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: | 20 June 2006 |
| 3.1 Calculate
Runoff Quantity | 3.2
Design of Storage/Settling Basin |
| 3.3
Determination of Discharge Rates from Runoff Storage/Settling Basin
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| 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 conveyance system is responsible for transferring the runoff from the collection/discharge system to the distribution system.
The two conveyance mechanisms available for the conveyance of runoff from the sump to the infiltration area within the VFS system are:
Gravity flow, the preferred and cheaper option, depends upon the grade differential between the runoff collection area or sump and the infiltration area. If sufficient grade differential exists to effectively move runoff via gravity, then this option should be pursued. However, experience in the northeastern United States has shown that a large majority of the systems rely on pumps to convey runoff from the storage/ settling basin to the infiltration area. A pump will be required if the runoff will not flow by gravity from the runoff collection area to the infiltration area. More than one pump may be required if the water from the runoff collection area is unable to flow to the external storage/ settling basin, particularly if an existing storage facility (such as an elevated tank) is being considered.
The preferred conveyance method for the VFS system runoff is via a pipe. Piping is required in most components of the VFS system and serves the dual function of dictating flow rates and routes. The opportunity to utilize an open channel (see Figure 3.4) is limited to farmsteads where there is adequate grade differential to support gravity flow from the storage/settling basin to the infiltration area. An open channel should be considered only when the infiltration area is situated directly adjacent to an integrated storage/settling basin. Runoff from the integrated storage/settling basin is discharged at the base of the containment wall and directed into an open channel. The open channel must be less than 20 m (65.6 ft) in length and discharge into a rock-lined distribution channel at the top end of the infiltration area. The conveyance channel may need to be properly lined to prevent infiltration to ground water. A culvert may be required along the open channel to accommodate crossing by farm equipment, livestock, or people.
The design, maintenance, and operation considerations associated with the development of an open channel include:
It is recommended that pipes are used, where possible, as experience with VFS systems in the northeastern United States shows that pipes operate more effectively than open channels.
Figure 3-4 Conveyance and Distribution Channel
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In the development of both the internal and external storage/settling basins scenario, discharge from the storage/settling basin is directed to a below-grade sump. The sump is the central collection area for all runoff to be conveyed to the infiltration area. The sump facilitates the creation of pulsed flow, an important characteristic of the flow stream that allows for optimum utilization of the infiltration area. With pulsed flow, a consistent flow of water is generated to the infiltration area when the pump is on, promoting even distribution of the flow. This is a critical aspect that improves the performance and efficiency of the infiltration area.
The runoff from the storage/settling basin will be directed to a
sump and discharged to the conveyance pipe/channel by a siphon or
pump. The siphon utilizes gravity flow to transfer flow to the conveyance
pipe. A pump is required where there is insufficient grade change
to accommodate gravity flow. Figure 3.5 illustrates the recommended
design of a gravity and pump flow mechanism.
With the use of gravity flow, pulsed flow to the infiltration area is generated by installing an automatic siphon system in the sump to purge or clear the flow (e.g., Flout™, www.rissyplastics.com) . Automatic siphon systems (see Figure 3.5) are activated when a certain elevation of the liquid is reached (high water level). The systems rapidly drain the tank to a low water level. The rate of flow varies based on the drawdown depth. When the low water level is reached, the siphon shuts off and stops the flow of runoff. The siphon repeats the cycle when the water level rises to the set maximum water level in the sump. The diameter of the discharge pipe that exits the sump is 100 mm (4 in.), and is consistent with the conveyance pipe diameter being proposed for gravity flow mechanisms.
There are several operating factors to be incorporated into the design of the sump to accommodate an automatic siphon system. The automatic siphon should shut off approximately 127 mm (5 in.) from the bottom of the sump. An additional 173 mm (7 in.) of dead storage should be provided to accommodate settlement of solids. The turn-on depth should be a minimum of 150 mm (6 in.) below the inlet pipe invert to the sump. The minimum operating depth of the automatic siphon system should be equal to the drawdown depth plus 300 mm (1 ft). The depth of the sump that is selected must accommodate the operating parameters associated with the specific automatic siphon device selected. Similarly, the length of the sump must be greater than the drawdown depth of the dosing unit plus 600 mm (2 ft). The location of the inlet pipe to the sump should be established so that it does not interfere with the operation of the dosing unit. Baffles may be required to deflect inlet pipe flow and control water movement in the sump.
