Gutter Spread & Inlet Interception Tool

Gutter Spread & Inlet Interception

FHWA HEC-22 Standards

Note: Spread ($T$) depends on flow & geometry. Changing inlet size only affects Interception Efficiency ($E$), not the upstream spread.

1. Hydrology & Flow ($Q$)
Direct Flow Input Rational Method & Spacing
Design Flow ($Q$)		m³/s
-+
Runoff Coeff ($C$)		ratio
Catchment Width ($W_c$)		m
Catchment Length ($L_c$)		m
Time of Conc. ($t_c$)		min
IDF Eq: $I = a / (t_c + b)^c$ Get a, b, c parameters
IDF Param ($a$)		-
IDF Param ($b$)		-
IDF Param ($c$)		-
Calc. Intensity ($I$)	0.0	mm/hr
Design Flow ($Q$)	0.00	m³/s

2. Gutter Geometry
Gutter Shape	Uniform (Triangular) Composite (Depressed)
Longitudinal Slope ($S_L$)		m/m
Cross Slope ($S_x$)		m/m
Gutter Cross Slope ($S_w$)		m/m
Gutter Pan Width ($W_g$)		m
-+
Manning's Roughness ($n$): 0.013 - Smooth Concrete 0.016 - Smooth Asphalt 0.020 - Rough Asphalt
Lane Width limit ($W_L$)		m
Curb Height ($H_c$)		m

3. Inlet Configuration
Inlet Type	No Inlet (Spread Only) Curb-Opening Grate Inlet Slotted Drain
Length ($L$)		m
Grate Width ($W$)		m

Approach Flow $Q$ 0.00 m³/s

Inlet Capacity $Q_i$ 0.00 m³/s

Bypass Flow $Q_b$ 0.00 m³/s

Max Spread $T$ 0.00 m

Max Depth $d$ 0.00 m

Efficiency $E$ 0%

Cross-Section (Looking Downstream)

Plan View & Water Spread (Top-Down)

Performance Rating Curve

Gutter Hydraulic Properties
Flow Area ($A$)	0.00	m²
Gutter Velocity ($V$)	0.00	m/s
Equiv. Slope ($S_e$)	-	m/m

Inlet Hydraulic Properties
Frontal Flow Ratio ($E_0$)	-	ratio
Side Flow Eff. ($R_s$)	-	ratio
Req'd Length for 100% Int. ($L_T$)	-	m

Gutter Spacing Analysis (Max Capacity)
Max Capacity ($Q_{max}$) at $W_L$	0.00	m³/s
Max Inlet Spacing ($L_{max}$)	-	m

Hydraulic Theory: HEC-22 Methodology

A. Gutter Flow & Advanced Shapes

Flow is calculated using an integrated Manning's equation for shallow triangles. The engine supports standard Uniform and Composite (Depressed) sections via iterative depth solvers.

• Uniform: $Q = \frac{K_u}{n} S_x^{1.67} S_L^{0.5} T^{2.67}$
• Composite: Calculates $Q_{depression} + Q_{road}$ requiring an equivalent slope ($S_e$).

B. Curb-Opening & Slotted Inlets

The length of a curb-opening inlet required to intercept 100% of the flow ($L_T$) uses the equivalent cross slope ($S_e$) for compound sections.

$$L_T = K_c \cdot Q^{0.42} \cdot S_L^{0.3} \cdot \left( \frac{1}{n \cdot S_e} \right)^{0.6}$$

If actual length $L < L_T$, efficiency $E = 1 - (1 - L/L_T)^{1.8}$.

C. Grate Inlets

Grate inlets intercept water through the front (frontal flow) and along the sides. The frontal flow ratio ($E_o$) calculates the specific flow directly within the grate's width ($W$) considering the chosen geometry.

$$E_o = \frac{Q_W}{Q} \quad , \quad R_s = \frac{1}{1 + \frac{0.15 V^2}{S_e \cdot L}}$$

Total efficiency is the sum of intercepted frontal and side flows: $E = R_f E_o + R_s (1 - E_o)$.

D. Hydrology & Gutter Spacing

The Rational Method correlates rainfall intensity ($I$) to peak discharge ($Q$). The maximum inlet spacing ($L_{max}$) is the road length that generates the exact capacity ($Q_{max}$) of the configured shape at the Spread Limit ($W_L$).

$$Q = C \cdot I \cdot A \quad , \quad L_{max} = \frac{A_{max}}{W_c}$$

E. Flow Duration & Hydrographs

Standard FHWA HEC-22 gutter spread criteria are evaluated using steady-state peak flow conditions rather than dynamic hydrographs. Here's why:

• Peak Spread Governs: Highway safety is dictated by the maximum spread ($T$) during the peak 5-10 minutes of the storm. If the roadway is safe during the peak, the receding limb of the storm is also safe.
• Continuous Grades Drain Quickly: On sloped roads, water flows continuously to the inlet without ponding. The duration water stays on the road after rainfall ceases is roughly equal to the Time of Concentration ($t_c$).
• When Hydrographs Matter: Dynamic routing (hydrographs) is typically reserved for Sag locations (where water ponds at the bottom of a hill and drawdown/drainage time must be calculated) or for sizing downstream detention basins.

