Environmental Engineering Reference
In-Depth Information
6 Seasonal expansion/contraction of open water
bodies, whose areas vary dynamically as a function
of rainfall and evaporation.
Points 1, 5 and 6 above illustrate the concept of
variablecontributingareas for surface runoff,which
stems directly fromGW-SW interaction and is ex-
tremelyimportanttothedistributionandseverityof
flooding, not only in the Northwest region of this
basin but also in all other lowland basins.
It was identified in the regionalmodelling study
that the key shortcoming of the uncoupled ap-
proachwas its inability to reproduce the dynamics
of surface water storage and conveyance, once
groundwater had exfiltrated. This indicated the
need to include within iSISMOD some represen-
tation of direct rainfall and evaporation from sat-
urated areas of ground. Figure 22.11 shows a
conceptual schematization of the functioning of
this feature of the model:
1 iSISMOD detects whether there is a situation of
Flooding or No Flooding, depending on whether
the water level in layer 1 is above or below the
ground surface. This check is performed for every
cell of the model, at every time step in the surface
water simulation.
2 For those cells where Flooding is detected, the
MODFLOW sub-model within iSISMOD does not
include infiltration in the continuity equation for
layer 2, but only the leakage originating from layer
1. At the same time, the continuity equation for
layer 1 will include terms for both rainfall and
evaporation.
3 For those cells where No Flooding is detected,
MODFLOWtakes the infiltration or recharge term
from a hydrological model (i.e. HYSIM in the case
of the R´o Salado study) and uses it directly as an
input stress to the aquifer.
The iSISMOD model was validated using runs
performed for a 33-year period of record and the
validated model was then used to generate FPMs
for the test area based on rainfall inputs for the
wettest period experienced in the test area region
(1985-1990). Runs were performed first for the
baseline condition and then with proposed flood
risk reduction measures in place.
The key question is whether the FPMs gener-
ated using the fully coupled GW-SW model
Local modelling approach
Localmodellingwas alsoundertaken for a test area
in the Northwest part of the Salado Basin (Region
A). For this application a coupled model was de-
veloped by linking an iSIS model for surface
flows with a MODFLOW groundwater model
(iSISMOD - see Box 22.4). The aim was to assess
the advantages and disadvantages of using a cou-
pled model to generate FPMs, in comparison with
the uncoupled modelling approach used in the
regional (broad-scale) approach. Predicted surface
water levels along longitudinal and cross-section-
al profiles within the relict dune fields were
examined to compare the model outputs with the
actual flooding patterns observed from satellite
imagery.
Schematization of the local, iSISMOD model,
consisted of two layers: a groundwater component
(layer 2) and a surface water component (layer 1).
Layer 2 was discretized in the horizontal plane
using cells of 500
500m, in place of the coarser
5000-m grid used in the regional model. Layer 1 of
the model was discretized using the units avail-
able in iSIS Flow in order to simulate as closely as
possible the flooding mechanisms that operate at
the surface, including those related directly to the
occurrence of groundwater-induced flooding and
waterlogging. These mechanisms are:
1 Exfiltration of water at the surface, due to
groundwater phreatic levels that exceed the land
surface elevation.
2 Ponding of exfiltrated water behind parabolic
dunes and/or spread of water over low areas in the
inter-dunal troughs between large, longitudinal
dunes.
3 Transfer of water from one low area to the next
down slope, once the topographic threshold creat-
ed by the crest of the intervening parabolic dune is
exceeded, creating a cascade effect and, occasion-
ally, a major stream of floodwater.
4 Transfer of water from low areas to existing
drainage infrastructure (canals) and flooding due
to insufficient conveyance capacity in the canal
system.
5 Saturation overland flow as rainfall falls onto a
variable area of saturated ground.
