Hydrology Alternatives
The hydrology alternative allows you to define hydrologic settings for catchments. The following columns are available:
| Column | Description |
|---|---|
| ID | Displays the unique identifier for each element in the alternative. |
| Label | Displays the label for each element in the alternative. |
| Groundwater Flow Coefficient | Value of A1 in the groundwater flow formula. |
| Surface Elevation | Elevation of ground surface for the subcatchment that sits above the aquifer. |
| Receiving Node | Lets you choose the node to which flow discharges from the currently highlighted element. To use this feature, click Select in the Outfall Node field. Move the cursor over the drawing pane and click the element you want to select for the outflow node. |
| Aquifer | Lets you assign a predefined aquifer to each catchment in the alternative, or create a new one by clicking the Ellipsis (…) button. |
| Area | Lets you define the area of the associated catchments. This column is only available for catchments using the Unit Hydrograph Runoff Method. |
| Initial Abstraction | Lets you define the initial abstraction (Ia) for each of the catchments in the alternative. The initial abstraction is a parameter that accounts for all losses prior to runoff and consists mainly of interception, infiltration, evaporation, and surface depression storage. This column is only available for catchments that use the Unit Hydrograph Runoff Method and the Generic Horton Loss Method. |
| Recovery Constant | Dry weather regeneration rate constant for the Horton curve. This attribute is active only when the Loss Method attribute is set to (Generic) Horton. |
| Maximum Volume | Lets you define the maximum volume for each of the catchments in the alternative. |
| Moisture Deficit | The value for Moisture Deficit, which is the saturated moisture content minus the original moisture content, for each of the catchments in the alternative that use the Green and Ampt Runoff Loss Method. This column is only available for catchments that use the Unit Hydrograph Runoff Method and the Green and Ampt Loss Method. |
| SCS CN | Allows you to define the SCS Cn value for elements that use the SCS Cn Loss Method. This column is only available for catchments that use the SCS Cn Loss Method |
| K | Lets you define the decay coefficient for each of the catchments in the alternative. This column is only available for catchments that use the Unit Hydrograph Runoff Method and the Generic Horton Loss Method. |
| fo | Lets you define the initial infiltration rate at the time that infiltration begins for each of the catchments in the alternative. This column is only available for catchments that use the Unit Hydrograph Runoff Method and the Generic Horton Loss Method. |
| fc | Lets you define the steady—state infiltration rate that occurs for a sufficiently large time period for each of the catchments in the alternative. This column is only available for catchments that use the Unit Hydrograph Runoff Method and the Generic Horton Loss Method. |
| CN Area Collection | Clicking the Ellipsis (…) button in this field opens the Cn Area Collection editor, allowing you to define the Cn area collection for each catchment in the alternative that uses the SCS Cn Loss Method. |
| Storage (Impervious Depression) | Lets you define the depth of impervious storage for each of the catchments in the alternative that use the EPA-SWMM Runoff Method. |
| Slope | Lets you define the slope of the associated catchments. This column is only available for catchments that use the EPA-SWMM Runoff Method. |
| Ks | The saturated hydraulic conductivity (the rate at which water travels through the soil when it is saturated) for each of the catchments in the alternative. This column is only available for catchments that use the Unit Hydrograph Runoff Method and the Green and Ampt Loss Method. |
| Capillary Suction | Lets you define the capillary suction value for the soil type associated with each of the catchments in the alternative that use the Green and Ampt Runoff Loss Method. This column is only available for catchments that use the Unit Hydrograph Runoff Method and the Green and Ampt Loss Method. |
| fLoss | Lets you define the fLoss absorption value for each of the catchments in the alternative. This column is only available for catchments that use the Unit Hydrograph Runoff Method and the fLoss Loss Method. |
| Loss Method | Lets you specify the loss method used by the associated catchments. Other columns become available/unavailable depending on the value chosen here. |
| Subarea Routing | Lets you define the type of subarea routing at the currently highlighted element. This attribute is available only when the Runoff Method attribute is set to EPA-SWMM Runoff. |
| Percent Routed | The percent of runoff routed between subareas. This attribute is available only when the Runoff Method attribute is set to EPA-SWMM Runoff. |
