§ II. Hydrology.
RUNOFF RATE COMPUTATION.
Storm drainage facilities for residential subdivisions and small commercial or industrial developments with watersheds less than 200 acres shall be designed on the basis of discharges calculated by the rational formula. In calculations for stormwater detention or retention basins the following computer implementations of the unit hydrograph are acceptable.
a.
SCS Technical Release No. 55, "Urban Hydrology for Small Watersheds" 2 nd Edition, June 1986
b.
SCS Technical Release No. 20, "Project Formulation Hydrology" 2 nd Edition, May 1983
RATIONAL FORMULA.
The Rational Formula is expressed as:
Q = CiA, where:
Q = Peak runoff rate in cubic feet per second (cfs).
C = Runoff coefficient which expresses the ratio of peak runoff to rainfall and which is dependent upon the pervious characteristics of the surface receiving the rainfall.
i = Rainfall intensity in inches per hour for a duration equal to the time of concentration.
A = Tributary area in acres.
DRAINAGE AREA.
The area, A, is the only parameter in the rational formula which is subject to accurate determination and represents the total area tributary to any point under consideration for which runoff is being determined. Since runoff is subject to various characteristics of the area, investigation of the drainage area should include a compilation of principal factors, which may affect runoff. These factors include land use, areas of impervious and pervious surfaces, character of soil and vegetation cover, and the general magnitude of ground slopes. A current topographic map with contours should be obtained for use in drainage area calculations. All upstream tributary areas are to be considered as fully developed as zoned at the time of design or as shown in the city's comprehensive plan, whichever is greater. The city may require that storm drainage systems for tributary areas upstream of existing storm drainage facilities include on-site stormwater detention facilities limiting the peak discharge to that which would have occurred for the existing land use type prior to a zoning change or prior to changes in development of the area. The city may waive requirements for detention facilities when the owner and/or the developer make satisfactory arrangements to improve or provide a downstream drainage system of adequate hydraulic capacity for peak rates of discharge to the system, including discharge from the owner's site, to a point downstream where the rate of total runoff from the site is ten percent or less of the total runoff rate conveyed by the downstream system measured at the time of system peak rate. The city may, at its option, also permit downstream system improvements and detention combinations that provide the same level of control.
RUNOFF COEFFICIENTS.
Of the parameters in the rational formula, runoff coefficients are the least subject to accurate determination. Surface runoff is affected by vegetation, and evapotranspiration. The most significant factor is percolation into the soil, which is greatly influenced by soil type and surface vegetation. It is common practice to use general overall runoff coefficients, which are based upon either land uses or surface characteristics. Runoff coefficients from Table 1 and Calculation Form B are to be used for runoff calculations.
TIME OF CONCENTRATION (T c )
Time of concentration is the period of time required for runoff to become established and for water to flow from the most distant point of the drainage area to the point under consideration. Time of concentration consists of an overland flow component and a channel flow component.
A minimum time of concentration of five minutes shall be used for runoff from pavements and paved swales. A minimum of ten minutes shall be used at inlets intercepting runoff from grassed swales.
TIME OF CONCENTRATION, OVERLAND FLOW.
The selection or computation of the overland flow component of the time of concentration, T c and the determination of the variables, which affect it, is a very critical and important step in computing the peak rate of runoff. For a particular drainage area, the length, slope, and composite runoff coefficient must be determined. The length is the distance from the extremity or most remote point in the catchment area, in the direction parallel to the slope, until a defined channel or inlet is reached. To the first inlet, gutter flow will be considered as channel flow and handled as such. The total time of concentration will be the time of overland flow plus the time within the channel.
The slope, in percent, is the difference in elevation between the most remote point in the catchment area and the downstream edge of the overland flow area, divided by the horizontal distance between the two points. A composite runoff coefficient will be determined for the tributary area where overland flow will occur. A nomograph, Figure 1, is included for estimating overland flow time.
TIME OF CONCENTRATION, CHANNEL FLOW.
The time of concentration within the channel can be determined by solving the Manning Formula for discharge:
Q = 1.49/n AR <?xpp frax/2 3?> S ½
and the continuity equation.
Q = AV
in which:
Q = The quantity of water flowing in the channel (cfs).
