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 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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