Assignment EDMA3-1
Follow the steps in the Excel worksheet (These steps may not be convenient for everyone to view in the Excel worksheet, so they are also available below. It may be beneficial to print this page if this is the case.): dynamic_streams.xls. Make sure that you select "Enable Macros" when the file is opened. More detailed instructions will be coming soon.
Introduction
Stream Balance
Stable mobile bed streams are balanced between the rate bed material is supplied and the rate it washes down stream. This tool shows how the driving variables affect the balance. Start by assuming an initial stable condition. Use drainage area and slope to estimate stable initial values or enter your own then zero the balance. Next change values to see how stream stability is affected.
The relationship between the variables is:
(slope)2(flow rate)2
is proportional to
(particle size)1.5(bed load supply)
Most stream channels are not static or rigid. They move, erode, redeposit again and again. If they maintain their size, shape, and character then they are in dynamic equilibrium.
To be in equilibrium a balance must be maintained between bed material supply and transport. Transport depends on three things; the particle size, how steep the channel is, and the amount of water flowing. So together an equilibrium is maintained between the:
- gravel supply
- particle size
- water flow rate
- channel slope
Supply = Transport
Specifically the relationship of the variables is:
(slope)2(flow rate)2
is proportional to
(particle size)1.5(bed load supply)
By assuming Particular Creek is in dynamic equilibrium or at least has been until recently and that these four variables have been in balance we can then use this relationship to think about what might cause them to be out of balance.
Step 1. Channel Slope
Particular Creek has an oxbow that is to be cut off. The flow path now 1000 feet long will be 833 feet which will steepen the slope from 0.3% to 0.4%. More channel slope should tip the scale to the degradation side but by how much?
- Start by setting the stable initial values in the shaded box to 20 sq.mi. and 0.3% slope and clicking the Set button.
- Move the Channel Slope Bar to 0.4%. QUESTION 1: What is the relationship between bed load sediment supply verses transport?
Step 2. Flow Rate
Particular Creek's headwaters are on the urban fringe. While some development has occurred over the years much more is likely to happen relatively quickly. One headwater of concern has a drainage area of 1.7 sq.mi. and a slope of 0.6%. Storms that produced peak flows of 6 cfs are expected to have flows of 8 cfs.
- Set the Flow Rate bar to 6 cfs, then click Zero Balance.
- Increase the Flow Rate to 8 cfs. QUESTION 2: How much more bed material is being transported than supplied?
Step 3. Bed Material Size
A new pond has been constructed on a steep unnamed tributary to Particular Creek. It used to be the best recruiter of the biggest gravel particles to Particular Creek. Now without the supply of big gravel from the unnamed tributary the average size of bed material in Particular Creek is smaller. It was 34 mm now its 24 mm. The watershed size is 10 mi.sq. and the slope is 0.4%
- Set estimated values for the watershed or just set the Bed Material Size to 34 mm and click Zero Balance.
- Decrease the Bed Material Size value to 24 mm. QUESTION 3: How much more bed material is being transported than supplied?
Step 4. Bed Load Sediment Supply
A property owner along Particular Creek has a new business idea. For her new driveway she just took a gravel bar from the creek. To her surprise the gravel bar reappeared. She is thinking, why not do this again, and again and sell the gravel? Well, why not?
- The watershed is 4.2 sq. mi. and the channel slope is 0.51%. Set the balance to those conditions and see that the bed load supply.
- By estimating the channel width and guessing the duration of big storms each year we suspect that rebuilding the gravel bar takes about 1/2 of the annual bed load supply. QUESTION 4: What does reducing the supply by half do to the supply vs. transport?
It seems from previous examples on Particular Creek that channels are very sensitive and prone to down cutting. Fortunately, as one variable changes others respond positively, compensating or adjusting back toward stability.
Step 5. Quick Compensation
In the first example, the steepening of the oxbow cut-off would drive incision, fortunately, Particular Creek compensates and dampens the effect. (Stable 20sq.mi. and 3% slope went to degrading 4% slope)
- The steeper section cut its way upstream distributing the problem over more length. Also, the eroded material increased the bed-load supply just down stream from the impact causing it to aggrade, further diluting the problem. QUESTION 5: What happens when the cutting upstream and depositing downstream cause the 0.4 % to become 0.35%?
- Also, more of the fine material washes downstream leaving disproportionately larger gravel armoring the bed. QUESTION 6: What happens if the average bed material size increases from 31 mm to 38 mm?
Step 6. Long Term Problem
If degradation does occur what happens to the flow rate? The total flow rate does not change but if the flow is no longer spread out across a floodplain but more of it is contained in the deeper channel there is in effect more flow over the channel bed. Set estimated stable values for 10 sq mi 0.4%. Then increase flow rate to 13cfs.
Corsening of the bed material size compenstates some but not enough. Increase bed material to 40mm. Stream length tends to shorten in response to degradation which increases slope. Increase slope to 0.42%.
Step 7. Long Term Recovery
Fortunately a predictable sequence is set in motion that, while it will take years and does cause its own growing pains, will eventually lead to recovery.
- Unfortunately for now degradation continues, the channel becomes entrenched thus containing bigger flows. Set flow rate to 15 cfs.
- Everything is out of balance and further degradation causes the channel banks to become still higher and then unstable. However the bank instability, erosion and failure start to drive the bed material supply way up. Set bed load supply to 25.
- Bed material becomes finer. Set bed material size to 37 mm.
- The eroding banks cause flows to be wider and shallower reducing the flow per foot of width. Set flow rate down to 13 cfs per foot of channel width.
- Over time the bank faiure slows and channel slope lessens. Set bed load supply to 18 and channel slope to 0.4%.
QUESTION 7: Do you have any streams now somewhere in this long term sequence? Name one and discribe where it is in the sequence described in step 6 and 7.
Step 8
QUESTION 8: List 4 watershed management actions or strategies and briefly describe stream stability implications.
The End
Products to turn in for review:
Compile your responses to the questions in a Word document and submit the file.