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Subbasin Sediment
A subbasin element represents a catchment where precipitation falls and causes surface runoff. Erosion within the catchment will result from a combination of several different physical processes. Rain drops cause erosion when they impact the ground surface and break apart the top layer of the soil, dislodging soil particles to move with the overland flow. Overland flow also imparts an erosive energy to the ground surface that may further break apart the top soil layer. As the overland flow rate increases, the flow becomes concentrated in rills which focuses erosive energy and further erodes the surface. Total erosion is closely linked to precipitation rate, land surface slope, and condition of the surface. In some cases, soil eroded high in the catchment may deposit before reaching the outlet of the subbasin.
Certain simulation features are common to all of the surface erosion methods available for the subbasin element. Each erosion method computes the total sediment load transported out of the subbasin during a storm. This calculation process is repeated for each storm during the simulation time window. The sediment load must be distributed into a time-series of sediment discharge from the subbasin. The distribution of sediment is based on the computed hydrograph and the power function approach of Haan et al. (1994). The power function is given by:
ct=kiQta
where ct is the sediment concentration at time t, ki is the proportion of the load for the current event i to the total annual load, Qt is the subbasin discharge (flowrate) at time t, and a is an exponent entered by the user.
Also common to all of the surface erosion methods is the approach to grain size distribution. All of the methods first compute the bulk sediment discharge which includes all grain sizes. A gradation curve specifies the proportion of the total sediment discharge that should be apportioned to each grain size class or subclass. A gradation curve must be defined by the user and selected at each subbasin. A different gradation curve can be used at each subbasin to represent differences in the erosion, deposition, and resuspension processes within each subbasin. The combination of these processes is often represented by an enrichment ratio.
Selecting an Erosion Method
The erosion method for a subbasin is selected on the Component Editor for the subbasin element (Figure 1). Access the Component Editor by clicking the subbasin element icon on the "Components" tab of the Watershed Explorer. You can also access the Component Editor by clicking on the element icon in the basin map, if the map is currently open. You can select an erosion method from the list of three available choices. If you choose the None
method, the subbasin will not compute any erosion and all sediment discharges from the element will be zero. Use the selection list to choose the method you wish to use. Each subbasin may use a different method or several subbasins may use the same method.
Figure 1. Selecting the surface erosion method for a subbasin element.
The parameters for each erosion method are presented on a separate Component Editor from the subbasin element editor. The "Erosion" editor is always shown next to the "Baseflow" editor. The information shown on the erosion editor will depend on which method is currently selected.
Build-up Wash-off
The build-up wash-off erosion method is designed for urban environments. In these environments, sediment accumulates in street curbs due to wind deposit and erosion from pervious areas adjacent to the curbs. The sediment accumulates during dry periods between storms. During a storm, the accumulated sediment is flushed from the street curbs by stormwater runoff. The method may optionally include street sweeping operations designed to mechanically remove accumulated sediment. A typical example of source data for parameterizing the method is Breault et al. (2005) though data sources should always be recognized as highly local. The Component Editor is shown in Figure 2.
Figure 2. Build-up wash-off erosion method editor at a subbasin element.
The initial time is an initial condition for the method. It specifies the number of days since the last sweeping operation when the simulation begins.
The half time specifies the number of days required for half of the maximum solids to accumulate in the street curb under continuously dry conditions.
The maximum solids amount is the limit to the accumulated sediment in the street curb under continuously dry conditions. Sediment will not exceed this amount even if there is no precipitation for an extended period of time.
This erosion method includes four parameters to describe the street sweeping operations within the subbasin. The density specifies the total length of street curb whether or not the curb is subject to sweeping operations. The density should consider whether the street has curbs on one side or both sides of the street. The sweeping percentage specifies the percentage of the curb length subject to sweeping. The percentage should account for the possible presence of parked cars which result is missed curb. The efficiency percentage specifies the efficiency of the sweeping equipment at removing accumulated sediment. Finally, the interval specifies the number of days between scheduled sweeping operations.
The wash-off coefficient determines how quickly the accumulated sediment is removed from the street curb during a storm event.
The exponent is used to distribute the sediment load into a time-series sedigraph. A small value flattens the sedigraph compared to the hydrograph. A large value heightens the sedigraph compared to the hydrograph.
The gradation curve defines the distribution of the total sediment load into grain size classes and subclasses. The gradation curve is defined as a diameter-percentage function in the Paired Data Manager. The current functions are shown in the selection list. If there are many different functions available, you may wish to choose a function from the selector accessed with the paired data button next to the selection list. The selector displays the description for each function, making it easier to select the correct one.
Modified USLE
The modified USLE method (Williams, 1975) was adapted from the original Universal Soil Loss Equation. The original equation was based on precipitation intensity, and consequently could not differentiate between storms with low or high infiltration. With high infiltration, there is little surface runoff and little accompanying surface erosion. Conversely, low infiltration events have relatively more surface runoff and consequently more surface erosion. The modifications to the original USLE equation changed the formulation to calculate erosion from surface runoff instead of precipitation. The other components of the original formulation remained the same. The method works best in agricultural environments where it was developed. However, some users have adapted it to construction and urban environments. The Component Editor is shown in Figure 3.
Figure 3. Modified USLE erosion method editor at a subbasin element.
The erodibility factor describes the difficulty of eroding the soil. The factor is a function of the soil texture, structure, organic matter content, and permeability. Typical values range from 0.05 for unconsolidated loamy sand to 0.75 for silty and clayey loam soils.
The topographic factor describes the susceptibility to erosion due to length and slope. It is based on the observation that long slopes have more erosion than short slopes, and steep slopes have more erosion than flat slopes. Typical values range from 0.1 for short and flat slopes to 10 for long or steep slopes.
The cover factor describes the influence of plant cover on surface erosion. Bare ground is the most susceptible to erosion while a thick vegetation cover significantly reduces erosion. Typical values range from 1.0 for bare ground, to 0.1 for fully mulched or covered soils, to as small as 0.0001 for forest soils with a well developed soil O horizon under a dense tree canopy.
The practice factor describes the effect of specific soil conservation practices, sometimes called best management practices. Agricultural practices could include strip cropping, terracing, or contouring. Construction and urban practices could include silt fences, hydro seeding, and settling basins. It is difficult to establish general ranges for these practices as they are usually highly specific.
Only some precipitation events will cause surface erosion. The threshold can be used to set the lower limit for runoff events that cause erosion. Events with a peak flow less than the threshold will have no erosion or sediment yield.
The exponent is used to distribute the sediment load into a time-series sedigraph. A small value flattens the sedigraph compared to the hydrograph. A large value heightens the sedigraph compared to the hydrograph.
The gradation curve defines the distribution of the total sediment load into grain size classes and subclasses. The gradation curve is defined as a diameter-percentage function in the Paired Data Manager. The current functions are shown in the selection list. If there are many different functions available, you may wish to choose a function from the selector accessed with the paired data button next to the selection list. The selector displays the description for each function, making it easier to select the correct one.