With current technology including more precise surveys, aerial photography, GPS, GIS, you name it, it has become increasingly easier to design projects without spending any real time on the site. And more often than not when the project is done, the only time we go to visit the site is if there's a problem. I confess that I used to fall into this trap as well. But lately whenever we get a significant rainfall (0.5"-1" or more-which seems to be happening with greater frequency - but that's for another blog post) I try to make an effort to check out some of my projects and other areas that have stormwater challenges. My wife finds this behavior quite amusing, but it has provided me some insight on stormwater design and basic hydrology that I couldn't get from the digital information that I have. Here are some of the rather simple but powerful insights that I have gained from standing in the rain looking like an idiot:
- Lawns and other areas that we often consider pervious aren't - It's common practice to assign lower curve numbers or runoff coefficients to lawn and/or grassed areas, which is a reasonable assumption to make. However, I think that more often than not these areas create more runoff than we give them credit for. One only has to stand in the rain in a residential subdivision watching the runoff from lawns to realize that while these areas are certainly not as impervious as pavement or roofs, they certainly produce significant runoff.
- Topographical surveys miss a lot - Topographical surveys are an essential part of the design and planning process and are a great tool for determining runoff patterns, but they can't replace some time on the site, especially in the rain. A few minutes in the rain will usually reveal small nuances in the topography and runoff patterns that even a good topographical survey will miss. And those nuances can have a major impact on the stormwater performance of your site.
- Better pay attention to your erosion controls - In my experience erosion controls are usually compliance based and not performance based. That is to say that they are designed and installed to the minimum standard required by law and to keep from being fined. Unfortunately, the result of this is often under performing erosion controls and a quick visit to the site during a heavy rain will reveal this.
- Detention ponds rarely (never?) mimic pre-development hydrology - Have you ever watched the discharge from a detention pond during and after a rainfall event? It almost never resembles the pre-development hydrology. Most detention ponds are designed to meet municipal standards for discharge, which usually focus on flood control and do nothing else to match pre-development hydrology. Never is this more clear than when you watch the discharge from a pond during and after a storm.