In This Section |
Vegetated Filter Strip System Design Manual
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| Author: | Robert P. Stone, P. Eng., Engineer, Soil/OMAFRA |
|---|---|
| Creation Date: | 04 July 2005 |
| Last Reviewed: | 30 May 2007 |
| 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 |
The runoff collection/discharge system has several components. The flow is directed to one common collection point. Prior to being discharged from the runoff area, the runoff passes through a series of screens to remove solids and floatables. An orifice situated at the bottom end of a perforated riser pipe controls the rate of discharge from the runoff collection area to an adjacent below-grade sump. The runoff collection/discharge system will vary depending on whether an integrated or external storage/ settling basin configuration is utilized. The following outlines the runoff collection/discharge system being proposed for an integrated and external storage/settling basin configuration.
The integrated storage/settling basin runoff collection/discharge system is outlined in Figure 3.3A. The runoff collection/discharge system has the following primary components which serve the following functions:
Figure 3-3a Integrated Storage/Settling Basin Collection/Discharge System Layout
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The following descriptions provide the design criteria to be used to develop the integrated storage/settling basin runoff collection/discharge system components.
In an integrated storage/settling basin configuration, runoff flows by gravity to a common low point and collects in an area referred to as the collection/discharge bay.
Configuration of Collection/Discharge Bay The collection/discharge bay will be constructed outside of the runoff collection area and incorporated into the containment wall, as presented in Figure 3.3A. The collection/discharge bay extends to the full height of the containment wall. The collection/discharge bay has a width of 1.0 to 2.0 m (3.3 to 6.6 ft) to allow for easy access for cleaning. The floor of the containment bay will be flat across the front end of the bay where the screens are located. The perforated pipe will be located in the centre of the bay area, downslope of the screens. The bay area floor will slope down to the perforated pipe on all four sides.
Material The collection/discharge bay containment wall and floor will be constructed using reinforced concrete.
Screens will be incorporated into the opening width of the collection/discharge bay to reduce the potential for clogging of downstream components of the VFS system. Solids contained in the runoff collection area runoff will be removed by the screens before entering the collection/discharge bay.
Configuration of Screens A triple screen system is recommended because it progressively provides a finer level of screening as the runoff passes through the collection/discharge bay. Figure 3.3A illustrates the design of a triple vertical screen system for the integrated storage/settling basin. The screens will be situated at the inflow end of the collection/discharge bay, contained in a frame, and placed in c-channel slots attached to the face of the collection/discharge bay. A handle would be placed along the top edge of the frame to facilitate easy removal of screenings. Removing the screens allows direct access to the collection/discharge bay area and discharge orifice. Screens will be placed 0.5 m (1.6 ft) apart. Screens may also be installed on a 60-degree angle above the horizontal (see Figure 3.3B) with a small beach at the top. With this arrangement, the face of each screen can be maintained with a common rake to allow for cleaning. The small beach allows for a location for piling up accumulated solids so that they do not drop into the next screen area. The screens will extend the full height of the containment wall (top of spillway wall height). The following outlines the screen opening sizes for each of the three screens:
1. Coarse-vertical spacing of approximately 25 mm (1 in.)
2. Medium-vertical spacing of approximately 10 mm (3/8 in.)
3. Fine-vertical spacing of approximately 3.1 mm (1/8 in.)
Figure 3-3b Triple Angled Screen System
Material The screens may be constructed from a variety of materials including wood, galvanized metal, or other suitable materials. The material must be able to withstand the environmental conditions found in runoff. For example, the bars may be constructed of 50 mm by 25 mm (2 in. by 1 in.) timber in a solid frame, or fabricated out of standard 50 mm wide by 4.5 mm thick (2 in. wide by 3/16 in. thick) flat stock galvanized steel.
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A drainpipe, along with an orifice plate, will be installed to control the rate of discharge and direct the flow to the sump prior to being conveyed to the infiltration area.
Sizing of Drainpipe/Orifice Plate The drainpipe and orifice plate will be sized to accommodate the required discharge rate from the storage/settling basin. The minimum drainpipe size and orifice opening will depend on the storage level that is accommodated in the storage/settling basin, and will correspond to the following:
The drainpipe size may be increased up to two times the discharge rate from the storage basin to accommodate future expansion. The drainpipe should be sized to match a standard pipe size. The orifice plate will be integrated into the inflow end of the drainpipe and utilized to generate the desired discharge flow rate as specified above.
