Curb inlets (Side entry gullies) play a crucial role in urban drainage systems by capturing stormwater runoff from streets and directing it into underground stormwater systems. The Hydraulic Engineering Circular No. 22 (HEC-22), published by the Federal Highway Administration (FHWA), provides comprehensive guidelines for designing and analyzing these drainage structures. This Curb Inlet Calculation Summary explores the step-by-step process for calculating curb inlets according to HEC-22.
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Storm drain inlets are used to collect runoff and discharge it to an underground storm drainage system. Inlets are typically located in gutter sections, paved medians, and roadside and median ditches.
Grate inlets consist of an opening in the gutter or ditch covered by a grate. Curb-opening inlets are vertical openings in the curb covered by a top slab. Slotted inlets consist of a pipe cut along the longitudinal axis with bars perpendicular to the opening to maintain the slotted opening. Combination inlets consist of both a curb-opening inlet and a grate inlet placed in a side-by-side configuration, but the curb opening may be located in part upstream of the grate.
1. Overview of Curb Inlets
Curb-opening inlets are most effective on flatter slopes, in sags, and with flows which typically carry significant amounts of floating debris. The interception capacity of curb-opening inlets decreases as the gutter grade steepens. Consequently, the use of curb-opening inlets is recommended in sags and on grades less than 3%. Of course, they are bicycle safe as well.
Understanding curb inlet hydraulics is essential for effective stormwater design and flood prevention.
2. Key Parameters for Curb Inlet Design
Several factors influence curb inlet performance and capacity, including:
- Inlet Length (L): The length of the curb opening.
- Flow Depth (d): Depth of water approaching the inlet.
- Flow Spread (T): The width of water spread on the pavement.
- Cross Slope (Sx): The slope of the road surface.
- Longitudinal Slope (S): The slope along the road length.
- Grate Type: If a combination inlet is used.
3. Types of Flow Approaching Curb Inlets
There are three main types of flow conditions considered for curb inlet design:
- On-Grade Flow: Flow parallel to the curb.
- Sag Flow: Flow that accumulates at a low point or sag.
- Combination Inlet: A combination of curb and grate inlet.
HEC-22 provides equations for both conditions to ensure the inlet is adequately sized to handle design storm events.
4. Hydraulic Analysis of Curb Inlets
A. Curb Inlets on Grade
Curb-opening inlets are effective in the drainage of highway pavements where flow depth at the curb is sufficient for the inlet to perform efficiently. Curb openings are less susceptible to clogging and offer little interference to traffic operation. They are a viable alternative to grates on flatter grades where grates would be in traffic lanes or would be hazardous for pedestrians or bicyclists.
Curb opening heights vary in dimension, however, a typical maximum height is approximately 100 to 150 mm (4 to 6 in).
The full design is mentioned in the Curb Inlet Calculation Summary According to HEC-22.
5. Design Steps
- Determine Flow Rate (Q): Calculate the peak flow using rational or other hydrologic methods.
- Identify Roadway Geometry: Measure cross slope, longitudinal slope, and gutter geometry.
- Select Inlet Location: Determine whether the inlet is on grade or in sag.
- Compute Required Inlet Length (L_c): Use the equations provided for the type of flow.
- Evaluate Capture Efficiency (η): Compare the required length to the actual length.
- Iterate as Necessary: Adjust inlet length or location to meet efficiency targets.
6. Tools for Curb Inlet Design
Several software tools implement HEC-22 calculations:
- My own Curb Inlet Calculations excel sheets.
- StormCAD and sewerGEMS
- Hydraflow Storm Sewers
- FlowMaster
- SWMM (Storm Water Management Model)
Download Curb Inlet Calculation Summary According to HEC-22
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Conclusion
Proper curb inlet design ensures efficient stormwater management and minimizes road flooding. By following HEC-22 guidelines, engineers can optimize curb inlet performance for safety and reliability. Accurate flow rate estimation, understanding roadway geometry, and choosing the right inlet type are critical steps in this process.
