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Paediatric Analysis Case

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Location of pits The pits are placed at the lowest point or corners of intersection of every sub-catchment area in order to collect runoff water. Runoff water would flow to the pits due to gravitational force. Also wales and gutters are connected to those pits accordingly. The runoff water would travel from high contour profile to low contour profile through swales and gutters. Runoff coefficient estimation The runoff coefficient is the coefficient that represents the ratio of runoff against rainfall amount. In the design of an urban area, proportion of impervious area needs to be taken into account. Rainfall could not infiltration into impervious area and rainfall would immediately become runoff if rainfall water fall on those area. Therefore, …show more content…

The discharge or runoff of every sub-catchments are calculated with the equation as shown below. Q=fCIA Where Q = Flow (m3/sec), f = Unit conversion factor = 1/3.6, C= Runoff coefficient, I= Rainfall intensity (mm/hr), A = Area (km2) The rainfall intensity I is determined by the IFD data of Melbourne extracted from the Bureau of Meteorology department. Only the 5 year ARI is considered in our design as the pipe system is designed for 5 year ARI events. Applying the Melbourne IFD equation I=-26.91 ln⁡(t)+128.31 for 5 years ARI, the intensity could then be determined. Time of Concentration Time of concentration represents the time taken for the pits to reach maximum discharge. By knowing the maximum discharge, we could determine the required size of pits. And the time of concentration is determined as the sum of time of overland flow and time of gutter flow. Tc=To+Tg or Tc=To+Ts Where, Tc = Time of concentration, To = Time of overland flow, Tg = Time of gutter flow, Ts= Time of swale …show more content…

After that, the velocity of gutter flow is calculated from the cross-section area found and, then is used to determine the gutter flow time for each sub-catchment. Equations used in calculation are shown as below: Depth of gutter flow, d= ((Q n)/(0.375 F S_o^(1/2) Z))^(3/8) Velocity of gutter flow, v=Q/A (m2/s) Time of gutter flow, T_g=L_g/v (s) The cross section of the gutter is illustrated in Figure 5.1 below. In the design, an assumption on reciprocal cross slope is made which the value of for reciprocal of cross slope is set to be 4 for all gutters in all sub-catchments. Time of swale flow, t_s Manning’s equation is applied for the design of vegetated swales. Q=(AR^(2/3) S^(1/2))/n Where A=area of the sub catchment area (m^2), R=(Cross section of swale)/(Wet perimeter of swale), S=longitudinal slope of swale and n=roughness coefficient of swale The following fixed design perimeters are set in order to facilitate the calculation. Manning's Roughness Coefficient 0.3 Design Width of swale (m) 2.5 m Design Depth of swale (m) 0.3 m Slop of trapezoid

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