The Present Floods in Ogun state at Ogun river, water hyacinth and the blockage at Kara and Ogun state river Fundamentals and practical ;solutions;BY AUTHOR ENGR. ABDUL RAZAK DAWODU BABATUNDE Phone:(+234) 07040375883,234-08023540741; E-mail: firstname.lastname@example.org or untouchableking2010@gmail,com.
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The Ogun River is a waterway in Nigeria that discharges into the Lagos Lagoon.
The river rises in Oyo State near Shaki at coordinates 8°41′0″N 3°28′0″E and flows through Ogun State into Lagos State. The river is crossed by the Ikere Gorge Dam in the Iseyin local government area of Oyo State. The reservoir capacity is 690 million cubic metres (560,000 acre·ft). The reservoir abuts the Old Oyo National Park, providing recreational facilities for tourists, and the river flows through the park.The Ofiki River, which also rises near Shaki, is the Ogun River’s chief tributary. The Oyan River, another tributary, is crossed by the Oyan River Dam which supplies water to Abeokuta and Lagos. In densely populated areas the river is used for bathing, washing and drinking. It also serves as a drain for mostly organic wastes from abattoirs located along the river’s course.On the 18th of June 2020, at the Ogun river near Kara cattle market, there was the appearance of a thick mass of floating aquatic vegetation dominated by the invasive water weed called- water hyacinth (Eichhioniacrassipes). This incidence obviously surprised the residents, traders and passers-bys at the Kara market area. Some called it a strange and mysterious occurrence that has never happen before. Others said that it was a serious sign from the gods, that the Ogun river at Kara would suddenly become a dry land. Let’s look at some key factors of the event.
In the Yoruba religion, Yemoja is the divinity of the Ogun River. The catechist Charles Phillips, father of the Charles Phillips who later became Bishop of Ondo, wrote in 1857 that the Ogun River was generally worshipped by the people who live along its banks from its rise until where it empties into the lagoon. The river ran through the heart of the old Oyo Empire. Metropolitan Oyo was divided into six provinces with three on the west side of the Ogun River and three to the river’s east. At one time, the river formed an important route for traders carrying goods by canoe between Abeokuta and the Lagos Colony.
The study was carried out in lower Ogun River, Akomoje in Abeokuta, Ogun state. The river is located in Abeokuta North Local Government of Ogun state and lies between longitude 3°21’S and latitude 7°21’E North of Abeokuta with a size of 1000 hectares. Ogun River is a perennial river in Nigeria, which has a coordinate of 3°28’E and 8°41’N from its source in Oyo State to 3°25’E and 6°35’N in Lagos state where it enters Lagos Lagoon. The dry season is between November to March while the wet season is between April to October. The annual rainfall ranges from 900 mm in the North of the River to 200 mm towards the South. Total annual potential evapotranspiration is 1600 mm and 190 mm. Ogun river catchment area is located in South West Nigeria, bordered geographically by latitude 6°26’N and 9°10’N and longitude 2°28’E and 4°4’E. The land is about 230 km2. The relief is generally low, with the gradient in the North-south direction. The water source is from the Igaran hills at an elevation of about 540 m above the sea level and flows directly southward over a distance of 480 km before it discharge into the Lagos Lagoon. The major tributaries of the river are Ofiki River and Opeki River.
The Present Floods in Ogun state at Ogun river, water hyacinth and the blockage at Kara
The Ogun river is a major water course and drains through three states of Nigeria (Oyo, Ogun and Lagos) via the Lagos lagoon to the Atlantic Ocean. Throughout its length, the Ogun river is connected to tributaries, reservoirs and reserved areas, dams, national park, other rivers and rivulets across the three states. The river passes through densely populated areas and it is used for bathing, washing and drinking, a water source for many concerns including a water board. It also serves as recipient of an array of waste types from some small and medium scale industries, food and livestock markets and even an abattoir. All these are located and drain their effluents and waste streams into the river course. Since a river or lagoon is the lowest point in any vicinity. The river is important religio-culturally and historically for transportation, trading, navigation, fishing, among others. The river originates around Saki, in Saki east local government area of Oyo state.
