§ III. Storm drain design.
Recommended criteria and procedures for design of the major components of the storm drainage system are presented in the following sections. These criteria and procedures are applicable to the most common types of drainage problems, which are expected to be encountered in the city.
PRELIMINARY DESIGN.
Careful planning of storm drainage systems in the preliminary design phase offers the greatest potential for cost savings and for compliance with storm drainage objectives. The best time to prepare conceptual layouts of storm drainage systems is prior to finalization of street layout, easement location, and site grading. Options available to the drainage engineer are greatly reduced once the surface characteristics of the drainage basin have been set.
In storm drainage system design, a significant part of the construction cost is represented by small diameter storm drains. The longer that overland flow can be kept from reaching the street network, the further the distance from the ridge line that the storm drain system need begin, and the fewer the number of inlets that will be required. Various layout concepts should be developed and analyzed prior to selection of a final concept for detailed design.
Once the street layouts, easement locations, and site grading are finalized, the preliminary storm drainage system layout should be prepared. Whenever it is practicable, storm drains should be aligned to:
• Avoid meandering, off-setting, and unnecessary angular changes; angular changes in alignment should be limited to a maximum of 90 degrees.
• Be parallel with the center line of straight streets unless otherwise unavoidable.
• Be in a straight line between structures, such as manholes, inlets, and junction chambers, for all pipelines 30 inches in diameter and smaller.
• Make angular changes in alignment by a uniform curve between two tangents for pipelines 33 inches in diameter and larger, or by the use of hydraulically efficient junctions approved by the city.
Preliminary selection of storm drain conduits should be based upon providing a velocity when flowing full of between three and 15 feet per second. The minimum acceptable conduit diameter is 12 inches except for storm drains located beneath roadways, which shall be at least 15 inches in diameter. Conduit capacity shall be determined by the Manning Equation, using the appropriate "n" values listed in Table 2.
GUTTER AND STREET CAPACITIES.
The depth of flow in gutters during the minimum design storm shall be limited to that depth that will produce a 10′-0″ spread of flow onto the pavement of a new residential street having a two percent straight crown.
Gutter flows equal to curb depth will be allowed in residential areas for storms of greater magnitude than the minimum design frequency storm.
The following criteria shall be used for establishing the maximum allowable depth of gutter flow in other than residential streets and in older parabolic crowned residential streets.
Type of Street Minimum Width of Noninundated Pavement
(feet)Residential 10 Collector 10 Arterial 20 Figures 2 and 3 show the relationship for quantity of flow, Q, in cfs; velocity, V, in fps; width of water in street in feet; and street slope in percent for a street section with straight two percent crown. It was derived by using Manning's Formula and the continuity equation.
STORMWATER INLETS
Inlet location and design should be given careful consideration. Spacing and location shall be such that the depth of flow in the gutter is limited as discussed in the section on gutter and street capacities.
Figures 7, 8, and 9 are included to aid in determining inlet capacities of the various types of inlets. The figures are used for sizing curb-opening inlets on a continuous grade and at a low point in grade. Figures 7 and 8 apply only to curb or side opening inlets on continuous grades. The capacity of the inlet depends upon the length of opening and the depth of flow at the opening. This depth, in turn, depends upon the amount of depression of the flow line at the inlet and cross slope, longitudinal slope, and roughness of the gutter.
To use Figures 7 and 8, the following information must be known:
Length (L) of inlet opening.
Design flow in the gutter. Any carryover from a previous inlet must be included.
Longitudinal street grade in percent.
The procedure for using Figure 7 is as follows:
Enter Figure 7 with the design flow in the gutter and the street grade at the point under consideration and determine if the design flow exceeds the capacity of the gutter at the permissible spread. If the gutter capacity is exceeded, additional upstream inlets are required.
If the design flow does not exceed the gutter capacity, determine the length of inlet, L, required to intercept 100 percent of the gutter flow or determine the amount of by-pass flow if a smaller inlet is selected. By-pass flow must not be allowed to enter arterial streets and must be intercepted by downstream inlets without the downstream flow exceeding the maximum for the appropriate street classifications set forth herein.
Example: Design gutter flow - 7.0 cfs.
Street Grade - 4 percent.
From Figure 7 note that on a four percent street grade, a four-foot type A inlet without deflectors will intercept 6.0 cfs and that 1.0 cfs will bypass downstream. At the given street grade, the eight-foot inlet will intercept 100 percent of the design flow.
The procedure for using Figure 8 is as follows:
Use of Figure 8 is similar to that of Figure 7. Note that the 4′-0″ Type A curb inlet with deflectors will satisfy most flows, with reasonable bypassing, when located to maintain the maximum ten-foot flow spread. The 8′-0″ inlet would be needed approaching intersection with arterial streets or at other points where a complete interception of flow is required.
The nomograph on Figure 9 solves inlet capacity problems under the following conditions:
The curb-opening inlet is located at a low (no grade) point.
All flow coming to the inlet must eventually enter the inlet and will pond until sufficient head is built up so the outflow through the inlet will equal the peak inflow from the gutters.
For example:
Enter the nomograph with any two of the three values h, Q/L, H/h and read the third in which:
h = Total height of opening in feet.
