Addicks De-Silt FAQs

A maintenance project repairs or maintains what already exists.
A capital project expands or improves the current drainage system.

Desilt projects are a type of maintenance that removes accumulated sediment and other material. Erosion of bayous and creeks is a constant, natural process that can be accelerated by heavy rain events and other damage to a channel, such as that caused by a failed outfall pipe. Sediment caused by erosion, in additional to other material that ends up in a channel – such as vegetation and trash – can accumulate over time.

Engineered, man-made channels are typically built with an established/documented depth and width. Let’s call that Original Configuration A. Over time, a portion of that depth and width may be partially impacted with layers of sediment and other material that ends up in drainage channels, such as vegetative debris and trash. Channels can also erode over time. This erosion could be a source of material in a channel. Let’s call that Eroded Configuration B.

While sediment generally tends to shift each time there is a major rain event, causing little impact on flood risk, in extreme cases this accumulated material potentially could reduce “channel conveyance,” which is the channel’s ability to move stormwater. Erosion of the channel side slopes could also potentially impact channel conveyance.

A maintenance project removes the sediment and repairs erosion, restoring channel conveyance and returning the channel to its Original Configuration A. This is potentially better for stormwater conveyance than Eroded Configuration B, but it does not widen or deepen BEYOND the Original (engineered) Configuration A.

In other words: Let’s say a channel is originally designed and built with a 1 percent (100-year) level of service. Accumulated sediment or erosion reduces that level of service to something less than the 1 percent (100-year) level of service. Removing the sediment and repairing the erosion allows the channel to carry more stormwater than it did WITH the sediment or erosion, but it is still no more than the original 1 percent (100-year) level of service.

Maintenance is positive, but it has limits.

Maintenance projects, in general, do not require the same degree of environmental permitting and mitigation as channel widening/deepening/straightening/concrete-lining, because maintenance projects do not alter the channel’s original configuration.

Maintenance can also take place on a non-engineered, natural channel, but that is a much more rigorous, time-consuming process, because of environmental permitting and compensatory mitigation requirements.

The construction contractor has the responsibility to obtain permitted disposal sites for the excavated material.

Following the removal of sediment and other accumulated material, the contractor will return to repair failed outfall pipes, and fix erosion such as sinkholes and wash-outs. Repairs to concrete headwalls and other sections also will be included in this project.

Most Flood Control District projects bring truck and heavy machinery traffic to the construction area. Residents near construction sites will see dump trucks, excavators and other equipment. Portions of individual lanes near construction access points may need to be temporarily closed for short periods, for the safety of workers and motorists. Flag personnel will be employed as needed, to direct traffic.

Desilt projects do not increase flooding risks in any area, either upstream or downstream of the project site. These projects will improve stormwater conveyance by restoring the channel’s capacity to hold and convey stormwater.

The Harris County Flood Control District has always had a year-round maintenance program, financed out of its Operations and Maintenance budget. Because of funding restrictions, maintenance was prioritized and performed where most needed. Now that the Flood Control District has additional funding from the 2018 Bond Program, delayed maintenance is underway in many watersheds.

The Addicks and Barker De-Silt project is the first comprehensive project to remove sediment and repair erosion in all channels in Harris County that drain into the two federal reservoirs where the Flood Control District has property rights and/or licenses. It began construction in 2018 as a pilot project on four Addicks Reservoir channels.

Previous projects following major rain events have addressed spot repairs. Repairs in the Addicks watershed following Harvey in 2017 are described here: www.hcfcd.org/Recovery Other projects over the last decade focused on specific segments of channel to remove silt and repair erosion.

The 2018 Bond Program has provided funding that allows the Flood Control District to accelerate its maintenance program overall, and to work on maintenance that had been deferred in many areas due to a lack of funding. The Addicks and Barker de-silt work is such a project.

