Storage Unit Seepage and Infiltration Loss

Storage units are commonly used to temporarily store stormwater and reduce downstream flooding. When modeling the stormwater system, it is important to account for seepage and infiltration losses from the storage units, especially if the base or sides are in contact with permeable soil.

Seepage and infiltration losses refer to the volume of water that exits a storage unit and infiltrates into the surrounding soil through the basin floor or sides. Accurately modeling these losses is critical for detention ponds, lakes, reservoirs, infiltration trenches, and other storage units with permeable boundaries.

This article describes how GeoSTORM calculates seepage (or infiltration loss) within storage units.

Defining Infiltration Loss Parameters

In GeoSTORM, the user can define the infiltration loss parameters using the Storage Area Seepage data panel of the Storage Area Data dialog box. Refer to this article in our knowledge base to learn more about the Storage Area Data dialog box.

The following parameters are provided in the Storage Area Seepage data panel:

• Suction head
  This entry field defines the average value of soil capillary suction along the wetting front. This parameter reflects the capillary action and attraction of soil pores for water. A higher value indicates that the soil can draw water into the ground more strongly as the soil becomes wet.

• Hydraulic conductivity
  This entry field defines the hydraulic conductivity of the underlying soil. This is the base infiltration rate of the soil, which represents how quickly water can move downward through fully wetted soil.

• Initial deficit
  This entry field defines the initial deficit of the underlying soil. This parameter represents the degree to which the soil is unsaturated at the start. A soil with a higher initial deficit can initially accept water more readily until the pores begin to fill.

Infiltration Loss Calculation

Infiltration loss is calculated based on the relationship between water depth and surface area defined by the storage curve. This method estimates both bottom and side infiltration losses without requiring manual input for the shape or perimeter of the storage unit.

The following points describe how the software calculates infiltration loss:

• Storage Curves and Area-Depth Data
  The storage curve defines how the surface area changes with depth using depth-area pairs. The depth-area data can be specified in the Storage Area Volume data panel of the Storage Area Data dialog box. Refer to this article in our knowledge base for more information on how to define depth-area data to be used in infiltration loss calculation.
  

• Bottom and Side Contact Areas
  Infiltration occurs through the bottom and submerged side surfaces of the storage unit. GeoSTORM does not require a direct input for side slope, perimeter, or shape. Instead, the software estimates the side contact area from how the cross-sectional area changes with depth.

1. 1. Bottom Area: The surface area at a given depth represents the base over which infiltration can occur.
   2. Side Area (Inferred): As water depth increases, the software estimates the side contact area by analyzing how surface area changes with depth. If the area increases a lot with a slight increase in depth, it means the storage unit gets wider quickly, leading to more side contact. On the other hand, if the area does not change much with depth, it means the storage unit has more vertical sides or a narrow shape.

• Internal Computation of Side Areas
  GeoSTORM automatically calculates side contact areas based on the area-depth relationship. By comparing surface areas at successive depths, it approximates the wetted perimeter. This perimeter is multiplied by a depth increment to estimate side areas. Summing up these side areas from the bottom to the top of the water level results in the total wetted contact area for infiltration.

• No Need to Manually Define Shape or Perimeter
  The user does not need to enter the storage unit’s perimeter or side slopes manually. Instead, the software automatically defines the shape based on the area-depth pairs. For example:

• 1. If the surface area increases linearly with depth, it suggests consistent side slopes.
  2. If the surface area remains nearly constant, it indicates steep or vertical sidewalls.

• Impact on Infiltration Calculation
  Once the software calculates the total wetted contact area (including the bottom and estimated side area), it applies the defined suction head, hydraulic conductivity, and initial moisture deficit to compute the infiltration loss rate.
  
  Note: If either the suction head or initial deficit is set to zero, the software assumes a constant infiltration rate equal to the hydraulic conductivity, regardless of water depth.