| Percent Impervious Zero Storage | The percent of the impervious area with no depression storage. This attribute is available only when the Runoff Method attribute is set to EPA-SWMM Runoff. |
| Storage (Pervious Depression) | Lets you define the depth of pervious storage for each of the catchments in the alternative that use the EPA-SWMM Runoff Method. This column is only available for catchments that use the EPA-SWMM Runoff Method. |
| Manning’s n (Pervious) | Lets you define the depth of pervious storage for each of the catchments in the alternative that use the EPA-SWMM Runoff Method. This column is only available for catchments that use the EPA-SWMM Runoff Method. |
| Manning’s n (Impervious) | Lets you define the depth of impervious storage for each of the catchments in the alternative that use the EPA-SWMM Runoff Method. This column is only available for catchments that use the EPA-SWMM Runoff Method. |
| Percent Impervious | Lets you define the percentage of impervious storage for each of the catchments in the alternative that use the EPA-SWMM Runoff Method. This column is only available for catchments that use the EPA-SWMM Runoff Method. |
| Characteristic Width | Lets you define the characteristic width value for each of the catchments in the alternative that use the EPA-SWMM Runoff Method. This column is only available for catchments that use the EPA-SWMM Runoff Method. |
| Runoff Method | Lets you specify the runoff method for each catchment in the alternative. |
| Tc Data Collection | Clicking the Ellipsis (…) button in this field opens the Tc Data Collection editor, allowing you to define the Tc data for each catchment in the alternative that uses the SCS Unit Hydrograph Runoff Method. |
| Tc | Lets you define the time of concentration for each of the catchments in the alternative that use SCS Unit Hydrograph Method. This column is only available for catchments that use the SCS Unit Hydrograph Runoff Method. |
| Convolution Time Step | Lets you define the convolution time step. This column is only available for catchments that use the Unit Hydrograph Runoff Method and the Generic Unit Hydrograph Method. Convolution Time Step has two uses: It represents the time step by which the input unit-hydrograph data is subdivided when used in a calculation. It represents the duration of excess rainfall that generated the unit hydrograph. |
| Unit Hydrograph Method | Lets you specify the unit hydrograph method to be used with the associated catchment. |
| Runoff Method | This column contains list boxes that allow you to specify the runoff method that is used by the associated catchments. This column is only available for elements that are set to the Unit Hydrograph Runoff Method. |
| Unit Hydrograph Data | Clicking the Ellipsis (…) button in this field opens the Unit Hydrograph Data dialog box, allowing you to define Time vs. Flow unit hydrographs for each element in the alternative. |
| Outflow Node | Lets you specify the element to which flow the catchment outfalls |
| Apply Groundwater | Specify whether or not the associated element applies groundwater. |
| Fixed Surf. Water Depth | Fixed depth of surface water at receiving node (ft. or m) (set to zero if surface water depth will vary as computed by flow routing). |
| Surface-GW Interaction Coefficient | Value of A3 in the groundwater flow formula. |
| Surf. Water Flow Exponent | Value of B2 in the groundwater flow formula. |
| Surf. Water Flow Coefficient | Value of A2 in the groundwater flow formula. |
| Groundwater Flow Exponent | Value of B1 in the groundwater flow formula. |
| Low Impact Development Control | Select a previously defined Low impact Development Control. |
| Top Width of Overland Flow Surface of Each Unit | The width of the outflow face of each identical LID unit. |
| Percent Initially Saturated | For Bio-Retention Cells, this is the degree to which the unit's soil is initially filled with water (0 saturation corresponds to the wilting point moisture content, 100 saturation has the moisture content equal to the porosity). The storage zone beneath the soil zone of the cell is assumed to be completely dry. For other types of LIDs it corresponds to the degree to which their storage zone is initially filled with water. |
| Number of Replicate Units | The number of equally sized LID units. |
| Occupies Full Catchment? | If set to True then the LID will occupy the full area of the LID's owning catchment. |
| Area of Each Unit | Specify the surface area occupied by the LID. |
| Percent Impervious Area Treated | The percent of the impervious portion of the subcatchment's non-LID area whose runoff is treated by the LID practice. |
| Send Outflow to Pervious Area? | Select this option if the outflow from the LID is returned onto the subcatchment's pervious area rather than going to the subcatchment's outlet. An example of where this might apply is a rain barrel whose contents are used to irrigate a lawn area. This field is ignored if the LID takes up the entire subcatchment. |