On the other hand, Water hyacinth is an aquatic plant originally known to the Amazon basin, and often considered a highly problematic and invasive species outside its native range. It lives and reproduces freely in freshwater ecosystems and entered Nigeria via the Lagos lagoon system through the Badagry and Porto-Novo creeks from neighbouring Republic of Benin in September, 1984. In addition, it is a biological invader, widespread and can alter population dynamics and community structure of native ecosystems. It is also able to grow over a wide variety of wetland types from lakes, pond, stream waterways, ditches and backwaters. It is a very fast growing plant and can constitute an environmental nuisance because it can destroy native wetland and waterways, degrade water quality and alternative plant and animal communities, killing native fish and other wildlife. Water hyacinth usually serves as microhabitat for a variety of disease vectors even for humans and known to cause blockage of drainages, navigation, increase in breeding of mosquitoes and snails while interfering with fishing activities.
Your worrying question is this, why did the Ogun river Flood and what is causing flood at Ogun river and why dry out at the Kara area. The occurrence at Kara cattle and ram market was simply a large blockage of the river course from a massive flow downstream of a community of water hyacinth that have been dislodged upstream by floodwaters induced by increased rainfall volumes. This huge mass of floating vegetation were contributed by the many fresh water tributaries and connecting rivers. These waterways have been increasing polluted with nutrients from domestic and market waste discharges that has largely encourages the strong growth of the weed in the immediate past dry season. Water hyacinth is renowned to thrive well with corresponding high levels of nutrients. Additionally, wastes and debris from the cattle market, abattoirs and from the region, that have been continually dumped into the water channel over the years, have led to increasing obstructions and the reduction of free flow for the river at this point. The upstream pressure from the rainfall induced floodwaters, flowing by gravity compressed the blocked floating vegetation gradually over twenty four hours or so. Making a very thick mat at Kara that accumulated from the day before as noticed by some observant residents. The water hyacinth plant grows associated with other plants such as other aquatic macrophytes, including grasses, water lettuce, duck weed, etc. These plants have air pockets to aid survival by floatation. Compression of these weeds, will give a strong adhesive and abrasive force, hence, allowing people to “seemingly walk” on it. But this is very dangerous. The river course is still alive and active. People are able to walk on the weed mass because of the force of water from upstream compressing and pushing it tightly downstream. This “push”, will eventually “win” if not cleared. And the blockage will give way.
The bottom line is this. We must begin to take the environment more seriously by effective and continuous environmental education to the people and reduce waste and indiscriminate discharges. We can all make the world a better place to live in. It begins with us.
The surface water quality parameters measured within Lower Ogun River Akomoje could be clasified as class 1 (excellent) according to classification. The river was quite productive and with the information gathered towards the end of the data collection (at the start of the rainy season) it was shown that the river will be able to support diverse number of organisms from planktons, benthos to fishes and macrophytes going by the abundance of chemical ions needed for interconversion of energy and production of organic materials present in the river. The physico-chemical data obtained in this river could be used as a baseline and reference point when assessing changes caused by nature and man within the river. It could also be observed from catch statistics that there was a reduction in the fish catch during the peiod of no rain [6,37]. The water quality will however support aquaculture. The aims of carrying out this experiment were to determine the water quality and to investigate if the various human and ecological activities around the river have any effect on the physico-chemical parameters of the river’s resources with a view to effectively utilizing these resources. Water samples were collected from two stations on the surface water of Lower Ogun River Akomoje biweekly for a period of 5 months (Jan-May, 2011). Results showed that temperature ranged between 24.0-30.7oC, transparency (0.53-1.00 m), depth (1.0-3.88 m), alkalinity (4.5-14.5 mg/l), nitrates (0.235-5.445 mg/l), electrical conductivity (140-190 μS/cm), dissolved oxygen (4.12-5.32 mg/l), phosphates (0.02 mg/l-0.75 mg/l) and total dissolved solids (70-95). The parameters at the deep end (station A) accounted for the bulk of the highest values; there was however no significant differences between the stations at P<0.05
with the exception of transparency, depth, total dissolved solids and electrical conductivity. The phosphate value was relatively low which accounted for the low productivity and high transparency. The results obtained from the physico-chemical parameters agreed with the limits set by both national and international bodies for drinking and fish growth. It was however observed that during the period of data collection, catches was low and this could be attributed to low level of primary productivity due to the quality of physico-chemical parameters of the water. It was recommended that the agencies involved in the management of the river should put the right policies in place that will effectively enhance proper exploitation of the water resources. More research should also be carried out on the physico-chemical parameters since this work only studied the water for five months.
