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 مهنيو المياه والذين يحبون أن يقرءوا عن حلول سودانية فذة لم تجد التوثيق مدعوون للأطلاع علي ملحمة ال
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08:02 AM July, 22 2018 سودانيز اون لاين
بروفيسور محمد الرشيد قريش-
مكتبتى
رابط مختصر
مهنيو المياه والذين يحبون أن يقرءوا عن حلول سودانية فذة لم تجد التوثيق مدعوون للأطلاع علي ملحمة القاش
(3 من 4)
بقلم بروفيسور محمد الرشيد قريش
A CASE STUDY
ON
A PIONEERING WORK
OF
SUCCESSFUL DESIGN AND EXECUTION OF STREAM-TRAINING
AND
FLOOD DAMAGE ABATEMENT PLAN
In Memoriam
of
Mohamed El Rashid Sid Ahmed
by
Professor Dr. M. E. GOREISH
B.Sc., M. B. A., M. Phil., Ph.D., Ph.D.
S.M.A.I.I.E., S.M.S.M.E., M.A.S.C.E., M.A.I.A.A., M.C.I.T, M.A.S.Q.C., M.TIMS, A.M.A.S.A.E.
EXPERT Systems Development Centre
For
Water Resources,
Transport , Manufacturing and Energy Engineering
Prologue
Engineering education currently uses two basic learning techniques, viz.
1. Instruction, epitomized by the conventional lecture format.
2. Inquiry, which is usually associated with problem-solving situations, and conducted through traditional formats such as seminars, laboratory works and computer simulations.
Recently, the Case Study approach has been proposed as an addition to the traditional arsenal of inquiry learning modes, since the approach offers students a rich opportunity to test their analytical skills in open-ended problems drawn from real life. The approach was originally pioneered by the Harvard Business School (HBS) and as a leading advocate of the technique notes, “it provides the skills required to bridge the gap between theory and practice, (as) cases (typically) reveal the complexities of the environment in which decisions are made, and involve the evaluation of facts against unimportant information, the formulation of alternative courses of action, and their evaluation in terms of the goal of the undertaking” 8.
One major drawback of the method, however, is the considerable effort associated with the preparation of the cases, as can be seen from the fact that HBS found it necessary to create an institute solely for this purpose, or as can be judged from this paper, whose aims also include the exploration of novel didactic approaches to Engineering education.
I. Introduction:
This epic-like narrative— which brings into the limelight an extraordinary, though little known flood-fighting episode is evocative of the parable of the “Old Man and the Sea”, immortalized by Ernest Hemingway's allegoric (Nobel prize-winning) novel, depicting Man’s heroic and eternal struggle against the forces of nature and his incessant attempt at controlling them. In contemporary real-life, it is reminiscent of the successfully executed plans of the famous fire-fighter - Paul Red Adir -to quell runaway oil-well infernos under the most over-taxing and stressful emergency conditions.
This engineering undertaking, which took place early in the forties, features both river-training and watercourse stabilization, as well as flood works, with all the toil and trial-and-error experimentation involved in developing a successful scheme to tame and combat a wild and unruly stream, and arrest the attendant scour and side erosion problems. As such, it has an hydraulic design analogy in the empirical resolution of the energy dissipation problems associated with weir (or fall) regulation, and the inevitable experimentation needed to determine the appropriate positioning of blocks, or hydraulic jump, (from the downstream end of the flow), in order to minimize bed scour and side erosion, for a given water depth and discharge intensity. Needless to say, however, that weir (or fall) surplus energy dissipation problems are not typically fraught with similar high risk to the staff’s personal safety, nor with the urgency associated with an impending flood hazard.
When this narrative was summarily recounted at the fourth congress of the International Commission on Irrigation and Drainage (ICID) on flood control - held in Madrid, Spain in June 196018, the conferees seemed greatly fascinated by the fact that such a difficult engineering undertaking was successfully and exclusively tackled by an unabated team of nationals during the era of Sudan’s colonization.
II. A back Note on Regime Theory and River Morphology:
The basic function of a river channel is to effectively transport both water and sediment. For this reason, the basic river attributes (associated with flood-control) are usually described in terms of certain stream characteristics such as:
i. Discharge (e.g. maximum discharge, flood frequencies, flood discharge hydrograph)
ii. Sediment load (e.g. rate of suspended and bed-load transport).
iii. River stage (e.g. maximum stage, stage hydrograph during floods, propagation of flood wave along the stream and flood profiles).
