharge rate for maximum storage volume (m³/s)
v_max = conservative maximum storage volume (m³)
h_tm = holding time (s)

For example, the discharge rate for the conservative maximum volume of runoff of 69.1 m³ (2,440 ft³) released over a 4-hour period is equal to (69.1 m³)/(14,400 s) = 4.80 × 10^-3 m³/s (0.17 ft³/s). The discharge rate released over a 10-hour period is equal to (69.1 m³)/(36,000 s) = 1.92 × 10^-3 m³/s (0.07 ft³/s).

From Table 6.1, the discharge rate for a 4- hour period is 4.8 × 10^-3 m³/s (0.17 ft³/s) and 1.92 × 10^-3 m³/s (0.07 ft³/s) for a 10-hour period.

Discharge Rate of Volume Less than Conservation Maximum Storage Volume

There may be situations where:

• Less than the conservative maximum storage volume can be integrated into the outdoor livestock yard/confinement area or permanent manure storage facility but more than the minimum storage volume (total rainfall amount generated by a 25-yr/five-min storm event over a minimum 15-min holding time) can be retained.

In this case, the discharge rate will match the routed peak discharge rate of the 25-yr/24-hr storm event. This value can be determined by using Equation 3.6.

Equation 3.6 Flood Routing to Establish Storage/Settling Basin Discharge Rate

q_o = q_p[1.25 - ((1500V/RA) + 0.06)^0.5]

Where:

q_o = rate of outflow when pipe flows full (m³/s)
q_p = peak inflow = peak discharge from runoff collection area (m3/s)
V = available storage volume (ha-m)
R = return period runoff 25-year/24-hour (mm)
A = runoff collection area (ha)

Reference: Soil and Water Conservation Engineering
3rd Edition, Schwab et al 1996, see page 261

Equation 3.6 outlines the variables, units, and mathematical relationship to establish the discharge rate from the storage/settling basin. Flood routing describes the relationship between peak inflow (q_p), available storage volume (V), runoff amount (R) and drainage area (A), over time. The VFS system should ensure that the storage area will not overflow. This is important where less than the conservative maximum storage volume of the 25-year/24-hour design-storm event is accommodated in the storage basin.

3.3.2 Discharge from an External Storage/Settling Basin

In situations where an integrated storage/settling basin is not feasible and an external storage/settling basin is required, the discharge rate can be established using the principle of flood routing.

Use Equation 3.6 to determine the routed peak discharge flow rate.

For example, an external basin with less than the conservative maximum storage volume that has a peak inflow rate of 35 × 10^-3 m³/s (1.24 ft³/s), available storage of 3.2 × 10^-3 ha-m (0.026 ac-ft), runoff of 72.7 mm (0.28 in.) and a drainage area of 0.1 ha (0.24 ac) has a rate of outflow from the external storage/settling basin that is equal to (35 × 10^-3) {1.25 - [(1,500) (3.2 × 10^-3 m³)/(72.7 mm) (0.1 ha)+0.06]^0.5 } = 14 × 10^-3 m³/s (0.50 ft³/s).

In cases where the conservation maximum storage volume of the 25-year/24-hour design-storm event can be accommodated in the external storage, the discharge rate will correspond to the length of time desired to empty the external storage, i.e., 4-10 hours.

In cases where the existing external storage has capacity to hold 1.5 times the storm event, the storage can be emptied over a maximum duration of 24 hours.

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| 3.1 Calculate Runoff Quantity | | 3.2 Design of Storage/Settling Basin |
| 3.3 Determination of Discharge Rates from Runoff Storage/Settling Basin |
| 3.4 Runoff Discharge System | 3.5 Conveyance System |
| 3.6 Design of Distribution System | 3.7 Design of Infiltration Area | 3.8 Preparation of Design Package |
| 3.9 Operation and Maintenance |

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