Shape — the shape of basin that lends itself to most rapid drainage is circular. A Low runoff potential due to high infiltration rates even when saturated Deep sand, deep loess, aggregated silts 0. These hydrographs are then summed to form the final runoff hydrograph. Because the hyetographs are dimensionless, all of the percentile hyetographs have the same dimensionless storm depth but represent variations in the temporal distribution of rainfall during the storm duration. The dataset provided here is representative of what you will generally get from a rainfall gauge.
Use the location of the watershed to determine nearby study watersheds. Therefore, when using hydrograph techniques, the designer must determine a rainfall hyetograph: a temporal distribution of the watershed rainfall, as shown in Figure 4-13. Do not forget to use appropriate conversion factors in calculations to have uniformity in the units of various quantities involved. Subtracting the base flow components plot and obtain ordinates of direct runoff hydrograph. A channel does not have a single-valued storage-outflow curve. Initial and constant-loss rate model The initial and constant loss-rate model is described mathematically as: Equation 4-29.
B Forms of precipitation Heavy Storms - in such a situation, rainfall is often far in excess of the infiltration capacity of the soil leading to much overland flow, and rapid rises in river levels. The time to peak of the unit hydrograph is related to the lag time by: Equation 4-52. Selection of Routing Method Selection of an appropriate routing method depends on several factors. Originally developed by Keim 2000 they were first applied in hydrology by Koehler 2004 as a means of highlighting inter-annual and intra-annual changes in streamflow. Until the accumulated precipitation on the pervious area exceeds the initial loss volume, no runoff occurs.
This is also true in those instances where the channel profile or plan is changed in such a way as to alter the natural velocity or channel storage characteristics. The runoff can then be routed from subbasin to subbasin. This volume can be converted into cm of runoff by dividing the same by the area of the drainage basin m 2 x 100. The equations are valid for gradually varied unsteady flow. This step uses the data for Hall Creek in Indiana.
For elaborate analysis infiltration indexes and curves will have to be used. The peak discharge of the unit hydrograph is calculated by: Equation 4-55. The first step is to compute the total volume of direct runoff. Lengthy rainfall — leads to the ground being saturated and overland flow. The peak discharge of the unit hydrograph is given by: Equation 4-44. You are done with deriving unit hydrograph. For larger areas, point rainfall depths and intensities must be adjusted.
Water is also lost due to evaporation and transpiration from the vegetation. As C p increases, the magnitude of the unit hydrograph peak increases. The equation of continuity can be written as: Equation 4-61. The process of starting with a hydrograph at a location and recomputing the hydrograph at a downstream location is called hydrograph routing. The horizontal line ф line can be drawn at 2 cm ordinate. Factors Affecting a Storm Hydrograph The Drainage Basin The shape of a hydrograph is altered by a few different things. For a pond or lake or where the stage-storage relation is not available, a relationship between storage and outflow can be derived from considerations of physical properties of channel or pond and simple hydraulic models of outlet works or relationship of flow and water surface elevation.
Several different unit hydrograph methods are available to the designer. The falling limb is the opposite showing that the discharge is falling. The constant loss rate can be viewed as the ultimate infiltration capacity of the soils. The Bulletin 71 Huff and the built-in Synthetic distributions are good examples and have gained popularity over the years. Nevertheless, various graphical and empirical techniques have been developed to perform these hydrograph separations. Another enhancement to the level-pool approach, employed by the Muskingum method, is to represent the storage in each reservoir as a combination of prism storage similar to level-pool reservoir and wedge storage additional sloped water on top of prism. If the watershed is dry, then I a will increase to represent the maximum precipitation depth that can fall on the watershed with no runoff; this will depend on the watershed terrain, land use, soil types, and soil treatment.
Not immediately obvious is the type of storm or precipitation that takes place. Instead, solution by numerical methods is required. Step 2: The amount of effective rainfall can be calculated as shown in Table 4. Users can choose to plot streamflow actual values or log values , streamflow percentile, or streamflow class from 1, for low flow, to 7 for high flow , for Daily, 7-Day, 14-Day, and 28-Day streamflow. Because the cross section is measured in metres 2 and the velocity is measured in metres per second the discharge is measured in metres 3 per second. Return flow from groundwater increases along with overland flow from saturated or impermeable areas during and after a storm event; moreover, a particular water molecule can easily move through both pathways en route to the watershed outlet.
Typically for hydrograph computations the storm duration should be four or five times the. This is because the relationship between storage and discharge is unique single-valued. This method, which is described in Chow 1988 and Fread 1993 , has the advantage that it will incorporate the shape of the cross section into computations. As a comparison, the adjusted curve number from Hailey and McGill Figure 4-23 can be used. The one-hour storm lasts long enough so that the entire drainage area contributes to flow to the most downstream point. The resulting delay in flood peak is the travel time of the flood hydrograph. The following figure demonstrates this concept.