The study was carried out between January and May 2011 by collecting water samples every two weeks. A total of four samples were taken using a 1 litre bottle with two samples per station and average value recorded. The sample bottles were rinsed thoroughly with river water. Water samples were taken on board from canoe to the sampling stations. The samples were taken from 10cm depth from the surface by holding the bottles upward and taken to the laboratory of the Department of Environmental Management and Toxicology, College of Environmental Resource Management, University of Agriculture Abeokuta, for further analysis of dissolved oxygen, nitrates, phosphates and alkalinity. The temperature, pH, electrical conductivity were measured in-situ using Combo metre by Hanna, model HI 98130. The metre was used by submerging the probe into the water and switching it on the values of the water temperature, electrical conductivity and pH were measured while the metre’s probe was submerged. The value of the electrical conductivity read on the metre was in millisiemens/cm, hence the value was multiplied by 1000 to convert it to μsiemens/cm. The total dissolved solids values was derived by multiplying the value read for the electrical conductivity by 0.5. The depth was measured with the aid of calibrated bamboo stick of height 6 m while the transparency was measured using a secchi disc. The bamboo was lowered into the water depth and the depth at which the lowered end reached the river’s base was recorded. The transparency was measured by using a secchi disc of 20 cm in diameter, painted in alternative black and white colours was lowered into each pond until it just disappeared and pulled up until it reappeared again. The two readings were recorded and an average value was calculated.
Measurement of Dissolved Oxygen (DO): This was achieved by Winkler’s method using Manganese II sulphide (MnSO4) solution, alkali-iodide-azide, concentrated sulphuric acid (H2SO4), Sodium thiosulphate (Na2S2O3.5H2O) solution, starch solution were used as reagents.
250 ml of the sample was poured into a dark bottle, 2 ml of MnSO4 and alkali-iodide-azide was added to the sample, to preserve the dissolved oxygen within the sample. The sample was allowed to settle for about 2 minutes and it was observed to have coagulated. In order to dissolve the coagulant, 2 ml concentrated H2SO4 was added to the sample and shaken. The sample was observed to give a clear solution.
203 ml of the sample was taken into a conical flask and titrated with 0.0125M of the Na2S2O3. 5H2O to give a pale straw colouration, 2 ml of the starch solution was also added and the titration continued, the colour became green and added thiosulphate in drops till a colourless solution was observed.
Measurement of Nitrate: A nitrate (NO3-) stock was prepared by dissolving 0.7222 g of potassium nitrate (KNO3) into 1000 ml of distilled water giving 100 ppm of NO3-N.
After preparing the nitrogen stock, standard preparations were made by adding 40 ml of distilled water to 550 ml centrifuge tubes. 0, 0.5, 1.0, 1.5 and 2.0 ml were then subtracted from the 5 tubes respectively, and the same amounts of 100 ppm of the NO3-N initially prepared. The individual tube will contain 0, 1.25, 2.50, 3.75 and 5.00 ppm of the NO3-N.
1 ml of the sample to be tested was taken into a cuvet and 4 ml of distilled water is added. The same is also done for the standard samples. Both the sample and the standards are read in a spectrophotometer at wavelength 210 nm after which a standard curve is plotted and the reading gotten from the standard curve’s plot is multiplied by 4.
Measurement of Alkalinity: The reagent used was concentrated hydrochloric acid (Hcl) while the indicators used were phenolphthalein and methyl orange. 100 ml of the sample was poured into a conical flask. 3 drops of phenolphthalein indicator was added. No colour change was recorded, a few drops of methyl orange indicator was added and titrated using 0.1 M of conc. HCl. The sample was titrated till a pink end point was observed. The alkalinity was calculated as thus:
Alkalinity (mg CaCO3/l)=Volume N/10 acid (ml)×50 OR Volume N/50 acid (ml)×10
Measurement of phosphate: The phosphate was measured using the Vanado-molybdo-phosphate acid colorimetric method. The reagent used was vanadate-molybdate reagent .
