building a barn first curve ball

[dodge man]

Calculating the run off, rainfall amount, runoff coefficient are all pretty much simple and standard engineering practices. The same thing goes for modeling stream flows during a flood event, computer programs do it with all the inputs. What gets complicated is accurate topographic data on the stream and surrounding ground. If the topo map isn’t very good neither will the flood elevations. The truth is historic data is often the best from an old timer living in the area.

 

[Rockbadchild]

Calculating the run off, rainfall amount, runoff coefficient are all pretty much simple and standard engineering practices. The same thing goes for modeling stream flows during a flood event, computer programs do it with all the inputs. What gets complicated is accurate topographic data on the stream and surrounding ground. If the topo map isn’t very good neither will the flood elevations. The truth is historic data is often the best from an old timer living in the area.

I will challenges you on this I hope you don't mind.

The problem with historical data from a old timer is it's not a credible data for anyone other that persons, its purely anecdotal. I can't go to the town office and say this persons lived there for 80 years and he never got flooded therefore its impossible. One can be 100 year old and haven't seen a 100 year storm, never mind recording the data like antecedent precipitation or exacts water level and condition. Also things changes over time, like the forest got clear cut or they planted trees, next door farmer created drainages ditch's or they built a parking lot all these would affect his previous experience. I will stick to what I can prove and demonstrate on paper. My dad is the one who lived there the longest, we are talking about 3 house on that watershed and we are the lower so he would have the best knowledge but its is not provable or demonstrative and he only been there 40 years not 100.

Accurate topo data are the easiest it ever been to get nowadays with drone lidar survey although still can be expensive and time consuming over a large area.

I am sure you know but yes these two or three factors and coefficient are easy to find or calculate but if only these are used, but if used in a design it would result in a vast over design for an accurate calculation one would also need factor like soil abortion, overall watershead slope, roughness coefficients, and these over a large area can be hard to get right then one need to do it with three different antecedent condition variables, good, poor and bad and choose the risk you are willing to take.

 

[dodge man]

You’re right, some of the calculations are subjective. Before I retired I surveyed for a bridge replacement, I probably spent 3 or 4 weeks in the field surveying for it counting the boundary work. This was on a county road with a large stream rather than a river. It takes a lot of work for the flood calculations data in the field. I talked to the guy that had lived nearby and asked him how high the water had been, he was a long time resident. He pointed to a door sill on a garage and said the water had gotten to there. You correct, we can’t design a bridge based on him pointing to a door sill but it does help give us a lot of confidence in our calculations.

When they calculate a flood elevation we build a bridge above that number but not by much. I don’t know how much we build above it but it’s not like we build it 10 feet above the flood, maybe just a foot or two.

 

[Rockbadchild]

You’re right, some of the calculations are subjective. Before I retired I surveyed for a bridge replacement, I probably spent 3 or 4 weeks in the field surveying for it counting the boundary work. This was on a county road with a large stream rather than a river. It takes a lot of work for the flood calculations data in the field. I talked to the guy that had lived nearby and asked him how high the water had been, he was a long time resident. He pointed to a door sill on a garage and said the water had gotten to there. You correct, we can’t design a bridge based on him pointing to a door sill but it does help give us a lot of confidence in our calculations.

When they calculate a flood elevation we build a bridge above that number but not by much. I don’t know how much we build above it but it’s not like we build it 10 feet above the flood, maybe just a foot or two.

ha I see that make sense the old timer statement is to add credibility / confidences in your calculation and not as a replacement which I thought that's what you meant.

 

[dodge man]

Long time residents are actually a valuable resource. You can do all the math, calculations, computer simulations etc. but someone that saw it happen helps. The problem is what did they witness, a 100 year flood, a 25 year flood? Sometimes you can go back and find historical data on rainfall but you usually don’t bother.

Also what is a 100year flood? I live about 40 miles from the Mississippi River. I’m 61 years old and they’ve had three hundred year floods.

