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?
Showing posts with label plugged. Show all posts
Showing posts with label plugged. Show all posts
April 9, 2006
January 1, 2006
Is your road up to the big, fast water challenge?
By Thembi Borras
The winter storms are delivering big, fast water causing some stream crossings to fail, road rock to be lost, sink holes and road failures. Well designed roads and stream crossings take into account large storm events. Moreover, when well designed roads and stream crossings are properly constructed and installed, they are better able to resist the big, fast and infrequent water as we have been having. The best time to determine if your road is holding up to the big, fast water challenge is during the storm. If it is not safe, you can often project, where the high water level was, based on debris, mudlines and waterlines, after the storm.
Indicators that your road is not adequately drained include waving farewell to your road rock, the capacity of your ditch relief culverts and inside ditches are being exceeded and rills are forming in your road bed.
An indicator that your culverted stream crossing is not adequately designed is the capacity of the culvert is being exceeded. Plugging is often evidence that a culvert is undersized. When a stream crossing plugs it can spell bad news. In the winter of 1995/1996 an undersized culvert plugged on our private dirt road. The diverted water traveled 125 yards where it outleted carrying 30 cubic yards of the roadbed to the creek, facilitated by the well formed berm on the outside edge of the road.
At the same time your are evaluating your road during a storm you can do effective short-term maintenance, with a shovel or a hoe, that will protect your investment. Dig ditches to get the water off the road as quickly as possible and deliver it to a stable location. Remove any blockages from culvert inlets. Clear your inside ditches of debris but keep growing things in the ditch, to slow the water and meter sediment, unless you need the capacity. Long term fixes include sizing your stream crossings for 100 year storm events and where appropriate can include replacing culverts with rock armored fill crossings, installing more frequent cross drainage and reshaping roads to have outslope.
In the spring, remember the big fast water of 12-30-05 and ask yourself if your road could be improved, it may not be cheap, but it could save you money and inconvenience in the end. Remember the goals of road improvement are to reduce the chance of sediment delivery because of episodic events, reduce chronic delivery of sediment and reduce maintenance.
The Handbook for Forest and Ranch Roads by Pacific Watershed Associates will aid you in your road management decision making and is available through the Mendocino County Resource Conservation District (707-468-9223) and the Navarro River Resource Center (707-895-3230).
The winter storms are delivering big, fast water causing some stream crossings to fail, road rock to be lost, sink holes and road failures. Well designed roads and stream crossings take into account large storm events. Moreover, when well designed roads and stream crossings are properly constructed and installed, they are better able to resist the big, fast and infrequent water as we have been having. The best time to determine if your road is holding up to the big, fast water challenge is during the storm. If it is not safe, you can often project, where the high water level was, based on debris, mudlines and waterlines, after the storm.
Indicators that your road is not adequately drained include waving farewell to your road rock, the capacity of your ditch relief culverts and inside ditches are being exceeded and rills are forming in your road bed.
An indicator that your culverted stream crossing is not adequately designed is the capacity of the culvert is being exceeded. Plugging is often evidence that a culvert is undersized. When a stream crossing plugs it can spell bad news. In the winter of 1995/1996 an undersized culvert plugged on our private dirt road. The diverted water traveled 125 yards where it outleted carrying 30 cubic yards of the roadbed to the creek, facilitated by the well formed berm on the outside edge of the road.
At the same time your are evaluating your road during a storm you can do effective short-term maintenance, with a shovel or a hoe, that will protect your investment. Dig ditches to get the water off the road as quickly as possible and deliver it to a stable location. Remove any blockages from culvert inlets. Clear your inside ditches of debris but keep growing things in the ditch, to slow the water and meter sediment, unless you need the capacity. Long term fixes include sizing your stream crossings for 100 year storm events and where appropriate can include replacing culverts with rock armored fill crossings, installing more frequent cross drainage and reshaping roads to have outslope.
In the spring, remember the big fast water of 12-30-05 and ask yourself if your road could be improved, it may not be cheap, but it could save you money and inconvenience in the end. Remember the goals of road improvement are to reduce the chance of sediment delivery because of episodic events, reduce chronic delivery of sediment and reduce maintenance.
The Handbook for Forest and Ranch Roads by Pacific Watershed Associates will aid you in your road management decision making and is available through the Mendocino County Resource Conservation District (707-468-9223) and the Navarro River Resource Center (707-895-3230).
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