Stormwater management plays a crucial role in urban drainage systems to prevent flooding and ensure efficient water flow. One of the essential components of this system is the design and analysis of top entry gullies. The Hydraulic Engineering Circular No. 22 (HEC-22) published by the Federal Highway Administration (FHWA) provides detailed guidelines for hydraulic design methods, including inlet spacing, flow interception, and capacity of entry gullies. This guide will walk you through the process of calculating top entry gullies according to HEC-22 standards.
Top Entry Gullies Calculations According to HEC 22
The hydraulic capacity of a storm drain inlet depends upon its geometry as well as the characteristics of the gutter flow. Inlet capacity governs both the rate of water removal from the gutter and the amount of water that can enter the storm drainage system. Inadequate inlet capacity or poor inlet location may cause flooding on the roadway resulting in a hazard to the traveling public.
1. Understanding Top Entry Gullies
Top entry gullies are designed to capture surface water and direct it into underground drainage systems. Proper sizing and spacing are critical to ensure the gullies can handle expected flow rates without causing surface flooding or bypass flow.
2. Key Parameters in Gully Design
When designing top entry gullies, HEC-22 highlights several key parameters:
- Drainage Area (A): The surface area contributing to runoff, usually measured in acres or hectares.
- Runoff Coefficient (C): A factor representing the imperviousness of the drainage area.
- Rainfall Intensity (I): The rate of rainfall for a specific return period and duration, typically measured in inches per hour (in/hr) or millimeters per hour (mm/hr).
- Gutter Flow (Q): The total flow in the street gutter approaching the gully.
- Cross Slope (Sx): The slope perpendicular to the flow direction.
- Longitudinal Slope (S): The slope in the direction of flow.
- Curb Opening Length (L): The length of the inlet opening along the curb.
3. Step-by-Step Calculation for Top Entry Gullies
Step 1: Estimate Runoff
The Rational Method is typically used to calculate the peak runoff (Q):
Q = C × I × A
Where:
- Q is the peak runoff rate (cubic feet per second, cfs).
- C is the runoff coefficient.
- I is the rainfall intensity.
- A is the drainage area.
Step 2: Determine Gutter Flow Characteristics
For a street or gutter, flow capacity is determined using Manning’s equation:
Where:
- n is Manning’s roughness coefficient.
- A is the cross-sectional area of flow.
- R is the hydraulic radius (area/wetted perimeter).
- S is the slope.
Step 3: Calculate Inlet Interception Capacity
Inlet interception capacity, Qi, is the flow intercepted by an inlet under a given set of conditions. The efficiency of an inlet, E, is the percent of total flow that the inlet will intercept for those conditions. The efficiency of an inlet changes with changes in cross slope, longitudinal slope, total gutter flow, and, to a lesser extent, pavement roughness. In mathematical form, efficiency, E, is defined by the following equation:
E=Qi/Q
Where:
- E is the efficiency of the inlet.
- Qi is the intercepted flow.
- Q is the total approaching flow.
Step 4: Bypass Flow Calculation
Flow that is not intercepted by an inlet is termed carryover or bypass and is defined as follows:
Qb=Q−Qi
Where:
- Qb is the bypass flow.
The interception capacity of all inlet configurations increases with increasing flow rates, and inlet efficiency generally decreases with increasing flow rates. Factors affecting gutter flow also affect inlet interception capacity. The depth of water next to the curb is the major factor in the interception capacity of both grate inlets and curb-opening inlets. The interception capacity of a grate inlet depends on the amount of water flowing over the grate, the size and configuration of the grate and the velocity of flow in the gutter. The efficiency of a grate is dependent on the same factors and total flow in the gutter.
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Conclusion
HEC 22 provides robust methods for the hydraulic design of top entry gullies, focusing on efficiency, capacity, and safety. Accurate calculations ensure effective stormwater management, reducing urban flooding risks and improving infrastructure longevity. Always refer to the latest HEC 22 manual for detailed coefficients and design tables.
