A. Population Forecasting
Data of Historical population
Population has been forecasted by considering the base of historical population census data of every decade from 1981 to 2011 census data. Also Pune Metropolitan Region Development Authority (PMRDA) population data has been considered as base reference.
Different Population Forecasting Methods
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Arithmetic progression method
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Incremental increase method
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Geometrical progression method
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Urban Rural Growth Differential
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Logistical Curving Method
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Density Method
1. Arithmetic progression method
The first method relies on naïve projections of future populations based solely on the past growth rates of the villages. Two methods have been utilized in these projections – the arithmetic increase and the incremental increase methods. The arithmetic increase method is used to calculate the populations of areas that are considered “stable”, i.e., without any substantial increases in population over the training period.
Pn = Po + nx̄,
Were,
Po - last known population
Pn - population (predicted) after 'n' number of decades,
n - Number of decades between Po and Pn and,
x̄ - The rate of population growth.
2. Incremental increase method
The second method used is the incremental increase method. The incremental increase method provides more accurate estimations for villages whose populations are increasing at an increasing rate. It calculates the increments in arithmetic increase while establishing the growth trends of the villages. This increment is used as a multiplier, enabling more realistic population projections for villages which are growing rapidly.
Pn = (Po + nx̄) + ((n (n + 1))/2) * ȳ,
Were,
Po - last known population,
Pn - population (predicted) after 'n' number of decades,
n - Number of decades between Po and Pn,
x̄ - mean or average of increase in population and,
ȳ - algebraic mean of incremental increase (an increase of increase) of population.
3. Geometrical progression method
Pn = P (1 + Ig/100) ^n
Ig = average percentage increase per decade
4. Urban Rural Growth Differential
The second method used is the Urban Rural Growth Differential, used widely by municipal corporations and regional development authorities around the world (?). Since Pune is central to the villages, the economic and development effects of Pune on the population growth for these villages must further be considered into the projection calculations. Further, since all the villages are closely linked to each other and not isolated, there is also likely to be a conglomeration effect of economic development and population increase which spreads between villages. In such cases, the URGD is a more reliable method for population projections since its formula integrates the urban-rural growth differential in relation to the total population while projecting.
\({ \mathbf{R}}_{\mathbf{n}+1}= \frac{{\mathbf{T}}_{\mathbf{n}+1}+{\mathbf{d}\mathbf{U}}_{\mathbf{n} }}{{\mathbf{T}}_{\mathbf{n}}}\) X \({\mathbf{R}}_{\mathbf{n}}\)
Where,
\({\mathbf{R}}_{\mathbf{n}+1}\) - URGD projected rural population in the year \(\mathbf{n}+1\)
\({\mathbf{T}}_{\mathbf{n}+1}\) - The total projected population of the village and the urban center in the year\(\mathbf{n}+1\).
\(\mathbf{d}\) - Growth differential
\({ \mathbf{U}}_{\mathbf{n} }\) - Actual population of the Urban Centre in the year n
\({ \mathbf{T}}_{\mathbf{n}}\) - Total Population of village and urban center in the year\(\mathbf{n}\).
\({\mathbf{R}}_{\mathbf{n}}\) - Actual population of the village in the year\(\mathbf{n}\).
5. Logistical Curving Method
This method is used when the growth rate of population due to births, deaths and migrations takes place under normal situation and it is not subjected to any extraordinary changes like epidemic, war, earth quake or any natural disaster, etc.,
The population follows the growth curve characteristics of living things within limited space and economic opportunity.
If the population of a city is plotted with respect to time, the curve so obtained under normal condition looks like S-shaped curve and is known as logistic curve.
$${\mathbf{y}}_{\mathbf{n}+1}= \frac{\mathbf{M}}{1+\left(\mathbf{B}{\mathbf{e}}^{-\mathbf{k}\mathbf{l}}\right)}$$
Where,
Yn + 1 - population of the village in the subsequent year
M - Total maximum population that can fit within the village
B, -k- constants that define the shape of the curve
l - time period
e - Exponential function used on -k and l
6. Density Method
This Method uses the increase in housing supply as the key factor in projecting populations for villages.
The growth trend of housing supply for each village is determined using past area development and population growth data to effectively track the “saturation point” of each village.
Saturation point - the year in which all available developable land has been occupied.
This method relies on 3 key attributes – area, density, and natural growth rate.
This method accounts for the 2 major components of population growth – natural growth rate and migration.
$${{y}}_{{n}+1}= {\left[\right({y}}_{{n}} \times {N})+({\varDelta {L}}_{{n}+1}\times {{D}}_{{n}+1}\left)\right]$$
Where,
y = Population
n = year
N = Natural population growth rate
∆ = Net change in developed land
D = gross density in that year.
Criteria for selection of Population forecasting methods
Village growth
|
Growth rate
|
Method
|
Slow
|
Lower than Pune
|
Mathematical (Incremental/Arithmetic)
|
x < 3%
|
Medium
|
Between Pune and village average
|
URGD
|
3% < x < 5%
|
Fast
|
Higher than village average
|
Density
|
5% < x
|
B. Calculation of Water demand & Sewage Generation
Constraints
|
LPCD
|
Water Supply rate
|
150
|
Institutional, Industrial/Commercial Demand − 10%
|
15
|
Total Water Supply rate
|
165
|
Sewage Generation
|
132 (80%)
|
Add infiltration − 5%
|
7
|
Total Sewage Generation
|
139
|
C. Preparation of Base Map
QGIS/ArcGIS software used for preparation of base map
Method for preparation of base map:
Collection of Top sheet.
