(a) Why is the Colby 1964 method the preferred way to calculate the
discharge of sands in mid-valley alluvial streams?
Because it is a relatively simple method, with clear inputs, and it
is very stable, constrained within a graph.
Which is the most complete sediment transport formula
to date? Why?
The Modified Einstein Procedure, because it is based on the Einstein method,
but with a step-by-step procedure, endorsed by a federal agency.
It is the only method that computes bed load, suspended load, and wash load,
pointedly by individual size fractions.
(a) What exponent of velocity in the
sediment discharge-mean velocity relation
qs =
k ρ v n
renders the equation
dimensionally homogenous, with k being a dimensionless coefficient?
(b) What is the physical significance of this fact?
(a) n = 3.
(b) For n = 3, the sediment transport rate is directly a function
of stream power τov :
qs =
k ρ v 3 = k ρ v 2 v =
(1/f ) k (f ρ v 2) v
= (1/f ) k τov
Therefore, for n = 3, sediment transport rate is directly proportional to stream power τov.
The value n = 3 is the exponent associated with the ultimate sediment concentration,
which prevails at high sediment discharges.
At this point, the bed configuration approaches upper regime (e.g.,
plane bed with sediment movement), in which bottom friction is largely due to grain roughness.
How does form friction affect the shape of the sediment
rating curve?
For low values of sediment transport rate, form friction (ripples and dunes)
acts to reduce the sediment transport rate to values less than those that would be applicable
if form friction were non-existent, and high values of n (n = 7)
prevail. Conversely, for high values of sediment transport,
form friction is obliterated as the sediment concentration approaches asymptotically
an ultimate value, corresponding to a low value of n (n = 3).
Is the concept of ultimate sediment concentration correct, or flawed? Explain using
basic sediment transport reasoning.
In the total absence of wash load, the concept of ultimate sediment concentration is correct.
However, in practice, large suspended bed material transport
rates usually feature large values of wash load.
The latter interferes with the suspended load, and has the effect of increasing the actual amount of suspended bed material
load, thus, defying accurate calculation of the (hydraulic) suspended
sediment transport rate.
(a) What is the fundamental parameter in assessing (calculating) sediment
trap efficiency in a reservoir? (b) What is the secondary parameter?
(a) The fundamental parameter in calculating
reservoir sediment trap efficiency
is the ratio of reservoir volumetric capacity to mean annual inflow volume C/I.
For C/I small, the trap efficiency is small (most sediment goes through);
conversely, for C/I large, the trap efficiency is large (most sediment is retained).
(b) A secondary parameter is the sediment size, either coarse, medium, or fine.
Coarse sediments are more readily retained (trapped) than fine sediments.
(a) Why is the construction of a sediment retention basin
likely to be counterproductive in an alluvial stream left in its
natural state? (b) How was this fact confirmed by the experience of the
Aguaje de La Tuna flood of 6-7 January 1993?
(a) According to the Lane principle, if sediment is retained in a reservoir,
particulary during a large flood, the outflowing water will become hungry
for sediment and will have a tendency to erode the river course located
immediately downstream.
(b) During the flood of 6-7 January 1993 in Aguaje de la Tuna, in Tijuana,
two sediment retention basins, specially built to hold flood sediment,
retained a portion of the incoming sediment. This
caused the hungry water flowing downstream to erode the
stream to the underlying bedrock, a depth of about 3 m.
(a) Why is the assessment of
bridge scour remaining a problem
to this date? State all main reasons.
The main reasons are: (1) encroachment of the bridge superstructure on the river, due to design economies,
causing the river to reduce its cross-sectional width and therefore, to increase its velocity,
leading to contraction scour; (2) encroachment of one or more piers on the stream cross-section,
producing vortices around the piers, which lead to
local scour holes and eventual failure;
(3) our inability
to accurately describe and model tridimensional flow of water and sediment,
and the consequent reliance on empirical models, which often fail
to accurately describe the actual fluid mechanics.
(a) What was the cause of failure of the pedestrian bridge
Puente Solidaridad in Lima, Peru, in March 2017? (b) Could the
excessive bank erosion that occurred have been averted or controlled in a timely fashion
to avoid the collapse of the abutment?
(a) The bridge was a suspension bridge, structuraly held on one side only.
Even if the right abutment failed by excessive bank erosion,
the bridge structure should have held. The bridge's collapse
during the collapse of the abutment points to inadecuacy
in the structural design.
(a) The bank erosion of the type that happened is extremely difficult to control.
Protection of the bank against erosion is possible, but
expensive, because it generally comprises large portions of the river.
(a) How did the concept of good flood/bad flood originate?
Sediments are part of Nature. However, they are generally
good in rural areas, but bad in urban areas. Therein the dicotomy.
Bonus questions.
(a) In the Missippippi river flood event of May, 2011,
did the U.S. Army Corps of Engineers proceed appropriately
in blowing the levee fuse to enable the
Birds Point-New Madrid floodway to operate as designed?
(b) What was the apparent source of the conflict?
(a) Yes, the Army Corps of Engineers proceeded appropriately,
The Birds Point-New Madrid hydraulic structure ia a federal floodway, which means
that it can be used to relieve floods on the Mississippi river,
if and when society at-large deems it appropriate.
(b) The apparent source of the conflict is that the lessees of the floodway
did not know, or were not aware, that they had no permanent right
to the lands they were leasing.
Can societies around the world solve the sediment problem once and
for all?
No, sediment is part of Nature, and will always be with us.
Society needs to learn to manage sediments in a more effective way.
Artifically moving, transporting, settling, and otherwise using
sediment, ostensibly to mimic Nature, will have to be an intrinsic
part of Sedimentation Engineering in the future.