(A) Rational formula.
(1) The rational formula shall be used to determine the peak flow rate for the design storm:
Q = CIA where Formula 2.1
(a) Q = peak rate of flow in cubic feet per second.
(b) C = coefficient of runoff having a value between 0.0 and 1.0 depending on surface characteristics (23.31.041(1)).
(c) I = the average intensity of rainfall in inches per hour for a duration equal to the critical time, usually the time of concentration.
(d) A = the tributary area in acres corresponding to the critical time above.
(2) The procedure for determining the values of C, A and I are given in the following sections.
(B) Runoff coefficient, C. Table 23.31.041(1) shows the basic coefficient, C, to be used for various types of surfaces. Where a tributary area contains more than one type of surface, the value of C shall be the weighted average of the respective values shown on Table 23.31.041(2) based on percentage of each surface area to the total area.
Land Use | Runoff Minimum (Tci) | Residential (1) |
Land Use | Runoff Minimum (Tci) | Residential (1) |
Rural density (zones with 5 acre | .25 | 30 or larger minimum lot size |
Suburban density (0.5 - 4.99 acre min.) | .30 | 30 |
Low density (single family) | .35 | 25 |
Medium density (multi-family) | .50 | 20 |
High density (apartments) | .70 | 15 |
Business and commercial | .90 | 10 industrial (2) |
Light | .70 | 15 |
Heavy | .90 | 10 other (2) |
Community park | .25 | 30 |
Recreation | .30 | 25 |
Schools | .40 | 20 |
(1) Refer to general plan for description. | ||
(2) In lieu of using this table, composite coefficients may be determined for land uses within a development by using the basic coefficients shown in Table 23.31.041(1), subject to review and approval of the County Engineer. | ||
(C) Rainfall intensity, I. Runoff for the ten-year storm must be based on the rainfall zone map and the rainfall intensity - duration curve.
(D) Tributary area, A. The tributary area in acres for each point of computation shall be based on actual field reconnaissance or use of appropriately scaled maps that certainly depict the drainage boundaries. All of the area that will contribute runoff to the drainage system shall be considered, regardless of the limits of the particular development under consideration.
(E) Critical flow time.
(1) The critical flow time is the time that results in the maximum flow rate for a given point in a drainage system. Maximum flow occurs when the product of the intensity and the contributing area corresponding to the flow time is a maximum. The time required for water to flow from the most hydraulic remote point in the watershed to the point in question is called the time of concentration. Using the intensity corresponding to this time and the entire drainage area in the rational formula usually results in the maximum flow rate for the point.
(2) The nomograph shall be used to determine the time of concentration to the first inlet of the drainage system. If this time is less than the minimum inlet time, the values shown in the table shall be used.
(3) When any part of the storm waters are conveyed to the point in question by pipes or open channels, the flow time in these conveyances shall be added to the inlet time to compute the total flow time. In complex drainage situations more than one computation may be required to determine the combination of contributing area and flow time that results in the maximum flow rate.
(4) The rational method tends to result in more conservative values for peak flows as the time of concentration becomes quite large and when several branches of varying times of concentration are involved; therefore, the designer may consider using a hydrograph method for determining runoff when the drainage basin is larger than 100 acres.
(F) Off-site flows. Flows entering the proposed development from outside the property are off-site flows. The off-site storm runoff must be determined and included in the drainage system design. Available drainage reports for off-site developed areas affecting the property must be reviewed and considered in the drainage system planning and design. Runoff entering the site from off-site must be computed using runoff coefficients based upon existing development, or for undeveloped land, based on zoning, whichever is greater. Allowable discharge for the developed area shall be limited to the ten-year event pre-development discharge, or the channel capacity, whichever is the lesser.
(G) Figures. See figures on following pages.
