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During the design process, it is always challenging to select the proper material, especially when there are multiple constraints attached to the project. Professional engineers are constantly dealing with multiple issues that require in-depth knowledge of the engineering field that relates to the project.

Stormwater Detention -Nose Creek, Calgary

Management skills become more important nowadays as the project goes through the following five phases:

  • Initiating
  • Planning
  • Executing
  • Monitoring & controlling
  • Closing

Every project should be analyzed according to the demands, and more importantly the constraints. In the stormwater management world, as municipalities continue to grow and expand, engineers are responsible to ensure that a mechanism is in place to mitigate the excessive runoff that could potentially make its way into the sanitary sewer system that will ultimately increase the inflow\infiltration ratio. This could result in overloading the pipe network in terms of its capacity that was originally designed. Each site needs to have a system that allows for the stormwater to be retained and slowly delivered to the existing city’s pipes. Overland surface areas can be a great solution to maintain this volume of water using parking lots (for small areas) or retention ponds for larger areas such as subdivisions. However, the retention on surface areas are limited to different design elements. On multiple situations, these surfaces areas are more valuable since can be used for more construction that will generate more revenue for the developer rather than the usage of these large areas for retention systems. So, what should we do instead? If we have this problem, is there a way that we can use to control the runoff from hard surfaces, roofs, green areas? Well thankfully, for any difficult problem there is a solution in life except for two things according to wise folks: taxes and death itself, these two cannot be avoided. A little bit too extreme but it is a true statement.

Part of the solution would be the installation of a retention system that will go under the ground, This could simplify the issue of having ponding on the surface and can mitigate the problem of having massive areas just to store water, such as retention ponds. As an engineer, we come up with very sophisticated solutions for problems and as an engineer somehow, we foresee problems that are not in the present. Are we in some way fortune tellers? Probably not, but it is nice to think that way.

The selection of the material that will go under the ground will be the key of the design itself. Engineers are designing these projects based on the requirements of the Municipality/City and in most cases, the stormwater management calculations are based on a 50-100 yr. storm event. The selection of the material should reflect the design projection, and this is where the project manager/design engineer has the authority to estimate and analyze what system should be applicable to the project. The designer has different options for the retention system that will go under the ground, such as: Corrugated Metal Pipe (CMP), Plastic Arches (HDPE) and Precast Concrete.

The Tanger Outlet Mall in Lancaster, Pennsylvania, U.S. summer 2018 installed an HDPE underground detention system that quickly failed.

However, not all the systems behave the same. Flexible systems are extremely dependent of the soil envelope and installation techniques which can be an issue, considering the fact, that installation sometimes can be very tricky if the contractor does not have enough experience to execute the installation for plastic materials. Precast retention systems are not very dependent of the soil envelope, and structurally speaking offer a great solution if the site has live loads and dead loads that needs to be addressed. The retention precast concrete systems are manufactured in a controlled environment which can save time in the procurement process of the project since the manufacturer can control production; saving time to the owner in terms of the installation and more importantly, money.

It is imperative to recognize the difference on each material and its behavior. Selecting a product without knowing how the material will perform can be considered as negligence and can be punished by the association or any jurisdiction competent that can prove negligence.

Purchasing a good watch can be expensive, but in the long run, a watch that was bought with meticulosity and patience will be more reliable than a watch that “will do the trick”. At some point what cost less will be more expensive than the reliable watch that cost a little extra more.

The Tanger Outlet Mall in Lancaster, Pennsylvania, U.S. summer 2018 installed an HDPE underground detention system that quickly failed.

As the analogy of the watch, engineers and contractors have options to install something that can be cheap from the get-go, but if the whole system collapses after the installation; what options do we have? Blame the product? Blame ourselves or blame the contractor? Ideally no one, but that is part of the real world. The only solution is to select the right material from the beginning. Precast retention systems can be a powerful solution that can offer capacity to withstand heavy loads upon the structure (live loads, dead loads, lateral forces etc.) inclusive, this material due to its density can be more reliable to buoyant forces than its counterpart, the flexible materials.

Protect yourself as an engineer and take care of that seal that was given to you. Do it right the first time, it can be less expensive.


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