Thursday, September 8, 2011

Comment on Spillway Discharge

After our post "Spillway Discharge" a reader commented about the river level.

The comment:

Anonymous said...

Can somebody please explain why they want to raise the river to flood level while lowering the Res level to below summer pool? I thought flood control was the main reason for the spillway. So why are we causing it to flood?

A fair question. We passed it along to John Sigman, Executive Director of the Pearl River Valley Water Supply District, his response follows.

There was somewhere between 10 and 12 inches of rain over the drainage basin. It takes 3-6 days for that water to flow to the dam.

Just because there are no rain clouds in the sky does not mean the flood event is over. We are expecting the peak flow on Friday from the rain Sunday night. By lowering the reservoir a little now we can capture some of the peak flow and release a lower maximum flow through the dam.

Trying to pass the flood all in one gulp will cause a much higher release rate. We have to be sure we can pass the flood without allowing the lake to rise too much. When the elevation of the water approached 300 feet MSL, the emergency spillway activates and we have no control of the river.

The lake will start rising rapidly over the next few days as the larger flows from Edinburg and Philadelphia arrive.

Also, this results in less river rise downstream than if the water is released as it arrives. Despite the foregoing description of flood control, the Barnett Reservoir is not a flood control lake.

The reservoir has two missions; water supply and recreation. The reason for the spillway is to pass water through the dam.

This amount of water coming down the basin is going to cause a flood no matter what. Releasing the water in this manner will protect the lake and cause less of a flood downstream.

The Rez News
Ross Barnett Reservoir

2 comments:

Anonymous said...

Flood control is not the main reason for the spillway. The reservoir was not built to control flooding. It was build for water supply for Jackson and recreation.

RWX said...

If you read the act that caused the reservoir to be built, you will find that flood control was part of the reason it was built. That said, it control all flooding but can prevent minor floods and will reduce the damage from a major flood.