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Hydrological Analysis and Design of Check Dam for Water Supply
1R.S Patil, 2Yogesh M, 3Bharath D S, 4Mallappa S Naganoor, 5Saleem Pasha C A
1,2,3,4,5Department of Civil Engineering, R.R Institute of Technology,Bengaluru-560090 Abstract : Designing a check dam is one of the most serious
and risky projects of civil engineering. But with proper knowledge and data and with experience a stable check dam can be designed to meet the necessary requirements. Also while designing a check dam one should design it in an economic manner. Designing a check dam unnecessarily big may result in wastage of investment and resource. So a proper study on hydrological features is relevant. Analysis of stability of slopes is of utmost importance as its failure may lead to catastrophic consequences resulting in loss of lives and great economic losses. Failure of a mass located below the slope is called a slide. It involves both the downward and outward movement of entire mass of soil that participates in failure. So in this project a proper hydrological analysis of the Arkavathi River basin was carried out. The analyses were basically determination of average annual precipitation of the catchment, yield series and flood discharge. After obtaining a suitable data the cross section of the Check-dam was designed. Now in order to check the stability of the dam a series of hand calculation was done. It included stability of upstream slope, downstream slope, overall stability against shear and stability of foundation.
Keywords: Rain fall data, Flood Discharge, Contour, Planimeter, Auto CAD, Hydrological analysis, Stability, Arkavathi
I. INTRODUCTION
A check dam is a small barrier or dam constructed across a swale, drainage ditch or other area of concentrated flow for the purpose of reducing channel erosion. Channel erosion is reduced because check dams flatten the gradient of the flow channel and slow the velocity of channel flow. Most check dams are constructed of rock, but hay bales, logs and other materials may be acceptable.
Contrary to popular opinion, most check dams trap an insignificant volume of sediment.
This practice applies in small open channels and drainage ways, including temporary and permanent swales. It is not to be used in a live stream. Situations of use include areas in need of protection during establishment of grass and areas that cannot receive a temporary or permanent non-erodible lining for an extended period of time
Purpose of the Project
The project is proposed to benefit soladevanahalli blocks of Bangalore district, Karnataka, which are chronically drought affected areas. These areas are inhabited mostly by poor people belonging to Below poverty line people.
The only source of income for the inhabitants of this area is agriculture. Therefore the project is absolutely necessary to improve agricultural output, drinking purpose, ground water recharge and economy of the region to mitigate the misery of a sizeable population, mostly belonging to backward classes.
Location of the Project
The location is in Soladevanahalli Block of Bangalore District which is near the village Tarbanahalli and is about 5 km away from Chikkabanavara sub-division of BangaloreDistrict. The nearest railway station of South west Railways is Soladevanahalli, about 5 km from the dam site..
OBJECTIVES:
The broad objectives of Check Dams (In-stream Storage Structures) are :
• To provide drinking water facilities in the villages along both the sides of the river after monsoon period.
• Ground Water recharge
• To provide incidental irrigation during late Khariff and Rabi by storing water at the end of monsoon mainly through lifting devices.
Guidelines for Exploitation of Surface Water by construction of Check Dams
• Irrigation use of water flowing down drainage channels.
• To divert water from perennial / semi-perennial streams in hilly areas for irrigation purpose.
• Other uses by villagers like bathing, washing, fishing, recreation etc. depending on location and potentiality
II. LITERATURE REVIEW
Stability analysis of an earth dam under steady state seepage- 17 March 1996 Tien-kuen Hnang
The aim of the work described in this paper is to describe a numerical procedure for performing stability analysis of an earth dam after the filling of a reservoir. Firstly, the piezometric heads at
different points in an earth dam after the filling of a reservoir are obtained with a trial-and-error procedure.
Then, the numerical analysis of the dam is performed using the finite element method, with a cap model used for representing soil behaviors. A special technique to handle the effect of steady state seepage is introduced. An example of a reservoir completed recently in Taiwan is illustrated. The results indicate that the factor of safety against stability failure of the dam is adequate.