This summary depiction, however, obscures the various dynamic - though often conflicting — flow processes that take place, such as:
1. The erosion phenomenon, in which - for a given discharge - bed scour reduces water
velocity, but increases (channel) depth, whereas bank scour reduces both velocity and depth, while increasing (channel) width. 14
2. The Channel floor deposition phenomenon which increases water velocity (and slope) but decreases (channel) depth.
Within this view, a channel in which scouring and silting balances out is referred to as a “regime channel”, in reference to the celebrated “regime theory” originally developed by Lacey and others, which postulates a definite relationship between the key parameters of a stable channel, viz. width (b), depth (d), gradient (s) and flow (Q), as expressed — for example — by Blench Formula:
d = 3√(sQ)/B2
b =√(BQ)/s , and
s = (B 5/6 s1/12 Q -1/6) /C
where B is a factor related to bed material, s -- a factor related to the tractive forces, and C is a coefficient relating to viscous drag 19
or by Leopold -Maddoek Formula:
d = m1 Q n1
b = m2 Q n2, and
v = m3 Q n3
Where V = Q/bd and m1 , m2 and m3 are coefficients and exponents such that
m1 m2m3 = 1 and
n1 + n2 + n3 = 119
It is thus usual to consider rivers in regime (or equilibrium) if there has been little change in their characteristics over a long time span viz. rivers of constant discharge, silt grades and loads. The processes which determine the emergent stream morphology, however, are far from being fully understood, but it is generally surmised that channel morphology is a result of an intricate and complex interaction and balance of several factors, including:
i) flow hydraulics (viz. discharge, velocity, roughness, sheer stress etc.)
ii) upstream channel configuration (e.g. channel size, shape, slope etc.)
iii) Load
iv) the character of bed and bank material14
This typically leads to one of three basic channel planforms (types), viz.:
1) straight channels
2) meandering channel (or their sinuous variation).
3) braided channels (or their anastomosing variation).
Moreover, as Annandale (4) notes, " if the channel patterns are the result of natural processes, it is usually found that straight channels are more stable than meandering channels and both are more stable than braided channel flow"
4
III. The Peculiar Stream Characteristics as the Genesis of the Gash Problem:
The River Gash, which originates in the Eriterian highlands and runs westwards towards Sudan’s central plains where it fans out forming an inland delta, is an ephemeral Stream which flows continuously only during a portion of the year (namely, July, August and September) and almost dries up during the rest of the year. Its catchment area is approximately 21000 square kilometres.
The streambed is constantly either scouring or silting, the heavy bed deposition often building up high enough to nearly block the main water course and (according to Richards) 17 “This continues until a stage is reached when an avulsion occurs (and the channel flow changes suddenly to a) “balag” (swampy) flow. This (balag state) persists until the level... is raised (and) a new channel then develops from downstream. (Thus, for example, between 1927-1934 the deep channel eroded back... to make... (an) extremely unstable channel... (and) in 1938 the river breached its left bank and the flow (turned) “balag” 17.
If one views this peculiar avulsing behavior as a variation of the braided channel pattern, noted earlier, one may venture one —or more --of three-causal processes for an explanation6
i. A sudden sediment overloading, resulting in stream-bed aggradations.
ii. Steep slopes, resulting in wide and shallow channel.
iii. Bank erodiblity, particularly in view of the fact that "in the Gash soils,(unlike the stiff Gezira banks with its almost impervious soils), take up water greedily" 11
It is, however, such peculiar but not fully explained avulsing behavior that has very early drawn the attention of the Colonial authorities to the need for constant and timely maintenance of the channel banks, to avert avulsions, and to the need for river training works.
