Equations for Weirs in SWMM 5

Equations for Weirs in SWMM 5

In SWMM 5, weirs are classified into four types: Transverse, Sideflow, V Notch, and Trapezoidal. Each type has its own flow calculation method based on specific hydraulic principles.

• Transverse Weir: This type of weir, also known as a broad-crested weir, has a flow equation that typically accounts for weir length and head over the weir. It uses the general weir equation where the discharge is proportional to the weir length and the head raised to the power of 1.5.
• Sideflow Weir: Similar to the transverse weir but positioned along the side of the channel. When flow is reversed, it behaves like a transverse weir. The flow equation for sideflow weirs adjusts for the angle of approach and length of the weir, affecting the discharge coefficient.
• V Notch Weir: Characterized by its triangular shape, this weir's flow is calculated using an equation that reflects the increasing flow capacity as the head rises due to the widening of the notch. The discharge coefficient can vary, but generally, the flow is proportional to the head raised to the power of 2.5.
• Trapezoidal Weir: This weir combines characteristics of both sideflow and V-notch weirs. It features straight sides and a triangular bottom, allowing for a combination of flow behaviors. The trapezoidal weir's flow equation includes contributions from both the side sections (similar to sideflow) and the V-notch portion.

Key points:

• End Contractions: Weirs in SWMM 5 can have zero, one, or two end contractions which affect the effective weir length and thus the flow calculation. For transverse and sideflow weirs, this might reduce the flow due to end contractions (Figure 2).
• Weir Length: The actual length of the weir can be modified by settings within SWMM. This includes the "Weir Setting" which might adjust the effective length based on operational conditions like gate control or physical changes in the weir structure.
• RTC Setting: For V-notch weirs, if the Real-Time Control (RTC) setting is less than 1.0, it behaves like a trapezoidal weir due to the modification in the flow path or the effective opening of the weir.

 Figure 1.   Weir Equations in SWMM 5

 Figure 2.   Valid Number of End Contractions

Figure 3.  Weir Length Calculations

Figure 4.   Weir Equations in SWMM 5

How Does Green Ampt Initial Moisture Defiict in SWMM 5?

 How Does Green Ampt Initial Moisture Defiict in SWMM 5?

The Green Ampt method in SWMM 5 uses several parameters to calculate infiltration rates in pervious areas of subcatchments. Here's how these parameters relate to each other, particularly focusing on the Initial Moisture Deficit (IMD):

Key Parameters:

• Soil Moisture: This is calculated as: Soil Moisture=IMD Max−(FUMax−FU)Upper Soil Zone Depth
• • IMD Max: The maximum Initial Moisture Deficit, defined by the user as a fraction of the soil's capacity to hold water. It represents the initial state of dryness of the soil before the simulation begins.
  • FUMax: The saturated moisture content of the upper soil zone, which remains constant throughout the simulation, measured in feet.
  • FU: The current moisture content of the upper soil zone, which changes as infiltration occurs, also in feet.
  • Upper Soil Zone Depth: The depth of the soil layer where infiltration is considered, which influences how much water can be stored before reaching saturation.

Interpretation:

• Initial Moisture Deficit (IMD):
  • Definition: IMD represents how much additional water the soil can take before it becomes fully saturated. A higher IMD Max means the soil starts drier, allowing for more infiltration before saturation is reached.
  • Dynamic Change: As rain infiltrates, FU increases, reducing the soil moisture deficit. The equation above shows how this deficit is tracked during the simulation.
• Graphical Representation:
  • The graph would show how these parameters evolve during a rainfall event:
    • Soil Moisture: Starts as IMD Max and decreases as FU increases towards FUMax, indicating less available space for infiltration.
    • FU: Would rise as water infiltrates, approaching FUMax as the soil wets up.
    • IMD: As a fraction, it would decrease from its initial value towards zero as the soil becomes saturated.

Impact on Infiltration:

• High IMD Max: Suggests a very dry soil at the start, potentially allowing for more initial infiltration as there's more room for water absorption.
• Low IMD Max: Indicates the soil is closer to saturation initially, which could lead to quicker onset of runoff since less water can be absorbed.

Practical Use:

• Modeling Realistic Conditions: By setting an accurate IMD Max, users can model how different initial soil moisture conditions would affect runoff and infiltration in urban areas or natural landscapes.
• Calibration: This parameter might need calibration based on local conditions or historical data to ensure the model reflects real-world behavior accurately.

Figure 1.  How Soil Moisture changes over time.

 

Figure 2.  Soil Moisture and IMD are related – the Soil Moisture has a maximum of IMDMax.