Note that the inlet opening is used much more efficiently than the inlet illustrated in Figure Inlet Designed with No Carryover. Inlet Designed with Carryover. Bypass flow is normally captured at some other location. The gutter between the two points must accommodate the additional flow.
Bypass flow at these locations would be crossed by vehicular traffic and may pose a traffic hazard. Bypass flow is also not recommended to be allowed to flow where there is no outfall or designated capture point. The design of on-grade curb opening inlets involves determination of length required for total flow interception, subjective decision about actual length to be provided, and determination of any resulting carryover rate.
Gutter Cross-Section Diagram. Use Equation to calculate conveyance, K W and K 0. Use Equation to calculate the area of cross section in the depressed gutter section. Use Equation to calculate the wetted perimeter in the depressed gutter section.
Use Equation to calculate the area of cross section of the gutter section beyond the depression. Use Equation to calculate the wetted perimeter of the gutter section beyond the depression. If no bypass flow is allowed, the inlet length is assigned a nominal dimension of at least L r , which should be an available nominal standard curb opening length. The exact value of L r should not be used if doing so requires special details, special drawings, structural design, and costly construction.
If bypass flow is allowed, the inlet length is rounded down to the next available standard nominal curb opening length. Bypass flows usually should not exceed about 0. Greater rates can be troublesome and cause a significant departure from the principles of the Rational Method application.
In all cases, the bypass flow must be accommodated at some other specified point in the storm drain system. The capacity of a curb inlet in a sag depends on the water depth at the curb opening and the height of the curb opening. Two types of inlet grates are shown in the standard plans. The curved vane grate is used in roadway and shoulder applications. The parallel bar grate may be used only in areas outside the roadway and shoulders, such as in grassy medians or other unpaved areas.
See Standard Plan details of inlet grates Std. Plans Plan Curb opening inlets consist of a longitudinal opening located in the face of the curb. Details of the curb opening inlets may be found in Std. The Type T inlet has a minimum length of 2. The Type T inlet has a local depression of 3 in. Curb opening inlets are recommended for use at all low points. In locations where heavy debris is expected, consideration should be given to doubling the calculated length of curb opening inlet to provide a safety factor against clogging.
Only curved vane type grates are used in pavement drainage installations. The capacity of grate inlets is a function of the inlet location. The capacity of grate inlets located on a continuous grade is influenced by the depth and velocity of the approaching water, the length and width of the grate, and the geometric configuration of the grate.
The capacity of grate inlets located at low points in grade is influenced by the perimeter of the grate or by the clear waterway area of the grate. The capacity at low points in grade is independent of geometric configuration of the grate.
Grate inlets provide an effective means for drainage of highway pavements when clogging by debris is not expected to be a problem. The capacity of the grate depends to a large degree upon how large a portion of the total flow passes directly over the grate. The portion of the total flow which passes directly over the grate is known as frontal flow. The remainder, which flows along the side of the grate, is known as side flow. The quantities of frontal flow and side flow can be determined through application of the modified form of Manning's equation given above to appropriate triangular sub-sections of the pavement and gutter cross-section.
Grate inlets will intercept all of the gutter flow passing directly over the grate if the grate is sufficiently long and the flow velocity is low. When the grate is too short or the flow velocity is too high, splash-over occurs, and the grate does not intercept all of the frontal flow.
Some portion of the side flow will be intercepted, depending on the length of the grate, the flow velocity and the gutter cross-slope.
The velocity at which water begins to splash over the grate depends on the length and geometric configuration of the grate. If the flow velocity exceeds the splash-over velocity, the ratio of frontal flow intercepted to total frontal flow, R f , can be determined using the equation:.
The ratio of side flow intercepted to total side flow, R s , can be determined using the equation:. If the total flow Q is considered to be the sum of frontal flow Q f and side flow Q s , the amount of flow intercepted by a grate inlet on grade Q i can be determined using the equation:.
A grate type drop inlet located at a low point in grade will behave either as a weir or as an orifice, dependent upon the depth of water at the grate. The capacity is independent of the geometric configuration of the grate. For depths of water less than 0. Tags: Drainage Design. Digital technologies have the chance to change the way the world uses water. Jay is a Content Manager at Innovyze and writes about how the top challenges faced by water management professionals can be overcome using smart analytics.
Back to blog home August 21, Jay Nelson. Share Facebook Linkedin Twitter. Assessing what really happens when it rains One procedure to follow for accurately designing inlet capacity and gutter spread is to leverage HEC22 methodology to determine an expected inlet capacity and forecast on how well a gutter will convey flow while minimizing spread into roadways. Those calculations are described here. Cross-section of Inlet Spread. Gutter spreads for inlets on a continuous gutter slope are based solely on the gutter properties basically an open channel are not affected by the inlet itself.
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