25 ml of the sample was poured into a 50 ml volumetric flask. 10 ml of the vanadate-molybdate reagent was then added to the sample giving a mixture of 35 ml. 15 ml of distilled water was added to give 50 ml. The dilution factor (the volume initially diluted to give 50 ml i.e 25 ml). A blank sample was prepared using 25 ml of distilled water to substitute for the sample in another volumetric flask. Both the standard and blank samples were then read in a spectrophotometer after 10 minutes at wavelength 470 nm and the curve plotted. The phosphate values were gotten from the calculation below:
Where D=dilution factor.
Statistical analyses were performed with SPSS 16.0 (1997). The probability level (α) for rejection of the null hypothesis was 0.05. The difference between the two sampling stations was compared by using paired sample t-test (Figure 1 and Plate 1).
The objectives of Ogun river Morphological report are:-
i) To identify the problems – location specific and morphological, works taken up and their performance.
ii) Interpretation of factors responsible for different type of problems, causes of failure of the measures taken and the necessary corrective measures.
iii) Understanding of river mechanics to facilitate mathematical/ hydraulic modelling.
iv) To identify further studies to be carried out. v) To identify additional data required.
vi) To evolve criteria for planning and design of structures for efficient river management.
The contents of a Morphological report are given in Annex I. The specific information to be included in each Chapter is given in Annex II.
APPRAISAL OF THE PROBLEMS
A detailed description of the problem, both location specific, reach specific or over entire system is a basic requirement as this would lead to a choice of the proper methodology for study and identification of the requisite data. The typical problems which are normally encountered in river management are those arising out of natural causes or those caused by man made structures or encroachments into the river bed.
These may be one or more of the following:-
(a) NATURAL PROBLEMS
i) Frequent changes in river course.
ii) Avulsion of one river into another (beheading).
iii) Heavy shoal formation (as in Ogun river) causing diversion of the main current towards the banks. iv) Development of natural cut-off in meandering rivers. This, some times, changes the meandering pattern.
v) Heavy landslides in the catchment causing sudden and steep rise in silt load. This causes instability, as was witnessed in the river in 2017.
vi) Heavy aggradation of the river bed. This causes high flood levels resulting in overtopping of banks/embankments even during floods of relatively moderate intensity.
vii) Heavy erosion of banks by hill streams due to flash floods (as in West Bengal where large tracts of tea gardens are affected).
viii) River instability due to changes in bed slopes as a result of seismic activity (as in many rivers in Assam).
ix) Changes in river channels due to changes in rainfall pattern.
x) Erratic behaviour of rivers in deltaic areas where they have numerous spill channels.
xi) Erratic behaviour of braided rivers. xii) Navigational problems due to shoal formations.
xiii) Formation of sand bars at river out-falls into sea, due to reduction in upland discharges as in Ogun River.
xiv) Morphological changes in a river due to changes in its base level i.e. the levels of the out-fall into another river or sea.
b) MAN MADE PROBLEMS
i) Degradation of river bed downstream of a dam or a barrage.
ii) Effects of constriction of river width due to barrage/bridge construction. iii) Effects of flood embankment on the regime of rivers. iv) Effects of extraction of sand and boulders from the river beds and banks. v) Effects of spurs and bed bars of different types on river behaviour. vi) Effects of inter-basin transfers of water on the regime of rivers. vii) Effects of river bed cultivation and construction by farmers in a river reach.
viii) Effects of dredging/channelisation of river bed. (This is usually done near big cities to keep deep channel near Power House or Water Works).
ix) Effects of pucca bathing ghats in big cities and places of pilgrimage. x) Effects of heavy urbanization along the river banks.