Perforated Riser Pipe A perforated riser pipe will be placed ahead of the orifice plate so that all flow passes through the openings in the riser pipe. The riser pipe primarily functions as a screen. The perforated riser pipe will be the same size as the drainpipe to simplify the connection. Check flow rate through the holes or slots in the perforated pipe to ensure that this estimate of flow rate is greater than that of the orifice. The area of the perforated riser pipe openings must be at least 25 per cent greater than that of the orifice plate opening at maximum head. The additional area of the riser pipe openings will compensate for potential clogging of the riser pipe.
Type of Pipe There are many types of piping suitable for moving wastewater. PVC piping is an appropriate pipe type that is readily available.
Runoff discharged through the drainpipe outlet from the collection/discharge bay will be directed into a sump.
Configuration of Sump The sump will be installed beside the collection/discharge bay and will be constructed below grade to facilitate year-round operation. The sump's primary function is to provide sufficient height to support the transfer mechanism being considered for the VFS system-a siphon (gravity flow transfer mechanism) or a pump (pumped flow transfer mechanism). The sump size will depend on the size and space requirements of the siphon/ pump. Consult siphon and pump manufacturers to establish an appropriate sump size.
Type of Sump There are several different types of pre-fabricated sumps, composed of various materials (concrete, metal, plastic, fibreglass, etc.), that can be used. A pre-fabricated concrete manhole box may also be considered because of its durability, versatility, and access capabilities. An access hatch must be provided to the sump that can be locked to restrict access to the sump.
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An alternate discharge system design involves an integrated storage/settling basin that is able to directly discharge to an infiltration area within 20 m (65.6 ft) of the discharge location. The drainpipe will be placed into the base of the containment wall, and runoff is discharged directly to an adjacent conveyance channel that flows via gravity to the distribution channel at the top end of the infiltration area. The use of this design means that a sump is not necessary.
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In an external storage/settling basin design, the runoff that is collected is transferred to the storage/setting basin first. The runoff collection area directs flow (via gravity) to a common low point where the runoff is transferred to the external storage/settling basin. Runoff can be transferred through a pipe or conveyance channel system to the external storage. In all cases the transfer system must be designed to carry the peak discharge flow from the runoff collection area for a 25-year/5-minute duration storm event. The runoff collection/discharge system has the following primary components that serve the following functions:
The following describes the design criteria to be used in the development of the external storage/settling basin runoff discharge system components.
Only a coarse screen, e.g., wood picket fence, should be installed at the runoff exit point of the collection area to prevent coarse materials from entering the external settling/storage basin.
Drain Pipe/Orifice Plate The drain pipe is situated at the discharge elevation equivalent to the minimum 1 m (3.2 ft) dead storage level of the storage basin. An orifice plate is placed ahead of the discharge pipe to control the rate of discharge from the storage basin to the sump. The drain pipe conveys runoff stored in the storage basin to the sump.
Sizing of Drain Pipe/Orifice Plate The drainpipe/orifice plate will be sized to accommodate the required discharge rate from the runoff storage/settling basin. The minimum pipe size/orifice opening will depend on the storage level that is accommodated in the settling basin and will correspond to the following:
The drainpipe size may be increased up to two times the discharge rate from the storage basin to accommodate future expansion. The drainpipe should be sized to match a standard pipe size. The orifice will be integrated into the inflow end of the drainpipe and utilized to generate the desired discharge flow rate specified above.
Type of Pipe The type of pipe will be the same as that used for an integrated storage/settling basin collection/discharge system.
Sump
Runoff discharged through the drainpipe outlet from the external storage/settling
basin will be directed into a sump.
Configuration of Sump The sump will be installed directly adjacent to the external storage/settling basin and will be constructed below grade to facilitate year-round operation. The sump characteristics will be the same as that used in the integrated storage/settling basin.
Type of Sump The type of sump will be the same as that used in the integrated storage/settling basin.
Note: In some cases an external concrete storage may already exist on the farm. Provided that the existing storage has capacity to contain either a 25-year/24-hour storm event or a 25-year/5-minute storm event over a 15-minute period, the storage may be used as a sump to support the pump or gravity flow mechanism. The storage tank must be situated below ground to facilitate year-round operation. The tank must be adequate in size to accommodate the settled solids, and access to the tank is required to remove the settled solids on a regular basis.
3.4.4 Calculating Orifice Size Based on Orifice Discharge Capacity
The following section describes the process to be followed in establishing an appropriate orifice opening size that will accommodate a specific discharge rate. Equation 3.7 outlines the variables, units, and mathematical relationship to establish the orifice discharge capacity.