III. METHODOLOGY:
HYDROLOGICAL ANALYSIS
Estimating spatial distribution of rainfall over the catchment
The available rainfall data are point values but we need data cover the whole Upper junk catchment. The objective is to determine how the rainfall distribution is influenced over the catchment by the different rain gauge stations. There are many suitable methods. In this project two methods were being used: Arithmetic Average Method,Thiessen Polygon Method and the incorporation of the elevation effect on rainfall distribution(Not dne).
These are introduced below.
Determination of average annual rainfall Arithmetic Average Method
The advantage of Arithmetic Average Method is that it needs the simplest calculation. But it shows a big disadvantage over the others. The method is suitable if the climate and the relief is near uniform throughout and the regional distribution of rain gauges is homogenous. So in this instance this method have appreciable inaccuracy, therefore it was used to compare this result with the other methods results.
The arithmetic average of the rainfalls is given by the equation:
Where: spatial average of precipitation - Rain gauge precipitation value
I - number of rain gauges n - Total number of rain gauges Arithmetic Average Method Max annual rainfall= 839.51 mm Min annual rainfall=325.24 mm Average annual rainfall=540.557mm Thiessen Polygon Method
The Thiessen Polygon Method was also used to find the spatial distribution of rainfall.
In this method the Upper junk catchment was divided into 4 parts performing the following construction: drawing straight lines between the rain gauges and constructing perpendicular bisectors for each line. Accordingly we obtain required sub regions which areas in the catchment are closest to each rain gauge station.
Each sub region belongs to one of the rain gauges. The spatial average precipitation in each region assumed to be identical with precipitation value of the regions rain gauge.
I have used Thiessen polygon method for the determination of average rainfall as it is easy and reliable method. Advantage of this method over arithmetic mean is that, in this method weightage is given to all measuring gauges on the basis of their aerial coverage on the map ,thus reducing discrepancies in their spacing over the basin.
Procedure
1. All the gauges in and around the basin were accurately marked on a map drawn to scale.
2. Consecutive stations were joined by straight to for triangles.
3. Perpendicular bisectors were drawn to these lines such that the bisectors formed a polygon around each stations.
4. Each stations on the map were thus enclosed by a polygon. A polygon represents an area for which the station rainfall is the representative.
5. Area of each polygon was measured by counting the unit boxes of the graph over which map was drawn.
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6. Thiessen weights were computed by dividing the area of each polygon by the total basin area and checked for the sum of weights of all stations to be equal to unity.
7. Finally the average precipitation was calculated by using the relation.
Fig 1: Determination of weighted mean SL
NO
STATIONS INFLUANCE
FACTOR
1 HESARAGATTA 0.198
2 SOLADEVANAHALLI 0.118
3 YELAHANKA 0.120
4 NELAMANGALA 0.071
5 DODDABELAVANGALA 0.1
6 KUNDANA 0.108
7 DODDABALLAPURA 0.147
Where , , , represents precipitation and , , , area of 4 stations respectively and , , , their Thiessen weight or influence factor given by , , , .
Thiessen Polygon Method
Max annual rainfall= 820.858mm Min annual rainfall=309.908mm Average annual rainfall=526.605mm Determination of Flood discharge
Hydrograph A hydrograph is a graph showing the rate of flow i.e. dischargeswith time in a river, or other channel or conduit carrying flow. It is the total response or the output
of a watershed beginning with precipitation as the hydrological exciting agent or input. A hydrograph is a result three phases namely base flow, subsurface and surface flow The rate of flow is usually expressed in cubic meters or cubic feet per second.
