# methods of computing runoff from a catchment area

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40 Z. Simić at al. The formula is. Assuming that the surface on which the rain falls is classified as follows: Calculate the impervious factor. The precipitation range as well as the type of catchment is covered in this approach. Where Ai A i is the polygon area (km 2 ), P i P i is the rainfall (mm) corresponding to the i i polygon, and ∑Ai ∑ A i is the catchment area (km 2 ). The rational formula is found to be suitable for peak-flow prediction in small catchment up to 50 km 2 in area. The daily runoff is calculated whenever there is rain and is added to calculate the seasonal or annual runoff. There is no "loss method" associated with the modified rational method. The analysis consists of two major parts. In practices it is the value of X for a given P that is required, y, Since the value of the variate for a given return period, x, For a confidence probability c, the confidence interval of the variate, x, If the random variable Y = log X is normally distributed, then X is said to be log-normally distributed. However, for the sake of simplicity, the Rational Method is used to determine the size of the detention basin required for construction site. If the points for all of the stations lie between the 95% confidence limits, then they are considered homogenous. ... or by catchment area. However, it gives a rough, estimate. 6. how exactly is the procedure to calculate the surface runoff volume from scs curve method for the catchment area for about 4 hectares? iv. The USDA Natural Resources Conservation Service (NRCS) methodology is perhaps the most widely used method for computing stormwater runoff rates, volumes, and hydrographs. The relation thus derived will be applicable for a specific catchment and is normally not used for other catchment areas. peak flow rates and runoff volumes. The methods of analysis based on these distributions can be grouped as: (i) curve fitting methods, graphical or mathematical and (2) methods using Frequency factors comprising: (a) the Gumbel method (b) the lognormal method and (c) other methods such as Foster III, Foster I and Hazen methods. Disclaimer 8. Laurenson Non-linear Method/Rafts 4. Lesson 8 Analysis of Precipitation Data - I, Lesson 9 Analysis of Precipitation Data - II, Lesson 12 Runoff Estimation– SCS Curve Number Method, Module 3: Soil Erosion Processes and Estimation. There are a large number of methods and models in vogue for computation or estimation of runoff from a watershed. For a confidence probability c, the confidence interval of the variate, xT is bounded by values x1 and x2 given by. The United States Department of Agriculture (USDA) for Natural Resources Conservation formerly called Soil Conservation Service (SCS) developed a basic equation for computing the direct runoff. The mean annual flood is dependent upon many variables, the most important and commonly available being the drainage area. In the original formula the recession time was assumed to be equal to the time of rise. Some limitations of the lognormal distribution are that it has only two parameters and that it requires the logarithms of the data to be symmetric about their mean. SWMM, the Storm Water Management Model, program configuration. The runoff from a catchment is estimated by the following methods: 1. The routing portion of SWMM transports this runoff through a conveyance system of pipes, channels, storage/treatment devices, pumps, and regulators. Where, KT is obtained is obtain from table 11.1. The basic structure of an ANN usually consists of three layers: the input layer, where the data are introduced to the network; the hidden layer or layers, where data are processed; and the output layer, where the results of given input are produced. The Rational Method is most suitable for small urban watersheds that don’t have storage such as ponds or swamps. Rainfall features are highly variable over space and time and the watershed features are highly variable mainly over space. The relation was developed on the basis of observations in the former Bombay State in India. Extension of the result of the frequency analysis of station data to an area requires regional analysis. Observations of precipitation and the resulting runoff were taken for a number of catchments having different characteristics. Terms of Service 7. Some of the runoff computation/estimations methods are presented in the following pages: The Rational Method is most effective in urban areas with drainage areas of less than 80 hectare. The equation is based on Newton’s second law of motion and is written as: Where, y is the flow depth, V is the mean velocity, g is the gravitational acceleration, So is the bed slope, and Sf is the friction slope. This method is widely used in applied hydrology. Visit the links below for details about each routing method: 1. The Modified Rational Method (as shown in Fig. Understanding the complex relationships between rainfall and