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Table 1: Manning's n for Street & Pavement

Type of Gutter or Pavement	n Value
Concrete gutter, troweled finish	0.012
Asphalt pavement: Smooth texture	0.013
Asphalt pavement: Rough texture	0.016
Concrete gutter with asphalt pavement: Smooth	0.013
Concrete gutter with asphalt pavement: Rough	0.015

Table 2: Suggested Allowable Spread

Road Classification	Design Spread limit
High Volume / Freeway	No encroachment on travel lane
Collector / Arterial (< 45 mph)	1/2 of outer driving lane
Local Streets	1/2 lane (or up to crown)

Table 3: Recommended Cross Slopes ($S_x, S_w$)

Surface	Recommended Range
Pavement (Driving Lanes)	1.5% to 2.0% (0.015 - 0.02)
Paved Shoulders	2.0% to 6.0% (0.02 - 0.06)
Composite Gutter Pan	Often 5.0% to 8.3% (0.05 - 0.08)

Table 4: Recommended Longitudinal Slopes ($S_L$)

Condition	Recommended Limit
Absolute Minimum	0.3% (0.003)
Preferred Minimum	0.5% (0.005)
Maximum	Dictated by road class (5%-8%)

Table 5: Typical Runoff Coefficients ($C$)

Surface Type	Recommended Range
Asphalt & Concrete Pavement	0.70 - 0.95
Roofs	0.75 - 0.95
Lawns, Sandy Soil (2% - 7% slope)	0.10 - 0.15

* Source: Typical Urban Hydrology (Rational Method)

Table 6: Time of Concentration ($t_c$) Limits

Design Condition	Recommendation
Absolute Minimum limit	5 minutes
Typical Urban Minimum	5 to 10 minutes

Using $t_c < 5$ mins results in unreasonably high intensity ($I$) estimates.

Table 7: Typical Standard Inlet Dimensions

Inlet Type	Typical Width ($W$)	Typical Length ($L$)
Grate Inlets	0.6 m (2 ft) or 0.9 m (3 ft)	0.6 m (2 ft), 0.9 m (3 ft), or 1.2 m (4 ft)
Curb-Opening	N/A (Recessed in curb)	1.5 m (5 ft), 3.0 m (10 ft), or 4.5 m (15 ft)
Slotted Drains	45 mm (1.75 in) wide slot	Variable (e.g., 3.0 m to 6.0 m pipe sections)

* Note: Actual available standard sizes vary significantly depending on local State/City DOT specifications and manufacturer castings.

References & Resources

• FHWA HEC-22 (3rd Edition, 2009): Urban Drainage Design Manual. Chapter 3: Hydrology & Chapter 4: Gutter Flow and Inlets.
• AASHTO (2014): Drainage Manual, 1st Edition.
• CivilSheets: Rainfall Frequency Analysis & IDF Curves (for deriving local $a, b, c$ intensity parameters).
• Local DOT / Municipal Drainage Manuals: Always consult local governing authorities for specific allowable spread limits, standard inlet casting dimensions, and regional IDF curve data.

Version 1.0

CivilSheets by Sothearith Seak

Highway Drainage Inlet Types (Cross-Section View)

Grate Inlet

Water drops vertically through a flush horizontal grate directly into a catch basin. Performs well on continuous grades but susceptible to clogging.

Curb-Opening Inlet

Water flows horizontally into a vertical opening in the curb face. Highly resistant to debris clogging. Best for flat slopes or sag locations.

Slotted Drain

A pipe cut along its length with a narrow grate. Intercepts sheet flow over a wide continuous area, especially where curb space is tight.

Roadway Geometry Design Criteria

Cross Slope ($S_x$)

Surface	Recommended Range
Pavement (Driving Lanes)	1.5% to 2.0% (0.015 - 0.02)
Paved Shoulders	2.0% to 6.0% (0.02 - 0.06)
Composite Gutter Sections	Often 5.0% to 8.3%

* Steeper cross slopes increase gutter capacity but can cause steering drift and be hazardous if icy.

Longitudinal Slope ($S_L$)

Condition	Recommended Limit
Absolute Minimum	0.3% (0.003)
Preferred Minimum	0.5% (0.005)
Maximum	Dictated by road class (5%-8%)

* Flat slopes (< 0.3%) lead to poor drainage and ponding. Avoid placing inlets exactly in sag vertical curves without flanking inlets.

Allowable Spread ($T$) & Depth ($d$) Limits

The design spread is usually limited by the road's classification, design speed, and traffic volume. Depth at the curb ($d$) should generally not exceed the curb height ($H_c$) to prevent water from overtopping the sidewalk.

• High-Speed / High-Volume: No encroachment on travel lanes.
• Arterials / Collectors: 1/2 of outer driving lane.
• Local Streets: Up to 1/2 of the street (or to the crown).

Hydrology Design Criteria (Rational Method)

Time of Concentration ($t_c$)

The time required for runoff to travel from the hydraulically most distant point of the catchment to the inlet.

Condition	Recommendation
Absolute Minimum	5 minutes
Typical Urban Min.	5 to 10 minutes

* Note: Using a $t_c$ of less than 5 minutes will result in unrealistically high rainfall intensities and oversized infrastructure.

Runoff Coefficient ($C$)

Surface Type	Range
Asphalt & Concrete	0.70 - 0.95
Roofs	0.75 - 0.95
Lawns / Grass	0.05 - 0.35

* Represents the fraction of rainfall that becomes surface runoff.