APPRAISAL OF MEASURES TAKEN
A detailed account of different measures along with sketches/drawings of structures constructed, basic design criteria and design conditions considered etc. is given. Performance of the existing works is given with the details of the extent and type of failure, if any, noticed from time to time. These details are likely to lead to an understanding of the possible causes of malfunctioning/failure of the work which, in turn, would dictate the choice of morphological parameters to be studied in detail.
The data requirement would depend primarily on the morphological parameters that are to be studied for a particular river. However, following minimum data is required:
i) Topographical data such as topographical maps, aerial photographs, satellite imageries etc.
ii) River cross-sections up to the highest recorded water level for different years at
– Existing gauge discharge sites (pre-monsoon and post-monsoon).
– The reaches affected by bank erosion and/or erratic river behaviour.
iii) Daily discharge data for the existing discharge observation sites.
iv) Daily gauge data for the existing sites as well as for the study reaches. v) Daily sediment load data for the existing sites.
vi) Grain size distribution of the bed material for existing sites as well as for study reaches.
vii) Hydrographic charts in the vicinity of the existing sites and in the study reaches.
viii) Grain size distribution of the suspended load and bed load.
ix) Vertical velocity and sediment load distribution at the existing sites. x) Dimensions of the dunes and/ or ripples.
xi) Geo-morphological map of the basin with particular reference to the flood plain and deltaic plain.
MORPHOLOGICAL TIME SCALE:
Various flow and channel parameters are interdependent. Identification of independent and dependent variables is an important step in a scientific study of river morphology.
Viewed in a geological time scale of millions of years, a river is an open channel system undergoing continuous changes and there are no definite relations between different parameters as they change with time. On the other hand, over a small time scale of a few days or weeks, a river may be in a ‘steady state’ in which no significant change in channel characteristics occur. The cause – effect relationship in the two cases may be quite different, which if documented quantitatively may be a source of serious error in the interpretation of the mechanics of river flow and thereby, in the understanding of the river behaviour.
In the graded time span, arbitrarily defined as a few hundred years, a graded condition or a dynamic equilibrium exists. During this time span, the variables which appear as constantly changing in ‘geologic time’ and as static in ‘steady time’ appear to fluctuate in a cyclic manner. Geology, hydrology, initial relief and valley dimensions may be considered as ‘independent variables’ and the channel morphology as ‘dependent variable’. It is this time span which is of relevance to the river engineer. In river morphological studies, therefore, these inter-dependencies have to be kept in view.
Since the river channel is the result of flowing water, magnitude and frequency of run-off events are major factors in determining the character of the river channel. It is, therefore, possible to show qualitative relationships between river flow on one hand and different aspects of channel morphology like channel dimensions, shape, gradient etc. on the other.
For a short term (steady-time) evaluation of changes in river parameters, the variables are considered as follows:-
Water discharge – Dependent variable. Sediment discharge – Dependent variable.
Hydraulics of flow – Dependent variable.
Channel morphology – Independent variable.
For the evaluation of parameters in the graded time scale, the variables are as follows:-
Hydrology (mean discharge of water and sediment) – Independent variable.
Hydraulics of flow – Indeterminate
Channel morphology – Dependent variable
IDENTIFICATION OF REACHES
After detailed appraisal of the problems, the river reaches to be studied are identified and sub-divided into a number of study reaches such that a precise profile of the energy line may be established for the study. Rigid criteria for the length of a reach obviously can not be laid down, but as a general rule, length of about 10 km is considered adequate. In meandering reaches, the study reaches are so demarcated as to identify the curved and straight portions. Braided reaches of the river are sub divided in such a way that all major and minor channels separated by shoals and bars are covered for study. Divided flow (twin channels) and long straight reaches are included in separate study reaches. Estuarine reaches needs to be treated separately especially when the flow is bi-directional.
DEMARCATION OF CROSS SECTION LINES
In each study reach, cross-sections are laid out normal to the direction of flow at an interval of about 5 km distance being measured along the centre line of the main channel.
FIELD SURVEY DATA
While para 4.0 gives the data requirement, following data is necessary for specific morphological studies. Some of the data is, therefore, common.
(i) Daily gauge, discharge and sediment data are collected for each Gauge and Discharge (G&D) site in accordance with the procedures laid down in the relevant codes of the Bureau of Indian Standards (BIS). Daily gauges are observed for each study reach. For morphological study, sediment samples are taken from each segment used in discharge measurements.