Equation 3.7 Orifice Discharge Capacity
Q = (C)(A)(2gh)0.5
Where:
Q = orifice discharge capacity (m3/s)
C = orifice constant (0.61) varies by type of orifice, value considered
conservative
A = orifice area (m2)
g = acceleration due to gravity 9.8 m/s2
h = head on orifice (m)
If the equation is rearranged to solve for the orifice area (A) then:
A = Q/[(C)(2gh)0.5]
The following equation is for the area of a circle (the orifice opening):
A = (3.14)D2/4
Rearranging the equation to solve for the diameter (D) of the orifice:
D = (4 x A/3.14)0.5
For example, the orifice area A required to discharge the conservative maximum runoff volume from the integrated storage/settling basin over a 4-hour period with a target discharge capacity of 4.80 × 10-3 m3/s (0.17 ft3/s), an orifice constant of 0.61 (varies by type of orifice, value conservative), gravity at 9.8 m/s2 (32.2 ft/s2), and a head of 0.21 m (0.69 ft) is equal 4.80 x 10-3 m3/s/[(0.61)(2 x 9.8 m/s2 x 0.21 m)0.5] = 3.86 × 10-3 m2 (0.013 ft2). The area of the orifice is equal to 38.6 cm2 (6.18 in2). Solving for the diameter D of an orifice with an area A of 3.86 × 10-3 m2 is equal to the square root of (4 × 3.86 × 10-3 m2/3.14)] = 7 × 10-2 m (2.77 in.).
If the target discharge capacity of 1.9 × 10-3
m3/s (0.07 ft3/s) was used to determine the
orifice diameter required to discharge the conservative maximum runoff
volume from the integrated storage/settling basin over a 10-hour period
the result would be as follows: an orifice constant of 0.61 (varies
by type of orifice, value conservative), gravity at 9.8 m/s2
(32.2 ft/s2), and a head of 0.21m (0.69 ft) is equal to
1.9 x 10-3 m3/s/[(0.61) (2 x 9.8 m/s2
x 0.21 m)0.5] = 1.54 × 10-3 m2
(0.017 ft2). The area of the orifice is equal to 15.4 cm2
(2.4 in2). Solving for the diameter of an orifice with
an area of 1.54 × 10-3 m2 (0.017 ft2)
is equal to the square root of (4 × 1.54 × 10-3
m2/3.14) = 4.4 × 10-2 m (1.7 in.).
Table 3.1 lists the orifice diameters and areas for a variety of discharge rates for a range of head values.
3.4.5 Perforated Riser Pipe Design
A perforated riser pipe is placed ahead of the orifice plate so that all flow passes through the openings in the riser pipe. The riser pipe primarily functions as a screen. The perforated riser pipe will be the same size as the drainpipe to simplify the connection. Flow rate through the holes or slots in the perforated pipe should be checked to ensure that this estimate of flow rate exceeds that of the orifice. The area of the perforated riser pipe openings must be at least 25 per cent greater than that of the orifice plate opening at maximum head. The additional area of the riser pipe openings will compensate for potential clogging of the riser pipe.
The minimum opening in the riser pipe is determined by using the orifice discharge equation below.
Q = (C)(A)(2gh)0.5
If the equation is rearranged to solve for the minimum opening area (A) in the riser pipe then:
A = Q/[(C)(2gh)0.5]
The area of the perforated openings in the riser pipe must be at least 25 per cent greater than that of the orifice plate opening; thus, minimum riser pipe opening area is A × 1.25.
For example, the minimum riser pipe opening area for a discharge
rate (Q) of 4.8 × 10-3 m3/s (0.17 ft3/s)
and a maximum head (h) of 0.7 m (2.3 ft) is equal to 4.8 × 10-3
m3/s/[(0.61)(2 × 9.81 × 0.7)0.5]
= 2.1 × 10-3 m2 (0.023 ft2).
Increasing the opening area by 25% results in 2.1 × 10-3
m2× 1.25 = 2.63 × 10-3 m2
= 26.3 cm2 (0.028 ft2) minimum riser pipe opening
area. Using 2 × 2 cm (0.75 in. × 0.75 in.) slots will
result in 26.3 cm2/4 cm2 = 6.6 or 7 slots in
the 0.7 m (2.3 ft) height of riser pipe above the orifice plate.
| Top of Page |
| 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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