Unit hydrograph: An Unit Hydrograph (UH) or unit graph of a watershed is defined as the hydrograph of direct runoff hydrograph resulting from a unit depth of 1 cm of excess rainfall of constant intensity generated uniformly over the basin or drainage area occurring for a specified duration of D hour. The term unit depth of rainfall excess means excess rainfall above and over all the losses (like evaporation, transpiration, interception, depression storage and depression storage) in the basin for which hydrograph is to be obtained.
Snyder’s synthetic unit hydrograph: When a catchment is ungauged, the established empirical formula or relation between the catchment characteristics and unit hydrograph parameters may be used to synthesize a unit hydrograph for a basin. A synthetic unit hydrograph has all the features of the unit hydrograph, but it does not require rainfall-runoff data for a particular flood. A synthetic unit hydrograph is derived from the theory and experience, and its main purpose is to simulate basin diffusion by estimating the basin lag or lag time based on a certain formula or procedure. The first synthetic unit hydrograph model was developed by Snyder in 1938 and is accepted as a standard practice for the derivation of a unit hydrograph for a basin where rainfall and runoff data’s are not available.
Determination of elevations of Ground level
Fly levelling: The levelling instrument was placed on the tripod and levelled accurately, the station points A,B and C .The levelling instrument was placed at a convenient distance from the station point C and B,A back sight was taken on C and fore sight was taken on B.The points A and B were not intervisible in a single set up. The intervisible point A' was taken at a convenient distance from A and B.The instruments placed between B and A',A back sight was taken to B and for sight was taken to A’, then instrument shifted to a convenient distance from A' and A,A back sight to A’ and fore sight to A was taken.
Profile levelling: First of all a chain was stretched through the centre of rode points marked at an interwell of 3m .one point was marked on either sides of the points on centreline. The levelling instrument was setup from which all points are visible A back sight was taken on the bench mark after levelled the instrument the height of collimation was calculated .Then sights were taken to the of collimation was calculated .then sights were taken to the points which were marked previously (points on centre line of rode in 3m interwell, one point to right and one point to left at 15m distance in each 3m chainage)the last point was marked as fore sight, the distance was
marked on distance column as left ,right and centre line points and corresponding sight reading were marked, the reduced level of each points was calculated.
By above methods R.L of site is found to be 841.430m Contour capacity
Plane table was setup at a convenient station(o),Draw radial lines towards the boundary of the area (ie A,B,C,D and E)by pivoting the alidade at O CHAIN WAS Stretched to A, marked the interwell along the radial lines ,placed the levelling instrument at a convenient station such way that all points were visible from it, then the levels of various points were founded, marked the points with same reduced level and interpolated ,then the contour map were plotted
Fig 2: storage valley Graphical method
By graphical method considering 1 unit=2500m2 Calculated contour area=1,30,000m2
Average ground level in the valley=840.05m F.S.L=844.50m
Average storage height=844.50-840.05=4.45m
Volume of valley storage= 0.13 X 1000 X 1000 X 4.45 = 5,78,500m3
Design of check dam Data Catchment area=1,30,000m2 Nature of catchment=good
Average annual rainfall=540.557mm T.B.L=846.00m
F.S.L=844.50m G.L =840.00m F.L =839.00m Design of flood By Ryve’s formula Q=1000A3/4 =1000X(0.0501) ¾
=105.89 cusecs =3.0010 cumes Design of weir Q=1.84(L-knH)H3/2
3.001=1.84(L-0.1X2X1)13/2 L=1.83m
Therefore provide 6m Discharge intensity=3.001/6 =0.5cumes
Normal scour depth(R)=1.35(q2/f)1/3 =1.35(0.52/1
=0.85m
Therefore provide 1m F.S.L=844.50m
T.B.L=844.500+1.5=846.000m Top width of weir
B1=0.55(√H +√h) =0.55(√4.5+ √1) =1.71
Provide 1.8m Bottom width B2=H+h/√(£-1)
=4.5+1/√(2.25-1)=4.9m Provide 5m
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Design of abutment
Height of abutment=846.00-840.00 H’=6m
Length of abutment=bottom width of weir =5m Provide top width 0.5m
Bottom width=0.4XH’=0.4X6=2.4m Provide 2.5m
Bottom width
B2=H+h/√ (£-1) =4.5+1/√ (2.25-1) =4.9m
Provide 5m Design of abutment
Height of abutment=846.00-840.00 H’=6m
Length of abutment=bottom width of weir =5m Provide top width 0.5m
Bottom width=0.4XH’=0.4X6 =2.4m
Provide 2.5m
Design of return wall
Provide same c/s for the u/s and d/s return wall as the C/S got from wing wall of junction return wall
Provide length of return wall up to suitable length to protect the bund with a minimum distance of 2m
Design of apron
LD =2.21C√H”/13 take C=4
=2.21X4X√4.5/13 =5.2m
Adopt 6m.
