Enable Your Drainage Design Toolbox to Tackle Gutter Spread and Inlet Capacity

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August 21, 2018 | Jay Nelson

Excel spreadsheets, hand calculations, or simple calculator utilities have long been trusted tools used by engineers to forecast road inundation due to runoff and how well storm inlets will work. But are they the most effective tools available? Even more, flow paths can be complex and difficult to accurately forecast with manual tools or calculations.

With the right technology, we can not only simplify the workflow but also improve the accuracy of drainage designs for a better understanding of what really happens to stormwater systems when it rains.

Gutter spread and inlet capacity – what’s the issue?

Two issues that have been especially tricky when designing drainage systems are forecasting gutter spread and inlet capacity. During rainfall, gutters convey flow along streets to storm inlets, which serve the purpose of capturing flow to prevent road inundation. When an inlet reaches capacity, however, it can result in water backing up not only beyond the gutters but across the roadway, making for unsafe driving conditions.

gutter flows diagram

Effectively designing for gutter and roadway spread is an important factor to consider. As runoff is collected in gutters and conveyed to inlets, road inundation needs to be minimized to reduce risks associated with water in traffic lanes or pedestrian walkways.

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.

HEC22 inlet types include: Grate, Curb-Opening Inlet, Combination Inlet, Slotted Drain Inlet. Selecting an inlet type and assessing grate length, width, curb length, height, depression, and clogging % can provide a clearer insight into inlet performance.

sketches of inlet types

Source:  http://www.plainwater.com/water/hec-22-pavement-drainage/

Appropriate use of these inlets can be evaluated for their cost-effectiveness as well as their ability to successfully capture approaching flow before reaching capacity.

Additionally, existing plan data or dynamic analysis can help determine gutter flow and how well stormwater will be conveyed into inlets, as well as what point water spread into roadways becomes a risk.

Manual tools or spreadsheets may provide an educated guess into these factors but lack the agility and power to quickly evaluate multiple design options, their cost-effectiveness, performance, and ability to meet regulatory requirements.

Conversely, by evolving your design toolbox to include advanced technology solutions, this job not only becomes easier and faster but also minimizes the risk of human error. As such, engineers can have a higher level of confidence in their designs and can adopt techniques for a more resilient stormwater drainage design.

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Tags: Drainage Design

About the Authors

Jay Nelson

Jay Nelson

Marketing Content Manager


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.