An Important Question

1) Norrish Creek
2) Illecillewaet River
3) South fork of Thor Creek



How often do some of the visible clasts in these streams move? Yearly? Once a decade?
If they are largely immobile, and hard to measure using the photogrammetric method, then is the channel-forming discharge the discharge at which they move and not the effective discharge that moves finer sediment through without moving these large clasts?

Slow Time

I'm taking time off school to do work!

It's financially rewarding but I'm getting ready to come back to academia and finish this thing off.

One sentence synopsis of results to date: Most BC headwater streams have infrequent bankfull discharge but frequent effective discharge, and hence may be classed as degradational systems rather than equilibrium systems.

First Update In A While

I haven't posted in a while because I have been too busy with last minute calculations, not to mention writing articles!

The image above is an example of a sediment transport calculation and a flow frequency analysis combined to evaluate effective discharge. The sediment transport relation is from the surveyed channel cross-section (via WinXSpro and HEC-GEORAS), sediment photoanalyzed with Digital Gravelometer, and the Wilcock and Kenworthy (2002) transport equations, using a two-fraction method with the fine "sand" fraction <8.5mm, assumed Dsand of 2mm.

The flow frequency analysis is from WSC daily data, and divided into 25, 50 and 100 arithmetic flow classes.

The Qeff is c. 6.9 cms, from this and other cross-sections (peak in the blue line). Interestingly the Qbf is 27.6 cms and the Q2, 22.5. So this is a watershed where the Qe << Qbf. It didn't look unstable, and it seems that a lot of sediment is moving through but not spilling over the banks.

All the bankfull stages are calculated! WinXSPro worked like a dream.

Some of the streams have impressive variability between cross-sections, but overall, reasonably consistent values for reach-scale bankfull discharge can be computed. And the results so far are about what should have been expected: for instance, bankfull discharge magnitude below the large logjam on the Albert River (where degradation was occuring due to reduction in sediment supply by the jam) was about twice the magnitude of Qbf above the jam, where aggradation was occurring.

The unexpected result was that many, but not all of the streams have Qbf return periods significantly higher than the Q2. Explaining this should make a nice paper.

Right now I am working on estimating the effective discharges. I'm using, or trying to use, the Sedimetrics Digital Gravelometer. Having some problems generating results: the program keeps stalling in the report generation process. David Graham at U of Loughborough has suggested some potential fixes, which I am currently trying.

Results are starting to come in!

HEC-GEORAS processing was impressive. Bank elevations seem pretty consistent.

Next stage is estimating Manning's n. Now the fun starts. As far as I can tell, HEC-RAS wants you to already know Manning's n. But I don't know it, I'm trying to estimate it, in fact.

So WINXSPRO to the rescue. WINXSPRO will take a cross-section profile output from HEC-GEORAS, and if you give it a water level (or a range of water levels chosen to lie over the range of bankfull) will give you back, both Manning's n, and estimates of Q. Seems like exactly what I need, right?

The only catch is that you have to do it one X-section at a time. That's going to be about 400 cross-sections x 15 minutes each (estimate) = 100 hours of work.

Here I go, watch me grind.

In the sample image above the apparent drop in discharge with increased stage results from the bank discontinuity being hit - indicating increased wetter perimeter with minimal increased area as flows go overbank, thus decreasing hydraulic radius R (A/p) and increasing effective n, so resulting in the dip in apparent discharge. That's my bankfull stage right there.

Nose to the Grindstone

Deep in cross-section processing with HEC-GEORAS. Results soon.

Effective Discharge and Sediment Supply

Effective discharge is the discharge that transports the most sediment over the range of stream flows. I have been thinking about what distinguishes effective discharge from stream power, based upon some papers at the AGU. If sediment supply to the system is unlimited (transport-limited) then the peak on the plot of stream power vs. duration of flow should approximate the effective discharge; if the sediment supply is limited (ie. supply-limited conditions) then this is only a nominal effective discharge because the actual sediment transport will depend on the sediment supply. I think. More about this later - at the moment I don't really know enough about stream power, so I'll be hitting the books.