This is going to be long, so here is a teaser:

So that is why the letter I wrote yesterday is one of the most absurd letters I have ever written.

In writing my last post on schedules, I started thinking about another topic: Assumptions.

The same post over at The Shape of Days is what got me thinking about this topic.

I am a Civil Engineer. I know, I know, you already know that, but please be patient. See, it is possible that I could get a new reader in here….No really….stop laughing…. Anyway, as a Civil Engineer, one of the main things I deal with is storm drainage.

Some of you out there probably think that engineers are very technical, ridged people who follow precise formulas that give them exact answers…..*cough*….

Here is a dirty little secret: It is all in your assumptions.

One of my roommates in college was a Physics major. He would spend hours and hours doing homework problems that dealt with relativity, particle dynamics and other motion problems. He had one problem that dealt with a horse, his mass and velocity. I don’t remember the specifics, but he had to find the drag or some distance or something. He spent hours on it. When the professor went over the problem in class, he stated, “First, assume that the horse is a sphere…” The problem was then solved in 5 steps.

It’s all in your assumptions.

In storm drainage there are Rules of Thumb and more precise ways of quantifying the amount of runoff. Both give answers that are defendable in a court of law and they can give answers that are 25% off from one another. So which is right? They both are.

It’s all in your assumptions

First you have to define the drainage area. This is done with a topographic map and tracing the contours to determine the extent of that basin or using aerial photos or infrastructure mapping to do the same thing. This number is always expressed in acres. This is your first error. You may not get all the area or there may not be as much area in the basin because of some pipe that was installed 40 years ago changed the basin. Remember, there are 43,560 sq. ft. in an acre. So do you use 5.45 acres? Nope, I would use 5.5 acres.

Then you have to determine land use. If the entire basin is a homogenous land use, it is easier, but usually there are areas within the basin that vary widely in land use. You may have some heavy industrial, standard residential, wooded or pasture. If that is the case, then you break out each area as its own drainage basin. Again, what you call the land use is an assumption and where you re-draw the basins is as well.

Next you have to determine the time of concentration for each basin. This is the time it takes for the most hydraulically distant point to reach the area of interest. The theory is that by the time that water reaches the point of interest, the system is in steady state with the entire basin contributing flow. Time of concentration can be “scientifically” determined or more likely it is a Rule of Thumb. If it is a paved parking lot, then 5 minutes is in order. If it is a residential area, then maybe 10-15 minutes. Larger areas may be 30-40 minutes. But it is all built on assumptions of flow path, slope and velocity.

If you are using the SCS method, you take your land use and correlate it to a curve number, CN. But to do that, you have to know the soil type in the basin and what condition it is in: A, B, C or D. Once you have that, you pick a standard CN. If you have several different types in the basin, you make a composite CN. This is nothing more than a weighted average of the different CN’s.

If you are using the Rational method, you take your land use and come up with a C factor. It is similar to the SCS method, but without the soil types.

After you have all your factors, it is a simple matter of plugging them into the appropriate formula and there you have it, you now know the peak discharges for the various return storms you chose. With that peak discharge, you can size your culverts and pipes, but you can’t size your ponds.

To size holding ponds you have to have hydrographs to rout through the pond. If using the SCS method, hydrographs are generated automatically and are as accurate as your assumptions. If using the Rational method, a much less accurate hydrograph is can be generated as it involves even more assumptions about the shape of the hydrograph.

So, you have assumptions about the size of the drainage basin, the area is rounded up or down, the land use is assumed, a CN or C factor is assigned based on your assumptions about the land use and soil type and finally, the hydrographs are routed to a pond.

Assumptions.

The way you measure the volume of a pond is a big assumption. The computer program I use does an end area volume calculation. This is typical, but it is in no way “accurate”. It is a reasonable approximation. The same goes for the outlet structures. You are assuming the orifice or the riser will take the amount of water as measured in the lab. The equations for orifices and risers are themselves approximations of observed behavior.

In the end you have a peak discharge and a pond design. But it is based on assumptions, upon assumptions. It is almost like you made assumptions about each assumption.

So that is why the letter I wrote yesterday is one of the most absurd letters I have ever written.

The gist of it is that a few citizens were upset with a development behind them. They were sure that the development would adversely impact their property and would cause flooding. They have had flooding issues for years, but they felt the new development was going to make things worse. The engineer for the development met with me and we went over his calculations. He did the above exercise in drainage analysis and I agreed with his assumptions and methodology. His results show that the pond will improve the drainage situation not just for his development, but for the residents in the area. Only in the 100-year storm event (9 inches of rain in 24 hours) does the water back out of the pond and into one ladies yard. But, if we get a 100-year storm here, the entire city will have water issues. But in order to satisfy the City Manager, I wrote the letter and tried to explain how little water would back into her yard and that it would only happen in a 100-year event. The calculations show that 2.5″ of water will pond in a small portion of her back yard.

2.5″.

Think of the absurdity of that number. The engineer assumed drainage areas, land use, CN’s, time of concentration and pond volume. Based on those assumptions, I clearly stated that there would be 2.5″ of water in the lady’s yard.

*cough*

Like I said, it’s all in your assumptions.