C = Constant. (4 marks), Next, let’s look at the distribution of the rainfall data. The length of the catchment is 30 Km. SOI is calculated as follows. IOD and ENSO using the Southern Oscillation Index (SOI)). How “useful” are these statistics for your understanding of the system? Describe the values you got and what they mean. 3.4. is considered then the run-off is given by : Where P = Coefficient depending on the porosity of soil, vegetation cover, initial state of saturation of soil etc. The area is based on the “footprint” of the roof, which can be calculated by finding the area of the building and adding the area of the roof’s overhang. The average depth of rainfall over the entire catchment can be calculated using: where in this method \(A_{i}\) is the area between isohyets \(P_{i}\) and \(P_{i+1}\) (km2), \(P_{i}\) is the average rainfall (mm) corresponding to the area \(A_{i}\) between the isohyets, and \(\sum{A_{i}}\) is the total catchment area (km2). In the present case for catchment area composed of sandy soil with thick vegetation, A = 800 sq.km = 80,000 hectares ; P from table 3.3 = 0.10 ; f from table 3.4 = 0.60 ; Ic = 2.98 cm/hour. Are there statistically significant changes in rainfall? Porosity of soil; that is, whether sandy, clayey or rocky; degree of previous saturation; area covered by forest; presence of lakes, ponds, swamps, artificial reservoir etc. As discussed before, the run-off depends on the following factors: (i) Degree of porosity and degree of saturation of the soil in the catchment area. Calculate the maximum flood discharge if the river has fairly clean, straight banks but having some weeds and stones. The river's discharge at that location depends on the rainfall on the catchment or drainage area and the inflow or outflow of groundwater to or from the area, stream modifications such as dams and irrigation diversions, as well as evaporation and evapotranspiration from the area's land and plant surfaces. We then look at how rainfall may be changing over time and how it varies over space across Western Australia. This paper has studied the estimation of peak flood discharge of the Kunur River Basin, a major tributary of the Ajay River in the lower Gangetic plain. It is needless to mention that the full quantity of rainfall does not reach the water course as some quantity is soaked in the soil to form the sub-soil water strata, some quantity is absorbed by vegetation, some quantity is evaporated and the rest only flows to the channel or river. In case of catchment area with steep but wooded rock, P from table 3.3 = 0.80. Other indicators of temporal variability (look for cyclic patterns). The values of H and L can be found from the contour map of the catchment area. A … If V, is the velocity at surface, Vb is the velocity at bottom and Vm is the mean velocity then their relationship may be established in the following equation. 60 min of 10mm/h = 10 mm of rain in one hour We can the assume that we had 5 mm in the first 30 min and 5 mm in the next 30 min. The expo-nents for the drainage-area ratio were 0.85 for the winter season, 0.91 for the spring season, and 1.02 for the summer season. Clean up the plot as required and submit on your answer sheet (1 mark). In this exercise we will examine the nature of rainfall data and use some simple statistics to describe these data. This formula (originally devised for Northern India but can now be used in most of the states of India with the modification of the value of the coefficient C) is given by: The area of a catchment is 800 sq.km. The specified location results in a catchment with no area . (. (we need that as our unit hydrograph is per 30 min) From a record of the Meteorological Department, Govt. Before publishing your articles on this site, please read the following pages: 1. Due to unavailability of sufficient discharge data for many rivers, hydrologists have used indirect methods for deriving flood discharge amount, that is, application of channel geometry and hydrological models, for the estimation of peak discharge in the selected ungauged river basin(s) in their research/project works. Engineering, Designing, Highway, Bridges, Considerations, Flood Discharge from Rivers. : The wetted perimeter P at H.F.L. The calculation algorithm used may not result in a valid catchment for each selected discharge point. The average depth of rainfall (ADR) over the entire catchment can be calculated using: ADR (mm) = ∑Ai ∗P i ∑Ai (1) (1) A D R ( m m) = ∑ A i ∗ P i ∑ A i. (, Break into small groups of about 5 and discuss the results of your plots. m per hour. The flood discharge can be evaluated by using various empirical formulae involving area of the catchment and some coefficient depending upon the location of the catchment. Some values of the rugosity