(ii) Cross sections of the river at the identified locations are taken every year before and after the monsoon season.
(iii) Hydrographic survey of each study reach is done separately for different river stages. The survey charts are used for bed form studies.
(iv) Bathymetric survey data, velocity profiles, water surface slopes, longitudinal profiles etc. should be carried out.
(v) Measurements of the dimensions of the dunes in different parts of the river bed are also made for sediment transport modelling.
(vi) Grain size distribution of the suspended sediment load is determined.
(vii) Grain size distribution of the bed material is also determined.
(viii) Vertical velocity distribution and vertical sediment distribution at significant river stages are observed for each G&D site.
(ix) Engineering properties along with sedimentological studies (both physical and laboratory oriented) of the river bank materials are determined, especially for locations susceptible to river bank erosion/failure/collapse.
(x) Geomorphological map, covering the flood plain and showing important features like point bars, alternate bars, middle bars, ox-bow lakes, palaeo channels, channel plugs etc. should be prepared.
REMOTE SENSING DATA
Satellite imageries play vital role in monitoring the changes of rivers in study reach. Remote sensing data helps in studying inaccessible areas. Remote Sensing data like satellite imageries, digital satellite data (microwave data -Radar sat (Canada data)). Field survey data and remote sensing data may be needed in hydraulic /mathematical models like MIKE-21C, ANN Model etc.
MORPHOLOGICAL ASPECTS FOR STUDY
RIVER FLOW ANALYSIS
The main objectives of the river flow information are (a) the study and description of river morphology, (b) investigation of river bed forms, (c) study and prediction of sediment transport and (d) analysis of aggradation and degradation. These groups of river mechanics problems are mutually inter-related processes dependent on the river flow processes. Therefore, river flow data is analyzed, described and presented in such a way as to provide the best insight into effects on the various dependent processes.
Many variables in river mechanics are power functions of discharge. Greater values of the exponent in the relationship indicate that high river flows are more important and neglecting the low flows is justified in the study of a particular river problem. Some variables depend on the integrated effect of previous discharges, both low and high, so that all flows are relevant in the study, while some other variables depend not only on the discharge but also on the rate of its variation.
Field survey data and remote sensing data may be needed in hydraulic /mathematical models like MIKE-21C, ANN Model etc. Analysis of Temporal
Satellite Data while asserting the history of river course is necessary.
In order to understand the significance of different variables in river mechanics, and to identify the different types mentioned above, structural analysis of the flow series is carried out.
For a proper understanding of the processes of erosion and silting on which the morphology of the river depends, it is also necessary to view the fluvial landscape in a historical perspective. The variables influencing the river channels and river system can be broadly categorized as (a) Structure (b) Stage and (c) Process.
The term structure as used in geomorphology implies not only the effects of various kind of rocks, but also the differential erosional character of the rocks, the influence of various geologic factors like fractures, joints, faults and their distribution in a drainage basin.
The change of landform with time is referred to as stage of development of landform. The progression of erosion in a given region is marked by “Competition” between river systems for drainage area. The most aggressive river with the steepest slope or greatest discharge or an advantage of lower altitude may capture the drainage area of another river system, thereby, changing the course of the latter. The qualitative understanding of this aspect helps in undertaking the detailed investigations of specific problems of avulsion of one river into another.
Landform development is studied from hypsometry curve for the basin drawn between h/H and a/A where ‘h’ is the contour height above base plane, ‘H’ is the total height, ‘a’ is the area enclosed by a given contour and ‘A’ is the total area of the basin. A typical hypsometric curve shows the lines corresponding to the different stages of development of landform viz. youthful, mature and old stage. Relations between contour height & area and between contour height & percent area above the contour could also be developed. These studies afford a preliminary knowledge of the basin which helps in deciding the further exploratory work in the field. For instance, it would be worthwhile looking for old courses (palaeo channels) of a river in a landform that has reached the old stage.
Drainage pattern of a basin gives a fair idea about the geology of the basin i.e. the nature of rocks, faults, joints, folds, fractures, unconformities etc.
