Stormwater Management: A Zero-Sum Game?

Posted by Guest Author on 7/7/16 11:30 AM  |  8 min read time

By David Chadwick, Editor of CAD User

*Editor’s Note: This article originally appeared in CAD User's May/June 2016 issue 

Stormwater management using site analysis tools in Vectorworks Landmark software was the focus of a fascinating presentation at the recent Vectorworks Design Summit in Chicago.

Looking for safe areas to put new developments on a small and overcrowded island is going to get more and more difficult as time progresses. It is not only large scale rural developments that need to take greater account of the areas they hope to be building on either – small plots will also impact to some degree on an area’s ability to withstand increasingly cataclysmic wet events.

Proof of that in Somerset, for example, is the choice of crops adjacent to main roads. Sweet corn, or maize, left to mature as cattle fodder, compacts the soil so that it inhibits water absorption - which then runs off and floods sections of roads previously untroubled by flooding. Building a small residential development that similarly hinders water retention and safe removal will have the same effect. We should work to zero net effect - i.e. does the project we are working on materially alter the flow in and out of water compared to its undeveloped state?

There is also the other aspect of water management, the re-use of grey water by filtration, and the replenishment of precious water resources, which, it is calculated, will be rather scarce within the next fifty years - a fact of vital importance to burgeoning urban areas. This is something that we will all no doubt become more aware of over time, but here we are mainly concerned with achievable results in small-scale developments.

We can calculate water throughput using tools available in Vectorworks software, and manage it by making the development more stormwater-friendly, by dispensing with gutters and downpipes, and letting water flow naturally onto permeable pavements, as they do in some parts of Canada, which assist in ground absorption of water. We can also analyse predicted weather patterns and stormwater events over a number of years for a particular area, and install a system of drainage pipes, gulleys, stormtanks, swales and other features to handle the most extreme of them.

Having covered Stormwater management solutions provided by dedicated hydrological software developers in past issues of the magazine, I was very interested to see how the problem of calculating stormwater flows and designing drainage systems to handle them would be handled within Vectorworks software's broader range of design solutions. I was gratified to discover that the flexibility of design tools within the software covered all pertinent issues.

Site analysis in the Landmark module of Vectorworks is the starting point for stormwater analysis. This allows users to build up a DTM or 3D terrain model of a site from imported survey data, define the site limits and its watershed, show the flow of water off the site through gradients and arrows, and then use Vectorworks site modifiers and massing tools to carve out the site and position the proposed construction elements. And, of course, to design and lay out the water management features.

Image courtesy of Grey Leaf Design, Inc.

Each water management component or feature comes with dimensions, capacities and throughputs, smart objects which can be aggregated within Excel-like worksheets within the software. Using Layers and Classes to keep the worksheets simple, and easily created using either the Create Report feature in Vectorworks, which selects information from the selected objects in the model, or using the Resource Browser to create a blank worksheet and fill it appropriately. Once created, it is saved with the model within the Resource Browser.

Specific components, such as Brentwood Stormtanks, an American stormwater holding tank which is also available globally, and similar, obviously come with performance capabilities attached.

Worksheets are also used to calculate other critical aspects of stormwater management. Site Impervious Cover Calculations provide localised existing and proposed imperviousness conditions - how fast water will dissipate through the ground. Phosphorous Export Calculations provide the data for determining pre- and post-development pollution levels, and what is required to minimise pollution.

There is a handy tool that can take all of the data from Vectorworks worksheets and perform the numerous calculations required to satisfy the projects requirements. The downloadable MIDS Calculator (Minimal Impact Design Standards), developed by the Minnesota Stormwater Manual team that is a part of the Minnesota Pollution Control Agency, is designed to keep the raindrop where it falls in order to minimise stormwater runoff and pollution and preserve natural resources.

The manual provides volume and pollutant reductions based on the assumption that the Best Management Practice (BMP) is properly designed, constructed and maintained. It also gives copious guidance and recommendations for design, construction and maintenance of stormwater management systems.

Best Management Practice represents the best proven solution for each type of water management system. BMPs are created by looking at a number of solutions - designing by worksheets - and coming at a solution obliquely. This provides data that sets the boundaries of what might and what will not work. Through not trying to get the right solution the first time, a range of trial solutions will be recorded, incidentally providing some planning authorities with the reports and background calculations to support planning applications if they ask for them!

Image courtesy of Grey Leaf Design, Inc.

That's the theory at least. Some more practical elements need to be considered though, such as the site's impervious coverage limits and the amount of water retention to aim for - say 80% for a level 1 event, and 60% for a level 2 event - an event being an expected storm once every 10 or 100 years in a given area (figures that I suspect need drastic upward revision judging by recent weather events).

Keeping Watershed Events in a separate Design Layer enables flow rate calculations to be performed merely by dropping the appropriate worksheets on the Design Layer. Multiple worksheets allow different aspects to be calculated - flow rates, phosphorous export rates, perc (percolation) rates, RR (Restoration and Recovery) rates, and post-construction stormwater run-off. Calculations specific to installed stormwater equipment is valid worldwide. Rainwater, soil imperviousness and other natural effects are local phenomena and appropriate data is available for download into yet more worksheets for adjusting local data.

If the selected BMP uses the correct design criteria - and it is suggested that BMPs are used for individual elements of the water course - users will be able to calculate the system capacity, water capture and drawdown times, which, when aggregated, will provide data for the correct sizing of drain pipes.

Stormtanks, such as the Brentwood Stormtanks mentioned earlier, can be combined to form underground temporary water storage areas below driveways or garden areas, but require additional infiltration and impervious membranes to retain water. Another stormwater management option that can, surprisingly, be assessed using Vectorworks worksheets, is bioretention - where contaminants and sediments are removed from rainwater run-off by creating natural drainage features, which allow sediment to be captured by plants in natural ponds, stormwater ponds and wild-life wetlands.

It appears then that designing a stormwater management system for any given area is entirely possible using Vectorworks software. The mathematic algorithms you will need are available within the MIDS calculator. So as long as you are familiar with the function of the different stormwater goods and features you would like to include in your model and their capabilities, can download local weather data, and are familiar with Vectorworks worksheets, then Vectorworks Landmark could be the solution for you.

Topics: Brentwood Stormtanks, Site Analysis, stormwater analysis, CAD User, David Chadwick, Grey Leaf Design, Inc., Landscape, Vectorworks Design Summit, Vectorworks Landmark

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