IV . River Regulation Systems:
River regulation is largely concerned with the construction of engineering works to guide water currents to scour the desired channel within a particular reach. It is mostly achieved either through the construction of training structures or through dredging works. These training structures range from spur dikes and bank protection works, to sills and bed-load traps intended to stabilize the river-bed and the bedload12,
• The bank protective work of longitudinal and traverse dikes is comprised of loose rocks
• The contraction work involve a training wall directing river currents to scour the bed and spur dikes – or groins — to reduce the effective width of the stream
• Thus banks of smaller streams are typically and satisfactorily protected by longitudinal diking. Such dikes should be anchored to the river bank to arrest failure by scour behind the dike. But the dike may still be vulnerable to undermining if the river-bed is readily erodible.8 For larger rivers, however, large-scale revetments (bank lining or pavement works) are usually needed for bank protection purposes. Such training structures also serve to establish the channel boundaries and thus confine the river to a definite water-course. Additionally, river regulation systems, in general, may serve to prolong the duration of water fiow in ephemeral streams (such as the Gash) but even more importantly (as in the case of the River Gash) is their role in protecting the people and the land from flood damages, by reducing the peak discharge.
(V) Flood Damage Abatement:
Floods occur when an stream channel fails to carry out its primary function (viz. the adequate conveyance of both the water and the sediment load), as the incoming load over-runs the channel capacity. Disastrous flood damages can then ensue due to water inundation and ponding (or merely due to water’s high velocity), or due to the heavy sediment depositions ( as for example on engineering structures). Such potential disasters made flood damage abatement a primary objective of regulation. For achieving this purpose, two routes are open to the planner7:
1. An structural approach, based on:
• decreasing peak flow through creating or augmenting upstream storage (e.g. through construction of upstream reservoir), or the diversion of peak flow, say, to a man-made channel.
• decreasing peak stage (for a given flow) through channel modification, e.g. by increasing the channel capacity (or cross-section), or by reducing bed friction to flow (e.g. through lining or clearing ) or through the construction of an addition on-site detention structures (such as levees and embankments).
• decreasing flood duration e.g. through watershed management techniques and run-off retardation measures (such as afforestation, and sound highway and railroad construction practices) or through improvement of the local drainage system.
2. A non-structural approach to flood damage reduction, based on:
• improved flood forecasting, which for ephemeral streams requires collection of discharge-duration-frequency information, including number of seasonal flow events and their duration and peaks, the duration of dry periods between these events (20), as well as the stream’s “persistence” (i.e. the tendency of high flows to be dovetailed by high flows, and low flows by low flows),or
controlling the use of the floodplain (e.g. through the enactment of proper zoning and land acquisition laws).
• An alternative way of classifying adjustments to floods is through distinguishing between efforts to modify floods and those aiming at modifying the flood damage “susceptibility”. The exhibit below gives an example of the matrix of options open to the decision maker:
• The Structural Measures that Modify the flood include:
• Dikes
• Channel improvements
• Floodways
• River diversions
• The Structural Measures that Modify the flood damage include “Flood proofing” of buildings
• The Non-Structural Measures that Modify the flood include Reforestation
The Non-Structural Measures that Modify the flood damage include Floodplain Management (e.g. zoning ordinates, statutes etc.)
(VI) The then (1940) Gash Regulatory System:
In the early forties, when the story described here took place, the Gash regulatory system initially consisted merely of a line of spurs with armoured ######### (set at 500 meters apart) and covering the first three kilometres13. An embankment (levee) was then added as a second line of defence, thus confining the river flow, throughout the levied section, to definite width, while providing protection of the surrounding land from overflow
(VII) The Genesis of the Crisis:
The Gash, true to its perilous and combative character, had continuously scoured—and gradually started to wash away—the foundation soil, breaching and ripping holes in the levied section, until it has totally annihilated the 20-year old embankment, and completely inundated the floodway, thus creating a highly perilous emergency situation and an impending disaster that had to be dealt with immediately.
(VIII) The Regulatory Mission TOR:
A fast remedial action was needed to:
1. Control bank erosion:
i.e. to protect the channel’s right bank through controlling the undercutting of channel banks by the stream and through controlling the rapid outpouring of water),
2. Control channel Scour:
i.e. to device a quick o effective mechanism for scour control.
3. Size the degree of channel scour:
of the foundation soils that would be subsequently needed for the design of a permanent replacement (embankment) foundation.
4. Protect the fertile flood plain:
from being destroyed by bank erosion while ensuring adequate distribution of water supply to agricultural plots
These terms of reference were then handed over to the (then) young engineer (El Rashid Sid Ahmed) who was to carry out the task with only a handful of semi-skilled helpers.
