SWMM 5 Threshold Groundwater Elevation

SWMM 5 Threshold Groundwater Elevation

SWMM 4's Limitation:

• Sequential, Disconnected Modules: SWMM 4's modular structure (Runoff Block for hydrology, Extran Block for hydraulics) meant that these processes were simulated sequentially. The Runoff Block would calculate infiltration and groundwater recharge and pass these results as an interface file to the Extran Block.
• No Dynamic Feedback: This sequential approach prevented dynamic, time-step-by-time-step feedback between the groundwater aquifer and the drainage network. The Extran Block couldn't directly influence the groundwater levels calculated in the Runoff Block during a simulation. This limited the accuracy of simulating systems where groundwater significantly impacts surface water flows, and vice-versa.
• Simplified Groundwater Representation: The groundwater component in SWMM 4's Runoff Block was relatively simple, often using basic empirical relationships to estimate infiltration and recharge.

SWMM 5's Advancement: Integrated Hydrology and Hydraulics

• Simultaneous Simulation: SWMM 5 integrates hydrology and hydraulics into a single computational engine. This means that surface water and groundwater processes are simulated simultaneously within the same time step.
• Dynamic Interaction: This integration allows for dynamic feedback between the groundwater aquifer and the drainage network. Changes in the water table elevation can instantly affect infiltration rates, exfiltration to drainage elements (pipes and channels), and baseflow in streams. Conversely, surface water levels can influence groundwater recharge.
• More Realistic Representation: This dynamic interaction provides a more realistic representation of systems where groundwater plays a significant role in the overall hydrology.

SWMM 5 Groundwater Threshold Elevation Options:

As you mentioned, SWMM 5 offers two options for defining how the groundwater elevation interacts with the drainage network at each node or for each Subcatchment:

1. Fixed Threshold Groundwater Elevation:

   • You can specify a fixed elevation for the groundwater table at each node. If the groundwater elevation in the aquifer exceeds this threshold, exfiltration (groundwater discharge) into the node will occur. If the water level in the node is lower the groundwater elevation then infiltration from the node to the Aquifer will occur.
   • This option is useful when you have a good understanding of the groundwater table's spatial variation or when you want to simplify the simulation by assuming a constant groundwater level across a specific area.

2. Receiving Node's Invert Elevation:

   • In this option, the invert elevation of the node is used as the threshold. If the calculated groundwater elevation rises above the node's invert, exfiltration will occur into the node. If the water level in the node is lower than the invert elevation then infiltration from the node to the Aquifer will occur.
   • This is a convenient way to simulate groundwater interaction without needing detailed groundwater elevation data. It assumes that the node's invert represents a relevant elevation for groundwater interaction.

Implications and Benefits of the SWMM 5 Approach:

• Improved Accuracy: The integrated approach leads to more accurate simulations, especially in areas with significant groundwater-surface water exchange.
• Better Representation of Processes: It allows for a more realistic representation of processes like:
  • Baseflow: The contribution of groundwater to streamflow during dry periods.
  • Stream-Aquifer Interaction: The exchange of water between streams and the underlying aquifer.
  • Impact of Drainage Systems on Groundwater: How drainage systems can lower the water table.
  • Wetland Hydrology: The complex interaction of surface water and groundwater in wetland ecosystems.
• Enhanced Modeling Capabilities: SWMM 5's capabilities are valuable for:
  • Urban Hydrology: Assessing the impact of urbanization on groundwater recharge and discharge.
  • Water Resources Management: Evaluating the sustainability of groundwater resources.
  • Climate Change Studies: Understanding how changes in precipitation and temperature might affect groundwater levels and surface water flows.

Summary

The shift from a sequential to an integrated approach for simulating hydrology and hydraulics in SWMM 5, along with the flexible threshold elevation options, represents a significant advancement in the modeling of groundwater-surface water interactions. This improvement leads to more accurate and realistic simulations, making SWMM 5 a more powerful tool for addressing a wide range of water resources challenges.

SWMM 5 Threshold Groundwater Elevation

Subject:  SWMM 5 Threshold Groundwater Elevation

A large difference between SWMM 5 and SWMM 4 is how the Groundwater Aquifer interacts with the drainage network.  In SWMM 4 since the hydrology was simulated in the Runoff Block, the results saved to an interface file and the hydraulics were simulated in the Extran Block it was not possible to have a time step to time step interaction between the Aquifer and the Open Channels.  SWMM 5 has integrated hydrology and hydraulics so it is possible to use either a fixed Threshold Groundwater Elevation for each Subcatchment or the Receiving Nodes Invert Elevation.