StandHyd is used to simulate runoff flows from urban watersheds. Two parallel standard instantaneous unit hydrographs are used to convolute the effective rainfall intensity over the pervious and impervious surfaces.
The losses over the pervious surfaces are calculated by one of three methods: 1) Horton’s soil infiltration equation; 2) SCS modified CN procedure; or 3) Proportional Loss Coefficient.
A baseflow can also be added to the total simulated hydrograph.
To obtain adequate results, the hydrologic object should be applied to areas with impervious ratios larger than 20%. For smaller impervious ratios, the watershed should be sub-divided into urban and rural basins.
TABLE 3-3 STANDHYD PARAMETERS
| Parameter Name | Description | Default Value |
|---|---|---|
| TIMP | Ratio of total impervious area. The value must be in the range of 0 to 1 and greater than or equal to XIMP. | 0.50 |
| XIMP | Ratio of total area directly connected impervious areas are those that form a continuous pathway from the point of runoff generation to the outlet point. For example, the area directly connected to the sewer system. The value must be in the range of 0 and 1. | 0.35 |
| LOSS | Rainfall loss method to be applied to the pervious area. It can be Modified SCS Curve Method, Horton’s Equation or Proportional Loss Method. | Modified SCS Curve Method |
| CN | Soil’s SCS or Modified Curve Number for the pervious area. Available when LOSS is set to Modified SCS Curve Method. | 85 |
| IA | Initial Abstraction (mm or in). Available when LOSS is set to Modified SCS Curve Method or Proportional Loss Method. | 1.5 |
| Fo | Initial infiltration rate (mm/hr or in/hr). Available when LOSS is set to Horton’s Equation. | 50 |
| Fc | Final infiltration rate (mm/hr or in/hr). Available when LOSS is set to Horton’s Equation. | 7.5 |
| DCAY | Decay constant (1/hr). Available when LOSS is set to Horton’s Equation. | 2 |
| F | Accumulated moisture in the soil at the beginning of the storm (mm or in). Available when LOSS is set to Horton’s Equation. | 0 |
| DPSP | Depression storage available over the pervious area (mm or in). Available when LOSS is set to Horton’s Equation. | 1.5 |
| C | Proportional loss coefficient ration (between 0 and 1). Available when LOSS is set to Proportional Loss Method. | 0.5 |
| SLPP | Average slope of the pervious area (%). Value must be greater than 0.0. | 2 |
| LGP | Overland flow length of the pervious area (m or ft) | 40 |
| MNP | Manning’s roughness coefficient for pervious surfaces. Note that coefficient should be selected based on sheet flow, not channel flow. | 0.25 |
| SCP | Storage coefficient for the linear reservoir of the pervious area (hr). Enter 0 to allow the program to internally select the value. | 0 |
| DPSI | Available depression storage over the impervious area (mm or in). | 1 |
| SLPI | Average slope of impervious area (%) | 1 |
| LGI Type | LGI calculation method. It can be Auto and Manual. Auto will calculate the LGI from AREA assuming AREA =  . Manual will read the LGI value from user input. |
Auto |
| LGI | The overland flow length of impervious area (m or ft) | Calculated from AREA |
| MNI | Manning’s roughness coefficient for pervious surfaces. Note that coefficient should be selected based on channel flow (i.e. sewer and/or road flow). | 0.013 |
| SCI | Storage coefficient for the linear reservoir of the impervious area (hr). Enter 0 to allow the program to internally select the value. | 0 |
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