(IX) The Remedial Action:
Under the imminent threat of wide-scale floods, there was little time for gathering material for the conventional constructions of flood abatement. For example, had there been sufficient time and resources a potentially appropriate bank diking technique9 for dealing with the situation would have involved the use of a rock-filled double fence comprised of steel rails driven into the stream bed and joined together by wire, so that the rocks would promptly drop into the created cavities of the scoured bed as soon as they develop . However, such rail-based bank protection work was not possible under the then existing circumstances and as the river bed was constantly scouring the mere lowering of unfenced rock cobbles, when it was tried, also turned out to be ineffectual, as the rocks proved to be vulnerable to the fierce eroding action of the water’s high velocity, and were thus continually broken up and washed out with every new scouring. An alternative structural flood emergency protection plan had to be quickly designed and experimentally implemented under the prevalent extraordinary natural hardships. Sandbagging seemed to be a natural choice, but the separate laying-out of individual sandbags - like that of the unfenced rocks could also not be consummated, as the unfettered sandbags would drift and get washed away too as soon as they were dropped into the cavities. It was then decided to stack and chain the sandbags together and then to lower them in chorus into the cavities in the same breath. Surprisingly enough, this strategy of chaining the sandbags together worked remarkably, precisely where that of lowering unrestrained stones or sandbags had failed, because of the faithful steadfastness and tenacious persistence of the chained bags to either stay put and intact, or to adhere to the stream bed and swiftly move down in unison with it after every new scouring. Thus as the stream scours the bed further, the fettered sandbags would drop down to fill the newly created cavities, hence blocking the way in the face of additional erosion. Moreover, the chained sandbags not only served to arrest further channel scour, but also served - throughout the mission - to provide an effective and inexpensive stream-training and floodwater detention and diversion tool that utilizes the free river power for channel cutting and for purpose of guiding the stream course in the desired direction. It was precisely this work which was largely responsible, at the time, for preventing the occurrence of an even much greater disaster. However, just while everything seemed to be going well, this latter task of flood water diversion suddenly turned into a highly hazardous episode for the flood-fighting squad: A loop of the unwieldy stream turned around the rear flank of the flood-fighting team, who then suddenly found themselves completely encircled and threatened by the treacherous river loop and would remain so for the next ten days (with water and supplies reaching them on camel backs) before the water finally receded and they were able to end their ordeal and break the besiegement. However, it was not until the flood hazard was completely abated and the water level subsided, that the engineer-in-charge of the squad and his team of helpers were able to accomplish the third task of their Mission TOR, viz. the determination of the precise depth and extent of the scouring. In the second phase of the assignment, the team-for approximately three weeks - kept monitoring the water spread in the floodplain to ensure that the flood-plain was not destroyed by bank erosion and that adequate supplies and optimal distribution of water have reached various cropping plots. The same flood-fighting squad would then tirelessly repeat this task for nearly every year in the four years to come.
(X) The Current Status of the Gash’s Problem-- An Update:
Since the forties the region has witnessed many flood disasters perhaps the most celebrated of them was that of 1975. Many preventive and remedial measures have been implemented, but the problem still remains just as grave, due to the many infractions on the stream’s watershed and floodplain. One particularly serious transgression was the recent clear-cutting of forests on the eastern bank. Clear-cutting, especially when accompanied by floodplain violations, is noted to bring down dramatic impacts upon drainage basins and their streams: For the drainage basin such actions would invite degradation of the patterns, increased runoff, increased sediment yield, accelerated land erosion and stepped-up frequency of landslides. For the stream in the vicinity, the response can be quite complex, encompassing both increased channel erosion and aggradations, thus making the channel wider and shallower. 5 The net effect of such coincident land and channel responses, is to dramatically increase the flood hazard to the region. And this may very well, perhaps, explain the stepped-up tempo of flood occurrences in the region during the recent years. The removal of forests also tends to decrease stream discharge at low waters. 9
(XI) Examples of Recently Implemented Flood Control Measures:
Over the years, the Ministry of Irrigation has implemented several river training and flood protection schemes. Recently, the Eastern Willaya (Province) embarked on an embryonic (10-kilometers long and 10 meters high) ring-road (out-circumferential route or ring-levee project, City’s Southern, Northern and Western flanks - whose primary objective is to protect the town against disastrous floods, but is endowed with the added (free-rider) benefit of helping to mitigate the traffic congestion problem within the Central business district (CBD)
XI. Conclusion:
The usual systematic steps followed in the design of a flood control scheme involve the following:
1. Determination of the design flood and the flood characteristics of the basin.
2. Evaluation of the potential flood damages associated with various
flood stages.
