stage-storage-discharge (ssd) table element

stage-storage-discharge (ssd) table element


© Clear Creek Solutions, Inc., 2010

The Stage-Storage-Discharge (SSD) Table element is a conveyance element. It can represent a pond, tank, vault, pipe, stream channel, river reach, dam, lake, or any flow conveyance pathway. Its flexibility in being able to represent a wide variety of different types of facilities makes the SSD element a very handy modeling tool.

One example of how the SSD Table element can be used in WWHM4 is in a situation where a pond has already been designed (and perhaps constructed) and there is the need to see if it meets the local jurisdiction’s flow control standard.

If the pond dimensions and orifice information are available then everything can be input in the appropriate pond element without using the SSD Table. However, there are times when all that is available is the stage, storage, and discharge information (often in the form of a table) for the pond. In these situations the user can input a SSD Table instead of the usual pond input information to route the flows through the facility.

However, it should be noted that a stormwater facility represented by a SSD Table cannot be automatically sized to meet flow duration standards by WWHM4’s AutoPond. The user must manually size a SSD Table facility by independently changing the values in the

SSD Table.

For this example we will set up a project with multiple SSD Table elements, with each element representing a different type of conveyance feature.

We will model a 10-acre site in rural King County, Washington, near the city of

Enumclaw. The first thing that we will do is to locate our project on the project map.


WWHM4 SSD Table Element

Our project site is located in southern King County, Washington. We click on the map to select the project location. Based on our project location WWHM4 selects the appropriate precipitation record and precipitation multiplication factor. We then have the option to fill in the Site Information boxes.

We will assume that the project site includes a farm pond and a stream channel. We will model each of these flow conveyance features with an appropriate SSD Table.


WWHM4 SSD Table Element

Our predevelopment land use is 10 acres of C, Pasture, Flat, plus existing farm pond and stream channel. Three acres drains directly to the farm pond; the farm pond drains to the stream channel along with the remaining 7 acres.


WWHM4 SSD Table Element

The only information that we have about the farm pond is from the original design and construction documents. This information is in the form of cross sections plus a drawing of the outlet weir.

We could have used the WWHM4 pond element to represent this farm pond, but the shape of the pond as shown in the cross-section drawings doesn’t fit our standard pond element dimension options. The SSD Table gives us greater flexibility to accurately represent these conditions.


WWHM4 SSD Table Element

To input the pond cross-sectional dimensions we first check the “Stage Computed” box.

This turns on the “Add Layer” button.


WWHM4 SSD Table Element

By clicking on the “Add Layer” button we get a new screen that allows us to input crosssection information.

The bottom elevation is the starting depth for this layer of information. If this is the bottom layer then this represents the bottom of the pond and the bottom elevation is zero.

If this is a higher layer (we will add a higher layer in a minute) then we put in a bottom elevation equal to the effective depth of the layer below it. This will make more sense when we input the second layer.

We input a bottom length of 10 feet and a bottom width of 8 feet. The effective depth is

2 feet (that is how high this layer goes up the side of the pond). All of the side slopes are

1 foot horizontal to 1 foot vertical.

After all of the information is added we click on “Update”.


WWHM4 SSD Table Element

WWHM4 automatically fills in the stage (from 0.0 to 2.0 feet) and corresponding surface area and storage for the farm pond based on the dimensions that we provided.

We now want to add a second layer to the pond configuration. We click on “Add Layer” again.


WWHM4 SSD Table Element

When we click on “Add Layer” we see the dimension input screen again. Now it shows us that we have already added a layer (Layer 0) that goes up to a depth of 2.0 feet. For our second layer we will set this layer’s bottom elevation at 2.0 feet to be consistent with the top of the first layer.

We input a new length and width at the 2-foot depth and a new effective depth of 3 feet.

The effective depth is the additional height about this layer’s bottom elevation, so the pond’s total depth is 5 feet (2 + 3 = 5).

For this layer we have vertical side walls (side slope H/V = 0). We hit “Update”.


WWHM4 SSD Table Element

First Layer

Second Layer

We have now entered two layers: the first layer is from zero to 2 feet; the second layer is from 2 feet to 5 feet. We can continue to add as many layers as we need to fully represent the dimensions of the farm pond.