Drainage density, expressed as the length of drainage channels per unit area of the basin helps in better interpretation of the hydrological data of a river. It also helps in identifying different categories of lands, for instance:-
Sand stone areas 3 to 4
Fractured igneous rocks 15 to 25
Bad lands 200 to 400
Other basin characteristics like form factor, circular ratio, elongation ratio, bifurcation ratio, stream order etc. are required to be discussed.
Distinctive characteristics of river flow are the “processes” which determine the character of a river channel. River flows in the form of daily discharge, gauge and sediment load, being the most pertinent time series, should be analyzed. This may be done by developing relationships between the three parameters and identifying the unique characteristics of the flow series. On the Yellow River in China, it has been shown that channel shifting varies with fluctuations in discharge, similar to Kosi river in India. A linear relation was found to exist between ratio between maximum discharge to bank full discharge and the wandering intensity in meters / day of the Thalweg.
The “Structure”, “Stage” and “Process” aspects of the river morphology afford a qualitative understanding of the nature of a river and its behaviour.
Lateral and vertical movement of thalweg at different locations in an alluvial river from year to year is described both in qualitative and quantitative terms from the study of river cross sections. The lateral/ vertical movements of the river bed, where possible, are co-related with the problem of bank erosion/collapse, if any. The description of secondary channels, especially those frequently shifting and causing significant changes in flow pattern near the banks are described.
(a) BANK FULL WIDTH
Bank full width is a subjective term. This is taken as the river width at water surface level corresponding to the dominant discharge or bank full discharge. This could also be decided by the visual examination of the river cross sections and confirmed with the study of gauge and discharge curve plotted on semi-log paper (the level at which the curve flattens out could be taken as the bank level).
Mean Depth (D) = Area of cross section at bank full stage (A) / Bank full width (B)
Width Depth Ratio = B/D.
It is also useful to study changes in channel parameters in the downstream direction for different frequencies of flow.
(b) REGIME STATUS
From the regime formulae of Lacey, width, depth, area and velocity are worked out and these are compared with the observed values at different river stages. Such a study would indicate the applicability or otherwise of the regime formulae of Lacey.
(c) RIVER BED CHARACTERISTICS
i) The qualitative and quantitative descriptions of lateral slope of the flood plain are given from the observed cross sections for different years.
ii) Aggradation and degradation of the river bed in a reach are studied through a comparative study of river cross-sections for different years. These cross sections for different years are superimposed and the area of each cross-section below a reference line is worked out. An increase in the area from one year to another would indicate degradation while a decrease in area would indicate aggradation at the site.
Aggradation and degradation of the bed may also be reflected in a shift in the G-D curve over a given period. For a meaningful study, it would be desirable to draw G-D curves separately for rising and falling stages of the river.
The above study from cross-sections and G-D curve would indicate aggradation/ degradation at a particular cross-section and not in a reach. However, such a trend between two sites could be broadly studied through a sediment balance study for the reach.
(d) SEDIMENT TRANSPORT
Though there are numerous sediment transport formulae developed by various investigators from time to time, none of these is considered suitable for all situations. Efforts are, therefore, made to develop relationships between observed values of sediment transport on one hand and different flow parameters on the other. The different flow parameters could be discharge, velocity and stream power.
(e) FLOW CHARACTERISTICS
Relations could be developed by plotting graphs on log – log scale between width, depth and velocity on one hand and discharge on the other. For low flows, critical velocity may be co-related with depth to develop an equation which will have the form of Kennedy’s equation.
(f) PLAN FORM
Shape of river in plan is very important in many design problems concerning location of bridges etc. In general, the plan forms of alluvial rivers can be classified as (i) Braided, (ii) Straight and (iii) Meandering. Meandering and braiding reaches of the river are separately studied. For meandering reaches, following relations may be developed on log – log scale:-
Meander length (also called wavelength) Vs. Bank full width
Meander width Vs. Bank full width
Radius of curvature Vs. Bank full width
Tortuosity, Sinuosity etc. of the river may be studied.
