Detention Pond Routing Basics

When performing detention pond routings it’s important that you understand the basic “calculation process” so that you can better troubleshoot the results. This article is not about the inputs for performing a pond routing in Hydrology Studio but rather helping you get a better feel for the procedures taking place behind the scenes, so-to-speak.

Probably the most widely used method of determining the required storage volume in detention basins is the Storage-Indication Method and is the method used by Hydrology Studio. This routing procedure consists of an iterative process based upon the Continuity Equation.

The basic premise is that, over a given time interval, the volume of water entering the pond minus the volume of water leaving the pond equals the remaining storage volume.

In simpler terms, at any given point in time along the inflow hydrograph, what goes In minus what goes Out is what’s left over… Storage. The process begins at time zero and progresses in steps equal to the Time Interval, usually around 1 to 6 minutes, until the entire time span of the inflow hydrograph has been reached.

The method begins with a stage-storage and a stage-discharge relationship (basically a pond you created with an outflow device). The stage-storage relationship indicates how much storage or volume is in the pond at a given depth or stage. Similarly, the stage-discharge relationship tells us how much flow is exiting the pond at a give depth or stage. To this we add an inflow hydrograph and the following equation:

I = inflow volume
O = outflow volume
ds/dt = change in storage volume

Performing a Simple Pond Routing

To perform a routing you need three things as described above:

  1. Inflow hydrograph (just one)
  2. Stage vs. Storage relationship
  3. Stage vs. Discharge relationship

Lets examine these one-by-one.

Inflow Hydrograph

Below is a simple hydrograph that will be used for a pond routing inflow. Now lets take a time span, for example from 0 hours to 3.25 hours depicted by the blue vertical line, and calculate the volume (blue shaded area) up to the blue line. It’s 8,000 cuft. In other words, 8,000 cuft of runoff has accumulated at 3.25 hours into the storm.

detention pond design software
8,000 cuft of runoff has accumulated at 3.25 hours into the storm.

Stage vs. Storage Relationship

To continue we need a physical detention pond and we need to know how much volume it can hold at any given depth or elevation, or what we like to call Stage. Stage is the absolute distance from the water surface to the pond bottom. Our question at this point is “What would be the Stage in the pond if we dumped that 8,000 cuft of water in it?” The Stage-Storage curve tells us that.

The 8,000 cuft of water filled the pond up 2 feet to elevation 102.00

Above is our Stage vs. Storage curve for this particular detention pond. The green line represents the storage provided at any given elevation or Stage. When we added the 8,000 cuft of water, the water surface rose to Stage 2 ft or elevation 102.00 ft.

If we poured 8,000 cuft of water into this pond, the Stage would rise to 2 ft.

Stage vs. Discharge Relationship

Now that we know how deep the water is from the 8,000 cuft, we next need to find out how much water will in-turn exit the pond due to this 2 feet of depth. Below is our Stage vs. Discharge curve for this pond which contains a single 18-inch culvert placed at the bottom.

Stage vs. Discharge curve shows that at Stage 2.0, the pond will release 8 cfs

It indicates that the outflow at this depth (head against the culvert) will be 8 cfs. So whenever the pond has 8,000 cuft of water in it, it will release 8 cfs. It takes both a Stage vs. Storage curve and Stage vs. Discharge curve to know this.

Detention Pond Basics

So once we know the relationship between the inflow volume from the runoff hydrograph and the flow out of the pond, we can monitor this across each individual time step, dt or Time Interval as its called, and produce the final outflow hydrograph. We will also know the volume in the pond at any point along the way. This is what the Storage-Indication calculation method provides.

The Final Outflow Hydrograph

Below is the final outflow hydrograph which peaks at about 10 cfs. The maximum volume stored in the pond is the difference between the inflow hydrograph (blue line) and the outflow hydrograph (red) from the beginning up to the peak of the outflow hydrograph.

The outflow hydrograph peaked at 10 cfs, where it intersects the inflow hydrograph.

If you cross-reference the 10 cfs peak Q on the outflow hydrograph with the Stage-Discharge curve, you’ll see that it corresponds to an elevation of about 102.50 ft. Cross-reference that on the Stage-Storage curve and you’ll see that the maximum storage used was about 10,000 cuft. Sure enough, this is what is shown (10,348) in Hydrology Studio’s Trial Routing results.

It’s important to note that the time to peak of the outflow hydrograph always occurs at a point ON the inflow hydrograph, where the two hydrographs intersect. At this point the flow entering the pond is exactly what is exiting. If at any time the flow coming in is more than what’s leaving, the pond is filling up. And if more is exiting than what’s entering, the pond water surface is lowering.

Troubleshooting Pond Routings

You may, from time-to-time, encounter errors during your routings. It is recommended to always inspect the stage-storage curve as well as the stage-discharge curve when adding your outlet structures. Make sure that the graph contains your target Qs. The following items are the most common culprits when pond routings fail.

  • There is water exiting the pond at Stage zero. Be sure that your outlet devices are not discharging at the pond bottom. Q must be zero at Stage zero. Remember, In – Out = ds. In this scenario, In – Out is negative and we cannot have negative change in storage (ds).
  • The Target Q you’re needing does not exist on the Stage vs. Discharge curve. Take a look at your Stage vs. Q curve. Does your Target Q appear on it anywhere? If not, you’ll never achieve your desired results. For example, in the Stage vs. Q curve above, the outflows range from zero to 26 cfs. If your Target Q is 28 cfs, you’ll never get a successful routing. The water surface will hit the top of your pond before releasing 28 cfs. It’s only capable of discharging 26. The same is true for storage.
  • The pond does not have sufficient storage. During the calculations the sum totals of ds exceeded the available storage in the pond. Try making your pond larger. Verify that your pond actually meets the estimated storage requirement.
  • The Time Interval is too large. This can cause too much granularity in the calculations when working with very small flows, < 1 cfs. This results typically in a Qp Out being larger than the Qp In. Try reducing the Time Interval to 1 minute.
  • No matter how large you make your pond, you can’t seem to make the routing work. This is more common with underground detention. Take a look at the pond’s total depth; translation… total head. If the total available head is small it will be difficult to achieve any significant discharge. Make your pond deeper to increase the head against your outlet device(s).
  • There’s no flow out of your pond during the routing. This may be due to the outlet devices being set above the pond bottom. The storage volume between the pond bottom and the outlet device(s) invert is greater than the volume of the inflow hydrograph.

More On Troubleshooting Pond Routings

More insights about how to troubleshoot a failing detention pond routing can be found in this article.

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