Improvement of Stormwater Module in MiTS 3 over MSMA Module

In MiTS 3, its Stormwater module is quite similar to the detention pond in the MSMA module. However, the Stormwater module is not exactly the same as the detention pond in the MSMA module. So, what are the differences between the two modules?

User Defined Hydrograph vs. Predefined Hydrograph #

In the MSMA module, you cannot input your own inflow hydrograph. You will have to analyze your detention pond project first, and then only you can get the inflow hydrograph from the report. The hydrograph created in MSMA was a predefined hydrograph that was calculated from the existing data based on the rain gauge location that was selected in the pre-development design process.

If you have new inflow data, you can’t use it in the MSMA module. However, you can use it in the Stormwater module because it now supports user-defined hydrograph. To use this feature, you will have to go to;

Stormwater > Input > Inflow Source > Tabular Flow > Node Inflow > Tabular

So, how to insert the inflow data inside the pop-up User Defined Hydrograph window?

Choose Node > Edit > Add ARI > OK > Selected ARI > Choose ARI > Input data

Multiple Pond Design vs. Single Pond Design #

In the MSMA module, you can only design one pond at a time. If you have two or more ponds to be designed, you will have to design them one by one. The example image below shows that you can only create one pond per Pond module. Such a limitation means that it’s very difficult to create ponds connected in series, or if you want to consider a drain sump for its node storage capability.

However, with the Stormwater module, you can design two or more ponds in your design. You can refer to this example project file of multiple pond designs. As shown in the image below, there are two ponds designed in the file, and they can be connected in series or in parallel. not only that, the ability to set a drain sump as a storage unit means that the drain sump can be designed for storage ability, which means that you can do more economic design because your simulation is closer to reality.

Correct Detention Drain Design vs. Manual Detention Drain Design #

A detention drain has the purpose of acting as a conveyance and storage at the same time, especially during the rainy period. Therefore, some sites may require the design to have multiple detention drains. However, in the MSMA module, the detention drain can only be applied to a single network drain for its dimension design and if the detention drain network consists of multiple branches, then you need to approximate the drain network into a pond with the same capacity. A lot of manual adjustments are needed and you can see the example in this blogpost.

Not only that, the MSMA module detention drain design is catering towards the storage purpose only; it’s as if the drain has no conveyance ability, and one needs to analyze the drain for conveyance ability according to the manning equation in the Drain module (which when we do it, it’s as if the drain has no storage capability). however, a proper detention drain design must be analyzed for both the conveyance and storage capability at the same time.

Manually syncing IL between MSMA and Drainage module

With the newly implemented Stormwater module, we solve this problem elegantly and entirely. You can design your detention drain with multiple branches directly in the module. Just analyze everything as one piece without any manual intervention. The software will consider both the conveyance and the storage requirements together because the software is doing the level pool routing (i.e.: the whole water flow simulation) on the entire drain system according to the designed hydrograph. This is the only way to ensure that the detention drain is designed accordingly.

Water level in detention drain

Flow and velocity in one of the drain

Catchment Routing Method – Exact vs. Approximate #

Catchment routing was calculated from the catchment defined in the designing process and also the overland flow time and drain time (tome of concentration).

In MSMA, the overland flow time and drain time can only be input once. Therefore, if you have multiple catchments, you will have to find out manually which catchment is the effective one. The image below shows that we can only input the time of concentration’s elements once, if you have two or more catchments, then you will have to find a way to “average” out the overland length, drain length and the catchment areas, runoff coefficient and fit them into this one input.

Meanwhile, in the Stormwater module, you not only can insert multiple catchments but also input different overland flow times for each catchment, and the software will handle the rest, with no need to explicitly compute the overland length or time. You don’t have to do much manual averaging, the software can compute for you.

The example image below shows the difference in results between having only one catchment and two catchments to one single drain. We can see that both graphs that have one catchment have a lower inflow than the graph with two catchments. Take note that both single catchment is the same catchment from the drain with two catchments. This means that the graph for the two catchments is the summation of both inflows. We also have different overland flow times and slopes for each catchment. The software will be able to calculate all the overland time and inflow by itself and sum them up accordingly.

C1 Max Inflow ≈ 2.65 m3/s

C2 Max Inflow ≈ 2.05 m3/s

Different overland flow time

Max Inflow ≈ C1 + C2 ≈ 2.65 m3/s + 2.05 m3/s ≈ 4.7 m3/s

The effect of the time difference between different catchments is even more pronounced when you have a series of pond, take a look at the below inflow hydrograph to Pond B, and note that the inflow hydrograph is not the designed shape (e.g.: Trapezoid, Triangle) that is specified in MSMA, because you can’t read it off a MSMA manual. You have to properly sum up, at each time interval, what is the inflow contribution from the previous orifice and the catchment flow. This is the only way to ensure that the inflow hydrograph is correct and to ensure that your pond design is accurate.

Conclusion #

There are many limitations when it comes to the MSMA module, simply because it’s just a textual calculation pad in essence, and contains very limited simulation ability. There is no easy way to enhance it, that’s why we rewrote the whole thing and integrated it within our GUI interface so that it’s more general, intuitive and easy to use. At the same time, we also ensure that the results are accurate by comparing them with the EPA SWMM engine, with MSMA module and drain manning calculation results under appropriate conditions.