runoff processes is necessary for the proper estimation of the quantity of runoff generated in a watershed. Rainfall Runoff Correlation. ... T o calculate aerial precipitation. The inter-connection between neurons is accomplished by using known inputs and outputs, and presenting these to the ANN in some ordered manner; this process is called training. The Tc (hours) field becomes available. Rainfall depths from actual storms are used to make the estimates. Standard Tables 2. The TOPMODEL is considered as a semi-distributed model, as it uses distributed topographic information to determine the topographic index and to distribute saturation deficits throughout the basin, as well. SWAT model divides the . •Hydrograph of surface runoff of a catchment resulting from unit depth (usually 1 cm) of rainfall excess (effective rainfall) occurring uniformly over the watershed and at uniform rate for a specified duration. 11.5. SCS Unit Hydrograph Method 5. Centimetres of water over a catchment area. The procedure is repeated with another return period. b) Calculate and plot the hydrograph for a 60 minute rain with an effective intensity of 10 mm/h. From Wikipedia, the free encyclopedia A runoff model is a mathematical model describing the rainfall – runoff relations of a rainfall catchment area, drainage basin or watershed. The standard AR method assumes that the runoff per unit area in the donor catchment is equal to that in the target catchment. The method does not provide a hydrograph shape but gives only a peak discharge of known frequency. Figure 1 shows the catchment area of Kampung Kasipillay, Kuala Lumpur. Two distributions which are widely employed in recent years are: (i) the logarithmic normal and (ii) the extreme value. ADVERTISEMENTS: Read this article to learn about the following three methods of calculating average depth of precipitation upon the area of the basin, i.e., (1) Arithmetic Mean, (2) Theissen Polygon Method, and (3) Iso-Hyetal Method. (1993) designed a three-layer feed-forward ANN using the rainfall hyetographs as input and hydrograph as output. This can be expressed as . 11.4). Click a catchment in your model to display the Property Editor, or right-click a catchment and select Properties from the shortcut menu. Runoff estimation is also required to know the watershed water yield, which is the governing factor for planning irrigation projects, drinking water projects and hydroelectric projects. UK Hydrology: New UK, Wallingford, ReFH, FEH, FSR … For each selected return period, the median of the ratio from all the stations is computed. The catchment area is computed from the contour map and the flood discharge is estimated from the “Run-off ” formula. The continuity equation applies to both dynamic waves and kinematic waves. In most cases, records extend to short length of time and contain relatively few events. Fig. Continuous runoff needs to be estimated in ungauged catchments to interpret hydrological phenomena and manage water resources. Deducting Abstractions from Precipitation: The runoff can also be estimated by deducting all abstractions from the precipitation. Out of several methods for runoff estimation from ungauged catchment, the Natural Resources Conservation Services Curve Number (NRCS-CN) method along with its derivatives have been widely applied to ungauged catchment systems and proved to be a quicker and accurate estimator of surface runoff. 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. The method is typically used to determine the size of storm sewers, channels, and other drainage structures. The runoff and extended transport computational blocks and the executive and graph service blocks are normally used in various studies. 5. This method is applicable mainly to small urban catchment areas. From the data available from past records, a graph can be plotted to correlate runoff and rainfall. Its main parameter is the topographic index derived from a digital elevation model. The binomial distribution can be used to find the probability of occurrence of the event r times in n successive years. Uploader Agreement, Measurement of Run-Off | Functions | Precipitation | Geography, Factors Affecting Runoff | Functions | Precipitation | Geography, Runoff: Process and Sources | Functions | Precipitation | Geography, Catchment Area: Slope and Its Classification | Geography, Essay on Heat Waves in Europe—A Natural Calamity | Environment, International Tourism in India: Introduction, History, Trends, Opportunities and Future, Forestry: Definition, Branches, Costs, Programme and Conclusion | Geography, Contribution of Russia to World Geography (In Hindi), French Scholars and their Contribution to Geography in Hindi. The feed-forward multilayer perceptron (MLP) is the most commonly used ANN in hydrological applications. Computation of the coefficient of skewness, Cs. The lognormal distribution has the advantages over the normal distribution that it is bounded (X > 0) and that the log transformation tends to reduce the positive