WWHM4 SSD Table Element

Just to show how it is done we will add a third layer. The bottom elevation starts at 5 feet with a length and width of 50 feet and a depth of 1 foot. Two of the four side slopes are 3

(H/V); the other two are 0 (vertical). We click “Update” to add this information to the stage, surface area, and storage columns.


WWHM4 SSD Table Element

With the addition of the third layer we now have a pond that has a maximum depth of 6 feet and corresponding surface area and storage volume. We still need to add discharge information (column 4 of the SSD Table).


WWHM4 SSD Table Element

We click on the heading for column 4 (“Not Used”) to view our options. We can either select “Manual” or “Outlet Structure”.

Manual means that we input the discharge (cfs) by hand (or actually keyboard) into column 4.

Outlet Structure means that we select an outlet structure and give it the appropriate dimensions and WWHM4 computes the discharge for each stage value.


WWHM4 SSD Table Element

We select “Outlet Structure” to represent the farm pond’s weir opening.

Although the farm pond doesn’t actually have a riser we will use the riser input to represent the weir with a notch. The weir is at 5 feet (the farm pond is 6 feet deep) and has a width equal to the riser diameter times pi (the circumference of the riser). This equals a weir length of approximately 38 inches (12 * 3.14).

The weir has a notch that has a height of 4.5 feet. This means that the notch starts 0.5

feet above the bottom (5.0 - 4.5 = 0.5) and is 0.5 feet wide.

We click on “Update” to add this information to the farm pond SSD Table.


WWHM4 SSD Table Element

Column 4 is filled with discharge values (in units of cubic feet per second) based on the outlet structure information that we provided. Note that there is no discharge below a stage of 0.5 feet. This is because the weir notch is 4.5 feet and the weir/riser height is 5.0

feet. Therefore, below a stage of 0.5 feet is dead storage.


WWHM4 SSD Table Element

We have now filled in all of the needed stage-storage-discharge information required to route runoff through this farm pond.

If we want to edit an existing layer or insert a new layer we only have to right click on the appropriate row number on the left side of the table to access the available options.


WWHM4 SSD Table Element

If we want to manually change any of the stage, surface area, storage, or discharge values in the SSD Table we can do that by first unchecking the “Stage Computed” box and then clicking on the selected cell that we want to change. We can then replace an existing value by typing in a new value.


WWHM4 SSD Table Element

The last thing that we want to do for the farm pond is to turn on precipitation on the pond and evaporation from the pond. We do this by checking the box for each. Make sure that both are checked (it doesn’t make hydrologic sense to check only one box or the other).

Let’s now move on to the SSD Table representing the stream channel.


WWHM4 SSD Table Element

Next is the SSD Table for the stream channel. We could have used the channel element to represent this length of stream, but we know the rating curve (stage-discharge relationship) for this stream plus this stream is a losing reach (in other words, there is infiltration through the bottom of the stream channel and during low flow periods the flow at the downstream end is less than the flow at the upstream end). The SSD Table gives us greater flexibility to accurately represent these conditions.

We have the choice of adding the stage, surface area, storage, and discharge data into the

SSD Table directly or entering the data first into another file and then uploading the file to WWHM4.


WWHM4 SSD Table Element

WWHM4 supports importing a SSD Table in either a text table format, an Excel spreadsheet comma delimited format (.CSV), or a WWHM Pond Table saved from another WWHM4 project. I find that the easiest way to set up a SSD Table is in an Excel spreadsheet.


WWHM4 SSD Table Element

Note that the SSD Table that we create must be in the same format and have the same units as the pond table created by WWHM4. Specifically:

 Column 1 is stage data (feet). The first value must be zero. Stage values must be in ascending order and the same stage value cannot be repeated.

 Column 2 is surface area (acres). The first value can be non-zero. Surface area values do not have to increase with depth.

 Column 3 is storage volume (acre-feet). The first value must be zero. Storage volume must increase with depth and should be computed based on surface area and depth. The same storage volume value cannot be repeated.

 Column 4 is the surface discharge (cfs). The first value must be zero. Discharge does not have to increase with depth, but usually does.

 Columns 5 through 8 are optional discharge columns. These columns are used if the facility has multiple outlets. These columns can represent a surface discharge

(cfs) and/or infiltration (cfs). The first value in each column must be zero.

