STAGE-STORAGE-DISCHARGE (SSD) TABLE ELEMENT

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STAGE-STORAGE-DISCHARGE (SSD) TABLE ELEMENT

© Clear Creek Solutions, Inc., 2010

The Stage-Storage-Discharge (SSD) Table element is a conveyance element. It canrepresent a pond, tank, vault, pipe, stream channel, river reach, dam, lake, or any flowconveyance pathway. Its flexibility in being able to represent a wide variety of differenttypes 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 situationwhere a pond has already been designed (and perhaps constructed) and there is the needto see if it meets the local jurisdiction’s flow control standard.

If the pond dimensions and orifice information are available then everything can be inputin the appropriate pond element without using the SSD Table. However, there are timeswhen all that is available is the stage, storage, and discharge information (often in theform of a table) for the pond. In these situations the user can input a SSD Table insteadof 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 cannotbe automatically sized to meet flow duration standards by WWHM4’s AutoPond. Theuser must manually size a SSD Table facility by independently changing the values in theSSD Table.

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

We will model a 10-acre site in rural King County, Washington, near the city ofEnumclaw. The first thing that we will do is to locate our project on the project map.

WWHM4 SSD Table Element

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

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


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


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

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


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To input the pond cross-sectional dimensions we first check the “Stage Computed” box.This turns on the “Add Layer” button.


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

The bottom elevation is the starting depth for this layer of information. If this is thebottom 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 bottomelevation equal to the effective depth of the layer below it. This will make more sensewhen we input the second layer.

We input a bottom length of 10 feet and a bottom width of 8 feet. The effective depth is2 feet (that is how high this layer goes up the side of the pond). All of the side slopes are1 foot horizontal to 1 foot vertical.

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


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WWHM4 automatically fills in the stage (from 0.0 to 2.0 feet) and corresponding surfacearea 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.


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


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

First Layer

Second Layer


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Just to show how it is done we will add a third layer. The bottom elevation starts at 5 feetwith 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 thestage, surface area, and storage columns.


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


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

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

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


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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 torepresent the weir with a notch. The weir is at 5 feet (the farm pond is 6 feet deep) andhas a width equal to the riser diameter times pi (the circumference of the riser). Thisequals 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 0.5feet 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.


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


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We have now filled in all of the needed stage-storage-discharge information required toroute 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 theappropriate row number on the left side of the table to access the available options.


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


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The last thing that we want to do for the farm pond is to turn on precipitation on the pondand evaporation from the pond. We do this by checking the box for each. Make sure thatboth 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.


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

We have the choice of adding the stage, surface area, storage, and discharge data into theSSD Table directly or entering the data first into another file and then uploading the fileto WWHM4.


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


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

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

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

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

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

Columns 5 through 8 are optional discharge columns. These columns are used ifthe 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 allof the values in this column are zero by default.


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To input discharge values in columns 4 through 8 first click on the column heading (thedefault 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 ontyping them directly into the column on the element form.


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The SSD Table element initially contains no stage-storage-discharge information in theSSD Table. We need to load the SSD Table information from a file. The Browse buttonallows 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 whenloading an external file for input to the SSD Table.


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After the file is loaded the stage-storage-discharge information is displayed in theWWHM4 SSD Table. Check it just to make sure that everything got in there okay andlooks like it should. You still have the ability to make changes on the SSD Table elementform, 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 thespreadsheet values will be shown in the column.


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For the stream channel SSD Table we still need to input the infiltration in the streamreach that corresponds to the information that we have about the reach being a losingreach. 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 moreoptions than column 4 does. This is because column 5 is the second outlet for aconveyance element and, by default, represents the infiltration outlet. However, in theSSD Table we can use this column in any of the above-listed ways. Let’s take a minuteand look at our options:

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

Outlet Structure: We used this option in the farm pond SSD Table. To refreshyour memory, we input riser and orifice data into the outlet structure form andWWHM4 computed the corresponding discharge.

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


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

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

Manual/Recharge: This option is the same as “Manual” (we manually input theinfiltration 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 measuredinfiltration rate of 10 in/hr and an infiltration reduction factor of 1. If we think that thechannel bottom will change over time by silting in we can lower the infiltration reductionfactor to a value less than 1.

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

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


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


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

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


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SUMMARY:1. Outside of WWHM4 create the SSD Table. One easy

way to create the table is to use a spreadsheet and savethe 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 andoutlet structure information.

5. Check SSD Table values to make sure everything looksokay.

6. Continue with WWHM4 project set up and analysis.7. Set up mitigated scenario.8. Finished.

Documents

STAGE-STORAGE-DISCHARGE (SSD) TABLE ELEMENT