1 The use of trademarks does not imply endorsement by OMAFRA, MOE and CH2MHILL
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Figure 3-5 Pump/Siphon/Channel Configuration
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Selecting the Appropriate Discharge Rate to the Conveyance Pipe The target discharge rate to the conveyance pipe should be 10 per cent greater than the discharge rate established for the integrated and external storage/settling basin. Calculate an appropriate drawdown depth that corresponds to the required discharge rate to the conveyance pipe. The maximum flow rate for the drawdown depth should be used and matched to the required discharge rate to the conveyance pipe.
Pump and Installation Requirements
A submersible sewage pump (see Figure 3.5) operates using a float switch that triggers the pump to start a discharge cycle at a specified water elevation. The float switch is adjustable so that the drawdown depth and duration of the flow can be modified. The pump should be installed in the sump approximately 300 mm (1 ft) above the bottom of the sump to allow for settlement of solids. The pump can be placed on an elevated platform made of concrete blocks or a series of stacked patio stones. The pump should be placed in a location that is easily accessible from the sump hatch. A power supply will be required to the pump.
Selecting the Appropriate Discharge Rate to the Conveyance Pipe The target discharge rate to the conveyance pipe should be 10 per cent greater than the discharge rate established for the integrated and external storage/settling basin. The discharge rate of a pump will vary according to the capacity of the pump motor (measured in horsepower) at various head differentials (change in grade from inflow point in sump to discharge elevation at distribution pipe merger). An appropriate pump size and corresponding discharge rate will need to be established. Contact a pump manufacturer to help determine (1) an appropriate pump size, (2) the appropriate operating parameters to meet the site-specific design requirements of the conveyance system and (3) the pump curves for the appropriate pump selected. The pump rate can be varied by adjusting the variable speed drive, changing the impeller, changing the motor to one of a different speed, or installing a gate valve to throttle the flow leaving the pump (essentially acting as an orifice). Every effort should be made to match the desired discharge rate with a reasonable infiltration area.
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The sump runoff will be discharged through the conveyance pipe, which is normally connected to the distribution pipe. The size of the conveyance pipe will vary depending on whether a gravity or pump flow mechanism is used. Under most conditions, a 100 mm (4 in.) diameter pipe will be adequate to accommodate the flow generated by gravity. Larger pipe sizes may be required if the pipe capacity is inadequate to accommodate the required discharge flow rate.
A pump, under most conditions, will require a 38.1 mm (1.5 in.) or 50.8 mm (2 in.) conveyance pipe. These are standard discharge pipe sizes for small sewage pumps. These two pipe sizes will be able to accommodate a large range of discharge rates. Additional flow can be achieved in these pipes by increasing the pump size and capacity (e.g., more horsepower).
In VFS systems that utilize gravity flow, the slope of the conveyance pipe dictates the ultimate capacity and velocity that can be achieved in the pipe. The slope of the pipe is determined by dividing the grade change between the inflow and outflow elevation of the conveyance pipe by the length of the pipe. The inflow elevation is obtained from the conveyance pipe elevation in the sump, and the outflow elevation is established at the connection with the distribution pipe. A minimum flow velocity of 0.6 m/s (2 ft/s) should be maintained when the 100 mm (4 in.) pipe is flowing full to prevent the settling of suspended particulate matter in the pipe. This is the fundamental limitation on the use of gravity flow. If adequate slope is not available to achieve the minimum flow velocities in the conveyance pipe, then a pump must be utilized. The conveyance pipe should be sloped such that the water will drain into the distribution pipe and infiltration area once the pulsed discharge event is complete.
In VFS systems that utilize a pump, the total head differential between the inlet and outlet elevation of the conveyance pipe, along with frictional losses associated with pipe run bends and curves, are factored into the selection of the pump size and associated pipe size. The distribution pipe is designed to work under a pressure head of 0.9 m (3 ft) to obtain uniform distribution through orifice holes onto the infiltration area. This pressure head has to be accounted for in calculating the total head that the pump must deliver against. All of the conveyance pipe should be sloped back into the sump. If the pipe can not drain fully or be buried below first level, it must be insulated and heat traced, e.g., pipe protected with heating cable installed around or along the pipe.
Design Process
In order to size the conveyance pipe, a general idea of the layout of the conveyance pipe run is necessary. The following information will need to be gathered to determine the pipe size:
A more detailed description of the above design elements of the conveyance pipe are discussed below.
Pump Discharge Rate to Conveyance Pipe The pump should be designed for target discharge capacity that is 10 per cent greater than the storage/settling basin discharge.
