§ IX. Design procedures.
STANDARD FORMS
Forms A, B, C, D, and E are included to aid the designer in completing computations for a storm drainage system consisting of street gutters, circular or circular-equivalent pipes, inlets, manholes or junction structures, and culverts. Design of less frequently utilized open channels shall be in accordance with procedures outlined in this manual utilizing the listed references applicable thereto. Submittal of open channel hydraulic calculations shall be in a form acceptable to the city.
BASE MAP PREPARATION
1.
Prepare a topographic base map showing locations of streets, buildings, lot lines, underground utilities, and natural drainage courses.
2.
Prepare a preliminary layout of a storm drain network to serve the area, including locations of inlets, storm drains, junctions or manholes, and swales and other routing for "major" runoff.
3.
Delineate drainage areas tributary to each proposed storm inlet.
CALCULATION FORMS A AND B
1.
For each subdrainage area, calculate the composite runoff coefficient, using Table 1, and record the values on Calculation Form A, or determine coefficients by using Calculation Form B.
2.
Select the appropriate rainfall return frequency from Table 3 for the predominant land use category to be protected in the drainage area. Record the value at the bottom of Calculation Form A or B.
3.
Complete Calculation Form A or B by checking or filling in the blanks for the planned street width, street cross slope, and storm drain material.
CALCULATION FORM C
1, 2.
Determine and record the length of overland flow, L, along the longest path to the inlet, and the average group slope, S, along the same path.
3.
Using the nomograph for overland flow (Figure 1), given L and S from the above steps and the runoff coefficient from Computation Form A; select the appropriate time of concentration for overland flow, T c .
4, 5, 6.
Enter the appropriate values for "C" and "A" from Calculation Form A or B and calculate "C" times "A" (or "CA").
7.
Using the rainfall return frequency from Calculation Form A or B, the value of T c from Step 3, and the rainfall-intensity-duration data from Table 4, determine the value of rainfall intensity, i.
8.
Calculate peak runoff from the subdrainage area, Q r , by multiplying CA by i (Column 6 × Column 7).
For each street gutter that will receive runoff, perform the following gutter and inlet hydraulics calculations, using the right half of Calculation Form C.
9—12.
Determine the depth of flow, Y g , and the width of spread, W, from the appropriate design chart (Figure 2), by entering the upper graph with the gutter flow, Q g , and the longitudinal street slope, S g , and then by projecting a vertical line down to the lower graph to its point of intersection with the lower curve. The coordinates of this point of intersection gives the values for Y g and W.
If the values obtained in the above step for Y g and W are less than the maximum values permitted by this manual, then proceed to the next step. If the maximum values are exceeded, then provide additional inlets and repeat the above design steps (beginning with Column 1).
13.
Select one of the city's standard inlets, using the following guidelines:
Condition Inlet Type Low point, sump condition Type A Curb Inlet, A Continuous grade, S g ≤ 5% Type A Curb Inlet, A Continuous grade, S g > 5% Type A Curb Inlet with Deflectors, A-D Determine the inlet capacity, Q;sub \sub;, for the selected inlet on continuous grade from the appropriate graph (Figure 7 or 8) by following the procedure outlined heretofore in the text of this manual.
Determine the inlet capacity per foot of length, Q<inf>j</inf>/L/sub<ital></ital>\sub ;, for inlets in sump conditions from the nomograph on Figure 9, by entering the nomograph with H h , based on the amount of ponding as permitted by the street cross-slope and the requirements for noninundated pavement as given in this manual and the height of opening as obtained from standard inlet details.
14.
The difference between gutter flow, Q g , and available inlet capacity, Q;sub ..... sub;, represents the flow that is bypassed around the inlet q.
Proceeding in a downstream direction, size each successive inlet by using the runoff discharge calculated by the Rational Formula for each individual subdrainage area (i.e. ignore cumulative effects) plus the bypassed flow from the upstream inlet.
CALCULATION FORM D
15—21.
Prepare a preliminary storm drain system, beginning at the upper-most storm inlet in the network. Required storm drain capacities are equal to discharges computed by the Rational Formula at each inlet, plus any carryover discharge from upstream inlets. Time of concentration, T c , at any inlet is equal to the larger of the following two quantities: (1) cumulative time of concentration to the upstream inlet plus travel time in the upstream storm drain reach, or (2) time of concentration for the subdrainage area immediately tributary to the inlet. Values for CA for storm drain design are cumulative values calculated from all upstream drainage areas. Select tentative lengths, slopes, and sizes for each segment of the storm drain network and record them on Calculation Form D.
22, 23.
Obtain the capacity and velocity of flow of the selected storm drain when flowing full by Manning's Equation (or using Figure 5 or Table 5, when applicable).
24.
Calculate travel time in the storm drain segment by dividing the length of the segment by the velocity calculated in the preceding step.
25.
Calculate system head losses, h, by using the graphs and procedures shown on Figure 10.
26, 27.
Record approximate ground elevations at each manhole or junction and select an initial downstream invert elevation for the uppermost manhole or junction. Proceeding in a downstream direction, successively set manhole or junction invert elevations, with downstream inverts being lower than upstream inverts by at least the distance h L . Prepare preliminary designs of connector pipes (laterals) between inlets and storm drains.
CALCULATION FORM E
For culvert design, it will generally be necessary to complete all steps on Calculation Form A or B and Columns 1 through on Calculation Form C, as described above.
For each culvert, beginning at the uppermost reaches of the drainage area, perform the following calculations and record them on Calculation Form E.
1—4.
Required culvert capacity is equal to discharge calculated by the Rational Formula, using cumulative values for T c and CA. Runoff calculations are to be recorded in the upper left-hand corner of Calculation Form E.
5—8.
Select a preliminary culvert type, slope (S o ), and diameter (D) and record them along with the culvert Length (L) on Calculation Form E.
9, 10.
Assuming inlet control, find HW/D from the appropriate nomograph for the type of culvert selected (Figures 11 through 14), entering with D and Q. Multiply HW/D by D to determine the value of HW for inlet control.
Assuming outlet control, perform the following calculations to find the value of HW for outlet control.
11.
Find the value for K e for the selected entrance condition and type of culvert from Table 6.
12.
Using the appropriate nomograph for the type of culvert selected (Figures 15, 16, and 17), connect the value for K e and L with the culvert dimensions with a straight line to establish a point on the turning line. Then draw a straight line from the required discharge, Q, through the turning point to the scale showing head, H. This value of H will be used in the equation HW = H + h o - LS o to find HW for outlet control.
13.
Calculate the critical depth, d c , for the selected culvert. Figures 18 and 19 give critical depths at various discharges for different types and sizes of culverts.
14, 15.
Determine the value of (d c + D)/2 and the tailwater depth, TW, downstream of the culvert. In most instances, culvert design will be such that TW = 0.
16.
h o is the greater of (d c + D)/2 and TW. Record the appropriate value under the column for h o .
17.
Multiply L by S o and record it in the column for LS o and record it in the column for HW.
18.
Calculate HW = H + h o - LS o . Circle the larger of the two values for HW. This value determines whether there is inlet or outlet control for the selected culvert and set of conditions.
(Ord. No. 4119, § 1(Exh. A), 6-16-08)