co-efficient, n, are given in table below for various types of surface conditions. It is also called catchment or drainage area. If the selection works you will see relevant stations listed, and then click “get data”. Delineate the boundary of a catchment/contributing area using a Digital Elevation Model and a given point (a specific point for which we want to find the contributing/catchment area). Figure 1: A depth of rain (\(P\), mm/day) is falling over a catchment area (\(A\), km2) to produce stream discharge (\(Q\), m3/day). The R.L. Highlight the station where there is a statistically significant slope. Drainage area was the only sig - nificant variable in the revised regression equations. Go to “Monthly climate indices” and then select DMI. For this we use simulation models. The annual rainfall varies from place to place and therefore, the recorded rainfall for a considerable period, say fifty years, is very useful in getting the maximum rainfall recorded during this period. age area to depend on the group. Individually (make sure you have at least one person per station but you can have more than one if needed). The values of P for various conditions of the catchment area arc given in Table 3.3: In addition to the coefficient, P, another coefficient, f, is introduced in the formula for calculating the run-off. Choose a new rapporteur to report back at the end of the discussion. What is the most appropriate type of graph for rainfall data? Input the data range. ; determine the percentage run-off. The most basic measure of average rainfall depth is the arthimetic average, whereby you sum the observed rainfall at each rain gauge inside the catchment and divide by the number of gauges. Break into small groups of about 5 and discuss the results of your plots. Rational Method: This method is also based on the principle of the relationship between rainfall and … The catchment area is the area from which rainfall is collected. The Rational method runoff coefficient (c) is a function of the soil type and drainage basin slope. 3.2 show two types of catchment. The general procedure for determining peak discharge with the Rational Formula is: Step 1 Determine the drainage area. Be able to source rainfall data, understand the nature of that data and describe it using simple statistics in Excel. where, R = Runoff in million m 3. Are they linear or non- linear and why? A major hazard to drinking water safety is presented by ‘precipitation’ events (rain, snowmelt), where large Introduction Many water-management programs require streamflow Choose a rapporteur to report back at the end of the discussion. Another important factor which determines the percentage of run-off is the shape of the catchment. If t = one hour and corresponding i is taken as i„ and the value of I is taken from equation 3.6. Determine the relationships between rainfall and runoff to evaluate the nature of the relationship between rainfall and discharge. What if any assumptions are there about these statistics? 4.1.2 Catchment protection Catchment protection is the second step in providing safe drinking water and where, for whatever reasons, source choice is limited it presents a key opportunity to minimise pathogen contamination. You will then be emailed a zip file of the station’s data. The methods are: 1. Roof Area (ft2) X Precipitation Amount (in) X 0.623 = Amount Collected (gallons) Easy Formula to Remember: 1″ of Rain on 1,000 sf Roof will Yield 623 Gallons. The area is located in Western India within 150 km. iii. In this exercise you are required to calculate the average rainfall (ADR) over the entire catchment using: You are then required to produce an estimate of catchment runoff, which we will assume is equal to the stream discharge out of the catchment (\(Q\)). \end{equation}\], \[\begin{equation} This can be done accurately by setting up two ropes stretching across the stream, 5 meters apart. may be found out by dividing the area into strips such as BPC, PCDO, ODEN etc. The time taken by the float to cover a fixed distance is noted by a stop watch and the distance travelled by the float divided by the time taken is the surface velocity of the stream. This is the maximum rate at which water will be allowed to discharge from the system. The formula is used to calculate the runoff for each month and then the values are added to arrive at the annual runoff. Make a name to save the data (the default should be fine). Does this match with your own rainfall station correlations (, Plot a scatter plot of discharge and annual rainfall, add a line to the chart. Then click correlate at the bottom and wait (the calculations are being done online and the graph returned for viewing). Example – Rooftop catchment area made of concrete; Mean annual rainfall in = 700 