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The Guidelines for Road Design 417 Appendix 10.A - Curbs and Gutters 10.A.1 CURBS AND GUTTERS Figure 10-A-1 shows the types of curbs and gutters that can be used To determine the hydraulic capacity of a gutter the modified Manning equation is considered to be applicable Figure 10-A-2 gives an explanation of the symbols and an example of the use of the equation 2.667 3.75 S0.5 o d Q nSx Where So = longitudinal slope d = depth of flow at curb n = manning’s number Sx = cross slope of gutter Figure 10-A-1: Curbs and Gutters 418 The Guidelines for Road Design The Guidelines for Road Design 419 Figure 10-A-2: Definition of Symbols B Calculate the depth of gutter flow, d T = 3m W = 0.6 m S = 0.005 Sw = 0.06 Sx = 0.04 n = 0.015 dw = W * Sw = 0.6 * 0.06 = 0.036 m Ts = T - W = 2.4 m = 3.0-0.6 ds = Ts * Sx = 2.4 * 0.04 d = 0.096 = ds + dw = 0.036 + 0.096 = 0.132 Find the total gutter flow QA+B (a) Divide the flow area into three parts: A (P1P2P5P6), B (P2P3P4), and C (P2P4 P5) (b) Calculate the flow in triangular area (A+C) QA+C = (0.375 * 0.0050.5 * 0.1322.667) / 0.015 * 0.06) = 0.133 m3/s (c) Calculate the flow in triangular area C QC = (0.375 * 0.0050.5 * 0.0962.667) / 0.015 * 0.06) = 0.056 m3/s (d) QA= 0.133 - 0.056 = 0.077 m3/s The Guidelines for Road Design 420 (e) Calculate the flow in triangular area B QB = (0.375 * 0.0050.5 * 0.0962.667) / 0.015 * 0.04) = 0.085 m3/s (f) Determine the flow in total area A + B QA+B = 0.077 + 0.085 = 0.162 m3/s 421 The Guidelines for Road Design Figure 10-A-3: Gutter Inlets The Guidelines for Road Design 422 Appendix 10.B - Design Example for Flow in a Roadside Ditch 10.B.1 DESIGN EXAMPLE OF ROADSIDE DITCH Required Check whether a grass-lined standard roadside ditch is adequate for the given design conditions Given  Drainage area and road details as shown in Figure 10-B-1  Standard ditch Grassed-lined Outlet to nearest watercourse No tailwater  Design for 5-yr and 10-yr storms Method This method is intended to provide a review of the basic design principle for readers who may be interested Divide the drainage area into sub-catchments along drainage divide lines Further subdivide each sub-catchment, if necessary, such that each subcatchment has a unique land use This design example has four subcatchments Select an applicable hydrology method for estimating design flow In this example, the Rational method is selected Select a suitable raingauge station Use the data given above Calculate the composite runoff coefficient, using Equation n C  C C= A i Ci At A1C1  A 2C  A 3C3  A 4C At 10 0.45  0.70  0.25  0.15 0.41 10    Where: A1 to An = areas of Sub-catchments At total drainage area = The Guidelines for Road Design 423 C1 to Cn = appropriate runoff coefficient for sub-catchment area See Table 10-B-1 and 10-B-2 Calculate time of concentration, Tc For runoff coefficient C > 0.40, use the Bransby - Williams formula Tc  0.057 * L 0.057 * 1025   45.8 0.2 0.1 0.750.2 * 200.1 Sw * A t The Guidelines for Road Design 424 Where Sw = watershead slope = 0.75 % L = watershead length = 1025 m At = A1 + A + A + A = 20 Figure 10-B-1: Design Example = A1 C1 = 0.45 = A3 = A2 C3 = 0.25 C2 = 0.70 C4 = 0.15 = A4 The Guidelines for Road Design 425 Table 10-B-1: Runoff Coefficients - Urban for to 10-Year Storms For flat or permeable surfaces, use the lower values For steeper or more impervious surfaces, use the higher values For return period of more than 10 years, increase above values as 25-year - add 10%, 50-year - add 20%, 100-year - add 25 % The Guidelines for Road Design 426 Table 10-B-2: Runoff Coefficients - Rural Calculate rainfall intensity from Table 10-B-3 Table 10-B-3: Rainfall Data Intensity, I =Coeff A *TC* *Coeff B Calculate design flow, using Rational method Q 0.0028 C I A t Qdesn 0.0028 * 0.41* 36.07 * 20 0.35 m / s