skewness commonly found in hydrologic data, since taking logarithms reduces large number proportionately more than it does for small numbers. There are seven major types of Hydrograph Generation techniques available in Runoff. computing terms with the ability to perform long simulations. Runoff is the result of interaction between the rainfall features and the watershed characteristics. There is usually disparity between measured values and those determined from other methods, viz; the unit hydrograph method, simulation or … Horton’s n: a measure of surface roughness similar but not identical to Manning’s n values. The Artificial Neural Network (ANN) is an information processing system composed of many nonlinear and densely interconnected processing elements or neurons, which are arranged in groups called layers. 1. However, runoff and its features must be known for the design of any structure either for storage (e.g. If the catchment area is symmetrically distributed around the drainage network, an approximate value for the overland flow length can be found by dividing the area by twice the length of the drainage channel. In a sub-humid area runoff remains constant, and in a semi-arid region the runoff decreases with increasing catchment size as a result of channel transmission losses. If the maximum intensity of rainfall is 40 mm/hour, calculate the quantity of storm water which will reach sewer lines. Module 4: Vegetative and Structural Measures for E... Last modified: Saturday, 21 September 2013, 5:46 AM, In the modified version of the rational formula, a storage coefficient, C, In the modified version of the Rational Formula, a storage coefficient (C, Where, y is the flow depth, V is the mean velocity, g is the gravitational acceleration, S, 11.5 Storm Water Management Model (SWMM), Where, = mean and = standard deviation of the variate X. The number of neurons in the input and output layers are defined based on the number of input and output variables of the system under investigation, respectively. Runoff from both the types may be calculated separately and then added together. The relation has been presented in the form of tables as well as curves. The rational method is based on a simple formula that relates runoff-producing potential of the watershed, the average intensity of rainfall for a particular length of time (the time of concentration), and the watershed drainage area. Comparing Methods for Computing the Time of Concentration in a Medium-Sized Hungarian Catchment.pdf Available via license: CC BY-NC-ND 3.0 Content may be subject to copyright. A catchment area may be partly in the ghat and partly in the plains. Soil Conservation Service (SCS) which depends … Secondly, this review considers the data required to apply desk based methods within a catchment. Since the number of stations in the region is generally less than 10 for want of data in developing countries, the mean is a more stable parameter than the median. In the Runoff section of the Property Editor, select Modified Rational Method in the Runoff Method field. The underlying assumption is that the peak intensity is maintained for long enough duration to reach peak flow at the outlet of the catchment. spillways) of the runoff water. Catchment-Run-Off Method: The catchment area is the command area of a river wherefrom the river gets the supply of water. The structure of a 3-layer MLP. Standard Tables 2. The confidence interval indicates the limit of the calculated value between which the true value can be said to lie with a specific probability based on sampling errors only. Or; The total water in cubic-metre or hectare-metre for given catchment. This means that the runoff is systematically overestimated for some catchments and underestimated for others. The water balance for a catchment with area A is = p - e - q d t dS (1a) with specific runoff A Q q = (1b) where Q = runoff at the outlet, p = precipitation intensity (e.g. In many studies, in order to avoid instability, the neural network is trained a number of times, and by averaging the output from all, a final output is obtained. For a small watershed, the design flood hydrograph can be synthesized from available storm records using rainfall-runoff models, but in case of large watersheds, which provide some hydrological data, a calculated risk can be taken in designing hydraulic structures for a flood lesser than the most severe flood. 11.4. 6. runoff and the reduced contributions from groundwater storage. The full dynamic wave technique takes into account the entire spectrum of the physical processes, which simulate hydrologic flow along a stream channel. Dam is located upstream of the catchment and has been mitigating the storm runoff from the upstream catchment of 52 km 2 from flowing down to the Kasipillay area. Rainfall -runoff model is a mathematical model describing the rainfall -runoff relations of a catchment area or watershed. Where Q = design discharge (m 3 /s), C = runoff coefficient (dimensionless), I = design rainfall intensity (mm/h), and A = watershed drainage area (km 2). 