The following article will guide you about how to estimate the quantity of sewage.
Estimation of Dry Weather Flow:
The dry weather flow (D.W.F) includes:
(i) Domestic or sanitary sewage which is the sewage or wastewater derived from residential buildings and from commercial, institutional and similar public buildings such as offices, schools, cinemas, hotels, stations, etc.;
(ii) Industrial sewage which is the sewage or wastewater obtained from manufacturing plants of the industries; and
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(iii) Groundwater infiltrating into the sewers through the pipe joints and other entry points.
The estimation of dry weather flow therefore involves the estimation of each of these components.
However, the quantity of dry weather flow is affected by several factors which are indicated below:
Factors affecting the quantity of dry weather flow:
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The quantity of dry weather flow depends on the following factors:
(1) Rate of water supply
(2) Population growth
(3) Type of area served
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(4) Infiltration and exfiltration
Estimation of Storm Water (or Rain Water) Flow:
When rainfall takes place a part of it infiltrates into the ground surface, and the remaining part flows over the ground surface. The part of rain water flowing over the ground surface is commonly known as storm water or runoff, which needs to be drained through the sewers, otherwise the entire area would be flooded.
The storm water (or rain water) flow through sewers is also known as wet weather flow (W.W.F.) in order to distinguish it from dry weather flow (D.W.F.) discussed earlier. For the design of sewers it is necessary to estimate the quantity of storm water (or rain water) that will reach sewers.
The quantity of storm water (or rain water) that will reach sewers depends on intensity and duration of rainfall, characteristics of catchment area or drainage area such as its shape, imperviousness, topography including depressions and water pockets, and the time required for the flow to reach the sewer.
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For estimating the storm water (or rain water) flow or runoff for the design of sewers the following two methods are commonly adopted:
(1) Rational method
(2) Empirical formulae
(1) Rational Method:
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In this method the storm water (or rain water) flow or runoff reaching a sewer is given by the expression –
Time-Area Graph:
In the case of large catchment areas or drainage areas the calculated time of concentration and the effective time of concentration differ from each other. Only in the case of equally distributed impervious area throughout the length of the storm sewer the calculated time of concentration practically equals the effective time of concentration and the maximum runoff is caused when the storm duration equals the time of concentration.
However, the determination of the time of concentration by calculating the time of flow through the sewers produces misleading results. This is because of the irregular distribution of impervious area such that for certain sewers or drains which may be collecting only little or no flow at all from the laterals, still the time of flow taken into account for such sewers and then included in the time of concentration is bound to give wrong results. In order to obtain proper results a method based on time-area graph may be adopted.
A time-area graph is obtained by plotting the time in minutes after the commencement of storm or rainfall along x-axis and the impervious area in hectares contributing to the sewer along v-axis as shown in Fig. 3.3. Such a graph shows the sum total of impervious areas contributing runoff to a selected point in a sewerage system at various periods of time after the commencement of a storm.
This will continue upto the end of the calculated time of concentration. Consider a drainage area as shown in Fig. 3.3 (a) provided with drainage lines along the streets and a main sewer line in the centre. Let P be the point of observation on the main sewer line. At the commencement of the storm, rain water will be arriving at P only from the area A1 which is in the immediate vicinity of P.
As the time passes, water from the respective areas A2, A3 and A4 will arrive and thus at the end of the time of concentration water from the total area (A1 + A2 + A3 + A4) will be arriving at P. The rate of runoff will reach its maximum value and as long as the rainfall continues at the constant rate the rate of flow at P will remain the same.
The time-area graph is represented by three straight lines. One line start from the origin (i.e., point of zero area and zero time) and it slopes upwards up to a point which represents the time of concentration (tc) and total hectares of impervious area Ac. At the end of tc the total area continues to contribute as long as rain continues to fall, and thus the impervious area remains constant with further increase in time, this being represented by a second line drawn parallel to x-axis.
When the rain stops, the time-area graph begins to fall as represented by a third line sloping downwards. The same method can be used for plotting the time-area graphs for a group of districts, each provided with an individual sewer line.
The time-area graph may be used for determining the maximum storm water (or rain water) runoff.
For this tangent method is commonly used which is discussed below:
The Tangent Method:
The time-area graph is plotted for the entire drainage area as shown in Fig. 3.4. From the origin the distances of minus 10 and minus 20 minutes are marked on the x-axis, depending on the value of the time of concentration for the equivalent effective area.
The tangents are then drawn to the time- area curve from these points and the angle 9 made by the tangent is measured. The runoff will then be proportional to tan 8 and the maximum value of 9 will give maximum runoff.
The actual value of maximum runoff is determined in the following way:
Mark the point on the time-area curve where the tangent touches. For this point measure the values of impervious area in hectares on v-axis and the time of concentration on x-axis. Using the appropriate formula determine the value of the intensity of rainfall. Then the maximum runoff will be equal to the product of the impervious area and the intensity of rainfall.
As shown in Fig. 3.4 the tangent from the point minus 20 minutes to the time-area curve gives an equivalent impervious area of 8.4 hectares and an equivalent time of concentration of 46 minutes. Using U.S. Ministry of Health formula an equivalent intensity of rainfall is obtained as –
(2) Empirical Formulae:
The use of rational formula for estimating the storm water (or rain water) flow or runoff for the design of sewers is usually limited to small catchment areas or drainage areas, say up to about 400 hectares. This is so because for large areas the selection of suitable values of runoff coefficient and intensity of rainfall requires extreme care and judgement.
For large areas empirical formulae are generally used for estimating the storm water (or rain water) flow or runoff for the design of sewers. However, the empirical formulae that are available for estimating the storm water (or rain water) flow or runoff can be used only when conditions comparable to those for which these formulae were derived initially can be assured.
The various empirical formulae involve the following variables:
(i) Catchment area or drainage area;
(ii) Rate or intensity of rainfall;
(iii) Relative imperviousness; and
(iv) Slope of ground.
Some of the leading empirical formulae for estimating the storm water (or rain water) flow or runoff are given below:
Inglis formula was derived by using the data of catchment areas or drainage areas of Maharashtra where it is commonly used.