Yes, even ditches can cause turbulence

[peek 21]

Yes, even ditches can cause turbulence when it gets windy enough! Yesterday we saw an experienced jumper land really hard (yet fortunately not get seriously hurt).

Winds were about 18MPH or so with no gusts. Canopy was a Spectre, wingloading was about 1.0.

There is a very shallow ditch adjacent to the landing area. Its geometry changes along its length, that is, it is not just a ditch that is a shallow depression along its entire length, but instead turns into more of a slight hill in this one area, which is the area that caused trouble for this jumper.

When the wind is coming from a particular direction, it can blow unobstructed upwind of the "hill" and then when it reaches the hill, create an upward flow which turns into a rotor and downward flow a bit downwind of that. (In other words, it creates kind of a ramp which makes the wind shoot upward.)

This jumper happened to land where the rotor was, and it caused her canopy to lose most of its lift right as she was flaring. She said she noticed this was happening but it was too late to do anything differently. She had to do the best PLF she could.

I see this kind of thing on a regular basis (and not just caused by this ditch), and it happens to all kind of skydivers, mainly with low to medium wing loadings, but this occurance was more extreme than most I see.

For a person with not a lot of jumps that thinks they are landing in a clear area with no obstacles upwind, having this happen is very confusing.

So hopefully what we can learn from this is:

1. Ditches can cause troublesome turbulence at some wind speeds.

2. We all need to continue learning how to visualize the wind and what it is doing in relationship to nearby objects.

3. We always need to be prepared to do a PLF at the last second because there may not be time to do anything else.

Hope this helps.

[Gene03 0]

I've been trying to teach people about turbulence for years.
The most effective way, with permission from DZO/neighbors, etc. is a smoke grenade. When I tried to have them "see" the wind all I got was huge amount of laughter and questions on what type of substance I was abusing that allowed me to "see the wind".
Smoke grenade, great visualation tool.

“The only fool bigger than the person who knows it all is the person who argues with him.

Stanislaw Jerzy Lec quotes (Polish writer, poet and satirist 1906-1966)

[billvon 3,111]

>2. We all need to continue learning how to visualize the wind and
>what it is doing in relationship to nearby objects.

I'd recommend the book "Understanding the Sky" by Pagen for a great overview of what happens to wind near the surface, and what trees/hills/valleys/open water etc does to wind.

[ChrisL 2]

From your description, it sounds to me like you are saying slight hill
can cause turbulance.

__

My mighty steed

[LouDiamond 1]

Good points Gary and I concur with what you have said. I personally watched a jumper under a 370 sq ft Military canopy have his canopy completely collapse while passing over what could be called a ditch, gully or an arroyo in Yuma Arizona several years back. The jumper was landing off the intended DZ and was approximately 20-30 feet above the ground when he passed over the ditch. The sudden collapse of the canopy caused the jumper to fall almost straight down from that height and he was knocked unconscious as well as dislocated his hip upon landing/impacting the ground. The turbulence created by those ditches could be felt under canopy at higher altitudes as well as a series of ditches ran parallel to one another about 10-30 meters apart and had to be flown over to make it back to the intended landing area. With the speed his canopy collapsed and his relatively low altitude I don't think there was anything he could of done to make the situation better. IMO, the only thing one can do is be cognizant of this fact and make all possible attempts to not fly over or land in close proximity to ditches when landing off the intended landing area.

"It's just skydiving..additional drama is not required"
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SKYMONKEY PUBLISHING

[DJL 235]

http://www.forestry-suppliers.com/product_pages/View_Catalog_Page.asp?mi=6206

These are really good for that application.

"I encourage all awesome dangerous behavior." - Jeffro Fincher

[Gene03 0]

Sweet, thank you.
Air flow studies, that would be it.

“The only fool bigger than the person who knows it all is the person who argues with him.

Stanislaw Jerzy Lec quotes (Polish writer, poet and satirist 1906-1966)

[strop45 0]

Thermals(rotors) are caused by differential heating and ditches provide an ideal place for thermals to form. A ditch is sheltered from the wind, thereby allowing hot air to accumulate, then be sucked out forming a thermal. This can also occur in the lee of buildings, especially if the area is asphalt.

Thermals and wind are not mutually exclusive, i.e. even on quite windy days there may still be thermals about. Stand on any hill top with a town upwind on a windy day and note the temperature and the wind speed. The 'hot' spots when the wind dies are not just due to an absence of cooling, they are caused by thermals passing over.

The difference between stupidity and genius is that genius has its limits." -- Albert Einstein