Alabama Soil and Water Blog Spot

Off-set   and   on-the-contour .  If you install a silt fence sediment barrier, these are two terms you should understand.   Research at the Auburn University - Erosion and Sediment Control Test Facility has found that the most effective silt fence trenching technique is an “off-set” installation.  The posts and wire are placed 6 inches behind the geotextile trench or slice.  By installing with this separation, less undermining occurs, and the trench is easier to backfill and compact. The Alabama DOT has transitioned to this installation technique and the next time the Blue Book is revised, it will recommend this too. No one likes to change, but this is a good change and will become commonplace soon. A silt fence must be placed mostly “on the contour” in order to pond runoff and cause sediment to settle from suspension.  So what are silt fences that are not on a contour?  Well, they are not a sediment barrier.  Maybe a diversion or a work-lim...

  There are a few practices that require an open-channel auxiliary (emergency) spillway to allow excess runoff to safely bypass a small dam.  Such practices would be a Drop Structure with a dam, a Sediment Basin, or a Stormwater Detention (or Retention) Basin. An open top principal spillway is NOT considered an auxiliary spillway . Sometimes all that is shown on the plans is the elevation for the auxiliary spillway, but there should be details in the construction plans that show the cross-section and profile all the way through the auxiliary spillway including a stable outlet. The best auxiliary spillway is a vegetated spillway (pictured) located around the end or abutment of the dam on undisturbed soil.  The Blue Book contains design guidance for this type auxiliary spillway. However, a vegetated auxiliary spillway is not always possible, and a spillway “over the embankment” is often used.  In this case, the exit slope on the back of the embankment (dam) is usua...

Turf Reinforcement Mats (TRMs) are used to reinforce the roots of vegetation to protect the soil for higher water velocities and tractive stresses.   One way to almost get “instant” armoring is to use a TRM with block sod on top.  Using three-dimensional TRM can create a void between the block sod and the soil. A key component of this application is to ensure that soil “in-fill” is used to fill the voids in the 3-diminsional TRM.  So here’s the process: Prepare the waterway (channel) to grade and dimension. Incorporate soil amendments if required (lime and fertilizer). Install TRM per job or manufacturer’s specifications. Add soil “in-fill” with just enough to cover the TRM. Lay Block Sod Secure the block sod.  One way is to place netting over the sod to help hold it in place. Irrigate as needed until sod has formed new roots.

When installing Rolled Erosion Control Products (blankets) you often hear the term “intimate contact”. Blankets must have soil surface contact and that contact must be maintained until vegetation is established in order for the blanket to be successful.  Rainfall goes thru the blanket material to the ground surface.  If not infiltrated, the rainfall then travels downslope as runoff on the soil surface.  If the blanket is not in full contact with the soil surface, the runoff will erode the soil as if the blanket doesn’t exist.  Also, for extreme runoff events, the blanket could float as most blankets are lighter than water. So, there is a reason that manufacturers specify the minimum stapling for their product, and it never hurts to have more staples than required.  Staples are cheap insurance.  

  In Alabama, we are now getting into the time of year where it is good to be seeding grass or laying sod.  But we are also at a time when soils can be dry due to hot and/or windy conditions, and we can have several days without rain.  So to encourage germination of seed and growth of grass or sod, we turn to irrigation to overcome dry soil and drought conditions. The question then is often asked “how much water is needed?”  And the answer is probably more than you thought. Newly seeded areas need water for germination and then water for growth.  Newly sodded areas need immediate water for sod survival, and then watering very often until the sod has formed a good root system.  You should follow quality guidance documents (like Extension Service) to determine how much water is needed.  One inch of water would probably be a minimum for initial watering.  That equates to about 27,154 gallons of water for an acre. Can you do that with a hydroseeder?...

  We often discuss the importance of vegetation to control erosion.  In order to establish permanent vegetation and ensure that it is long-lived, the root zone in the soil must be "right".  That is, the pH must be within an acceptable range and the plant community must have access to the plant nutrients  needed for  a particular species to thrive. So, what if your soil test calls for Ag. Lime and plant nutrients (NPK Fertilizer)?  How do you ensure these soil amendments are incorporated the best way for the roots? Whenever possible (generally slopes 3:1 or flatter), lime and fertilizer should each be applied in split applications crossing each other to ensure uniformity.  Once applied, the soil amendments should be incorporated (heavy discing or roto-tilling) to the depth needed to create an   adequate zone for root development (generally 6-8 inches).  Incorporation of lime increases pH at a much faster rate in...

  Have you ever seen a Check Dam (some say Ditch Check) stay on a site long after construction is over? Check Dams can be rock, wattles, sand bags, silt fence, and other manufactured products.  But, my main point here is that a Check Dam is a temporary erosion control practice used to help stabilize a concentrated flow channel during construction. The Check Dam can also capture a small amount of sediment due to the ponding effect.  Check dams should be removed and the final stabilized surface (vegetation or armoring) should be installed prior to construction finalization. This leads me to two more thoughts.  Was the Check Dam really needed?  If the final stabilized lining of the concentrated flow area can be achieved without a check dam, is there really a need for a check dam? The small amount of sediment caught by check dams should definitely not be the main reason for using the practice. Upstream erosion control practices sh...