L = Total length of opening in feet.
H = Depth of water at the entrance in feet. (Depth of water, H shall not exceed nine inches, 0.75 feet, for Type "A" inlets on streets with two percent straight crown).
Q = Total peak rate of flow to the inlet in cfs.
Normally, Q, L, and h will be known and the nomograph can be used to determine the depth of water, H, at the inlet. The spread of the water on the street will depend upon the cross slope of the pavement.
MANHOLES AND JUNCTION STRUCTURES.
Manholes and junction structures provide access to a drain for cleaning and act as a junction box for tributary drains. Therefore, a manhole is required wherever a drain changes size or slope, at an abrupt change in alignment, where tributary drains join a main line, and at intervals of not more than 500 feet if the conduit is too small for a man to enter. The designer should check the head losses through manholes and junction structures by using the charts provided in Figure 10, since the losses can be quite significant.
The curves are from "Water Supply and Pollution Control" by Clark, Viessman and Hammer, second edition. They were developed for surcharged pipes entering rectangular structures, but may be applied to manholes as pictures on the curves. The "A" curve is used to find entrance and exit losses; the "B" curve is to evaluate the head loss due to increased velocity in the downstream direction. The loss is defined as the difference between the head losses for downstream and upstream pipe (V h-2 —V h-1 ). In cases where the greatest velocity upstream, the difference will be negative and may be applied to offset other losses in the structure. The "C" loss results from change of direction in a manhole. The "D" loss is related to the effect produced by the entrance of secondary flows into the structure.
CLOSED CONDUIT FOR STORM DRAINS
Storm drainpipe shall generally be reinforced concrete. Bituminous coated corrugated metal pipe may be used for culverts and drains under 200 feet where specifically permitted by the City. Pipe should be sized for the amount of water calculated to be in the pipe as determined by inlet analysis under design conditions. The pipe is assumed to flow full under conditions of steady, uniform flow. The minimum design velocity shall be 3 feet per second. Pipe sizes 30 inches in diameter or less shall not decrease in the downstream direction even though increased slope may provide adequate capacity in the smaller pipe. The top of opening of inflow pipes to manholes or inlets shall be at the same elevation or higher than the top of the outflow pipes. Pipes may be sized by using the nomograph in Figure 5 or by calculation using Table 5.
Rectangular or square concrete box structures with one or more barrels may also be used as storm drains. In areas where this type of structure appears feasible, comparative cost studies with prefabricated or preconstructed sections should be made to determine which is most economical. The possibility of using the top slab as a driving or walking surface should also be investigated. Capacities for structures of this nature should be determined using the Manning Formula.
OPEN CHANNELS
Open channels for storm drainage may be either natural or man-made channels. Closed conduits flowing less than full can be considered to be open channels. Wherever possible, a channel should be left in its natural state. Normally, a man-made channel, lined or unlined, transports water faster than a natural channel; the increased velocity reduces the time of concentration and increases the intensity of rainfall to be used in the design. This, in turn, increases the design peak runoff for the area and could result in higher costs for structures throughout the drainage area.
Flow in open channels should be analyzed by using the Manning Equation (Figure 6) and the continuity equation. The depth at which uniform flow occurs in an open channel is the normal depth. For a channel cross section with a specified discharge, Q, uniform flow may occur at critical depth, at less than critical depth, or at more than critical depth depending on the channel slope. Flow at or near critical depth, d c , is highly unstable and channel sections giving depth of flow near the critical depth should be avoided. Subcritical velocity will prevail at normal depths greater than the critical depth and will occur on mild slopes. Supercritical velocity will prevail at normal depths less than the critical depth and will occur on steep slopes.
Critical flow is characterized by a Froude number, F, equal to unity. If F is less than 1.0, the flow is subcritical and if F is more than 1.0, the flow is supercritical. The Froude number, F, is defined as:
F = V √gd m in which:
V = velocity in feet per second.
g = gravitational constant, 32.2 feet per second per second.
d m = A/b w
where:
b w = width of water surface.
A = cross-sectional area of flow.
For a rectangular channel, d m = d.
Flow that passes from supercritical to subcritical may result in a hydraulic jump and should be investigated in areas where problems could result.
It is rare that uniform flow will occur in all reaches of a channel. There will normally be interconnected reaches of uniform and nonuniform flow. The determination of water surface profiles for a given discharge in the area of nonuniform flow may be necessary to insure against extensive property damage. Computations should begin at a known point and extend upstream for subcritical flow and downstream for supercritical flow. Standard flood profiles have been developed by the Corps of Engineers, U.S. Army for the Missouri River. Studies of development in the lower portions of drainage basins intersected by these streams should include the computation of backwater curves for the design storm using the 100-year flood elevations at the intersections as the starting point.
To insure against costly errors in the design of conduits, it is good practice to use the energy grade line as a basis for calculation, rather than the flow line.
There is an abundance of reference material available concerning the various problems that can be encountered in the backwater analysis of open channels. Since the possible variations in conditions make each problem unique, this manual does not discuss specific solutions.
(Ord. No. 4119, § 1(Exh. A), 6-16-08)