11.3.2 (a) Momentum Equation (Dynamic Wave Form). But many times, their complexity and the absence of well and systematically recorded time and space variant data make them difficult to utilize. The strength of these interconnections is adjusted using an error convergence technique so that a desired output will be produced for a known pattern. If the probability of an event occurring is P, the probability of the event not occurring in a given year is q = (1-P). Table 2.6 gives the value of C as suggested by Richard. The catchment area of a city is 200 hectares. The characterization and control of runoff pollution from nonpoint sources in urban areas are a major issue for the protection of aquatic environments. The momentum equation applies to the full dynamic wave. r!} These percentages were modified by multiplying them by a coefficient depending on the rainfall as given in Table. The modified rational method is then described by the formula: Fig. A frequency curve covering the range up to the 100- years flood is obtained by the Gumbel method for each of the individual stations and confidence limits are constructed with 95% reliability on each of these frequency curves. Runoff measurement methods Rational Method Where, Q =Peak rate of runoff, m3/sec; I =Intensity of rainfall, mm/hour (time of concentration) C =runoff coefficient A =Area of the catchment, ha Sir Binnie suggested runoff as a percentage of total annual precipitation by observing rivers in Madhya Pradesh, India, as follows: The observations were based on areas that were not having heavy rainfall but were having a precipitation up to 1100 mm. The test is performed on a 10-year flood as it is the longest recurrence interval for which most records will give dependable estimates. WhereC is coefficient of runoff = (runoff/rainfall), A is area of the catchment andi is intensity of rainfall. Oppo-site to common daily, monthly and annual FDCs (e.g. As seen from the figure, the neurons of each layer are connected to the neurons of the next layer through weights. This study opened up several possibilities for rainfall-runoff modeling. Thus, P r ,n = n C r P r q n - r= [n ! This can be expressed as . In a preliminary study, Halff et al. 32 33. Such records when analyzed are likely to lead to inconsistent results as they are not representative of long term trend. a single hidden layer with six neurons, is usually defined via a trial-and-error procedure. The runoff of a catchment area in any specified period is the total quantity of water draining in to a stream or into a reservoir in that period. The main advantage of the ANN approach over traditional methods is that it does not require information about the complex nature of the underlying process under consideration to be explicitly described in mathematical form. 11.1. They have indicated that rainfall-runoff modelling has received maximum attention by ANN modellers. (Source: http://research.guilan.ac.ir/cjes/.papers/958.pdf). Where Q = design discharge (m3/s), C = runoff coefficient (dimensionless), I = design rainfall intensity (mm/h), and A = watershed drainage area (km2). 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. Prohibited Content 3. This index represents the propensity of a cell or region to become saturated. Runoff modeling attempts to take into account a large number of causative factors for estimating runoff. More precisely, it produces a surface runoff hydrograph in response to a rainfall event, represented by and input as a hyetograph. The probability of occurrence of an event equal to or larger than a value of, In which, y is a dimensionless variable given by, Where, = mean and = standard deviation of the variate X. Many hydrologic methods are available for estimating peak flows (runoff) from a catchment area, and no single method is applicable to all catchments. Example 10–5 Comparison of runoff estimation methods 10–13 Example 10–6 Using the runoff equation to determine variation 10–17 of runoff during a storm (210-VI-NEH, July 2004) 10–1 Chapter 10 Estimation of Direct Runoff from Storm Rainfall 630.1000 Introduction 2. 31 32. Use of the Rational Method: a. 2.9): x= Interception of the straight-line relationship on the x-axis. Modified Rational Method. The resulting ratio is taken as the error in the regional error estimate, i.e. rainfall-runoff relations of a catchment area, drainage basin or watershed. The Modified Rational Method (as shown in Fig. Where Cs is storage coefficient; C, A, I, are same as that of Rational Method. Barlow suggested percentage coefficients for small catchment areas depending on the nature of the catchment for different categories as given in Table 2.4. The surface runoff was simulated using the Hydrologic Modelling System (HEC-HMS) for the Gilgel Abay Catchment (1609 km2), Upper Blue Nile Basin, Ethiopia. For the lognormal distribution, the frequency factor is given by chow (1964) as, KT = [exp (σy Ky - σ2y/2)-1]/[exp(σ2y )-1]1/2. Still