Thalweg length – Valley length
Tortuosity = ×100
Sinuosity = ×100
For a more logical and quantitative description of braiding phenomena, following indices have been proposed by Dr. Nayan Sharma (1995):
Plan Form Index (PFI) = B
where, T = T1+T2 = Flow top width B = Overall flow width
Flow Geometry Index (FGI) = Σdi.xi × N
where, di and xi are depth and width of submerged sub-channel R = Hydraulic mean depth of the stream
Cross Slope = (L/2)/ (Average Bank level-Average bed level)
Plan Form index represents the percentage of actual flow width per braided channel. Obviously, this index reflects the fluvial landform disposition with respect to a given water level and its lower value is indicative of higher degree of braiding.
Flow Geometry Index reflects the underwater sub-channel disposition and the hydraulic efficiency of a braided stream. Its higher value signifies occurrence of higher degree of braiding. Cross slope is basically a form ratio indicator and its higher value indicates higher braiding intensity.
(g) BARS AND SHOALS
Bars and shoals in the river bed may be identified as point bars, middle bars, alternate bars, islands, etc. The changing features of bars/islands associated with meanders/braiding, etc. may be ascertained from the sequential satellite images of different resolutions as per requirement to identify the probable direction of flow pattern/shifting of river courses for consideration in correcting river alignments. Temporal Satellite data may be used to study the river dynamics and area statistics to be derived from the temporal data to find the erosion and deposition characteristics. Unwanted shoals are identified for removal.
(h) BED FORMS
There are several methods of bed form study. However, those by EngelundHansen and Garde – Raju could be used.
(i) BED AND ENERGY SLOPES
A comparative study of valley slope, bed slope, water surface and energy slope is made. Excessive energy loss in a reach is associated with significant shoal formations.
Valley slope could be taken from the Survey of India topo sheets. Bed slope observed for reaches may be given. Water surface slope in different reaches, if available, may be given otherwise the same at the G & D sites may be given. Energy slope is drawn with respect to the river-bed taking into account the hydrostatic head
(depth) and velocity head, .
(j) CHANNEL MIGRATION
Channel migration is believed to be dependent on the variation in discharge from season to season. High variation is usually associated with significant shift in the deepest channel while low variation is associated with comparatively stable channel. There may be some exceptions to this general observation. Relation may however be established between the ratio of maximum discharge to bank full discharge on one hand and lateral channel shifting on the other. In some cases, there is a very good relation between these two parameters. There should be clear distinction between temporary (chute cut-off) and permanent cut-off by using temporal satellite data.
The above morphological parameters are studied to explain the unique characteristics and behaviour of the river and their bearing on various problems. The channel parameters, which need to be corrected / modified to find the solution to the problems, may be identified and the extent, to which such corrections / modifications are required, may be spelt out.A summary of various studies carried out, conclusions arrived at and recommendations for necessary remedial measures should be given at the end of the report. It should also summarize the behaviour of the river and practical usefulness of the morphological studies in future planning and design of remedial measures for flood control/ erosion control.
Ogun state river Fundamentals and practical ;
stabilization etc of the Erosions and floods Rivers
Is Rivers engineering methods and River training and guide rules in rivers on focus on both river banks enhancement for protecting the banks from Ogun river erosion,floods. By contrast, submerged river training structures function by effectively guiding eroding currents away from the banks and hence lowering shear stress in the bank area. Such installations known as “micro groins” and “meandering ramps” are built of natural boulders. They operate from within the river bed, not from the banks. Accordingly, these designs are covered under the term Instream River Training (IRT). A further key feature is the generation of helicoidal secondary flows which have an effect on sediment transport as well as on velocity and shear stress distributions. As opposed to conventional groins, IRT structures protrude only marginally from the river bed (i.e. 0.15 m), yet they operate efficiently during storms floods. In this paper we present fundamentals as well as monitoring results of recently built IRT structures. The monitoring results are promising. In 2011 micro groins and meandering ramps were installed in the river, canton Freiburg, Switzerland. These submerged groins proved to successfully protect the outer banks from erosion during a 30-year flood and to diversify bed morphology. The monitoring program for the river will be continued until 2017 to further investigate IRT structures.
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