Implementation of Macrophyte Effects on Hydrodynamics in EFDC+
Introduction
Submerged macrophytes are part of an essential relationship between the physical habitat and the biological community. The EFDC+ code in EEMS 12 has been enhanced to simulate macrophyte growth through the vertical layers of the model and simulate the impact of the drag induced by the vegetation.
Submerged aquatic vegetation (SAV) or submerged macrophytes form a critical link between the physical habitat and the biological community by significantly contributing to the total primary production and nutrient cycling in the water systems. SAVs also bind the sediments to the bed, stabilize sediments, buffer the shoreline, and minimize erosion by dampening the energy of incoming waves. By retarding water currents, they allow suspended sediments to settle, and water clarity is improved. As a result, macrophytes are often a desirable component of the ecosystem.
In the existing water quality module in the EFDC+ code, the macrophyte class is attached to the bottom layer and only coupled to other water quality components such as phytoplankton, dissolved oxygen, nutrients, etc. The hydrodynamic impact of macrophytes is not considered. The macrophyte module in the EFDC+ version 12.0 has been enhanced to improve the simulation. This enhancement has two primary objectives:
- Allowing the macrophyte to grow upwards from the bottom layer through model layers.
- Modeling the hydrodynamic effects produced by the macrophyte drag.
Model Description
Macrophyte Growth through Model Layers
The upward macrophyte growth from the bottom through model layers is illustrated in Figure 1. A layered threshold value of biomass concentration is specified for the simulated macrophyte group. Upward growth is accomplished by moving the biomass of a layer to the layer above in the case where the concentration in the layer is greater than a threshold value, and the concentration in the upper layer is less than the same threshold value.
Macrophyte Impact on Hydrodynamics
The resistance of flow through the macrophyte is dependent on the flow velocity, macrophyte distribution, and hydrodynamic properties associated with stems and leaves. Like the vegetation module, to model the additional flow resistance of macrophytes, the drag of individual stems and leaves is summed to determine the total drag force in a model cell. Here, the drag force on a rigid obstacle has been introduced as a sink term into the Navier-Stokes equations and can be calculated as:
F_D = \rho \frac{U^2}{2}C_D \lambda
Where U is the velocity averaged, C_D is the experimental drag coefficient, which corresponds to the shape and diameter of the macrophyte, \lambda is the stem density.
Application
The extension of macrophyte on hydrodynamic feedback was tested on a model of a river in North America, where the extensive macrophyte population is found along the bank. Two model runs were performed, in which the second run included the macrophyte hydrodynamic feedback option. The objective is to demonstrate the impact of macrophyte growth on the hydrodynamic behavior of the river. Model parameters of macrophyte include Nominal Stem Diameter: 0.02 (m), Stem Density: 30 (stems/m2), Threshold to Allow Growth Between Layers: 40(g C/m2/m)
The results of velocity magnitude with and without macrophyte hydrodynamic feedback, together with the temporal evolution of macrophyte height at a cell, are compared in Figure 2. From January to March, the macrophyte does not grow, and macrophyte concentration and height stay at a small value. Hence, no significant impact on the water velocity was observed. However, a substantial reduction in the flow velocity can be observed from April. This reduction can be attributed to the rapid growth of the macrophyte, and its increased dimension causes resistance of flow.
In conclusion, the upgraded macrophyte module was able to predict the increase in macrophyte growth and simulate the hydrodynamic impacts on the water system. Together with the water quality module, this can be used as a tool to investigate water resource issues related to macrophytes.
Do you want to try these options for yourself? You can start by downloading EEMS and activating in the free demo mode and then running our demonstration model. To see these features in action, head over to our YouTube page.
Please get in touch with us if you have comments or questions. For more information on EFDC+ capabilities, contact the DSI team today.
