The following points highlight the three main types of storm water regulators. The types are: 1. Side Flow Weirs or Overflow Weirs 2. Leaping Weirs or Jumping Weirs 3. Siphon Spillways.
Type # 1. Side Flow Weirs or Overflow Weirs:
A side flow weir or overflow weir is provided along one or both sides of the sewer channel constructed in a manhole or a masonry chamber constructed along a combined sewer. If the weir is provided only on one side of the channel it is known as single sided overflow weir and if weirs are provided on both sides of the channel it is known as double sided overflow weir.
Fig. 6.17 shows the plan and section of a single sided overflow weir provided on the outer side of a curved sewer channel constructed in a masonry chamber. The crest of the weir is set at such a level that so long as the flow in the combined sewer A is less than or equal to the maximum flow of domestic sewage (i.e., about 3 times the average dry weather flow) the entire flow passes through a small sewer C and the weir, therefore, does not come into action.
When the flow through the combined sewer A increases due to addition of storm water (or rain water) during rainfall, the flow level in the sewer channel increases above the crest level of the weir and the excess flow passes over the weir crest and enters the sewer B. The sewer B carries the excess sewage to a water course or any other point of discharge, whereas the sewer C, in which the normal quantity of sewage continues to flow, carries it to a treatment plant.
The length of the side flow weir is given by the following formula devised by Babbitt:
L = 7.6 x 10-3 VD loge (h1/h2) … (6.2)
Where
L = required length of the weir in m;
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V = velocity of approach in m/s;
D = diameter of the combined sewer in mm; and
h1 and h2 = heads in m above the crest of the weir upstream and downstream.
The above formula is limited to conditions in which the weir is placed in the side of a circular pipe at a distance above the bottom greater than d/4 and less than d/2, where d is the diameter of the pipe, and the edge of the weir is sharp and parallel to the invert of the channel. Its usefulness is limited in that it was devised for pipes between 450 and 600 mm in diameter and where the depth of flow above the weir should not exceed 3d/4.
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In the case of single sided overflow weir the required length of the weir as given by equation 6.2 is to be provided on one side of the channel only. This will require a storm water overflow chamber of relatively large length.
However, by adopting a double sided overflow weir half of the required length may be provided for the side flow weir on each side of the sewage channel, and hence it is possible to halve the length of the storm water overflow chamber.
Fig. 6.18 shows the section of a double sided overflow weir provided in a sewage channel made in a manhole. In order to prevent the escape of floating matter from the sewage channel adjustable plates, are sometimes provided as shown in Fig. 6.18.
In another arrangement, openings at suitable height above the invert are provided along the length of the combined sewer as shown in Fig. 6.19. These openings are connected to storm water channel or storm water sewer which is laid near the combined sewer. The excess sewage is thus diverted and conveyed to the natural stream or river or to any other point of discharge. However, this arrangement is not commonly used on account of practical limitations.
Type # 2. Leaping Weirs or Jumping Weirs:
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A leaping weir or jumping weir is formed by a gap in the invert of a combined sewer. The dry weather flow through the combined sewer being small falls through the weir (or gap) directly into a small intercepting sewer.
The intercepting sewer runs at right angles to the combined sewer and it carries the sewage to the treatment plant. When the flow in the combined sewer increases due to the addition of storm water (or rain water) during rainfall, a considerable portion or all of the storm water leaps or jumps across the weir (or gap) and it is carried through a storm water sewer to a natural stream or river or to any other point of discharge.
The width of the gap may be determined by trial and error, i.e., a small width is provided in the beginning and its performance is watched. If it is found insufficient, it is expanded or enlarged. Alternatively an adjustable crest may be placed to control the width of the gap.
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The advantage of leaping weirs is that there are no moving parts but they have the disadvantage of concentrating grit in the low flow channel.
Type # 3. Siphon Spillways:
A typical siphon spillway is used for diverting excess sewage from a combined sewer. The short arm of the siphon is connected to the combined sewer. The long arm of the siphon is connected to an overflow channel which carries excess sewage to a natural stream or river or to any other point of discharge.
The crest level of the siphon is kept exactly same as the overflow level in the combined sewer. An air pipe connects the throat of the siphon to the combined sewer. The opening of the air pipe is held at a level slightly above the overflow level in the combined sewer.
The siphonic action does not start as long as the level of sewage in the combined sewer is lower than that of the crest of the siphon. Since the dry weather flow occupies only the lower portion of the combined sewer, corresponding to which the level of sewage is lower than that of the crest of the siphon, the siphonic action does not take place as long as there is only dry weather flow in the combined sewer.
When the flow in the combined sewer increases due to the addition of storm water (or rain water) during rainfall, and the level of sewage rises above the crest level, the opening of the air pipe in the combined sewer gets closed, the level of sewage in the short arm of the siphon rises and the sewage overflows the crest of the siphon.
Since the opening of the air pipe in the combined sewer gets closed, the supply of air through the air pipe is cutoff and the air contained in the siphon is carried away by the flow. Due to suction thus produced the siphon gets primed and a continuous flow of sewage through the siphon takes place. This removes the excess sewage into the overflow channel.
When the sewage level in the combined sewer falls below the overflow level the opening of the air pipe gets exposed thereby air enters the siphon and it breaks the siphonic action.
The siphon spillway is the most effective device for diverting excess sewage from the combined sewer because of the following reasons:
(i) It works on the principle of siphonic action and it operates automatically. The diversion of the excess sewage from the combined sewer is thus carried out in a better way.
(ii) It has a higher discharging capacity because in this case the effective head causing the flow is the difference between the overflow level in the combined sewer and the surface of sewage flowing in the overflow channel.