Material of Construction of Pipe A PVC pipe will be suitable for most VFS system applications. However, several other piping options are suitable, depending on the VFS system design. Always check with pipe manufacturers to ensure that the selected pipe is suitable for:
If the pipe is expected to bear stress from traffic or other sources, provide adequate protection to ensure that the pipe won't be damaged.
Conveyance Pipe Slope in Gravity System or Head Differential in Pump System For a gravity system the slope of the conveyance pipe dictates the capacity and velocity that can be achieved in the pipe. The pipe slope is determined by dividing the inlet/outlet elevation difference of the conveyance pipe by the pipe run length. A minimum flow velocity of 0.6 m/s (2 ft/s) should be maintained when the conveyance pipe is flowing full.
In pump systems the total head differential between the inlet and outlet elevation of the conveyance pipe, along with frictional losses associated with the pipe are factored into the selection of the pump size and associated pipe size. The distribution pipe pressure head of 0.9 m (3 ft) is also accounted for in the calculation of the total head.
The Darcy-Weisbach Equation is used to calculate friction losses in the piping system.
Equation 3.8 Darcy-Weisbach Equation
hf = f (L/D)(V2/2g)
Where hf = head loss due to friction (m)
f = friction factor (use 0.020)
L = length of pipe (m)
D = inside diameter of pipe (m)
V = average velocity of flow (m/s)
g = acceleration due to gravity (m/s2)
Localized friction losses due to pipe bends, valves, etc. should
be accounted for, although in most cases they will represent a small
percentage of the total losses due to friction.
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Manning's Equation is the most widely used formula for determining the hydraulic capacity of a pipe for gravity and pressure flows. Equation 3.9 outlines the variables, units, and mathematical relationship to establish or confirm a pipe size. The intent is to confirm that with the use of gravity flow, sufficient slope is available to generate the required flow velocity in the conveyance pipe. A 100 mm (4 in.) diameter pipe size will be evaluated based on a target discharge flow rate that is 10 per cent greater than the orifice opening required to transfer the conservative maximum storage volume in an integrated storage/settling basin over a four-hour period (value established in a earlier example). The process should be repeated again to ensure that the 100 mm (4 in.) pipe is capable of accommodating the longer discharge period of 10 hours and the associated 10 per cent higher discharge rate. The equation is rearranged to solve for the slope of the pipe. If the slope available is greater than the slope calculated, then the pipe size is acceptable.
Equation 3.9 Manning's Equation
Q = (AR2/3S1/2)/n
which can also be written as
V = (R2/3S1/2)/n
since
Q = VA
Therefore
S = [Vn/R2/3] 2
Where:
Q= flow rate (m3/sec)
A= area of pipe (m2)
S= slope of pipe (m/m)
R = hydraulic radius = area of water flow in pipe divided by the wetted perimeter of pipe
n =Manning's n, PVC Pipe (smooth inner walls) = 0.009
V= velocity of flow m/s
For example, if the integrated storage/settling basin is to be drained in a 4-hour period then the following assumptions can be made: a 0.1 m (4 in.) diameter pipe will be evaluated (pipe radius = 0.05 m), a flow rate of 5.3 ×10-3 m3/s (0.19 ft3/s) (orifice flow rate for desired drain period of 4.8 ×10-3 m3/s (0.17 ft3) increased by 10%), a flow velocity of 0.6 m/s (2 ft/s) (minimum velocity to reduce solids buildup, also derived from Manning's equation Q = VA and solved for V). The above variables were incorporated into Manning's equation and the equation was solved for the slope. The slope = {(0.6 m/s × 0.009)/[(3.14 × (0.05 m)2)/(3.14 (2) (0.05 m)]2/3}2 = 0.004 m/m (0.004 ft/ft). A slope of 0.4% is adequate to accommodate the 100 mm (4 in.) diameter pipe for the specified flow to maintain the minimum velocity of 0.6 m/s (2 ft/s).
However, if the same exercise is carried out for the 10-hour drainage period, there is insufficient velocity generated by the 100-mm (4-in) pipe for the lower discharge rate. The result would indicate that a pump may be required if the full range of the flows (within the four- to 10-hour drainage period) is to be accommodated. An iterative process would need to be carried out to verify the drainage period and a corresponding discharge rate accommodated by the 100-mm (4-in) diameter pipe, before the velocity in the pipe decreases below the target 0.6 m/s (2 ft/s) required.
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