Pond Sizing
While our intent is to not produce a manual detailing ever aspect of the User Interface (most of which no-one ever reads) we will spend some time on the Pond Sizing features. Basically, for each FTable, the program will allow the association of a storage structure and some combination of discharge structures. There really isn't much to say about the storage structures, but since we are going to talk about the discharge structures, we will start with storage.
Your storage structure (pond) can be trapezoidal or vault shaped. It can be a shaped as a circular tank, or and arched shaped tank, it can be defined as a depth area relationship. In addition to the standard shapes, we support a sand filter and gravel trench. There is nothing unusual about how any of these structures are defined. However, you can specify a void ratio for the structure. A void ratio of 1 means that it is 100% voids, a void ratio of 0.3 means there are 30% voids. The bottom elevation is optional. It is set to zero, but can be set to the bottom elevation of your storage structure. The effective depth is the maximum stage that the program will make the FTable. However, if you intend to use Saturated Conductivity (KSat) as the basis for infiltration, then a real bottom elevation will be needed because KSat needs the elevation of the ground water table.
Sand filters are nothing special, but requires that you first define the shape based on the TRAP storage structure. After that is defined, change the storage type to Sand Filter and specify a filter material depth. Before computing the rating curve or FTABLE, you must be sure that the storage shape is Sand Filter and not Trap. We will discuss the particulars to the Sand Filter in our discussion on Discharge structures. The Sand Filter is a water quality BMP whose purpose is to infiltrate or treat a certain volume of runoff. The volume of runoff is determined by Best Engineering Practices, or in the case of Washington State, 91% of the runoff. That represents the water quality event. When a time series is passed through the filter, the toolkit reports the percent infiltrated.
Gravel Trenchs are also supported. This is a not a water quality BMP. It is different from other storage structures in that the user is able to specify up to three layers of materials along the bottom of the trench. Each layer can have a different porosity. When the Gravel Trench is selected, the void ratio field is disabled to ensure that the porosity take precedent in the formulation of the storage rating curve. The rating curve for the gravel trench assumes that the water surface is parallel to the bottom of the trench. This means that it assumes flat slopes.
Discharge Structure:
The discharge side of the detention pond is much more interesting. The program will automatically recognize a combination of (starting from the bottom):
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Infiltration (optional). If it is selected, the infiltration can be a straight in/hr OR a KSat value (more on this later). If can also be applied only to the wetted bottom or to the top area.
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Up to three intermediate orifices. All are optional. If a low orifice is not selected, then the lowest outlet can either be a custom rating curve or a Thirsty Duck™! Incidentally, the Thirsty Duck (3/2008) has just received approval from the WSDOT and WSDOE as an approved structure.
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Next there can be either a rectangular, vee or sutro shaped weir.
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Finally there can be an overflow riser.
If you are going to use a control, you need to specify a height (in ft) from the bottom of the pond to the control. The riser height should be BELOW THE effective depth of the storage structure.
Infiltration
The program supports a simple infiltration rate in in/hr. If you have the soils data, you can specify a Saturated Conductivity value (KSat). If you specify a KSat, you can also indicate whether the pond has a high potential for biofouling and whether there is an average to high degree of long term maintenance. It will also ask you to enter the elevation of the groundwater table below the pond.
What does the program do with all this data? It computes infiltration using Darcy's law as outlined in great detail in the 2005 Washington State Department of Ecology Volume III Section 3.3.8.
Based on the above reference, the Toolkit extends the functionality by allowing you to create a database of Soils Logs, where the program can then compute the KSat directly from the log instead of requiring you to manually compute the KSat for the log for entry into the program. Since the program supports a database of soils logs, a different KSat can be computed for each pond!
The benefit is of using the soils logs is in the detail. If you have a soils profile with numerous soils layers, the KSat will change depending one how much below the surface is excavated to reach the proposed pond bottom. Doing it by hand (or checking) each time you change the bottom elevation of the pond can be tedious.
If the storage structure is a sand filter, infiltration is only applied to the bottom area. In addition, infiltration is adjusted based on the water depth above the sand filter. The adjustment is ((depth+filter thickness)/(filter thickness))* infiltration rate.
Rectangular Weir
One would think rectangular weir computations are fairly straight forward, but apparently not. The toolkit uses the standard implementation where the weir length is adjusted by 0.2 times the depth over the weir. The methodology is described in the 3G documentation. It has come to our attention that some programs, such as WWHM3 uses a constant Cd of 3.33.