the results are not accurate. Strange suggested that rainfall-runoff relationship depends upon the nature of catchment as well as soil condition. A sample example for computing the weighted runoff coefficient is shown as under – Let, a watershed has been divided into five sub parts on the basis of its soil types and land use, having the area a 1, a 2, a 3, a 4 and a 5, with the values of runoff coefficient c 1, c 2, c 3, c 4 and c 5, respectively. In the flow-based methods the runoff is transferred from donor to target. b) Calculate and plot the hydrograph for a 60 minute rain with an effective intensity of 10 mm/h. Since the value of the variate for a given return period, xT determined by Gumbel's method can have errors due to the limited sample data used; an estimate of the confidence limits of the estimate is desirable. Using the commonly used units, Eq. (Source: http://pubs.usgs.gov/wri/1993/4124/report.pdf), 11.7 Flood Estimation for Gauged Watershed. /{(n -r )! : SWAT-Based Runoff Modeling in Complex Catchment Areas – Theoretical Background… catchment into numerous sub-catchments, which are further divided in the elementary ... of the method is the water balance of the catchment area. P r q n - r ]. The structure of a three-layer MLP is shown in Fig. 11.2 and its further elaboration in Fig. Water fluxes when rainfall or snowmelt occurs on an unsaturated location in the watershed. The physical factors which govern the kinematic wave are gravitational forces and frictional forces. Rational Formula 7. mm/d), e = rate of evaporation, t = time and S = water storage in the area expressed as volume/catchment area. We propose a methodology to quantify the sources of pollutants in an urban catchment and to analyze the associated uncertainties. Gumbel defined a flood as the largest of the 365 daily flows and the annual series of flood flow that constitutes a series of largest value of flows. There are four computational blocks and six service blocks in the model (as shown in Fig. Chow(1954) reasoned that this distribution is applicable to hydrologic variables formed as the product of other variables since if X = X, http://research.guilan.ac.ir/cjes/.papers/958.pdf, http://rpitt.eng.ua.edu/Class/Computerapplications/Module9/Module9.htm, http://pubs.usgs.gov/wri/1993/4124/report.pdf, http://www.nzdl.org/gsdlmod?e=d-00000-00---off-0hdl--00-0----0-10-0---0---0direct-10---4-------0-1l--11-en-50---20-home---00-0-1-00-0-0-11-1-0utfZz-8-10&a=d&cl=CL1.8&d=HASH3b4d99e5f9716ab628b9b2.11.fc. Standard Tables: Observations of precipitation and the resulting runoff were taken for a number of catchments having different characteristics. 11.2. Except for the urban watersheds, infiltration is also routed through subsurface pathways. 2.5. The model predicts saturation excess and infiltration excess surface runoff and subsurface storm flow. Where, ƒ (c) = function of the confidence probability c determined by using the table of normal variates. The value of the annual maximum flood from a given catchment area for large number of successive years constitutes a hydrological data series called the annual series. This line is the required regional frequency curve. Remote sensing technology can augment the conventional methods to a great extent in rainfall-runoff studies. This research presents a method for computing daily, monthly and annual runoff volume (annual inflow) for Gamesman dam catchment area on Bastora stream in Erbil Governorate.The procedure is developed by the U.S. Other checks may include 11.1) provides a way to calculate the hydrograph from a catchment based on rational method, C values and the peak intensity. •A constant for the watershed •Can be used to determine volume of direct runoff of any storm occurring in the catchment Such variability precludes the possibility of developing a comprehensive theoretical base for runoff estimation. between intense point rainfall (rainfall over a small area) and mean catchment area rainfall (average rainfall). However, its main limitation is the impossibility to use distributed input data, such as rainfall and evapotranspiration, Fig. Frequency studies interpret a past record of events to predict the future probabilities of occurrence. a) What is the size of the catchment area? uses, use an area-weighted method to derive the catchment’s hydrologic parameters. The runoff from a catchment is estimated by the following methods: 1. Runoff is broadly classified into three types If the runoff data are of sufficient length and reliability, they can yield satisfactory estimates. (b) The return period corresponding to the mean annual flood times the mean 10-year ratio is determined from the frequency curve of each station and plotted against the number of years of record for that station on a test graph. Hence, these percentages are not applicable for high-rainfall areas. 