Fig 3: Drawing of check dam Stability analysis
This is the most important part of this project. Designing a dam is not only of the prior importance, designing it safe against failure criterion is the main deal. The constructed dam should be safe against adverse meteorological condition and the geological feature of the location. The following stability condition were taken into consideration for analysis as mentioned below:
• Stability of the downstream slope during steady seepage
• Stability of the foundation against shear
• Overall stability of the dam section
Fig 4: Earthen bund
CONCLUSION:
The primary function of an engineer is to design a structure economically without compromising on its strength. So one should never compromise with the strength even though the cost is high. The fury of nature should never be underestimated. A thorough knowledge on the hydrological analysis is therefore relevant for designing such structures safely and economically. While determining the average precipitation of basin, two methods were used. There is not much variance between
the two results, but still we choose the Thiessen polygon method because here the distribution is much spatial as compared to average mean method and so give much better results.
While determining the centroid of the basin cardboard and thread method was used which is always not accurate.
Because the cardboard may not be uniform at every point.
But still it the easiest and can be calculated easily by anyone as compared to modern softwares.
In case of design of slopes of dam, steep slopes require less earth work hence, lesser cost. But, the factor of safety is compromised.. Another, option is to provide reinforced slopes or retaining walls. These slopes have greater factor of safety than corresponding non-reinforced or unsupported slopes. Although, they decrease the amount of earth work involved the cost is significantly increased due to the addition of these structures. But, the cost of construction of slopes also depends upon the cost of land.
Therefore, in urban areas where the cost of land is high steeper slopes may be provided with adequate reinforcement or retaining walls in order to minimize cost.
In case of small earthen dam horizontal filter can be used.
But while designing bigger dams vertical chimney as well as rock toe should be provided to considerably reduce the seepage
REFERENCE
[1] Hydrology and Water Resource Engineering by K.C. Patra
[2] Irrigation Engineering and Hydraulic Structures by S.K. Garg.
[3] Dam Safety Code Requirements for Dams Design
& Construction.
[4] IS 2720 : Part 17 : 1986 Method of Test for soils – Part 17 : constant head test and falling head test to determine permeability of soil, Bureau of Indian Standards, New Delhi.
[5] Dam Safety Code Requirements for Dams Design
& Construction.
[6] IS 2720 : Part 15 : 1986 Method for Test for soils – Part 15 : Consolidation test ,Bureau of Indian Standards, New Delhi.
[7] IS 2720 : Part 10 : 1991 Method for Test for soils – Part 10 : Unconfined compressive strength. , Bureau of Indian Standards, New Delhi.
[8] SUBRAMANIAM,P(2011), “Reliability Based Analysis Of Slope Foundation And Retaining Wall Using Finite Element Method”, Submitted to National Institute of Rourkela Rourkela, India [9] Henderson F.M 1996 open canal flow in New
York
[10] Ponc.V.M and Yevjevich.V 1978, Muskingm-Cung method with variable parameters, journal of the hydrological division.
[11] Ponce. V.M 1986 Diffusion wave modelling of catchment dynamics, Journal of the hydrological division.
