Hi, this is Megan Poff and I'm the Field Office Chief at the USGS in Las Vegas, Nevada.
I'll be talking briefly about the survey requirements for a multiple-component indirect measurement.
A multiple-component indirect measurement could go two ways.
The first is simpler from a computational standpoint
and it just involves adding up separate indirect measurements on multiple channels.
I did one several years ago that was the combined sum of one road overflow indirect measurement
performed at a low water crossing, two slope-area computations on a main channel and a side channel
and a slope-conveyance on another side channel.
If you have a situation like this, check out the videos on each different component.
The second type of multiple-component is the focus of this video and involves
flow going through a structure and over another structure at the same time.
For example, you could have flow going through a culvert and over the road on top of the culvert.
The key difference is that you are using a common approach area, versus completely separate channels.
The computational approach for a divided flow is discussed in TWRI Book 3 chapter A4, page 33.
First things first - let's flag the high water marks.
Flag the marks upstream and downstream exactly as you would for a regular culvert computation
and make sure you had ponding on the upstream side.
Flagging for a culvert computation is discussed in detail in the culvert computation videos.
Be sure to survey high water marks upstream and downstream of the culvert
you need both the approach and tailwater water-surface elevation conditions.
When you survey these flagged marks,
take the time to plot the high-water mark profile in the field to verify that you had ponding.
It can be tempting to try to save time by not plotting those high-water marks in the field
but you will save a lot of time in the long-run and prevent future headaches by verifying that
you had the correct conditions in the first place. Nothing is worse than that sinking,
"Uh oh, I messed up" feeling!
Save yourself the hassle and just plot up your high-water marks in the field.
You've now plotted your high-water marks and verified
ponding on the upstream side and you have documented the tail water conditions.
Time to survey the rest of the indirect measurement reach!
You will need to survey the following items:
1) approach cross section
2) full culvert measurements and geometry, and
3) a cross section on the crest of the road.
Let's talk about surveying the approach cross section first.
the approach cross section is located at least one culvert width upstream.
We know that for a regular road overflow computation
the approach is three to four times the depth of water flowing over the road.
Hmm...seems like a conundrum, right?
Where do you place the approach cross section with two different sets of guidance?
Think of it this way - take the larger requirement based on the site conditions and use it.
Let me give you an example to help clarify:
If the depth of water flowing over the crown of the road was one foot,
and the culvert is 10 feet wide,
your cross section should be located at least 10 ft upstream from the culvert and road.
You can use the same approach cross section for both the culvert flow and the road overflow.
You'll apportion the conveyance appropriately when you actually do the computation.
but you could also use a contracted opening and a road overflow.
Keep in mind that you still want to verify ponding on the upstream side and
make sure you didn't have scour going through the contraction.
Otherwise, the procedures will be similar to what I described for the culvert.
You will have four major items to survey:
1) an approach cross section,
2) a cross section in the contracted section at the point of maximum contraction,
3) bridge geometry and details regarding the opening, and
4) a cross section on the crest of the road.
Don't forget to survey high water marks both upstream and downstream of the structure.
Contracted openings are described in the contracted openings computation videos,
so be sure to check those out if you are planning
to survey an indirect measurement that involves a contracted opening.
I just want to briefly highlight where the approach cross section will be located
for a multiple component situation with a bridge opening and a road overflow.
Just like for a culvert, just use the larger requirement based on the site conditions.
Most likely you will use the approach section spacing based
on the contracted opening and not the road overflow
because the contracted opening requirement is likely a longer distance.
Therefore, the approach cross section will be located one bridge opening upstream of the contraction
unless you have fully eccentric conditions
and in that case the approach cross section will be two bridge openings upstream from the contraction
If you need help in the field, call your supervisor
surface-water specialist, or indirect measurement specialist.