Beyond Human Influence

By Thembi Borras

A road in which I have an interest recently became impassable due to a slide blocking it. In 2003, the same road had been upgraded to the tune of $35,000. The work encompassed reshaping (outsloping with rolling dips), upgrading wet crossings, improving drainage and installing rock in a few key locations. Soon after, I went out to walk the slide.

The story begins 600' upslope from the improved road in grassland. In the grassland, an unstable area is now clearly defined by a crack in the ground in the shape of a horseshoe that is 300' wide and 600' long. Approximately 4 acres of the hillside is moving, upon which clumps of redwood trees are now leaning. Near the upper margin of the unstable area is the headwaters of a swale, 15' by 30' of which became active within this much larger land feature. The watercourse that connects the swale to the fish bearing watercourse below became a high speed highway of liquefied soil. The watercourse passes through three culverts under three roads, a legacy logging road, a driveway and the improved road. The first culvert immediately plugged with a slug of sediment that could not be dug out. Fortunately the watercourse did not divert and stayed in the channel, the second culvert plugged but was subsequently cleared before it reached the third culvert that plugged with the liquefied soil. The bulk of the liquefied soil is now sitting on the improved road. Now as far as I could tell there was no direct human cause for this event. At least there was nothing obvious. There is a possibility that past road building activities or logging may have collapsed soil pipes or caused compaction that may have contributed to a change in drainage that in turn contributed to the activation of this feature.

The moral of this story is that although you may spend $35,000 dollars to prevent sediment from reaching the creek simultaneously improving the road, an occurrence beyond human influence can in the space of hours or minutes deliver as much sediment as you saved and block the road until further notice. However, there are questions you can answer when planning road placement to avoid inherent instability. Do you absolutely need the road? Can you gain access to your destination from the ridge, thereby having to cross fewer watercourses? Can you tie existing stable legacy road cuts together? Can you locate the road on gentle ground at a gentle gradient? Can the road be located in forest soils, which are typically more stable then grassland soils?