Where will that water go?

A Sword of Damocles is hanging over the Green River Valley.

Thank the Greeks for this image, which, generally speaking, refers to a sense of foreboding that’s present in any precarious situation.

A massive flood of the Green River would certainly put the Green River Valley and the second-largest warehouse district on the West Coast in a precarious position.

The good news is that the sword is getting duller all the time.

The chance of a major flood occurring in the Green River Valley now stands at about 1 in 25, or 1 in 32 if you count all the supersized sandbags that are going up on and along the banks of the river.

So how did we get to this precipice? Look to Mother Nature and how she pounded the Howard Hanson Dam last January.

Behind all the emergency plans and flood-insurance purchases and hand wringing and public meetings and flood-protection structures going up along banks of the Green River is the Howard Hanson dam’s right abutment or support — and two holes.

Those two holes opened last January after the accumulated mass of days of rain washed downstream until it smacked against the abutment. Ultimately, the roiling water reached the highest point on the structure that it had ever reached, pounding the face — until something gave.

Dam monitors soon noticed rocks and dirt, the structural components of the abutment, in the drainage tunnels below the dam. If actual parts of the abutment were washing away, engineers realized, then the structure could be weakened. And if it were weakened, it could imperil the dam it supported.

The U.S. Army Corps of Engineers, which operates the dam, dug out the depressions, installed monitoring equipment and conducted tests while water was held at the traditional summer conservation pool elevation.

Nothing they found restored their confidence in the dam’s holding capacity. So last summer the corps hired two contractors to work on projects aimed at improving the integrity of the abutment in time for the start of the flood season on Nov. 1.

An $8.9 million contract went to Nicholson Construction for construction of a seepage barrier, a 450-foot-long, 90-to-160 foot-deep “grout curtain” in the right abutment. Final testing was done earlier this month, giving the corps reason to say a flood now only has the 1 in 25 chance of occurring.

Previously, the estimate of a flood was 1 in 4.

The grout curtain slows the water but does not stop it.

Corps spokeswoman Casondra Brewster likened the grout curtain to something modern – a Botox injection for the hillside.

The grout replaces the water, and while some water remains, the water is not moving as fast. The speed of the outflowing water causes a “heartache” for the corps, she said.

The corps also awarded a contract to install filter drains to help prevent water from undermining the structural integrity of the abutment.

A more permanent solution in the form of a barrier wall is at least three to five years off.

The corps has stressed that the dam is not in danger of failing. However, to reduce the stress on the abutment or support, it can’t fully fill the reservoir behind the dam.

The corps makes the point it has never lost a dam.

However, also hanging over the corps – and the Green River Valley – is Katrina, the hurricane that devastated New Orleans. The corps was criticized for how well it had protected the city against a massive hurricane.

Katrina’s sword is sharp.