Main Types of Storm Water Relief Works | Sewer | Sanitary Engineering

Storm relief works are of the following three types: 1. Storm Water Overflow 2. Siphon Spillways 3. Jumping Weirs (or Leaping Weir).

Type # 1. Storm Water Overflow:

These are very common devices, which are used in storm water relief works. It essentially consists of a masonry chamber having one overflow weir. The extra quantity of sewage from main sewer flows over the weir and goes into another overflow sewer, which carries it directly to the natural watercourses.

Usually in the chamber the top of the main sewer is open and the surplus water falls out over the weir. The overflow weirs may be single acting or double acting depending on the quantity to be overflown. Generally the top of the weir is fixed in such a way that six times D. W.F. flows in the main sewer and only the extra quantity flows over the weir.

Fig. 7.12 shows some most common types of overflow weirs. By providing overflow weir at suitable places the size of the main sewer is reduced, which results in the overall economy. Overflows are not recommended in separate system, because in such cases the overflow will pollute the water of natural water courses. In case of combined system, the rain water has already diluted the sewage, which is further diluted by mixing it in water courses and there are no chances of stream pollution.

A side flow weir constructed along one or both sides of a combined sewer delivers excess flow during storm periods to relief sewers or natural drainage courses. The crest of the weir is set at an elevation corresponding to the desired depth of flow in the sewer. The weir length must be sufficiently long for effective regulation.

The length of the side flow weir is given by the formula devised by Babbitt.

Where L is the required length in metre

v is the velocity of approach in m.p.s.,

d is the sewer dia in mm.

h₁ and h₂ are the heads above the crest of the weir upstream and downstream.

This formula is limited to the 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 dia. and in that the depth of flow above the weir should not exceed 3d/4.

Type # 2. Siphon Spillways:

Storm overflow weirs require considerable length of sewer to be placed in the masonry chamber, resulting in high initial cost. To overcome all these difficulties siphon spill-way is used which is very effective and has high capacity than storm overflow weirs. The siphon spill-way is very sensitive and automatic in its action and it removes more volume of extra sewage in less time.

Fig. 7.13 shows the section through a siphon spillway. To the main sewer a siphon is connected, the lower end of which is connected to a sewer, which carries the surplus quantity of sewage to the natural water courses. It consists of a starting pipe or priming pipe ‘p’ connecting the combined sewer with the throat of the siphon. When the sewage level rises in the sewer ‘A’, it also rises in the rising leg B.

The end of priming pipe in the sewer is kept slightly above the crest of the siphon C. The rising level of sewage when crosses the crest C, it seals the entrance of air in the pipe, due to which the level rises simultaneously both in the combined sewer and above the crest.

When the level reaches the fixed mark, the siphonic action takes place and the sewage which is above the point F rushes towards pipe D and goes in overflow sewer. The siphonic action stops as the sewage level falls below that of F.

Type # 3. Jumping Weirs (or Leaping Weir):

Fig. 7.14 shows this type of storm relief work, in which the D. W.F. falls in the sanitary sewer through an opening provided in its crown. The excessive quantity of sewage leap or jump across the opening. The opening of the crown can be adjusted by using movable crests depending upon the theoretical analysis.

When the discharge is small, the velocity is low due to which all the sewage drops into the sanitary sewer. When the discharge increase, the velocity increases, due to which more quantity of sewage jumps over the gap and goes to the outfall.