mm/year (0.7m) Surface area of catchment = 2629 m 2 (See Figure 2) the arithmetic mean (considering only stations within the catchment boundary), the Thiessen polygon weighting system, and, the Isohyetal method. Select the last 5 years of data and in DATA ANALYSIS start a REGRESSION. How the rain water reaches the channel or the river from the catchment area is shown in Fig. Delete the rows of data for the last partial year (2019) and any partial years at the start. The average depth of rainfall (ADR) over the entire catchment can be calculated using: Where \(A_{i}\) is the polygon area (km2), \(P_{i}\) is the rainfall (mm) corresponding to the \(i\) polygon, and \(\sum{A_{i}}\) is the catchment area (km2). In order to calculate the flow discharge produced by a large storm (convective type) in the catchment in, it is necessary to calculate the average depth of rainfall (ADR) over the catchment. Clean up the plot as required and submit on your answer sheet (2 marks). In normal single catchment, the watershed is long and narrow having a number of short tributaries joining the main stream. The possibility of intensive rainfall falling simultaneously over the entire area of a big catchment is less and therefore, a lesser percentage of run-off may be taken. Therefore, a rainfall of Ic cm per hour over an area of A hectare will cause a run-off of 100 A Ic cu. The velocity in this case is determined at site by direct measurement of the velocity in place of theoretical calculation from bed slope etc. Image Guidelines 5. Highlight any months that have a significant p-value (<0.05). In general, river discharge is computed by multiplying the area of water in a channel cross section by the average velocity of the water in that cross section: discharge = area … How would you describe the seasonal pattern of rainfall (hint look at the climate type)? Once we have computed ADR, we can now estimate the likely amount of runoff. Use a PIVOT table again. Below is a link to a first flush volume and length calculator based on last rule of thumb \tag{1} Therefore, while estimating the flood discharge from the catchment area, the aforesaid factors shall be duly taken into consideration. Report a Violation, Top 6 Methods for Estimation of Flood Discharge, Design Flood: Definition and Computation of Design Flood Hydrograph. From equation 3.9, io (One hour rainfall) can be worked out if the total rainfall R and duration of the severest storm are known. area = 254900000 m2area =54.9 km Time (min) Discharge (m3/s) 0 0 15 4.5 30 10 45 12.5 60 11 75 9 90 6.5 105 4 120 2.5 135 1 150 0 a) What is the size of the catchment area? If land use and soil cover are homogeneous over the drainage area, a … Plot the annual sums as a timeseries. A … The image below shows how differences in roof slope do not change this building’s catchment area. In order for this command to produce a catchment area, the surface triangle edges must form a channel that slopes toward the discharge point. In such catchment, storms of shorter duration which cause the maximum flood discharge, will not reach the bridge site nearly at the same time and as such run-off in such catchment area will be less than that in a fan-like shape of catchment. We then want to correlate each grid cell on the earth with our climate index (SOI and DMI) to see how the correlations very spatially. Calculate potential supply of rainwater from the catchment area. It is advisable to consider a number of heavy storms spread over a prolonged period and io may be calculated for each case and the maximum value of U shall be taken as the one hour rainfall of the region for the estimation of flood discharge. The previous exercise focussed on estimating rainfall and runoff at catchment scales during a storm event. the distance from the rain-gauge to the centre of the representative area. If you are using the roof of a building for this purpose, calculate the square footage of this area by taking the length of the building, including the overhang(s) and multiply this by the width measurement plus the overhang(s) of the building. (2 marks), Use the Data Analysis add-in to calculate descriptive statistics (mean annual rainfall, max annual rainfall, min annual rainfall, Standard deviation of annual rainfall, Skewness) for the whole data (you can install by going to Tools->Add-ins->Analysis ToolPack). How do we frame the hypothesis? Are they consistent across WA? Km. The formula is used to calculate the runoff for each month and then the values are added to arrive at the annual runoff. First you will need to register so you can save data series. A calculator with the option ‘y to the power x’ (y x) Preferably the IDF-curves (intensity-duration-frequency curves) of the zone studied Analysis of the catchment area First the