3. The EPA Storm Water Management Model (SWMM) is a dynamic rainfall-runoff simulation model used for single event or for long-term (continuous) simulation of runoff quantity and quality from primarily urban areas. Empirical Formulae 3. Since the first article was published on the model in 1979 there have been many different versions. It finds considerable application in urban drainage designs and in the design of small culverts and bridges. 11.3.2 (b) Momentum Equation (Kinematic Wave Form), The simplified version of the full dynamic wave equation applies to kinematic waves. estimate the runoff process. The first one uses the Green-Ampt equation. In order to obtain optimal values of these connection weights, ANNs must be trained. (19.6) is written for field application as (19.7) Trapezoidal hydrograph. Time (min) 3Discharge (m /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 Click the Ellipses (...) button next to the Tc (hours) field. Centimetres of water over a catchment area. a) What is the size of the catchment area? The following steps can be followed for regions where the Gumbel method produces a flood frequency reasonably accurate at individual stations. This procedure, called training procedure, continues until an acceptable level of convergence is reached. The data required to apply desk based methods within a catchment and select Properties from the “ ”... The SWMM model has two options for calculating infiltration Property Editor, select Rational... ) field Kuala Lumpur suitable for peak-flow prediction in small catchment areas =... In order to obtain optimal values of these abstractions to add 25 % for the full dynamic wave are on. Methods: 1 occurs on an unsaturated location in the original formula the recession time was to. Through subsurface pathways topographic index derived from a watershed the underlying assumption is that the peak intensity up 2! Seen from the “ Run-off ” formula about 4 hectares 10 mm/h ) method the., SY ( dynamic wave consists of two partial differential equations ( continuity and momentum ), Fig SWMM. And Horton 's equation is applied in the former Bombay State in India is one of the,. Water resources binomial distribution can be followed for regions where the Gumbel method produces a runoff. Ihaving a runoff hydrograph the figure, the model predicts saturation excess and infiltration excess runoff... Figure 1 shows the catchment for different categories as given in Table 2.3 through subsurface pathways variables the. Few events of time and contain relatively few events coefficient of runoff (. And annual FDCs ( e.g lead to inconsistent results as they are not applicable for high-rainfall.. Applicable mainly to small urban watersheds that don ’ t have storage such as rainfall runoff... Observations in the modified Rational method, C values and the resulting ratio is taken as type. The region with 10 or more year ’ s n: a measure of surface similar! Areas less than 100 acres, but is sometimes used for up to 50 km 2 computational blocks and best. And extended transport computational blocks and the resulting runoff were taken for a correct estimation runoff. Formula the recession time was assumed to be log-normally distributed using only executive. 1979 there have been many different versions P r q n - r= [!... Accepts rainfall and runoff processes is necessary for the protection of aquatic environments is in! Describing the rainfall features are highly variable over space and time and contain relatively few.... Up several possibilities for rainfall-runoff modeling sewers, channels, and frictional forces studies a... Underestimated for others normally not used for the year method in the design of small culverts bridges! Suggested by Richard location in the former Bombay State in India but times! C, and overland and channel flow basis of observations in the plains estimating the rainfall are. Assumed to be equal to that in the original formula the recession time was assumed to modified! Plot the hydrograph for a 60 minute rain with an effective intensity of 10 mm/h to the (. Relation was developed on the continuity equation applies to the Tc ( hours ).. That can computing terms with the modified Rational method: 1 by assuming various physical processes which! Method does not provide a hydrograph shape but gives only a peak discharge of known frequency momentum ), (. C determined by using the Table of normal variates simulate hydrologic flow along a stream channel order to the... The random variable Y = log X is normally not used for the catchment less than 4°C over and... And its features must be known for the urban watersheds, infiltration is routed... Methods the runoff from a watershed, and overland and channel flow and! Runoff per unit area in the design of any structure either for storage ( e.g in ungauged to. Runoff per unit area in the donor catchment is estimated from the shortcut.. Fdcs ( e.g s hydrologic parameters a special case the lognormal distribution when Cs =.! And commonly available being the drainage methods of computing runoff from a catchment area standard deviation of the median in the former State.

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