Discharge does not have to increase with depth. If there is no discharge then all of the values in this column are zero by default.


WWHM4 SSD Table Element

To input discharge values in columns 4 through 8 first click on the column heading (the default setting is “Not Used”) and then select “Manual” or “Outlet Structure”. Use

“Manual” if you have already put the discharge values into a spreadsheet or plan on typing them directly into the column on the element form.


WWHM4 SSD Table Element

The SSD Table element initially contains no stage-storage-discharge information in the

SSD Table. We need to load the SSD Table information from a file. The Browse button allows us to search our file folders for the SSD Table of our choice.

Note that the “Stage Computed” box (used for the farm pond) is left unchecked when loading an external file for input to the SSD Table.


WWHM4 SSD Table Element

After the file is loaded the stage-storage-discharge information is displayed in the

WWHM4 SSD Table. Check it just to make sure that everything got in there okay and looks like it should. You still have the ability to make changes on the SSD Table element form, if needed.

If you don’t see any values in column 4 you need to change the column heading from

“Not Used” to “Manual” by clicking on the heading and selecting “Manual”. Then the spreadsheet values will be shown in the column.


WWHM4 SSD Table Element

For the stream channel SSD Table we still need to input the infiltration in the stream reach that corresponds to the information that we have about the reach being a losing reach. We will use column 5 for the stream reach infiltration data.

When we click on the “Not Used” heading of column 5 we see that column 5 has more options than column 4 does. This is because column 5 is the second outlet for a conveyance element and, by default, represents the infiltration outlet. However, in the

SSD Table we can use this column in any of the above-listed ways. Let’s take a minute and look at our options:

 Manual: We already know that this means that we manually enter the discharge values either by typing them directly into the column’s cells or by putting them first into a spreadsheet and then loading the spreadsheet file.

 Outlet Structure: We used this option in the farm pond SSD Table. To refresh your memory, we input riser and orifice data into the outlet structure form and

WWHM4 computed the corresponding discharge.

 Infilt/Recharge: In this option we are given an infiltration input form in which we enter the measured infiltration rate (inches per hour) and an infiltration reduction factor. The measured infiltration rate is multiplied by the infiltration reduction factor to determine the model’s infiltration rate. The model’s infiltration rate is


WWHM4 SSD Table Element then multiplied by the bottom area and converted into an infiltration flow rate in units of cubic feet per second (cfs). The recharge part comes into play in later computations if the user is using that data to make other decisions. (As a side note, we have used this information in our modeling of groundwater recharge to the Edwards Aquifer in Texas.)

 Infilt (cfs): Same as the Infilt/Recharge option above, except we don’t track recharge.

 Manual/Recharge: This option is the same as “Manual” (we manually input the infiltration or channel loss values in cfs) and recharge is tracked for later use.

For our stream channel we are going to use the “Infilt (cfs)” option. We input a measured infiltration rate of 10 in/hr and an infiltration reduction factor of 1. If we think that the channel bottom will change over time by silting in we can lower the infiltration reduction factor to a value less than 1.

The measured infiltration rate will be multiplied by the infiltration reduction factor to determine the model’s infiltration rate so don’t leave the infiltration reduction factor at zero or you will not get any infiltration in column 5.

We change the “Use Wetted Surface Area (sidewalls)” from “No” to “Yes”.


WWHM4 SSD Table Element

We click “Update” to add the infiltration discharge. Note that the infiltration starts at zero and then increases in proportion to the values in the surface area column (column 2).


WWHM4 SSD Table Element

As with the farm pond SSD Table, we need to make sure that we turn on precipitation and evaporation.

Now we can run the scenario before next going on to the Mitigated scenario. (I will leave that exercise to you to do at your leisure.)


WWHM4 SSD Table Element


1. Outside of WWHM4 create the SSD Table. One easy way to create the table is to use a spreadsheet and save the file as a comma delimited file (CSV format).

Remember appropriate columns and units.

2. Locate project site on map.

3. Input Predeveloped land use information. Connect the downstream SSD Table to POC 1.

4. Browse/Load SSD Table file to WWHM4 or create stage-storage-discharge data by inputting layer and outlet structure information.

5. Check SSD Table values to make sure everything looks okay.

6. Continue with WWHM4 project set up and analysis.

7. Set up mitigated scenario.

8. Finished.


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