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<br />3. Estimate the ratio of the summation of distances between <br />rows of buildings, LlLo' to the total length of reach, LT, <br />parallel to the direction of flow for the reach in step 2. <br /> <br />4. Select a Manning's roughness coefficient, nO' for the <br />open area between the buildings. Open areas include <br />features such as trees, shrubbery, and streets. <br /> <br />5. Compute the urban roughness coefficient n from the <br /> following equation: u <br /> [ i (~,H " ) LlLO ~ ] , (7) <br /> n = n LlWO LT <br /> u 0 <br />where <br /> <br />n is the Manning's n value for urban areas used as part of the total <br />c~oss-sectional area; nO is the Manning's n value for the area between the <br />buildings on the overbank; WT is the total width of cross section, in feet; <br />LlWO is the summation of ind1vidual widths between buildings of a cross <br />section through a row of buildings perpendicular to the direction of flow, in <br />feet; LlLO is the summation of longitudinal distances between rows of <br />buildings, in feet; and LT is the total length of the reach along a profile <br />parallel to the direction of flow. <br /> <br />Use the roughnEss coefficient for the subsections including buildings <br />directly in the hydraulic analyses, and use the total flow area of cross <br />section. <br /> <br />In many cases, the idealized conditions described by Hejl (1977) and <br />shown in figure 13 will not exist, and modifications such as the following <br />will need to be made: <br /> <br />1. For buildings not in line with each other or not at a right angle <br />to the direction of flow use approach number 1 on page 42. <br /> <br />2. Fences and other obstructions may need to be evaluated as part of <br />LlWO and LlLO and; <br /> <br />3. Conditions may warrant computation of n values for different <br />depths of flow. <br /> <br />44 <br />