Underground Chambers

When space is abundant, the most common approach to stormwater retention/detention systems is to design above ground storage ponds. Above ground ponds occupy large amounts of space and can create attractive nuisances, not to mention becoming a breeding ground for insects and other pests. When space is at a premium, underground systems provide an alternative means of stormwater detention storage. They reside underground and free up valuable land above which can still be used for parking lots, green areas or even playgrounds.

There are numerous solutions to provide underground stormwater detention storage but high density polyethylene (HDPE) pipe often provides the most economical choice. HDPE pipe (Chambers) and associated fittings are easy to install and provide a long service life with minimal maintenance.

Hydrology Studio offers an easy way to setup and model these types of systems. The design process is very similar to that of traditional above-ground designs. It’s the same three-step process of:

  1. Estimating the maximum required storage
  2. Creating a Pond (in this case using chambers) to accommodate the required storage
  3. Designing the Outlet Structure to satisfy discharge requirements

To start, select UG Chambers as the storage type.

You’ll then have two options for your chamber definition:

UG Chambers by Manufacturer

Here you can select chambers from a variety of manufacturers whose models and products are built-in to this software. Their sizes and incremental storage will automatically be filled in for you as well as suggested stone encasement dimensions. This will be your easiest choice for system layout.

Use this option for when you already have an idea as to what specific type of chamber you’d like to employ. Once you have determined the amount of required storage for the pond, you can simply select your preferred manufacturer and model from a drop-down list. The software will automatically fill in most of the required input fields provided by the manufacturer’s specifications. All you need to supply is the total number of chambers and the number of rows which best satisfies the project’s site requirements. The example plan layout shown below has a total of 12 chambers divided over 3 rows.

Hydrology Studio offers a robust Plan and Section Layout drawing to assist you in this effort.

The inputs for the Manufactured Chambers are simple and straightforward. Perhaps the most difficult decision is the actual chamber selection. As of this writing there are over 30 models from several manufacturers to choose from, and the list is growing.

Required Data for Manufactured UG Chambers

Once a model has been selected, its dimensions and bare chamber storage for a single chamber is filled in in the input grid. In addition, suggested dimensions for the stone encasement (spacing, above, below, sides and ends) are filled in. Any of these can be overridden if needed.

Storage per chamber
Enter the bottom elevation of the encasement and click [Apply] to instantly see what the total, single-chamber, storage is. This is displayed on the output table on Row 20. Note that some chambers include end caps that contain storage. From this you can quickly estimate the total number of chambers to use.

How many chambers?
Now specify the total number of chambers; select the number of rows that best suits the site restraints; and you should be very close to a final layout. Adjust any or all parameters as needed.

Click on the Layout Chart radio button to view a Plan and Section of your chamber layout as shown below.

Underground Chambers - Hydrology Studio
Notice the Plan and Section view options below the chart

Following are descriptions of the required inputs.

Select a chamber model from the drop-down list.

Chamber Height, Width and Length
These values are read-only and will be filled-in based on the Model selected.

Total No. Chambers
Enter the total number of individual chambers. In the Plan view shown above, there are 38 chambers.

Chamber Storage (read only)
The total storage for the number of chambers is displayed and is read-only. This is the bare chamber storage. No end cap storage or stone is included in this.

Number of Rows
Enter the total number of rows. The Plan view diagram shows 5.

Space Between Rows
This will typically be filled in based on the manufacturer’s recommendation but can be overridden.

Stone Above, Stone Below
These will typically be filled in based on the manufacturer’s recommendation but can be overridden.

Stone Sides, Stone Ends
These will typically be filled in using a default of 12 inches and can be overridden.

Voids (%)
Refers to the Encasement stone. This value defaults to 40. It allows for the reduction in storage due to a stone-filled encasement.

Bottom Elevation
Enter the elevation of the encasement bottom.

Click [Apply] when finished. Then click the [Next] button to proceed to Step 3 – Outlets.

Click on the Layout Chart radio button to see a plan and section view of your chambers. Below is a sample Section view.

Generic Chambers

This choice offers you the most flexibility in chamber shapes, sizes and dimensions. Here you can enter your own data as well as specify headers and stone encasement dimensions.

Use this option for when you need to model a specific type or shape of underground chamber that is not offered by the Manufacturer option. You can specify underground storage chambers in the shape of circular, arch or rectangular vessels laying either flat or on a slope. Headers as well as a stone encasement are also optional. Hydrology Studio will automatically calculate storage volumes for you based on a given chamber size, shape, slope, length and stone encasement properties.

Note that the Generic inputs will differ somewhat from the Manufactured Chamber inputs. For example, Rise and Span are entered in feet (meters) rather than inches (mm). You will also enter the entire length of a row, or barrel as it’s called here, rather than the length of an individual chamber section.

To start, select the UG Chambers button. Then select “Generic” as the Chamber Definition.

Then double-click the Invert Elevation Down input on the input window.

Required Data for Generic Chambers

The following is a description of each of the required input items. Click the Help option button at the top of the input table to view a help diagram.

Invert Elevation Down
Enter the elevation at the lowest point in the chamber. If an encasement is not used, this will be the elevation at stage zero.

Chamber Rise
Enter the diameter or height of the of the chamber vessel in feet (m).

Chamber Shape
Specify Arch, Circular or Box.

Chamber Span
Enter the width of the chamber vessel in feet (m).

Chamber Length
Enter the length, single barrel, of the chamber in feet (m).

No. Barrels
Enter the number of chamber rows or barrels.

Barrel Slope (%)
Enter the slope of this pipe in percent.

This option will add connecting header chambers at the upstream and downstream ends. They are assumed to have the same shape and size as the primary chambers. Check the box to add headers.

Stone Encasement
Check the corresponding box to indicate stone encasement is being used.

Bottom Elevation
Enter the elevation of the encasement bottom. This must be below or equal to the chamber Invert Elevation Down.

Width per Chamber
Enter the encasement width for a single barrel. Must be greater than or equal to the chamber Span. The total encasement width will be computed as (Width per Chamber x No. Barrels). For example, to achieve a 6-inch spacing between chambers, enter the Span plus 0.5 feet.

Enter the depth of the encasement in feet (m).

Voids (%)
Refers to the stone encasement. This value defaults to 40. It allows for a reduction in storage due to a stone-filled encasement.

Click [Apply] when finished. Then click the [Next] button to proceed to Step 3 – Outlets.

Click on the Layout Chart radio button to see a plan and section view of your chambers. Below is a layout with 5 barrels including Headers and Encasement.

Using the Interactive Tool

This will sure become your most favorite and productive way to fine tune your Storage Chambers. This tool contains buttons as shown below and basically allows you to edit most of the input items without having to type their values. Your edits will be applied automatically. You’ll see instant results on your output table, Layout Charts and Stage Vs. Storage graphs.

Here’s how it works: Put your cursor on any input item, for example, Total No. Chambers. Then click on either the Arrow Up or Arrow Down buttons. This will increase or decrease the number of chambers by 1.

The default increment is typically set to 1. It is 0.01 ft. for the Bottom Elevation. Holding the [Shift] key down increases the increment to 0.1 ft. Holding the [Ctrl] key down makes the increment 1. Certain inputs are not adjustable using the Interactive Tool. For example, Voids (%). In these cases, the the Interactive tool will show to be disabled or beep.

Stage-Storage Chart

You can quickly view a chart of the Stage-Storage by clicking the Stage-Storage radio button.

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