NASH RANCH ROAD ASSOCIATION

The Martinez Culvert - Background and observations

Bruce Wicinas  July 30, 2013 based mostly on July 15 measurements and observations.

 

Key points

 

Maximum water flow is relatively small, and only intermittent.  (Bill Seekins testifies in an e-mail 7/15/2013) “Last winter, I checked the culvert and the stream bed up stream of it, once or twice a week during the wet season.  All the creek water went through the pipe.  The year before, much of it went in to sink holes.  That's why I consider the apron, leading into the pipe, to be a success.  Last winter was fairly dry, with few heavy rains.  I don't recall seeing water levels in the pipe over 6 inches deep.  But, I think this was enough water to wash silt, sand and clay out of the pool below the pipe.  I've been told that steel head and salmon need pools with clean gravel bottoms to breed.  In your picture, you can see a water stain on the floor of the pipe.  That is probably a typical amount of flow if there had not been any rain in the previous 2 or 3 weeks, [last winter, anyway].”

 

The width of the earthen “dam” is 65’ at its base. See the scaled cross section below. Hundreds of yards of earth which would have to be washed away for the crossing to fail catastrophically.

 

The inlet. Note the large log above the culvert running parallel to the road. Did this once support the easternmost lane of the road, allowing the travelled way to be located much father east than it is today? July 15, 2013

The outlet. Note the peaceful scene, including delicate branches. This is not scenery which is yearly raked by powerful water flows.   July 15, 2013

 

The natural fall of the watercourse is about 11 degrees.  The fall of the land perpendicular to the water course – the axis of the road – is about 4 degrees for several hundred yards approaching the crossing. It gets steeper to the south as the road climbs the hill beyond, hundreds of yards from the crossing.

 

A zone of almost-mature and mature trees parallel the road on the upstream side. The lattice of roots of these mature trees form a stronger-than-concrete subterranean structure. Even the water force of Mill Creek would be challenged to overwhelm this subterranean root structure.

 

Based on all we know, what seems most likely is recurrence of subsidence of the westward lane like that Bill Seekins recalls 20 years ago. But this is less likely than before. That previous time the fill settled it filled subterranean voids and assumed a more stable angle of repose.  In other words, the subsidence that occurred about 20 years ago probably left us a more stable earthen barrier. The fill is unlikely to move downhill as a single mass. The “downhill” is only 11 degrees –not an angle of instability. The planes of contact between fill and original terrain are a very large surface area. Large earthen surfaces don’t slip easily.

 

I heard that “the culvert had separated,” that the culvert was “hanging in the air far above the floor of the ravine”, etc.  These statements were exaggerations.  The culvert separated, according to the testimony of Bill, about 20 years ago. You can plainly see the seam inside the culvert. Bill and his son did an excellent job of joining it and sealing it. Even insects cannot slip through the joint. There is no sign of recurrent separating.  The culvert is not hanging in air far above the ravine. The intake is a couple inches above the floor of the streambed as presently observed. The outlet is at the correct elevation.

 

View of the crossing looking northward. Note Martinez’ PG&E drop which presently prevents us from moving the travelled way away from the westward (downstream) drop-off.

 

Bill Seekins observed that the maximum flow in the culvert has a depth of 6”. This is attested by the stain on the culvert – see picture above. I did a cross section calc. A 6” depth of water in a 4’ culvert is 2 sq ft/12.6 sq ft or 16%. It would appear that the culvert is oversized by 600%. A culvert with huge capacity awaits a flood which can never come. This enormous culvert gives the impression that enormous pulse events have occurred.

 

This diagram illustrates the volume of water we’re “fearing” based on Bill’s “6 inch” observation.

 

The blue arrow points to the blue sliver which is the cross section of maximal winter water flow, to scale.

 

In order for water to wash out such a crossing it must be of sufficient volume and speed to dislodge and carry away many yards of dirt. This ravine hosts a battle between only a very small amount of water and an earthen dam which is 65’ wide at its base consisting of hundreds of yards of earth. Even to ream a sinkhole into an underground void large enough to be alarming,  the water flow has to be significant and steady.

The volume of water which would approach this culvert in a "100 year flood" must be calcuated by a credible expert.

 

Hazard to traffic, and a suggestion

 

We all worry about the hazard to traffic of the proximity of the south-bound (westerly) lane to the long, steep drop-off on the west side. There’s no shoulder and insufficient space to erect a guardrail.

 

Why not move the road – the travelled way – at least 10’ to the east? The diagram below illustrates the idea.

 

The immediate obstacle to this remedy is the location of Martinez’ PG&E drop. It’s dead in the way of a relocated road.

 

It’s an option.

 

Showing suggested relocation of the travelled way to the east. To relocate it this much – about 18’ - would require extension of the culvert to the upstream, removal of some of the smaller trees, relocating of the PG&E drop, and additional fill on the upstream side.  A relocation by less than this might be just fine.