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

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3. Design Guidelines of VFS System Components

• 3.1 Calculate Runoff Quantity
  • 3.1.1 Establish Extent of Runoff Collection Area Contributing Runoff
  • 3.1.2 Range in Storage/Settling Basin Volume Requirements
  • 3.1.3 Defining Runoff Coefficient
  • 3.1.4 Calculating Runoff Storage/Settling Basin Volume Requirements
    

• 3.2 Design of Storage/Settling Basin
  • 3.2.1 Integrated Storage/Settling Design Basin
  • 3.2.2 External Storage/Settling Basin Design
  • 3.2.3 Emergency Overflow
    
    
• 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
    
    
• 3.4 Runoff Discharge System
  • 3.4.1 Integrated Storage/Settling Basin Discharge System
  • 3.4.2 Integrated Storage/Settling Basin with Direct Discharge to Infiltration Area
  • 3.4.3 External Settling/Storage Basin Discharge System
  • 3.4.4 Calculating Orifice Size Based on Orifice Discharge Capacity
  • 3.4.5 Perforated Riser Pipe Design
    
    
• 3.5 Conveyance System
  • 3.5.1 Conveyance Mechanism
  • 3.5.2 Open Channel Conveyance
  • 3.5.3 Managing Flow to Conveyance Pipe
  • 3.5.4 Design of Conveyance Pipe
  • 3.5.5 Manning's Equation for Establishing Pipe Size
    
    
• 3.6 Design of Distribution System
  • 3.6.1 Raised Distribution Pipe
  • 3.6.2 Design of Raised Distribution Pipe
  • 3.6.3 Distribution Channel
    
    
• 3.7 Design of Infiltration Area
  • 3.7.1 Determining Infiltration Area Size
  • 3.7.2 Infiltration Area Design Elements
    
    
• 3.8 Preparation of Design Package
  • 3.8.1 Engaging Professional Services
  • 3.8.2 Contents of Design Package
  • 3.8.3 Specifications
  • 3.8.4 Construction of VFS System
  • 3.8.5 Record Keeping
  • 3.8.6 Inspection
    
    
• 3.9 Operation and Maintenance
  • 3.9.1 Runoff Area and Storage (Internal or External)
  • 3.9.2 Discharage Bay
  • 3.9.3 Pumps
  • 3.9.4 Piping
  • 3.9.5 Infiltration Area
  • 3.9.6 Winter Operation
  • 3.9.7 Accessibility
  • 3.9.8 Safety
  • 3.9.9 VFS System Inspection Sheet

This section of the manual outlines the step-by-step procedures for determining and defining the individual components that make up the VFS system. The overall makeup of the VFS system will vary from farmstead to farmstead based on site-specific conditions and, in some cases, individual user preferences. Economic viability is an important factor in the development of the VFS system components. The aim of this manual is to outline a process that generates a functional VFS system at a practical cost. Each step of the design process will outline development alternatives based on site-specific conditions. Criteria will be defined to establish minimum design objectives for each component of the VFS system. The following are the primary components that will be discussed in detail:

• runoff collection area
• storage/settling basin
• collection/discharge system
• conveyance pipe
• distribution system
• infiltration area
  

3.1 Calculate Runoff Quantity

The first step in designing a VFS system is to determine the quantity of runoff generated from an outdoor livestock yard/confinement area or permanent solid manure storage facility, so that the runoff storage/settling basin can be sized appropriately.

3.1.1 Establish Extent of Runoff Collection Area Contributing Runoff

The extent of the runoff collection area is determined through field observations. The runoff collection area involves the outdoor livestock yard/confinement area or permanent solid manure storage facility that contains manure where runoff is generated. Upslope sources that contribute clean water (e.g., roof drains) and dirty water (e.g., milking centre washwater) to the runoff collection area should be isolated, rerouted, and dealt with appropriately.

3.1.2 Range in Storage/Settling Basin Volume Requirements

The preferred conservative approach establishes a maximum storage volume that accommodates the entire rainfall amount generated within a 25-year storm return period for 24-hour storm duration. The stored volume of runoff must be emptied within four to 10 hours following the storm event. The selection of the appropriate length of time for the runoff volume to empty will vary based on site-specific factors of the livestock yard/ confinement area or manure storage facility. The primary factors involve the extent of flooding during storage periods and the resulting lack of sufficient dry area or access being restricted, requiring a reduced time to empty.

However, there may be instances where there is:

• insufficient existing storage volume capacity available
• a need to reduce the costs associated with the establishment of an external storage/ settling basin
• sufficient suitable land to accommodate a larger infiltration area
  

At this point, a minimum storage volume equivalent to the peak rainfall rate generated by a 25-year storm return period for 5-minute storm duration with a minimum holding time of 15 minutes may be established.