A = The cross-sectional area of water at a given channel section (sq. ft.).
V = The mean velocity of water through a given channel section (fps).
R = The hydraulic radius which is the cross-sectional area, A, divided by the wetted perimeter of the section W (ft.).
S = The slope of the energy grade line which is equal to the water surface and the channel bottom in uniform flow (ft/ft).
N = The roughness coefficient from Table 2.
Figure 6 can be used in the general solution of the Manning's Formula.
Since the quantity of stormwater reaching an open channel will normally increase continuously along its length, and the velocity will vary from the upper to the lower end of a channel section, it will be necessary to determine the average velocity in the channel section in order to compute a value for T c .
Typical channel cross sections near the upper and lower ends of the channel section can be plotted and the average slope determined from field survey information or from a contour map of the area. A value of "n" can be selected from Table 2.
The quantity of water at the upper end of the channel can be computed. Various water depths on the typical section can be tried and corresponding values for A and R determined until the Q computed by the Manning Formula is equivalent to the design Q. The values of A and Q can then be substituted into the continuity equation to determine the velocity, V, at the section being considered.
By using the same steps, a velocity can be computed at the lower end of the channel section and an average velocity determined. Velocities for channelized gutter flow can be taken from Figures 2, 3, and 4. Time of concentration within the channel is then equal to the length of the channel section, L, in feet, divided by the average velocity, V a , in feet per minute.
T c = L/V a
The time of concentration within the channel must be estimated so approximate Q and V values through the typical cross section at the lower end of the channel can be computed and used in determining the actual time of concentration in the channel.
This principle can also be applied if a closed conduit is involved instead of an open channel, except that the quantity of water and velocity will normally not vary from the upper to the lower end of the conduit length under consideration. Therefore, only one value of velocity will need to be determined. In the case of a pipe, it will be sufficient to use the velocity when the pipe is flowing full to find the T c . Figures 5 and 6 can be used in computing velocity of flow in closed conduits.
FREQUENCY INTERVAL.
The frequency, or return period, is the statistical average interval between rainfalls having an equal magnitude. Storm drain facilities in residential areas shall be designed as a minimum for a 10-year frequency storm. In the central business district and connected business areas, industrial areas, and new business and industrial areas, drains shall be designed for a 25-year frequency storm. Storm drain facilities in new residential areas located below an existing or projected business or industrial area in drainage basins shall be designed to handle the 25-year frequency storm drainage through the business or industrial area.
Although storm drain systems are to be designed for storm frequencies as discussed above, new systems for undeveloped areas as stipulated by the city shall include provisions for handling the design runoff, without threat to personal safety or undue property damage, in accordance with the design storm return relationships shown in Table 3.
Provisions for handling the runoff in excess of the minimum design storm may include, but not be limited to, temporary detention facilities, gutter flows to curb depth, drainage easements for overland flow, or simply limiting development to elevations above the flood plain. If provisions cannot be made otherwise to handle the excess runoff, the storm drain system shall be designed for the total runoff, with the approval of the city.
RAINFALL INTENSITY.
Selection of a rainfall intensity is made from rainfall intensity-duration-frequency data prepared from historical rainfall data for the area, which is provided in Table 4. The storm return frequency to be used is dependent upon the land use category, which is being protected, and is selected from Table 3. The storm duration is equal to the total time of concentration at the point under consideration.
FLOWS.
Two separate design flows must be developed for most storm drainage projects. First, design flows for gutter capacity and inlet location and capacity are developed. For these calculations, time of concentration is equal to time of overland flow for the incremental drainage area under consideration plus travel time for gutter flow. The design discharge rate is equal to that discharge calculated by the rational formula plus any flow bypassed around upstream inlets. Resulting design flows are used to locate and size inlets and connecting laterals to storm drains.
Second, design flows for storm drain capacity are developed. For these calculations, time of concentration is equal to the cumulative time of concentration from the uppermost reaches of the drainage area to the point under consideration. The runoff coefficient is a composite or weighted average coefficient for the entire area upstream of the point under consideration. Design flow is equal to the discharge calculated by the rational formula using the above cumulative values for time of concentration and the composite runoff coefficient.
(Ord. No. 4119, § 1(Exh. A), 6-16-08)