Release Rates May Be Looked At in Future
March 13, 2016
It was a dark and stormy night. Thunder boomed and heavy rain pelted the roof as a small group of us sat nervously in the middle of the rising Addicks Reservoir in far west Houston listening to the district commander of the Army Corps of Engineers tell us Addicks and Barker dams are safe.
Flanking Col. Richard Pannell last Wednesday, March 9, was a panel of officials from the Corps’ Galveston District office, as well as representatives from the Harris County Flood Control District and Granite Construction, the company awarded the $71.9 million contract to replace the leaky conduits in the 70-year-old earthen dams. The audience was a small crowd of about thirty hardy citizens who had braved the heavy traffic and heavy rain and managed to find the Bear Creek Community Center in the middle of a dark, wet forest despite the wrong address in the Corps’ public notice.
Col. Pannell made a thorough and convincing presentation of the history of the dams, the current problems, and what is being done about them. Among other work, the conduits in the two dams will be entirely replaced by new conduits in separate locations. This process will take about three years. He emphasized that the dams were in no immediate danger of failing and explained that the dams are labeled “extremely high risk” because of the potential consequences to Houston, the nation’s fourth most populous city, in the unlikely event that the dams failed.
Dams Operating Normally Despite Leaks
There were several questions about the capacity of the dams and the impact of repairs on the rate of future releases of water impounded behind the dams. The pattern of extended high releases and rapid drops has been criticized for damaging property on the bayou and killing trees and vegetation that control erosion.
After the enormous Memorial Day storm last year, the Corps began releasing impounded stormwater at 3000 cubic feet per second (cfs) for the first time. Normally the release rate is limited to 2000 cfs as measured by the Piney Point gauge. The Corps does not release water from the dams during a rain event, and the high releases began on June 1 and continued for ten days, finally dropping on June 12. The banks were already saturated from the flood, and the damage from the extended high flow was plainly visible up and down the bayou.
The extended high release was necessary because another tropical storm (“Bill”) appeared headed toward the Houston area and the reservoir pools had to be emptied in preparation, explained Steve Fitzgerald, chief engineer for the Harris County Flood Control District and Richard Long, Corps resource manager for the Addicks and Barker dams.
Since then the Corps has resumed limiting releases to the normal rate of 2,000 cfs, said Long.
The release rate is a combination of water from the dams and local runoff. The rate of flow in the bayou solely from runoff during rainfall, when the dam gates are closed, frequently exceeds 2,000 cfs at Piney Point. On May 26, 2015, during the Memorial Day storm, for example, the Piney Point gauge registered more than 7,000 cfs.
Flooding in Buffalo Bayou is the result of the rapid accumulation of runoff from buildings, highways, parking lots, and other paved and impermeable surfaces below the dams. Trees, plants, permeable surfaces, detention ponds, etc. help absorb, deflect, spread out and slow rainwater and prevent flooding.
Flooding of rivers and streams, however, is a natural and necessary phenomenon, vital for replenishing, replanting, and fertilizing the floodplain. Native trees and plants along the waterway help maintain the health of our waters and environment and control erosion, among other benefits. Such riparian vegetation, naturally planted by rivers and streams, including bayous, is generally adapted to being submerged and helps transform sandy banks into soil for growing forests.
Buffalo Bayou flows for fifty-three miles from its source in the Katy prairie, through Barker Reservoir, past private property and public parkland, through downtown Houston and the ship channel into Galveston Bay. Unlike Brays or White Oak bayous, for example, the 18,000-year-old bayou has never been channelized with concrete.
Misconceptions about Dam Operations and Capacity
There has been a public misunderstanding about the impact of the dams’ structural problems on the way the dams are operated, said Long.
There have been no changes in the level of the reservoir pools behind the dams and no changes in the release rates, other than the extraordinary episode following the Memorial Day flood, said Long.
While a 2010 Interim Reservoir Control Action Plan did establish the possibility of “evacuating pools” more quickly by releasing impounded water up to 4,000 cfs, the Corps has never done that, emphasized Long. Flood damage occurs to low-lying homes 6.5 miles downstream from the dams when the flow reaches 4,100 cfs, according to the Interim Plan.
The limit to levels of the reservoir pools set in that Interim Plan corresponds to the pools of record set in 1992, which amounts to about 30-35 percent of the total capacity of the reservoirs. There has been no reduction in capacity, said Long.
Goal Is to be Prepared for the Next Big Storm
Both Barker and Addicks reservoirs are dry reservoirs, essentially almost empty most of the time. They both contain large parks and recreational facilities.
But neither the presence of the recreational facilities nor the dams’ structural problems have anything to do with keeping the pool levels so low, said Long in a later interview.
Water has to be released immediately and quickly from the dams because “we don’t know when that big one is coming,” said Long about a major storm. “It could come next week.” And if the pools were filled to capacity, “we couldn’t get rid of [enough water] fast enough without flooding downstream.”
According to the 2009 Addicks and Barker Dams Master Plan (see page 12) it would take about 34 days to empty Barker Reservoir at a release rate of 1,000 cfs when the reservoir pool level is 93.4 feet, which is about the 30-35 percent capacity limit set in the Interim Plan. Median flow in Buffalo Bayou is generally between 100 and 200 cfs.
Said Long, “Our goal is to be prepared.”
But the issue of how or how fast reservoir water is released into the bayou is something that could be considered in the Section 216 study required by the federal Water Resources Development Act of 2000, said Col. Pannell at the public meeting.
That study is scheduled for completion in 2018, he said.