Depth to Weir and Initial Fill Value
MICHAEL SPINA
Stephen Stone
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MICHAEL SPINA
| Month | Elevation | depth to weir (in) |
| 6 | 95.59 | 32.03 |
| 7 | 94.18 | 15.09 |
| 8 | 93.91 | 11.89 |
| 9 | 93.91 | 11.91 |
| 10 | 94.02 | 13.17 |
| 11 | 94.32 | 16.80 |
| 12 | 95.68 | 33.12 |
| 1 | 95.03 | 25.32 |
On Friday, December 23, 2016 at 12:09:05 PM UTC-5, sston020 wrote:
Hi Micheal,
You basically right in your interpretation.We define the 'depth to weir' as the difference between the 'wetland bottom elevation' and the outlet, or elevation at which water will leave the site.For example, if the average elevation of the area that could become ponded at your site is 235' and your outlet is at 235.5' then your depth to weir would be 0.5'.Building on that same example, if at the start of your modeled period the elevation of the water in your site is 235.25' then your initial fill would be 0.25'. The 'initial fill' would be how much water is retained behind the weir at the start of the period you are calculating a water budget for.It's important to keep in mind that the basic scenarios in Wetbud calculate everything relative to the wetland bottom elevation. Sometimes it helps to remind yourself of that reference point when thinking about how to represent your site and how to interpret the model output.Hope this information is helpful.Happy holidays!Stephen
On Dec 23, 2016 11:51 AM, "MICHAEL SPINA" <michae...@gmail.com> wrote:
Hi and happy holidays!I have a question about how to calculate depth to weir and initial fill value. It would e helpful to understand how these values are defined and how they may be interrelated. In my specific project site,I am looking at an existing ag. field as the site of a potential wetland mitigation project. We have 3 monitoring well on site and there is a long narrow drainage ditch running down the middle of the site with a culvert/ outfall on the downstream end.Is depth to weir the difference between the observed groundwater elevation and the outfall weir elevation? Seems like this might result in a negative value. Is initial fill value the volume of water that would fill that difference in elevation?Any help is much appreciated. Thank you!
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Stephen Stone
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Rolband, Mike
Michael:
I thought I would chime in with some user perspective – as we did develop the predecessor model that the Research Team has taken a leap forward with – as the “Depth to Weir” concept stems from that and the Original “Pierce” model.
The depth to weir is a positive number that is the distance from the weir invert elevation to the ground elevation if the wetland is level. When its irregularly shaped – we use the average grade of the wetland. To be precise we use a weighted average of the wetland surface grade. In the old days I used a planimeter to measure the area at each 6-inch contour and did a weighted average. My staff now uses AutoCAD Civil 3D to do the same thing way more rapidly.
If you are using the Basic model and setting the weir invert at the bottom of the ditch invert – it will assume that the surface water is always at the bottom of the ditch. Normally in our area we are “plugging the ditch” and thus ranging the weir invert above the ditch invert. The Basic Model does not model ground water except for a user input for inflows and a soil Ksat derived rate for outflow.
I will defer to Stephen on the Advanced model – as that will allow you to account for the groundwater way better – and allow for a wetlands system above the weit invert.
Hope this helps. (added Lee also in case he can help more).
Mike
Michael S. Rolband, P.E., P.W.S., P.W.D., LEED®
AP
President
Wetland Studies and Solutions, Inc.
5300 Wellington Branch Drive, Suite 100
Gainesville, VA 20155
direct: (703) 679-5602 | main: (703) 679-5600
mrol...@wetlandstudies.com
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MICHAEL SPINA
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Stephen Stone
Stephen Stone
Michael,
We have been working on a solution for your situation, and I think we have something that will work.
As I said earlier, I believe that solution involves creating two scenarios, one for each side of your ditch. Use Mike’s suggestions for determining a weighted wetland bottom elevation for each side of the ditch. If the weighted wetland bottom elevations on each side of the ditch are the same, you can develop two ‘Basic Scenarios’ within the same ‘Project.’ However, if the weighted wetland bottom elevations are not the same, create two Projects, as wetland bottom elevation is set at the ‘Project’ level.
This new approach would have all surface water leave the model at the end of each month, as the weir elevation would be the same as the wetland bottom elevation, and the depth to weir would be zero (i.e. no surface water is stored in the wetland, which appears to be the existing condition). The model would predict groundwater levels on either side of the ditch as water levels below the wetland bottom elevation. When you interpret the model output, be sure to refer to the ‘Actual Water Level (J),’ as this is the predicted water level adjusted for available storage space in the soils and it would be the more accurate representation of the water table elevation. See screenshot for reference. A ‘Water Level’ of 0 in this example would be equal to the wetland bottom elevation.
The screenshots, at the bottom of the message, show (1) how I think Wetbud is interpreting the information you have entered, (2) the alternative approach for modeling the existing conditions, and (3) a sketch of one way you could approach the restoration design model.
To incorporate a groundwater component in either the alternative model approach (2), or the restoration design model (3), you could:
(a) Do a hand calculation of the groundwater flux using the stream gage data and nearby well data, or an assumed constant elevation equal to soil elevation on either side of the stream (if it is persistently saturated, otherwise, you may have to use another justifiable constant elevation), to determine your gradient for groundwater flux. The flux you calculate will be volume/time. Wetbud’s Basic Model does everything in length/month. To convert, divide by the area of the wetland. If you have two scenarios, divide by the areas unique to the scenarios to determine your flux for each scenario. To enter your hand calculated GW fluxes, go to Basic Scenarios > select your scenario > Inputs and Outputs > Water Outputs, you will see ‘Groundwater OUT Options’ near the bottom of the page.
(b) If you have well data, you could use the well data and the stream stage data to calculate groundwater flux in Wetbud using the Effective Monthly Recharge (WEM) model. Let me know if this is the road you want to go down. There are instructions in the manual for using WEM, but I can certainly help with that as well.
(c) If your restoration design model plugs the ditch at the same elevation as the ground surface, or creates a retention structure with an elevation greater than what is now top of bank, you can use the default groundwater out value of 1 in/mo. If you put the plug elevation lower than what is now top of bank, you can recalculate your GW flux estimate based on the elevation of the plug relative to the wetland elevation to reflect the designed, lower, hydraulic gradient.
Here are the figures I mentioned earlier:
