Hydrologic Based Construction Site Silt Fence Sediment Barrier Design Tool

This spreadsheet tool can be used by  designers to size silt fence sediment barriers while determining the design volume and design peak flow rates.  In addition, the tool allows for estimating the number of maintenance cycles expected for each segment of silt fence.  The tool is divided into multiple ‘Drainage Area’ (DA) sheets and a single ‘Summary’ sheet.  The tool allows users to develop individual drainage area worksheets for each silt fence segment, accommodating up to 30 drainage areas.  For large projects, multiple workbooks may be used.

The ‘DA’ sheets allow designers to input hydrologic characteristics of the contributing drainage area for up to three phases of construction.  The sheet calculates runoff volume and peak flow rates using the NRCS TR-55 methodology.  In addition, RUSLE analyses is incorporated into the ‘DA’ sheets to calculate annual soil loss for each drainage area.  The LS-factor is calculated based on inputs of slope length, slope gradient, and information about mulching or established vegetation.  The C-factor is selected from a drop-down list corresponding to different erosion control practices.  The K-factor and P-factor are manual entries.  The R-factor is determined based on geographic area.  Results of the hydrologic and RUSLE analyses (i.e., runoff volume, peak flow, and annual soil loss) are automatically transferred to the ‘Summary’ sheet.

The ‘Summary’ sheet includes project information including location, phasing schedule, and design rainfall depths.  In addition, the sheet provides summary of the individual ‘DA’ sheets embedded within the workbook.  This sheet allows a user to select the type of silt fence installation configuration used and input corresponding design dimensions for each drainage area.  The storage volume for each silt fence segment is automatically calculated with these input design parameters.  The tool then compares against the drainage area runoff volume and determines the corresponding safety factor for each segment.  If a silt fence segment has a safety factor less than 1.0, a red marker prompts the user to make modifications.  These modifications may include resizing or adding upstream practices to reduce the contributing drainage area.  The ‘Summary’ sheet is also used to design overflow weirs for each silt fence segment.  The designed height of silt fence is used to determine the weir height from which the weir flow rate capacity is calculated.  The user is prompted in the event that the weir capacity does not meet the required peak flow conditions.  Using the phasing calendar included in the ‘Summary’ sheet, the workbook determines the interval of maintenance schedules required to remove captured sediment from the silt fence segments.  Maintenance requirements are triggered once 50% of the storage volume upstream of the silt fence is occupied.  Output results of the tool are exportable as a printable report, which can be supplied as a reference for both designers and contractors, and can assist in the communications during the implementation on the jobsite.