Site preparation for construction of the P removal structure

As discussed in the post on site assessment, water must be channeled into a single location for a P removal structure inflow point.  That means that if you are working with a drainage ditch or a single subsurface drainage pipe, then it is pretty easy.  The flow in that case is already concentrated for you. 

For our site however, the flow was only somewhat concentrated along the gravel road in front of the poultry houses and on the East-West gravel road.  The flow from the field also combined with the gravel road into a natural drainage ravine.  We simply had to build some earthen berms to concentrate the flow into the P removal structure.  The figures below show the approximate location of the berms (in orange) and the P removal structure (in blue) for both an aerial view and a contour map.  The eastern berm was very short since it butted up against a steep hill, which acted as berm for channeling water.  

After the berm was constructed, we seeded with tall fescue and covered with an erosion-control mat:

Since then, the grass has been well established.

Also at this time, we cut the foundation for where the structure would be located.  As discussed on the previous post, we chose to use ¼” sized slag that was treated, which means that our structure required 40 tons of material, and would be approximately 33 ft long by 13 ft wide with a 20 inch depth of material.  This depth of material was chosen based on the slope of the site (see the contour map).  We simply cut into the ground 20 inches deep on the upslope side, and made a flat surface for 33 ft.  At 33 ft, the total change in elevation will be about 20 inches.  The cutaway view below shows the original ground surface relative to the “box shape” cut into the ground:

A word on drainage ditches

As discussed in the previous two posts, flow rate through the material is critical as it controls how much water will be able to be treated during a large runoff event.  Flow rate is controlled by the depth of the PSM, hydraulic conductivity of the PSM, and hydraulic head.  Hydraulic head is a function of the slope of the site.  Some sites have extremely low slope, and therefore, hydraulic head will also be naturally low.  This is the case on the Delmarva Peninsula where drainage ditches carry much of the surface water away from agricultural fields. 

Dr. McGrath has got around this problem by incorporating flow control structures to increase hydraulic head for the P removal structure, thereby increasing flow rate through the PSM and maintaining a more buffered and constant flow rate.  Our friends at Agri-Drain supply ideal flow control structures for achieving this task.  Essentially, the flow control structure acts as a miniature adjustable dam in the ditch.  The picture below show some of the P removal structures constructed by Dr. McGrath in drainage ditches and also some of the Agri-Drain flow control structures.

However, one must be cautious in the use of flow control structures to increase hydraulic head for the P removal structure because it can decrease the drainage upstream by backing up excessive water.