3. Delineation of flood-prone areas that need protection
4.Choice of the method of flood protection that offer the desired
protection at minimum costs and selection of suitable sites for
engineering works if the structural approach is adopted.
5. Preparation of detailed design of the works.
6. Conduct of a cost-benefit analysis of the scheme.
7. Development of a flood forecasting and warning system.
It appears though, from this case study most of the earlier Gash flood damage abatement plans have oscillated between project purposes that are either not always reconcilable or contributing only marginally to the primary objective of flood control, and the optimal subsequent use of flood waters for irrigation purposes. Clearly, there still remain several untried, but imaginative remedies that may-someday-be explored even though their costs currently seem to be prohibitive e.g.:
• those involving reduction of peak overflow through the construction of a
relief canal to which excess flood waters may be temporarily diverted and then returned to the stream at low waters, thus prolonging the stream’s water flow duration. A second possibility to be explored involve
• the creation of an upstream flood moderating reservoirs (e.g. through
regional effort to control the stream by a joint- project with Eretria involving the damming of one of the gorges of the upstream mountain valleys), supplemented by rational management of the catchment to reduce erosion (and hence reservoir siltation), and reduce the runoff (e.g. by improving the efficiency of drainage system). Similarly,
• a scheme of peak stage reduction (e.g. through increasing channel capacity),
though seems forbidding under the current economic difficulties, may someday be seriously considered. Alternatively,
• one may opt for a more ambitious strategy of prolonging the stream’s flow
duration: for an ephemeral channel, like the Gash, this has the effect of transforming it to a perennial stream. Such a goal may have well been one of the objectives of the proposed scheme of interbasin transfer involving the building of the Setit Dam (near the juncture of Setit and the Atbara rivers), and the construction of a Canal (off-taking from the then created reservoir) to transfer water from the Atbara-Setit watershed to the Gash watershed. Such an ambitious scheme, however, must prove acceptable not only economically, but socially and environmentally as well.
In all cases however, the pioneering work of those courageous members of the flood-fighting squad described in this paper would remain a great source of inspiration for many generations of engineers to come.
References:
1. Abstracted from Raghunath, H. Hydrology, 1985.
2. Adapted from Ahmed, N. (ed.).1962. Irrigation Research Institute (India).
3. Adapted from Hermes, R. et al .1975. Fundamentals of Transportation Engineering.
4. Annandale, G. "River Mechanics: A Universal Approach", IAHS Pub. # 10, Water for the Future: Hydrology in Prespective Systems, April 1987.
5. Botkin, D. and E. Keller. 1987. Environmental Studies.
6. Chang, H.“Geometry of Rivers in Regime” J. of Hydraulics Division June 1979.
7. Duncan, W. et. al. "Development of Flood Management Plan" J. of Water Resources, Planning and Management, Vol. 111# 4 Oct. 1985.
8. Haynes, W.1969. Managerial Economics
9. Hennes, R Fundamells of Transportation Engineering 1975 Ra H Hydrology 1985
10. Hennes, R. et. al 1975. Fundamentals of Transportation Engineering .
11. Inglis, C. op cid.
12. Mamak, W. 1964. River Regulation
13. MOI .1957. Sudan Irrigation
14. Morisawa, M. 1985. Rivers: Forms and Process
15. Raghunath, H. op cid
16. Raghunath, H. 1985. Hydrology.
17. Richards, C. H., as cited in Inglis, C. 1954. Irrigation Problems in the Sudan and Recommendations as to How to Deal with them
18. See the Transaction of the Fourth Congress of ICID.
19. See, for example, Hennes, R. et. al.1978. Fundamentals of Transportation Engineering, p.425
20. UN. 1976. River Basin Development.
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بروفيسور محمد الرشيد قريش
فيس بوك
تويتر
انستقرام
يوتيوب
بنتيريست