This document summarizes a thesis that studied variations in groundwater seepage lines on meso-tidal dissipative beaches. Key findings include that beachface volume is more correlated with groundwater seepage lines during low tide. Field data and video images showed decoupling of seepage lines and shorelines during tides. Numerical models could predict tidal groundwater changes and seepage line positions. The thesis concluded that beach slope, hydraulic conductivity, wave setup, and rip currents most influence seepage lines and suggested further study of their effects.

1. Variations to the Groundwater Seepage Face on
Meso-tidal Dissipative Beaches
Amir Emami
University of Waikato, New Zealand

2.  Thesis aims and objectives
 Key findings
 Conclusion
Presentation outline

3. Mean Sea Level Swash zone
Groundwater Exit point
Seepage face
Tidal Range
Set-up
Hydrological components of the beachface
The aim of this thesis is to study the factors that control the variation of the
groundwater seepage line on gently-sloping dissipative beaches using statistical
models, field data, video images and numerical models.
Thesis aims and objectives
Which processes: rip currents, hydraulic conductivity or beach slope, cause the
greatest variation in seepage on meso-tidal dissipative beaches?

4. Developing statistical model at Muriwai beach Using video images at Raglan beach
Collecting
field data
at Raglan
beach
Developing
numerical model at
Muriwai and
Raglan beaches
Groundwater seepage line (GWSL)
Shore line (SL) Seepage face
Methods

5. Seepage line
Groundwater
seepage line
Cross-Shore
Alongshore
Set-down
Rips
Landward
 What is the relationship between the observed groundwater seepage line and the intertidal
beach volume?
 In most regions of Muriwai beach, there is a clear
correlation between the intertidal beach volume
(Vo) and groundwater seepage line (GWSL)
Key findings
 The correlation between Vo and SM is not as easy
to interpret as Vo and GWSL
GWSL Vo (erosion)
SM (Rip currents) Vo
SM (Sand bars) Vo
Set-up
Bars
Seaward
or
SM (Rip currents) Vo

6. (1) : V=a1+a2GWSL
(2) : V=a1+a2SM
(3) : V=a1+a2GW+ a3SM
(4) : V=a1+a2GW+ a3SM + a4GW.SM
(5): V=a1+a2GW+ a3SM+a4GW2+ a5SM2 + a6GW.SM
 What is the best statistical model which can describe the importance of the groundwater seepage line
and surfzone morphology in changing beachface volume?
Key findings
 Model 3 shows the greatest R-square at significance level of 95% (between 0.65 and 0.75 in
four months)
 Groundwater explained more variance than rips in winter (GWSL in winter is more correlated
with the Vo than summer.)
 Vo is more correlated with GWSL at low tide rather than high tide. Beachface volume reduction
is more influenced by the low tide GWSL.
Hydraulic head at L.T. Stronger seepage flow Sediment remobilization (erosion)

7.  How does the groundwater seepage line on a dissipative meso-tidal beach change over tidal
cycle? How can video images be used to observe the decoupling of the GWSL from the SL?
Decoupling between
GWSL and SL
during rising tide
Key findings
 On high tide, there is not
much decoupling between
the GWSL and the SL.
 At low tide, the GWSL is
completely decoupled from
the SL and the seepage face
can be identified along the
beach.
 Both GWSL extracted from
time-averaged images and
SL extracted from variance
images showed the
decoupling process in both
incoming and outgoing tide

8.  How well can video images be used for extracting groundwater seepage lines and shorelines at a
dissipative meso-tidal beach? What is the accuracy of this technique in comparison with
surveying data?
 Time-averaged images can be used well for extracting the groundwater seepage line in a gently
slopping dissipative beach.
Key findings
 The shoreline extracting
algorithm using variance
images is not always
accurate. It works better
at the high tide rather
than the low tide.
difference
between the
detected SL
and the
surveyed data
Incoming Tide
Outgoing Tide

9.  How does field measurement show the infiltration and exfiltration process of the beach
groundwater?
Key findings
During rising tide: Infiltration from tide WT el. (Beach GW increases much more rapidly than tide rises)
During falling tide: GW exfiltration WT el. (Beach GW decreases more slowly than tide falls)
 The beachface acts
as a non-linear filter
 Pattern of exfiltration and
infiltration on gently-sloping
dissipative beach is different
from on steeper low-energy
intermediate and reflective
beaches.
Raglan beach fills more easily than it drains

10.  What are the main parameters controlling the groundwater seepage line on a dissipative, meso-
tidal beach? Which driver is the most important in explaining changes to the seepage line?
 The main parameters controlling the GWSL on a dissipative, meso-tidal beach:
Inland groundwater table, Tide variation, Wave set-up, Intertidal beachface geometry, Beach sediment
porosity and the Hydraulic conductivity.
Inland Watertable
GW E.P.Tidal range
Set-up
Elevation(m)
Hydraulic conductivity
Beach slope
Key findings
GW E.P.

11. Key findings
 A steeper beach profile and
higher hydraulic conductivity
are two important factors in
decreasing the groundwater
exit point elevation and
shortening the seepage face
width across the south of the
beach.
 Transect 3 in south of the
beach located in the part of the
beach with rip currents. The rip
current may have an effect on
lowering the groundwater exit
point and shortening the
seepage face width.
 What are the main parameters controlling the groundwater seepage line on a dissipative, meso-
tidal beach? Which driver is the most important in explaining changes to the seepage line?

12.  The seepage line calculated by the 2D non-linear Boussinesq model is more compatible with the
surveyed groundwater seepage lines rather than linear model. The non-linearity effect of the hydraulic
conductivity and the groundwater depth may play an important role in accuracy of the results.
 Can numerical models (both linear and non-linear) accurately predict the tidal groundwater
changes across the beachface and determine the position of the groundwater exit point?
Linear model Non-Linear model
Key findings
 Both numerical models based on the linear and non-linear Boussinesq equation can predict the tidal
groundwater changes across the beachface and determine the position of the groundwater exit point.

13. Beach morphology
Set-up
K
S
GWEP El. & GWSF W.
Lower or higher
beachface
volume
Surfzone morphodynamics
Rips
GWEP El. & GWSF W.
Lower or higher
beachface
volume
Morphological
feedback loop
Surfzone morphodynamics
feedback loop
Overview

14. Overview
 Implication of shellfish living across the beachface due to fluctuation of the beach ground water
 Effect of higher groundwater table on accelerating the beach erosion
 Delineation of the area which is prone to beach erosion that guide to restrict urban development
 To help the community for better understanding of what causes beach erosion and its consequences
 Emphasising the role of the beach morphology and non-linear distribution of the hydraulic
conductivity in changing beach groundwater seepage line and consequently beach volume reduction and
erosion due to the higher groundwater and beach dewatering
 To suggest more studies on the morphological feedback loop in terms of groundwater variation
Achievement of this research in terms of biological and environmental aspects:
Achievement of this research for community:
Suggestion for future studies:
 To suggest further studies on the effect of the spatially-varying wave set-up and the presence of rip
currents on changing the beach groundwater elevation