Storm Drainge

STORM DRAINAGE

Disadvantages of Combined Sewers: (Sewage + Drainage).

(i) Larger size of sewer section is required.

(ii) Hydraulic performance is unsatisfactory during Dry Weather Flow (DWF).

Hence, it is frequently preferred to carry the storm water through storm water drain.

Surface Drains / Storm Water Drains:

(i) Surface drains are used to carry sullage (kitchen waste water) and rain waters.

(ii) Surface drains are less hygienic since they are open and exposed to atmosphere.

(iii) Surface drains are normally laid along either sides of the street facing boundary walls of houses and buildings.

(iv) In surface drain, it is difficult to maintain self-cleansing velocity and they require frequent cleaning.

(v) Surface drains flow under gravity.

Requirements for efficient surface drains:

(i) The inner surface of the drain should be smooth.

(ii) It should have sufficient carrying capacity and should have reasonable free board.

(iii) It should be laid at sufficient gradient to achieve self-cleansing velocity during dry weather flow.

(iv) All the joints should be properly and neatly finished.

(v) It should be structurally safe and stable.

(vi) It should be constructed with non-corrosive material and should be resistant to erosion.

Shapes of Surface Drains:

Following shapes are commonly used:

(i) Rectangular Section

(ii) Trapezoidal Section

(iii) Semi-Circular Section

(iv) U-section

(v) V-section.

Design of Drainage System

 (i) Contour maps of the area is collected.

(ii) In the map, position of major/link drains and disposal source is marked to achieve gravity flow. Providing flat gradient is economical over the cost of pumping..

(iii) Alignment of drains-laterals, branches, mains is done.

(iv) Catchment area of each drain is marked.

(v) Based on population of catchment area, the peak discharge expected in each drain is calculated.

(vi) Information on underground structures (water lines, existing sewer lines, electric and telephone cables, gas lines, etc.,) location of streets, subsoil conditions, strata type, ground water level, fluctuations of the drain are assessed.

(vii) Longitudinal section of the entire drain line is drawn fixing FSL. The NSL (Natural Surface Level) of catchment area is checked with FSL of drain. NSL should be higher than FSL to prevent water logging.

(viii) The bed level (depth) of drain is fixed based on following criteria:

(a) Bed of drain should be higher than bed of discharge source at outfall point.

(b) Depth should be less than Man's height to prevent drowning.

(c) Depth depends on the land area available for providing sufficient drain width. If space is limited, width is reduced and alternatively depth is increased to accommodate the desired flow discharge.

(d) The drain so designed should be economical section and the velocity achieved should be non-silting/non-scouring.

Empricial Formula - Width/Depth of Drains:

(i) Drains upto 15 cumecs

Y = 0.5 √B

B-width of drain

Y - depth of drain.

(ii) Drains with discharge between 15 to 30 Cumecs.

(iii) CWC's recommendations (Central Water Commission).