 

[Rockbadchild]

Long time residents are actually a valuable resource. You can do all the math, calculations, computer simulations etc. but someone that saw it happen helps. The problem is what did they witness, a 100 year flood, a 25 year flood? Sometimes you can go back and find historical data on rainfall but you usually don’t bother.

Also what is a 100year flood? I live about 40 miles from the Mississippi River. I’m 61 years old and they’ve had three hundred year floods.

yeah when I was in school they where talking about that lately the frequency of 100year flood is increasing and what number should they used when some storm exceed it, like in my case the storm precipitation I used in my calc was bigger then the 100 year storm which is odd if you had that big of a storm in the last 100 years should it be that number?

 

[paulsharvey]

I could be very mistaken; so want to say that first; But typically I thought you could build most places in a 100 year storm plain, but not in a 25 year?

Also, a 100 year event, is going to happen. in average, like every 50 years. A 10 year storm should average about every 5 years. A 100 year event is a 1% chance per year. I know that's not really how statistics work; it's not like once you reach 50 year, then you have a 50% chance, then a 51% chance; but in theory, you have roughly a 1% per year. A 25 year event; roughly 4% chance per year. One of the things that make it almost impossible to really design flood proof features; there is always the chance of a 25 year, followed immediately by a 100 year event. Or multiple minor saturating events; followed by a heavy event. Also, I dont know how frozen ground affects infiltration.

All of that disregards weather cycles; like La Nina, or El Nino, tropical storms, ect.

Guess what I'm saying; a barn in a 100 year plain, I wouldn't worry about; a 25 year plain; I would look at ways to raise the grade (if allowed).

A lot of drainage design is about minimizing catastrophic damage, but assuming some damage.

Note: not a drainage engineer; although I have listened to them speak to other engineers many times; most of the actual storm events, models, ect are over my head.

 

[Rockbadchild]

I could be very mistaken; so want to say that first; But typically I thought you could build most places in a 100 year storm plain, but not in a 25 year?

Also, a 100 year event, is going to happen. in average, like every 50 years. A 10 year storm should average about every 5 years. A 100 year event is a 1% chance per year. I know that's not really how statistics work; it's not like once you reach 50 year, then you have a 50% chance, then a 51% chance; but in theory, you have roughly a 1% per year. A 25 year event; roughly 4% chance per year. One of the things that make it almost impossible to really design flood proof features; there is always the chance of a 25 year, followed immediately by a 100 year event. Or multiple minor saturating events; followed by a heavy event. Also, I dont know how frozen ground affects infiltration.

All of that disregards weather cycles; like La Nina, or El Nino, tropical storms, ect.

Guess what I'm saying; a barn in a 100 year plain, I wouldn't worry about; a 25 year plain; I would look at ways to raise the grade (if allowed).

A lot of drainage design is about minimizing catastrophic damage, but assuming some damage.

Note: not a drainage engineer; although I have listened to them speak to other engineers many times; most of the actual storm events, models, ect are over my head.

that explanation makes sense

 

[Rockbadchild]

Slowly getting questions answered, my lawyer told me its not up to him to tell me if and where I can build on a propriety before purchases, which I find odd, like I buy a propriety with intention to built and he could let me purchases a propriety that I can't built on.... very strange. anyhow... also learn from the ministry that flood maps are only a tool to identify flood plains and municipality are entirely responsible for the mapping of them and its for the municipality to judge the best method to identify flood plains to minimize risk and hazard. So in other word they are the only responsible party and they did that map. I will stop sending emails to them now and will go there in persons and will put reason in this person in question.

 

[Rockbadchild]

so finally got a response from the city and their consultant, i would need a certified/ registered surveyor to confirm my work is empirical evidence but the only registered firm is 3h away and i doubt they will do it for cheep or even take my work for granted and slap their name on it …. i could sign a waver removing the city responsibility in that regard so I will just do that. It’s pretty frustrating that my data need to be verified but theirs doesn't.