Collection of Village map.
Collection of Satellite image from NRSA – Hyderabad & Pune Municipal Corporaation.
Geo-referencing of Top sheet & Village map.
Digitization of existing network from raster to vector form.
D. Surveying
Topographical Survey
The topographical survey is carried out with DGPS and total station instrument with original coordinates. The TBM has been set up in each village and has been linked to GTS bench mark. Topographic Survey includes existing roads, natural streams. Manhole survey covered geographic location of Manhole and topographic reduced Level (RL) of manhole top.
Topographical survey with Total Station/DGPS (Differential Global Positioning System)
Manhole Opening Survey and Network Mapping
So for each manhole, the depth was measured using measuring staff. The data top RL of manhole and manhole depth is used to calculated Invert Level (IL) of manhole. The diameter of sewers incoming and outgoing from the manhole, depth of manhole and other information is recorded in field survey format and network connectivity marked in field drawings.
Based on field drawing and survey data, sewage collection network drawing is prepared. Based on the field data, manhole number, manhole depth, sewer diameter (In and Out) and remarks are incorporated in the drawing. Any missing connectivity is discussed with respective engineers of PMC ward/project office and jointly verified on site. After incorporating the observations of joint field visit, a final network drawing is prepared, which is then used as a basis for hydraulic modeling.
D. Design of sewer network on Sewer-gems software
The task will be carried out using advanced sewer design software i.e., Bentley Sewer GEMS v8i.
Importing the survey data and existing details (Population, type of conduit and diameter, Contours/levels, base map for background) in the software.
Assigning population, sewage flow, IL levels of MH, Pipe diameters, peak factors for extreme flows etc.
The hydraulic modelling/analysis will be done for immediate, intermediate, and final stages, separately for sewage network in each drainage district.
Design Input Constraints for Sewer GEMS
Input parameters has been taken by recommendation of CPHEEO Manual.
Manning’s Formula is used for designing the sewage collection system.
V = 1/n R 2/3 S 1/2
Were
V = Velocity of flow in pipe in m/sec
N = Manning’s Coefficient. Of roughness
R = Hydraulic radius in m
S = Slope of hydraulic gradient
Minimum velocity at initial peak flow: 0.6m/s
Minimum Velocity at Ultimate peak flow: 0.8m/s
Maximum Velocity: 3.0m/s
As per CPHEEO manual, the Manning’s coefficient of roughness for RCC pipe 0.011
Slope Table
Rise
(mm)
|
Slope (Minimum)
|
Slope (Maximum)
|
300
|
450
|
50
|
450
|
850
|
200
|
600
|
1000
|
200
|
900
|
1000
|
200
|
1000
|
1000
|
200
|
1200
|
1000
|
200
|
1600
|
1000
|
200
|
Extreme Flow Factor
The flow in sewers varies from hour to hour and seasonally. However, for the purpose of hydraulic design estimated peak flows are adopted. The peak factor or the ratio of maximum to average flows depends upon contributory population as given in following Table.
Contributory Population
|
Peak Factor
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up to 20,000
|
3.00
|
Above 20,001 to 50,000
|
2.50
|
Above 50,001 to 7,50,000
|
2.25
|
Above 7,50,001
|
2.0
|
Cover constraint
Cover is the distance between Ground level to the top of the conduit.
Minimum Cover: 0.9m
Maximum Cover: 4.5m
Mapping & design analysis of Existing Network
Drafting of existing network sewer line by using AutoCAD software & importing in Sewer GEMS software by using Model Builder command.
Input data i.e. Ground level of Manholes are given by T-rex command & invert levels of existing Manholes are been taken by excel sheet containing invert levels.
Other input data i.e. velocity, cover, slope & material of conduit are provided by taking reference from CPHEEO manual of Sewerage system.
Thiessen polygon method is used to provide unit sanitary load for each manhole. This area having polygons are export as shape files to QGIS software to get each polygon area.
After running the hydraulic model of existing sewer network result is analysed to check weather existing network is adequate or not for 30 years design forecasted population.
Depth/Rise ratio should be < 80%. If ratio exceeds 80% then pipe diameter should be increased & again check for the same.
Mapping & design analysis of Proposed Network
Similar procedure should followed for mapping proposed network for same village which is having existing sewer network.
Design conduit of existing network should be off so that existing levels can be match to proposed level.
Care should be taken that where existing manhole & proposed manhole are connected existing manhole invert level should be fixed so that existing manhole invert level will not change according to proposed inert level.
Velocity of conduit should be between 0.6 m/s – 3 m/s.
Depth of conduit below ground level should < 4m so that excavation cost gets cheaper.
Depth/Rise ratio should be < 80%.
Design Steps Involved In Sewer GEMS
1. Import of Existing network by using Model Builder command & this above interface will be shown.
2. Then after providing unit sanitary loading & demand model is run. Above picture shows errors occurred in hydraulic model which should be minimize.
3. After minimizing errors profile/L-Section of conduits is generated to analyze adverse slope & hydraulic flow.
4. This above picture shows results having outputs like velocity, depth/rise ratio, slope, diameter, discharge & proposed invert levels of conduits.
STP Capacity & Location
According to forecasted population & water demand, sewage generation is calculated for each village.
By considering this STP capacity is calculated in MLD.
Location of STP