Gutter and Inlet Calculator

Gutter flow

Q = (0.56/n) S_x^1.67 S_L^0.5 T^2.67 for a uniform cross slope (HEC-22 Eq. 5.2, Izzard). A depressed gutter is split into the depressed width and the rest, with the frontal flow ratio E_o of Eq. 5.7; its spread is found by iteration, as in HEC-22 Example 5.2.

Curb openings and slots on grade

Length for 100% interception L_T = 0.6 Q^0.42 S_L^0.3 (1/(n S_e))^0.6 and efficiency E = 1 − (1 − L/L_T)^1.8 (Eqs. 7.10–7.13). A local or continuous depression raises the equivalent cross slope S_e = S_x + (a/W) E_o. HEC-22 notes that FHWA research finds Eq. 7.10 can underestimate L_T for long openings.

Grates on grade

Frontal flow is caught until the gutter velocity passes the splash-over velocity V_o: R_f = 1 − 0.09 (V − V_o), at most 1; side flow R_s = 1/(1 + 0.15 V^1.8/(S_x L^2.3)) (Eqs. 7.3–7.9). V_o is read from HEC-22 Figure 7.8, which is published as a chart only, so you enter it.

Sweeper combination

The curb opening ahead of the grate intercepts first; the grate takes part of what passes (HEC-22 Example 7.4). The curb opening beside the grate is neglected.

A run of inlets

The procedure of HEC-22 section 7.4.2: starting from the high point, each inlet takes the flow added since the last inlet plus the bypass from it, and the spread and depth approaching it are compared with your allowable spread and curb height. Added flow comes from the Rational Method (one C, one intensity and a tributary width, as in HEC-22 Example 7.9) or from values you enter. The run can end at a sag inlet, which must take everything that reaches it.

Clogging scenarios

On the grade, an allowance lowers each inlet’s interception by the percent you enter, and the extra bypass moves down the run; HEC-22 section 7.1.3 leaves this to local experience. In a sag, clogging takes away a grate’s perimeter and open area, or an opening’s length (HEC-22 section 7.3). Run it at 0 percent and at your agency’s value and compare.

Inlets in a sag

Weir and orifice equations 7.14–7.21 with the HEC-22 coefficients (grate C_w 0.37 on the perimeter L + 2W and C_o 0.67 on the open area from Table 7.5; curb opening C_w 0.37, or 0.29 on L + 1.8W when depressed and 12 ft long or less (longer openings use the 0.37 form, as HEC-22 advises), and C_o 0.67; slot C_w 0.31, C_o 0.8), g = 32.2 ft/s². Between weir and orifice flow the larger depth is used, as HEC-22 recommends. With an allowable spread, the flow the inlet passes at that spread is also given.

The calculator is tested against the cases below every time its code changes.

Compared with FHWA HEC-22 (4th ed.) Examples 5.1, 5.2, 7.1, 7.2, 7.3, 7.4, 7.6, 7.7 and 7.9 to the precision they print; other runs of inlets are checked by hand calculations. Examples 7.5, 7.8 and the depressed case of 7.6 are not used: their printed results do not follow from their own inputs (listed in the fixture file).

• A run has one longitudinal slope, one gutter section and one inlet type on the grade. It computes the inlets you place and does not choose their spacing; try spacings and compare the spread. Storm drain hydraulics are not computed.
• The route of water past the last inlet, or out of a clogged sag, is not traced. Note it in your report: the flow that bypasses is reported, its path is yours to show. Flanking inlets (HEC-22 section 7.4.3) are not computed.
• Grate splash-over velocity is entered from HEC-22 Figure 7.8. Grates narrower than a depressed gutter, curved and parabolic pavement sections, and median or roadside ditch inlets are not computed.
• Clogging is one percentage for an inlet; HEC-22 section 7.3 notes a real scenario can change the opening area and the perimeter by different amounts. Your agency sets the clogging allowance and the allowable spread. The TxDOT HDM states its allowable ponded width as a fraction of the outer lane by road class (Chapter 10 §4), not as a table, so there is no TxDOT preset; enter the width for your section.

What spread is allowed?

Your city, county or TxDOT criteria set it by road class and design frequency. The TxDOT HDM (Chapter 10 §4) limits ponding to half the outer lane on interstate and controlled-access main lanes, to the outer lane on major highways with two or more lanes each way, and on minor highways to a width and depth that leave one lane per direction passable; depth never exceeds the curb height. Enter the resulting width as the allowable spread and the calculator flags a wider spread; in a sag it also gives the flow the inlet passes at that spread.

How do I check inlet spacing down a grade?

Choose A run of inlets, enter the gutter, the inlet and the spacing, and read the table: the flow, spread and bypass at each inlet, with the bypass carried to the next. If the spread passes your allowable, move the inlets closer or change them and run it again, as HEC-22 section 7.4.2 describes. The calculator does not choose the spacing for you.

Why does a longer grate add so little?

Grates catch nearly all the frontal flow over their width; length only adds side flow, which is small at high velocity. Curb openings and slots gain much more from length (HEC-22 section 7.2.5).

Why use the larger depth in a sag?

Between weir and orifice flow it is not clear which governs, so HEC-22 computes both and uses the larger depth (the smaller capacity). A sag inlet must take all the flow that reaches it.

Can I attach the calculation sheet to a design report?

You can print it and include it as a supporting calculation that you have checked. It is a calculation aid, not a sealed engineering document: the engineer of record reviews it and is responsible for the design, and the criteria of the reviewing agency decide which methods and values apply.