C.4 - Inlets.
C.4.1
General.
A.
The primary purpose of storm drain inlets is to intercept excess surface runoff and deposit it in a drainage system, thus reducing the possibility of surface flooding.
B.
The most common location for inlets is in streets which collect and channelize surface flow, making it convenient to intercept. Because the primary purpose of streets is to carry vehicular traffic, inlets must be designed so as not to conflict with that purpose.
C.
The following guidelines shall be used in the design of inlets to be located in streets:
1.
Grated curb inlets are discouraged from use due to their increased tendency to clog and problems with replacement. In all instances where a curb inlet can be used in lieu of a grated curb inlet, it shall be required unless approval is given from the city engineer.
2.
Minimum transition for recessed inlets shall be ten (10) feet.
3.
All curb inlets (whether in a sump or on grade) incorporate a standard five-inch depression. Unless otherwise approved in writing by city engineer, all curb inlets shall be a minimum of ten (10) feet in length.
4.
When recessed inlets are used, they shall not decrease the width of the sidewalk. Also, it should be noted that the use of recessed inlets must be approved by the city engineer for all streets.
5.
Design and location of inlets shall take into consideration pedestrian and bicycle traffic. In particular, grate inlets shall be designed to assure safe passage of bicycles.
6.
Inlet design and location must be compatible with the criteria established in division 3 of this article.
7.
The use of slotted drains is discouraged except in instances where there is no alternative. If used, the manufacturer's design guidelines should be followed.
C.4.2
Inlet Classifications.
A.
Inlets are classified into two (2) major groups: (1) inlets in sumps where flow contributes from two (2) or more sides (Type S); and (2) inlets on grade (Type G). The following list references the various inlet types (See Figure C-5 through Figure C-11).
Table C-26 - Inlets in Sumps [and on Grade]
Inlets in Sumps (1) Curb opening Type S-1 (2) Grate * Type S-2 (3) Combination (grate and curb opening) * Type S-3 (4) Area without grate Type S-4 Inlets on Grade (1) Curb opening Type G-1 (2) Grate * Type G-2 (3) Combination (grate and curb opening) * Type G-3 Recessed inlets are identified by the suffix (R), i.e.: S-1(R).
* For the flow capacity through the grate inlets, the engineer should check appropriate vendor catalog.
Figure C-5 - Curb Opening Inlet in a Sump (Type S-1)
Figure C-6 - Grate Inlet in a Sump (Type S-2)
Figure C-7 - Combination Inlet in a Sump (Type S-3)
Figure C-8 - Area Inlet Without Grate (Type S-4)
Figure C-9 - Curb Opening, Inlet on Grade (Type G-1)
Figure C-10 - Grate Inlet on Grade (Type G-2)
Figure C-11 - Combination Inlet on Grade (Type G-3)
C.4.3
Storm Inlet Hydraulics.
A.
Inlets in sumps. Inlets in sumps are inlets at low points with gutter flow contributing from two (2) or more sides. The capacity of inlets in sumps must be known in order to determine the depth and width of ponding for a given discharge.
1.
Curb opening inlets (Type S-1) and area inlet without grate (Type S-4). Unsubmerged curb opening inlets (Type S-1) and area inlets without grates (Type S-4) in a sump function as rectangular Weirs with a coefficient of discharge of 3.0. Their capacity shall be based on the following equation:
Q = 3.0h 1.5 L Equation C-27
Where:
Q = Capacity of curb opening inlet or of area inlet, cfs
h = Head at the inlet, feet, = a + Y 0
L = Length of opening through which water enters the inlet, fee
a.
Curb opening inlets and drop inlets in sumps have a tendency to collect debris at their entrances. For this reason, the calculated inlet capacity shall be reduced by ten (10) percent to allow for clogging.
2.
Grate inlets (Type S-2). An area inlet with a grate (Type S-2) in a sump functions as an orifice with a coefficient of discharge of 0.60. Therefore, the orifice equation becomes:
Q = 4.82Ah 0.5 Equation C-28
Where:
Q = Capacity, cfs
h = Depth of flow at inlet, feet
A = Area of grate opening, square feet
a.
Area inlets with grates in sumps have a tendency to clog from debris which becomes trapped by the inlet. For this reason, the calculated inlet capacity of a grate inlet shall be reduced by fifty (50) percent to allow for clogging. Since the clogging problems require maintenance, grate inlets in sumps are discouraged.
3.
Combination inlets (Type S-3).
a.
The capacity of a combination inlet Type S-3 consisting of a grate and curb opening in a sump shall be considered to be the sum of the capacities obtained from Q = 3.0h1.5L Equation C-27 and Q = 4.82Ah0.5 Equation C-28. When the capacity of the gutter is not exceeded, the grate inlet accepts the major portion of the flow.
b.
Combination inlets in sumps have a tendency to clog and collect debris at their entrances. For this reason, the calculated inlet capacities shall be reduced by their respective percentages indicated previously (which are ten (10) percent for a curb opening and fifty (50) percent for grate inlets).
4.
Recessed inlets in sumps. (Type S-1(R), Type S-3(R)). Recessed inlets can be either curb opening or combination types. The clogging factors shall remain the same for recessed or nonrecessed inlets.
B.
Inlets on grade with gutter depression A.
1.
Curb opening inlets on grade (Type G-1). The capacity of a depressed curb inlet should be determined by the appropriate calculations. Because the inlet is on a slope and there is no grate to catch debris, the majority of the debris will be carried downstream; therefore, no reduction for clogging is necessary.
2.
Grate inlets on grade (Type G-2).
a.
The depression of the gutter at a grate inlet decreases the flow past the outside of the grate. The effect is the same as that caused by the depression of a curb inlet.
b.
The bar arrangements for grate inlets greatly affect the efficiency of the inlet. In order to determine the capacity of a grate inlet on grade, the appropriate vendor catalog should be checked.
c.
Grate inlets have a tendency to trap debris such as leaves and paper being carried by the gutter flows. This causes traffic problems from ponding water and requires maintenance. A reduction factor of thirty (30) percent to allow for clogging should be applied.
3.
Combination inlets on grade (Type G-3). Combination inlets (curb opening plus grate) have greater hydraulic capacity than curb opening inlets or grate inlets of the same length. Generally speaking, combination inlets are the most efficient of the three (3) types of inlets on grade presented in this article. The basic difference between a combination inlet and a grate inlet is that the curb opening receives the carry-over flow that passes between the curb and the grate. The reduction factor for clogging of this type of inlet shall be zero (0) percent for the curb opening and thirty-five (35) percent for the grate inlet.
4.
Recessed inlets on grade (Type G-1R, G-3R). Capacities for recessed inlets on grade shall be calculated as 0.75 times the capacity for nonrecessed inlets. The clogging factors shall remain the same for the various types of inlets.
(Ord. No. 2016-61, § 1(Exh. A), 10-13-16)