Street Flooding vs. River Flooding

During Harvey, most of the damage in Kingwood happened from river flooding. Yesterday, it happened from street flooding. What are the respective causes? Differences? Fixes?

*River Grove Park the morning after the street flooding during the previous day and night. The river was still well within its banks and is not predicted to come out.*

River Flooding Overview

River flooding happens when heavy rainfall exceeds the conveyance capacity of a river. In other words, the river comes up and out of its banks. This happened to the San Jacinto during Harvey. The river, normally a couple hundred feet wide, became three to four miles wide, inundating miles from the main current.

River flooding can be caused by heavy rain, upstream snow melt, dam/levee breaks, dam releases, and more. Most commonly, it takes hours to days for water to work its way through a river system and affect people downstream.

River Flooding Remedies

Fixes for river flooding include things like:

Building dams upstream to reduce the flow downstream.
Increasing the conveyance of the river through widening, deepening or dredging
Removing blockages such as the mouth bar on the West Fork of the San Jacinto
Increasing the outflow capacity, for instance by adding gates to dam
Building levees
Diverting water to tunnels.

Street Flooding Overview

Street flooding, on the other hand, is often much more local and happens over shorter periods of time. It is often referred to as flash flooding because it comes up quickly and goes down quickly. That’s what the Lake Houston Area experienced yesterday. The San Jacinto, its tributaries and drainage ditches were and still are well within their banks.

Flash flooding occurs when the rainfall RATE temporarily exceeds the drainage capacity of storm drains, sewers, swales, and ditches that lead to rivers.

Yesterday, we received 1/20th the amount of rainfall that we did during Harvey, but many people reported the water coming up higher. That’s because storm sewers could not handle the intense rainfall that happened during approximately a one hour period.

*Drains on Valley Manor could not handle the sudden surge.*

Streets Designed as Part of Flood Retention System

The streets in Kingwood (and most cities) are actually designed to be part of the flood retention system. When developers excavate streets, they often use the fill to build up homesites. By increasing the elevation difference between street level and your foundation, they reduce the chances that you will flood.

They size the sewers so as not to make drainage ditches overflow. Rainfall rates like we experienced yesterday don’t happen for very long. The storm passes. The water in the street goes down and life returns to normal. You just don’t want to be caught out on the road in your car when it happens. Nor do you want to have your car parked on the street!

Street Flooding Remedies

The Greater Houston Flood Mitigation Consortiums’ report on Harvey contains an extensive discussion of the different types of flooding. Their discussion of street flooding starts on page 14. Page 15 identifies solutions, including:

Communicate the role of streets in the drainage system. Many residents may not be aware that roads are intended to act as a secondary short-term storage system and/or conveyance pathway for water. This can lead to severe property damage and loss of life when residents do not move property out of harm’s way. For example, recent street flooding has caused severe flooding of cars, which are frequently parked in the street. It is also important to note that 12 inches of water can float a sedan and 18-24 inches can float a larger vehicle, creating hazardous road conditions during extreme events. It is critical to develop a strategy to communicate to the public that streets are designed to be a secondary short-term storage system so that people can take action to prevent loss of property and life.
Address debris in streets and waterways. Maintaining clear openings to storm sewer inlets is a great way to help reduce street flooding. Flooding in many neighborhoods can be made worse by debris and floating trashcans that clog inlets. Additional city, county and MUD maintenance, or organized community efforts may be required.
Ensure that developers have sufficient “upstream” information. Land development engineers typically focus on mitigating the runoff generated by their development project, and as such, flows entering onto their site from other sources may not be considered in their design analysis. Examining strategies to share information about upstream conditions with developers during the design analysis phase could help maximize site-based mitigation.
Maximize on-site retention. Green infrastructure may provide a viable alternative for managing stormwater and reducing nuisance flooding through implementing on-site retention or by providing additional in-line storage capacity within the street (examples include Cottage Grove and Bagby Street).
Identify & target high-risk areas. Determining the neighborhoods most vulnerable to flooding from local drainage challenges would be a pivotal step to targeting public education and mitigation strategies.

For More Information

The flooding yesterday happened so quickly that it scared people. For many, it brought back vividly the trauma of Harvey and the pain that followed. One person even died when her car ran into a downed tree that fell across Kingwood Drive.

The most damage happened, not to homes, but to vehicles caught in high water on flooded streets – streets DESIGNED to hold water. There’s a simple answer to that. Unfortunately many people did not get the warnings that could have kept them off the streets.

For more information on different types of flooding, see the first part of this FloodWarn Training Seminar that Katie Landry-Guyton of the National Weather Service presented last year at Kingwood College.

Posted by Bob Rehak on 5/4/19

613 Days since Hurricane Harvey