Ground & Surface Water Models

Groundwater models are useful tools for solving and predicting groundwater flow and transport problems in aquifers. Typically these problems might involve: calculating the radius of influence of a well or wellfield; modelling the movement of leachate, leachate production, transportation and attenuation; and, calculating the impact of large scale groundwater abstractions on other users and on available resources. Analytical or numerical models can be deployed for solving these issues. The selection of the most appropriate model depends on the complexity of the problem at hand.

Analytic Models:  Such as LANDSIM, CONSIM GFLOW or Visual AEM allow steady-state 2-D groundwater flow systems to be simulated using analytic functions. In these programs, a library of specialized “analytic elements” are superimposed to model a variety of features such as head of leachate, flow rates, engineered barriers, advective transport, contaminant concentration, drains, slurry walls, wells and reservoirs. The functions associated with these elements meet Darcy’s law and Mass Continuity equations everywhere in the domain. Some of these models such as LANDSIM and CONSIM are specially designed to produce estimates of possible contaminant concentrations using probabilistic assessment techniques (e.g. Monte Carlo Method).

Numerical Models:  Such as the U.S. Geological Survey’s (USGS) developed Modular Finite-Difference Flow Model (MODFLOW) provide the code for solving groundwater flow and contaminant transport in 3-D. Since its development in 1983, numerous codes or packages have been developed for MODFLOW. Packages can be stand-alone codes or can be integrated with MODFLOW. These packages include advective transport model (PMPATH), the solute transport model (MT3D), parameter estimation programs (PEST), solute transport model (MOC3D), inverse models (MODFLOWP) and seawater intrusion (SEAWAT).

Software, such as iMOD or the Groundwater Modelling System (GMS), can be deployed that integrates these packages with analysis and calibration tools and 3D visualization capabilities into a single software environment.

Hydrological models provide conceptual representations of the hydrologic cycle. For civil engineering projects, they are particularly useful in solving and predicting return periods, flood discharges, flood volumes, the impact of floods and the design of engineering solutions to mitigate these risks.

The Annual Exceedance Probability (AEP) describes the likelihood or chance a specific event will be exceeded in a given year. There are several ways to express AEP as shown in the table below.

Probabilities of Exceedance

Typical techniques for predicting Design Flood Discharges are: 
– Flood Frequency Curves (e.g. Creager (1945))
– Frequency Analyses (e.g. Peak Over Threshold (POT)).
– Empirical Methods based on a catchment’s geometry (e.g. Mean – Annual Flood (MAF)).
 – Rainfall intensity and area relationships (e.g. the “Rational Method”).

Design Flood Volumes can be calculated by creating a Unit Hydrograph (UH) for a particular storm event. A UH is the hypothetical unit response of a watershed (in terms of runoff volumes and timings) to a unit input of rainfall, which enables the time to peak-flood and time to flood-dissipation to be estimated for other storm events.

3D-Visualisation of flood inundation can be provided using HEC-RAS to see how these will impact a Project Site. HEC-RAS, developed by the US Army Corps of Engineers (USACE), is capable of modelling flow regimes (e.g. a network of channels, a dendritic system or a single wadi/river reach) along with the effects of bridges, culverts, weirs, and structures.

BREEAM is the world’s longest established method for assessing and rating the sustainability of infrastructure and buildings and is the method deployed by this consultant for flood risk accreditation.