In Line Detention Ponds

This tutorial discusses the procedure for designing a pond that is inline, possibly at the confluence of several trunk lines. The point is that there are multiple drainage areas contributing to this pond. This tutorial assumes that you are already familiar with the Simple Detention tutorial, so it skips steps related to setting up the pond design.  Also in the interest of procedure, we will take some short cuts.

What you want to do:

Before you can actually compute the layout, you must have the pond defined. Unlike the simple detention tutorial, there are four drainage areas here that comprise the post developed runoff. The strategy is to first compute all the flows to the would-be pond location (in this case N-004).  First, open the Simple Detention project (Step by Step Guide) and draw the layout shown above.  It is necessary to go one node past the would-be location of the pond (which is N-004) because otherwise the flows from reach P-003 and P-007 will not be combined. When the above layout is drawn (you need the practice), define B-002 from the previous project as the contributing node for each starting Node.  This is necessary because we want each run to have flow in it.   Before we can route flows through the layout, we need to size the pipes.  We will do it the easy way by having the program size the pipe diameters and slopes.  Select the Layout/Zero Diameters menu item and Zero everything as shown in the following dialog box. Now lets route the 100 year event through the layout.  We are doing the 100 year so that the program will size the diameters and slopes for the large flows.  After routing the 100 year event throught the layout, we can go back and route the 2 and 10 year events.  Remember, because we routed the 100 year flows through the layout FIRST, the pipes and slopes are now sized for that design event.  Go ahead and route the 2 and 10 year events through the layout.  The image below shows the 100 yr results. Since we want to insert a detention pond at N-004, we need to determine the outlet elevation.  It is the invert of pipe P-004.  Double click on P-004 and check the upstream elevation.  It should be 95.5 ft.  We will use the Pond structure in the Simple Detention example, so you need to adjust the starting elevation of the Trap node, Orif discharge structure and Pond node - in that order to 95.5 so that the Pond that will replace the N-004 node will be at the correct elevation.  Go ahead and make those changes. We will now size the Pond using the same steps as the Simple Detention tutorial. Use the hydrographs created from the above step and the inflow hyd for the pond.  The hydrographs are automatically named by the last node in the layout followed by the design event.  For example, outlet-100 year.   The above hydrograph ploit is an example of the 100 year routing.  Please note that the maximum stage for the 10 year event is 106.9156 ft.  We are not going to use that number in this example, but will use it in the Flow Diversion example that follows.  Not to worry about getting the same answer, I have probably used a different rainfall type in my routings.  You probably used a type 2 rainfall event.  The point is to understand the process, not matching the numbers in this tutorial. Now that the control structure is sized, we simply replace N-004 with Pond in the layout. That's all there is to putting an inline pond in a layout.  The key considerations are: 1.  Sizing the pond before inserting it.  This is done by generating your predeveloped and developed flows to the spot where the pond is suppose to be inserted and using them to size the pond. 2.  Inserting the pond into the layout by substituting it for a "carrier" node (like N-004 in this example). When you route the various design events through the new layout, the program will automatically compute an HGL through the pond.  There are two considerations.  The first is that the pond maximum water surface may not be the same as the HGL computed by the peak out from the pond.  The program will report the higher of the two at the pond and use that number to continue with the HGL computations upstream.  Most of the time the Pond water surface will be higher than the computed HGL.  If it is not, then there are problems with your design.  It is left to the design engineer to look at the HGL elevations of inline ponds to determine how the HGL computed by the peak pond release rate relates to the detention ponds peak water surface elevation.