Drain Channel Design Program

The drain channel is a concrete channel built to handle the flow of sediment,inadequately treated water, and drainage to a lagoon. The channel is positioned along the inlet channel side of the plant. Example preliminary sketches:

The purpose of this program is to design the dimensions of the drain channel based on the height of the water in the plant, length of the plant, and assumptions regarding desired drainage time and basic tank geometry. The Drain Channel is constructed as a separate channel built along the side of the flocculator and the sedimentation tank on the inlet channel side. The channel will have its lowest elevation near the bottom of the sedimentation tank extending in the ground. Drain valves will open the flow of water into the channel from the flocculation and sedimentation tanks. These drain valves are supported by a layer of two bricks that supports the valves. On top of this supporting layer is a cover for the drain channel that acts as a walkway for the operator, as shown in the plant blueprint below. This cover is removable to allow the operator easy access to the drain channel and the drain valves in the event of clogging or a need for general maintenance.

The Drain Channel must be built to accommodate the maximum flow through all the drain valves of the flocculation tank and sedimentation tank. This is calculated as:

Design Assumptions

Geometric constraints used in the design come from some basic assumptions. The depth of the drain channel at the beginning of the flocculator is equal to one half the head loss at the opposite end of the drain channel. Also, we will over-design the channel by calculating the height the cross-section as if it were a rectangle and then adding the triangular bottom, based on a user determined channel width.

Another assumption is that the slope of the drain channel is equal to this head loss at the sedimentation tank end per unit length of the inlet channel side of the plant.

In order to calculate the cross sectional area and depth of the drain channel, we assumed the channel can be treated as uniform open channel flow over a long distance in addition to being a rectangular shape. This allows us to use the friction coefficient and head loss equations for a rectangular channel in our calculations where we use an iterative function to determine the headloss for the channel, given a pre-specified channel width .

From this, the slope of the channel and its depth and the beginning of the flocculator and its depth at the end of the sedimentation tank can be determined.

Design Process

Two different methods were used to create the drain channel. The first method draws the channel as a rectangular channel with a rotated wedge unioned to the bottom of it. That object is then rotated an angle equals to the inverse tangent of head loss at the end of the channel over the length of the channel, relative to base of the plant. This more primitive method was used to ensure the creation of the design, regardless of coding efficiency. This method was scrapped once the loft function was completed.

The second method uses the AutoCAD Loft function. This method receives an input of two point arrays representing cross sections of either end of the drain channel at the flocculator side and the sedimentation tank side. The loft function then connects the two cross sections, filling out the structure to create the drain channel.

This method required calculation of 10 points, based on the origin of the channel (D.Channel.origin) if it were rectangular, that defines the shape of each cross section. Points 1, 10, 6, 5 have the same values for x and z for both cross sections. The rest of the points are lowered a distance of h/2 in the z-direction on the sedimentation tank end.

The design process takes a conservative approach in drawing the rectangular portion of the channnel to the necessary depth based on the headloss. The triangular addition to the cross-section result in an over-design in terms of depth, but ensures the desired volume necessary to accommodate the entire flow of the plant in the event of a full plant drainage.

Future changes to the design program of the drain channel should include:
-Code in the brick layers above the rectangular portion of the channel that support the operator walkway covering the drain channel.
-Upgrade valve drawings to include elbows that empty out into the drain channel at 45 degrees
-Code in the operator walkway above the drain channel