Calibrating Bank Full Measurements Using Regional Curves and USGS Stream Guage Data

Bankfull is important to fluvial hydrogeomorphology (HGM) because it often determines the shape of the channel by moving and depositing sediment. Bankfull (BF) is defined as the high water level that recurs every 1 - 2 years, but measuring it in the field involves using multiple indicators in a 'preponderance of evidence' detective-style approach.

Most plants that cannot tolerate saturated soil conditions for days at a time, like Alders, will not grow below BF, while willows and cottonwood can. Also, the top of point or side bars can indicate the height of BF, but on the Rio Embudo, near Dixon NM, BF indicators were contradictory and hard to find. Is BF just a few centimeters above the base-flow water, or are all the willow below BF?

From Rio Embudo at Dixon, NM Hydrology Analysis

A number of bars and scour features at different heights further compounded the mystery. It was time to seek out other clues. One source of potential indicators was our aerial imagery, which was taken during Spring runoff, 2008:

From Rio Embudo at Dixon, NM Hydrology Analysis

The point bars at bottom right are bisected by a side channel that is several feet above the base level today. That means BF must be at least that high, and would probably inundate most of the willows. Corroborating this, the landowner reports that the willows are indeed flooded almost every year. But exactly how high is BF? To gather more data, we surveyed three channel cross sections, or transects (TR), noting the heights of the major terraces.

TR-Upper

From Rio Embudo at Dixon, NM Hydrology Analysis

TR-Middle

From Rio Embudo at Dixon, NM Hydrology Analysis

Tr-Lower

From Rio Embudo at Dixon, NM Hydrology Analysis

On each of these cross sections we marked where the current base flow water level is, where we think BF is, and where we think Flood Prone (FP) might be. To check these guesses, we correlated those heights with flow data from a USGS gauge just downstream:

From Rio Embudo at Dixon, NM Hydrology Analysis

From this graph we could see that the high water level with recurrence every 1 -2 years is about 400 cubic feet per second (CFS). We could also see that the current flow was about 38 CFS. If the Rio Embudo is flowing with 38 CFS today, how high would a BF flow of 400 CFS be?

between the flow today and BF flow. To figure that out we might need to correct for any changes in the velocity (feet/second). Manning's Equation:

shows that velocity V is proportional to a constant, u, inversely proportional to a coefficient of friction, n, varies to the 2/3 power of channel cross-sectional area, R, and to the 1/2 power of slope, S. Since neither slope nor the constant would change, we can discount them and focus on n and R; n will likely increase because the willows will act like a series of giant combs, increasing friction, and R will also obviously have to increase. For example, doubling the height of the water would multiply that term by 1.6. Unfortunately, coefficients of friction need to be experimentally determined, so we can only guess at n. To make things easier, I decided friction would also increase by a factor of 1.6, to exactly cancel out R. In other words, I don't think the velocity would change by much.

So it is a simple matter of geometry to calculate the cross-sectional area that would correspond to 400 CFS on our cross sections (red lines on the cross-sections, above). Without exception, this height is higher than our field-determined BF (green lines on the cross-sections, above) and, at least for TR-L, even higher than our FP height.

But is this right? Are we getting closer to the truth? To check, we can calibrate our answers for the Rio Embudo against data published by Natural Channel Design on a large number of other Southwestern rivers:

From Rio Embudo at Dixon, NM Hydrology Analysis

I plotted both our field-determined BF cross-sectional area (green points) and the USGS-determined BF cross-sectional area (red points) on the regional curve above. The green points seem to fall on the line for New Mexico, while the red points fall on the Arizona line, corroborating our field measurements and casting doubt on the USGS. However, the watershed above Dixon is very impermeable and could behave more like AZ than NM. I think the true value is probably somewhere in-between the field and USGS values.

This line is probably as close as any to Bankfull:

From Rio Embudo at Dixon, NM Hydrology Analysis