catchment area with its boundaries will have to be identified on the map. In a stream having non-erodible banks and bed, the shape and the size of the cross-section remain practically the same during a flood as at normal times and therefore, the normal cross-section and the perimeter may be used in calculating the discharge. \[\begin{equation} Copy the results here and complete Table 4 for your site (and others when they are done). VVR111 – Hydrology and Aquatic Ecology. Select Year and Rainfall amount as the FIELDS. The best way to calculate discharge of a river is to measure it. the surrounding area: Most hydrological analysis at the catchment scale requires knowledge of the areal distribution of precipitation in the form of an average depth (mm) of water for the entire catchment over a specified time period, using information from a set of unevenly placed rain-gauges spread around the area. Find out the peak storm discharge by the Rational Method assuming that the rainfall in 5 hours is 20 cm. Go the Department of Water web site. Here you can easily find how much rainwater you can collect from you roof, how much you need to remove from a courtyard, or how much runoff you can expect from an area of land. is the bed line BCDEFGHI which is the summation of the length of line BC, CD, DE etc. (2 marks), What hypothesis can we make about changes in rainfall amount over time? The catchment area is computed from the contour map and the flood discharge is estimated from the “Run-off ” formula. Select Month and Rainfall amount as the FIELDS; drag the Month FIELD into the ROWS box. Calculate the arithmetic mean and write this in your summary table (Table 3). The analyses resulted in a relation that can be used for estimating storm peak discharge based on drainage area, basin-average rainfall, and impervious area… From the daily record of rainfall, annual rainfall for a zone is determined. Add the Pearson correlation coefficient (r2) value as well. the characteristics of the local storm patterns (convective versus low intensity events). To correlate the index and rainfall select “Correlate with other time series”. Q = Peak discharge, cfs c = Rational method runoff coefficient i = Rainfall intensity, inch/hour A = Drainage area, acre. After that we explore how different modes of climate variability affect annual rainfall. Select the required data across multiple columns (year C to rainfall amount F). Para 4.2.2 The design discharge (Q) normally shall be computed flood with a probable recurrence interval of 50 years. The catchment area (also known as flow accumulation), can be used to set a threshold for channel initiation. The catchment area of the stream or river upstream of the bridge site is obtained by marking the ridge line of the contour map and measuring the area enclosed by this ridge line with the help of a plan meter or tracing paper graphs. (vii) Intensity of rainfall; Run-off is more if the same amount of rainfall say 50 mm is within a very short period of, say, two hours than is spread for a larger period of, say, 24 hours in which case it is in the form of drizzling. 3.1 and Fig. of India, the values of io for various places of the Indian Union are reproduced in Table 3.2: Time of concentration is defined as the time taken by the run-off to reach the bridge site from the furthest point of the catchment which is termed as the critical point. Know how different modes of climate variability such as IOD and ENSO affect annual rainfall. Estimation of Design discharge (Q) (Para 4.2 S.S.C) Para 4.2.1 The estimation of design discharge for waterway shall preferably be based, wherever possible, on procedures evolved from actual hydro meteorological observations of the same or similar catchments.”. What may be causing the changes? Area of Cross-Section and Velocity As Observed At Bridge Site 6. Area based rule-of-thumb: 0.41 liters for every meter squared of roof [verification needed] OR 10 gals for every 1,000 foot squared of roof . You will then see graphs and options for further integrating the data. where, R = Runoff in million m 3. Content Filtrations 6. Note any trend and variability in the index. This can be done using the Channel network algorithm. You need two types of models, run sequentially: a hydrological (rainfall-runoff) model and a hydraulic (routing) model. A watershed is all the land & water which contributes runoff to a common point or outlet point. Privacy Policy 8. are the same for both types of catchment, the run-off at the bridge site will be more for fan shaped catchment than for normal single catchment. Rational Method 4. from coast. 3.3 since the mean intensity for a small time interval, t, is more than the mean intensity for the whole time period, T. 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