In addition, the Flood Control District is investigating the use of sedimentation basins in some projects and locations, such as near the point where streams flow into the Addicks and Barker reservoirs. Sedimentation basins are areas along a channel designed to slow the rate of stormwater conveyance so as to trap sediment and other materials. Stormwater slows and drops its sedimentation load, where it can be more easily removed in a maintenance project.

No, all watersheds in Harris County are at risk of flooding. That is because Harris County is flat, has impermeable clay soils, and is prone to severe rainfall, including tropical storms and hurricanes. While the $2.5 billion 2018 Bond Program will reduce flooding risks across the county – including in the Addicks and Barkers watersheds – it will not eliminate those risks. Any watershed in Harris County can flood, given enough rain over that watershed.

During Hurricane Harvey, operation of the federal reservoirs resulted in stormwater backing up behind the reservoir dams, flooding hundreds of properties outside of government-owned land.

For the Flood Control District, Level of Service refers to the level of storm event that can be carried within the banks of a particular drainage channel, before it overflows and begins to affect nearby structures. One channel might be able to handle a larger 1 percent (100-year) storm within its banks, while another can only handle a smaller 10 percent (10-year storm) without overflowing.

There is no single Level of Service for any watershed. Natural channels were generally created by nature and can change or be modified over time. Engineered drainage channels in Harris County were built at various times and by various entities over time. LOS is not the same across the county, within a watershed, or even along the entire length of the same channel.

In late 2019, the Flood Control District initiated a project to determine the LOS of all channels in our Harris County drainage network. Initial results are expected in 2020.

Additionally, the Flood Control District and FEMA are re-mapping all of the floodplains in Harris County.
www.hcfcd.org/Z-03
www.maapnext.org

The Flood Control District GIS database shows that we have historic property rights and/or licenses on several creeks and channels inside of the government owned Barker Reservoir, as we do on Addicks Reservoir. However, at the current time, neither the Flood Control District nor the Corps can document those property rights as they pertain to Barker Reservoir.

If proper documentation cannot be located, the Flood Control District would need to request a new license to perform maintenance on channels inside the reservoir. This request would follow the National Environmental Protection Act process, which could take from 2-3 years to complete.

For now, the Flood Control District will focus on channels for which we have documented property rights and licenses.

The Corps owns, operates and maintains the reservoirs, including the operation of outlet facilities that control discharges from the reservoirs into Buffalo Bayou. Most Flood Control District easements end at the limits of the federal-owned land.

There are several different reservoir options under discussion, with different goals. Most often, talk of a “third reservoir” refers to a third reservoir on the west side of Harris County that could be added into the existing Addicks and Barker federal reservoir system, which is owned, operated and maintained by the U.S. Army Corps of Engineers.

The purpose of this third reservoir option would be to allow the federal reservoir system to handle more stormwater. It is intended to address the natural overflow from upper Cypress Creek into the Addicks Reservoir watershed. It would not necessarily reduce flooding risks downstream along Cypress Creek because that overflow is already leaving the Cypress Creek watershed. It also would not INCREASE flooding risks along Cypress Creek, because the overflow would continue to leave the watershed. Various study efforts by the Flood Control District and others have examined components that could allow the reservoir systems to handle more stormwater. These studies include the Cypress Creek Overflow Management Study and a 1940 study by the Corps.

The Galveston District of the U.S. Army Corps of Engineers is currently working on the Buffalo Bayou and Tributaries Resiliency Study. This study will consider the third reservoir option, as well as other options such as increasing storage within the existing reservoirs, such as through buyouts of the pool fringe areas.

The Flood Control District is coordinating with the Corps on this study. Also, Bond Project F-56 is intended to study ways to manage the overflow of stormwater from the Cypress Creek watershed to the Addicks Reservoir watershed. It will not have an impact downstream along Cypress Creek.

Additional reservoirs in various locations also have been discussed in neighboring counties and municipalities, most notably in Montgomery County on the San Jacinto River and Spring Creek.

Please visit our website here:   www.hcfcd.org/Recovery