What is the specific gravity of a typical sediment particle?

Quartz and feldspar are the main constituents of sediment particles. Their specific gravity is generally considered to be about 2.65.

Under what circumstance would a sand-sized particle be considered washload?

Washload is the fraction of sediment load whose particle sizes are not significantly represented in the channel bed. Thus, in a gravel-bed river, sand-sized particles would be considered washload.

(a) What is Prof. Ponce's friction formula? (b) What is its main advantage, compared to the Manning equation?

(a) Prof. Ponce's friction formula is:

S_o = f F ²

in which S_o = bottom slope; f = friction factor, equal to 1/8 of the Darcy-Weisbach friction factor; and F = Froude number.

(b) The advantage of this formula is that it is dimensionless; therefore, applicable in any system of units. It clearly and directly links the forces of gravity and friction through a dimensionless number referred to as the Froude number.

How does Prof. Ponce's friction equation help justify Jarrett's principle?

In flows that are near critical, the bottom and energy slopes are generally steep and do not vary too much with changes in discharge. For a nearly constant slope, Prof. Ponce's equation establishes that an increase in Froude number can only happen with a concomitant decrease in friction. Thus, for near-critical flows in a natural channel, increases in Froude number would have to follow decreases in friction. The latter is typically not the case, due to the additional form friction provided by gravel beds under near-critical stages. Therefore, the Froude number is not likely to increase as the flow approaches near-critical stage in a natural channel.

What is the fundamental job of a cowboy? Explain in erosion and sedimentation terms.

The fundamental job of a cowboy is to feed his cattle without causing overgrazing. In extreme cases, overgrazing could lead to the destruction of the range through sustained sheet and rill erosion, gully development, and ecosystem failure.

Why is bunchgrass better able to reduce erosion that an annual weed?

Bunchgrass is a natural species, adapted to a range ebvironment, featuring a substantial root structure. In a degraded range, annual weeds take over due to the prevailing insufficient moisture, both as surface water (drought-produced lack of rain) and as subsurface water (the lowering of the water table due to gully development in the vicinity). Under a protracted lack of moisture, the bunchgrass dies, and this paves the way for the annual weeds to take over. Once the weeds are established, soil infiltrability is substantially impaired due to the reduced quantity of macropores, which carry moisture readily to the root zone. The net result is increased surface runoff and, consequently, accelerated erosion.

How does terracing lead to water conservation and erosion control?

Terracing is an agricultural practice of cultivation along the topographic contour. This maximizes water infiltration, reduces surface runoff, and minimizes sheet and rill erosion.

(a) What type of ecosystem and associated geomorphology would lead to the prevalence of black river waters, as opposed to sediment-laden river waters? (b) What is a good example of this situation?

(a) Very mild overall terrain slopes produce very little sediment movement. When this type of terrain is extensively covered with humid forests, the decay of organic matter (plant litter) produces humic substances, which owe their characteristic black color to their large content of carbon. A humic substance (humus) is an intermediate stage of the decomposition of organic matter, beginning as plant litter, eventually decomposing all the way to its elementary constituents.

(b) A good example of this unusual combination of geomorphology and ecology is the Hyelan forest of the central-northern Amazon basin, which demonstrably features the Rio Negro, or Black river.

(a) According to Einstein, what is the main cause of river meandering? (b) Is the control of meandering a viable endeavor across the timeframes?

(a) Einstein attributes the cause of river meandering to the effect of the Earth's rotation, which produces a Coriolis force, coupled with the balance of forces in a direction perpendicular to the direction of motion (friction, gravity, pressure gradient, and inertia).

(b) The control of meandering will prove to be fruitless over the long run. Thus, risk remains a fundamental issue affecting all engineering practice.

(a) Why is the Atachafalaya poised to eventually prevail over the Mississippi river as the main outlet to the Gulf of Mexico? (b) Who is standing in the way of this not happening in the near future?

(a) A large river, crossing a continent, carries large quantities of sediment and deposits it in its delta, as it approaches its mouth in the nearest ocean. The shape of a delta implies that the sediment is being spread out, fanning the river across its (delta) base, while distributing the sediment more or less equally. Thus, no delta should be fixed to eliminate its lateral movement. Given enough time, the Atchafalaya will prevail as the master stream taking the flow of the Mississippi to the Gulf of Mexico, albeit only for a few centuries at a time.

(b) The U.S. Arny Corps of Engineers is charged by Congress not to let this happen.

Why and how did the flooding of the Salton Sea happened in the winter of 1905-06? State the hydraulic and hydrologic reasons.

In 1905, the California Development Company, operator of the Imperial Valley irrigation, attempted to tap the Colorado river, in Baja California, Mexico, for much-needed water to irrigate the Imperial valley. Since the diversion structure was of a temporary nature, they cautiously waited to carry out the diversion until the winter had passed. The procedure seemed appropriate at the time, because the existing record of 27 years of Colorado river stages did not show more than one winter flood in any year. Unfortunately, that year the Colorado went on flood stage three times. The third time, the river breached the temporary structure, changed direction about 180°, and headed north, through a steeper downslope, towards the Salton basin depression, filling it, and creating the Salton Sea.

(a) What is hungry water? (b) Under what circumstances does it occur? (c) How can it be controlled?

(a) The phenomenon of "hungry water" is produced when a certain flow is forced to drop the sediment it normally carries. As it continues to flow downstream, this "hungry" flow will naturally entrain the sediment it wants to carry (its sediment load), either from the channel bottom, or from the channel banks.

(b) Hungry water typically occurs downstream of an impoundment, either natural, but more typically, artificial, such as a water reservoir or a sediment-detention basin.

(c) The only way to control the occurrence of "hungry water" is to avoid or minimize the use of artificial impoundments, but this is a "high call" in developed societies that rely heavily on storage reservoirs to optimize their use of water resources.