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We are closing on our new built house this week. One last issue that arrose was how the builder was going to handle the drop point in the back of the house. We do not have much of a back yard, and there is a 7 foot drop into a stream area about 3 feet back from where our deck foundation is. The drop point is about 15 feet back from our foundation.
To handle erosion the builder has packed soil at a 45 degree slope, starting about 3 feet from our deck posts and covered the slope with a Erosion Control Fiber Blanket. The stream (it's more like a storm drain) seldomly has water in it, usually only after it rains. I live in central North Carolina so we do get some pretty heavy rains around here. Is the Fiber Blanket going to be enough to handle any erosion problems from years to come? Will I need to do any additional work years from now? Should the builder install a retaining wall in situations like this when the slope is so close to a deck foundation?
There simply isn't enough info to make a reasonable determination. A lot depends on soil type, the maximum water height, the speed of the flow, the subsurface, and so on.
Our stream can go from a cross-sectional area of less than one square foot to a fifteen foot wide turbulent current that is quite capable of moving tree trunks. However, it sits in shale, and building as close as five feet from the bank would be safe.
Your big issue is not having the experience of a flood flow, where 3" or more of rainfall falls in the space of a few hours. If debris can get caught in the current, that blanket could be shredded or undermined in a few minutes. A 45 degree slope is not a natural angle of repose for soil, so it will slump and wash away if uncovered. Slumping may occur anyway, and more quickly if the area remains moist and the soil has a high clay component.
The more I think about it, unless there is only a minor amount of water being transported, you are probably in for the annoyance of some continuing bank erosion. Whether it affects your deck or not depends on how deep the foundation is.
First, is the stream close to straight, or does it meander? Are you on what would be the "cut bank" side? If there's never more than an inch or two of water, it probably doesn't matter, but if there is enough water to call it a "stream" it may well.
Does the county or city require permits to do erosion control? What would be the effect of the work on your property to "upstream" or "downstream" properties?
The grade of the slope is more like a 60 degree angle, than a 45 degree angle. The stream is a straight stream. When it rains is gets pretty full and fast. Probably no more than 2 feet in depth, and the water level subsides failry quickly. There is no cut bank, but my fear is that erosion that may occur in the future may cause one.
The type of soil is a red clay. We live in central north carolina so that type of soil is all I see here.
The foundation of the deck goes 6 feet into the ground, and the base of the deck is attached to the home. It is not free standing
My main concern is that problems with erosion will not occur until 2-3 years down the road where all responsibility that the builder has to correct any erosion problem have exhausted, and I am stuck with a large bill to fix any issue. The builder has indicated that the erosion control that they have put in place does meet the town's standard and it does pass inspection, but just looking at the slope and the stream close the home just does not sit well with me. Thank you for all the information!
The blanket you refer to is a temporary- at best. It is a means to reduce erosion and retain moisture so that seed (if any was put down) can germinate. Vegitation is what will fend off erosion. As far as your slope is concerned- you should have no greater than a 2/1 slope. A 45degree slope would be 1/1, and 60degree is obsurd. Have the slope checked, maintain a 2/1 slope or better (any civil engineer will tell you that).
This is an area that you obviously don't want to have to maintain (mowing, weed wackin', etc.), I would strongly recommend that something other than grass/ grass seed be installed. The bad thing is it's too late for most seed- and winter rye will only last until next spring. If thats all you can get out of the builder- make some kind of deal that they will re-address the issue come spring. The best grass I could recommend is Love grass. Great root system, and requires no maintenance.
But my best recomendation would be a low growing shrub like juniper; again, great root system and little or no maintenance.
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