There are several related consequences associated with the development of a storage/ settling basin that has less capacity than the conservative maximum storage volume. Some of the consequences associated with the smaller storage/settling basin volume relate to the resulting increase in the rate of discharge from the basin to the infiltration area, and the potential to exceed its infiltration capacity. As a result of the shorter holding time in the basin, the following impacts to components of the VFS system can be expected:

• greater capacity pumps
• larger diameter pipes
• greater electrical power requirements (voltage, amperage)
• significantly larger infiltration area
  

Rainfall amounts and peak rainfall rates are obtained from Rainfall Intensity-Duration-Frequency (IDF) tables. IDF tables and curves produced from historical rainfall data by Environment Canada (for various geographical areas) statistically determine the amount of rainfall in millimetres that would be expected to fall for a given design-storm return period and storm duration. IDF tables ($100 each) are available from Environment Canada, Ontario Climate Centre, 4905 Dufferin Street, Downsview, Ontario, M3H 5T4 or by calling (900) 565-1111 (this is a toll number, with a charge of $3/minute at time of publishing).

3.1.3 Defining Runoff Coefficient

The runoff coefficient is a function of the imperviousness of the floor of the runoff collection area. A runoff coefficient of 0.95 is recommended for VFS system design, meaning that 95 per cent of the rainfall entering the runoff collection area can be expected to be discharged to the VFS system. It is anticipated that approximately 5 per cent will evaporate or otherwise escape, or be held up within the VFS system, such as by being absorbed by the solid manure pack and removed with it during routine cleaning.

3.1.4 Calculating Runoff Storage/Settling Basin Volume Requirements

Conservative Method - Maximum Runoff Storage/Settling Volume

The conservative method is a basic method that can be used to calculate the maximum runoff storage/settling volume. Equation 3.1 outlines the variables, units, and mathematical relationship to establish a maximum storage volume. The maximum storage volume is established by multiplying the area of the runoff collection area (A) by the rainfall amount of a storm with a 25-year return period for 24-hour duration (a) by the runoff coefficient (C).

Table 6.1 provides maximum runoff storage/settling volume using the conservative method for selected Ontario centres.

Equation 3.1 Conservative Method to Calculate Maximum Storage Volume

V= CaA

Where:
V_max =maximum storage volume (m³)
C =runoff coefficient 0.95
a=rainfall amount 25-yr/24-hr storm event (m)
A= runoff area (m²)

For example, the IDF value for Oshawa shows that a total of 72.7 mm (2.86 in.) of rain will fall during a 25-year return period for a 24-hour duration design-storm event. Therefore, the runoff volume generated from a 1,000 m² (10,764.3 ft²) outside concrete livestock yard/confinement area using Equation 3.1 would be equal to (0.95) (72.7 × 10^-3 m) (1,000 m²) = 69.1 m³ (2,440 ft³).

From Table 6.1, the storage/settling volume for Oshawa WPCP centre is 69.07m³.

Rational Method - Minimum Runoff Storage/Settling Volume

The rational method is one method that can be used to calculate the peak discharge and, subsequently, a minimum runoff storage volume. Equation 3.2 outlines the variables, units, and mathematical relationship to establish a peak discharge rate. The peak discharge rate is established by multiplying the area of the runoff collection area (A) by the rainfall intensity of a storm with a 25-year return period for 5-minute duration (i) by the runoff coefficient (C).

Table 6.2 provides minimum storage/settling volume required and peak discharge using the rational method.

Equation 3.2 Rational Method to Calculate Peak Discharge

q_p=0.0027CiA

Where:
q_p = peak discharge (m³/s)
C= runoff coefficient 0.95
i =rainfall intensity 25-yr/5-min storm event (mm/h)
A= runoff area (hectares)

The minimum volume (V_min) of runoff required is outlined in Equation 3.3. The peak discharge rate determined by using Equation 3.2 (q_p) is multiplied by the minimum holding time (h_tm) of 900 seconds (equivalent to the 15-minute holding time required).

Equation 3.3 Minimum Storage Volume

V_min= h_tmq_p

Where:
V_min= minimum storage volume (m³)
q_p= peak discharge (m³/s)
h_tm= minimum holding time of 900 (seconds)

For example, the IDF values for Oshawa show that a 137.4 mm/hour (5.4 in/hour) of rain will fall during a 25-year return period for a 5-minute duration design-storm event. Therefore, the corresponding peak discharge from a 1,000 m² (10,764.3 ft²) outside concrete livestock yard/confinement area using Equation 3.2 would be equal to (0.0027) (0.95) (137.4 mm/hr) (0.1 ha) = 35 ×10^-3 m³/s (1.24 ft³/s). The required minimum storage volume is established using Equation 3.3 and is equal to (35 ×10^-3 m³/s) (900 s) = 31.5 m³ (1,112 ft³).Note that use of the rational method results in approximately half the storage volume as compared to the conservative method.

From Table 6.2, the minimum storage volume for Oshawa WPCP centre is 31.72 m³ (1,120 ft³) and peak discharge rate is 0.04 m³/s (1.41 ft³/s).

Related Links

• To request IDF Tables (Environment Canada)

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