Gully erosion is defined as the erosion process whereby runoff water
accumulates and often recurs in narrow channels and over short periods,
removes the soil from this narrow area to considerable depths (Poesen
et al., 2002). Permanent gullies are often defined for agricultural
land in terms of channels too deep to easily ameliorate with ordinary
farm tillage equipment, typically ranging from 0.5 to as much as 25-30
m depth (Soil Science Society of America, 2001). The term ephemeral gully
erosion was introduced to include concentrated flow erosion larger than
rill erosion but less than classical gully erosion, as a consequence of
the growing concern that this sediment source used to be overlooked in
traditional soil erosion assessments (Grissinger, 1996; Foster, 1986).
According to the Soil Science Society of America (2001), ephemeral gullies
are small channels eroded by concentrated overland flow that can be easily
filled by normal tillage, only to reform again in the same location by
additional runoff events.
Poesen et al. (1984, 1996, 2001) observed ephemeral gullies to
form in concentrated flow zones, located not only in natural drainage
lines (thalwegs of zero order basins or hollows) but also along (or in)
linear landscape elements (e.g., drill lines, dead furrows, headlands,
parcel borders, access roads, etc.). Poesen et al. (1984, 1996,
2001) distinguishes rills from (ephemeral) gullies by a critical cross-sectional
area of 929 cm2 (square foot criterion). Other criteria include
a minimum width of 0.3 m and a minimum depth of about 0.6 m, or a minimum
depth of 0.5 m (Imeson and Kwaad, 1980). Hydraulically any classification
that related erosion forms into separate classes, such as micro rills,
rills, mega rills, ephemeral gullies, gullies, is, to some extent, subjective
(Grissinger, 1996). Therefore, to the upper limit of gullies, no clear-cut
definition exists (Poesen et al., 2002).
Purpose of this research is the study and investigation of morphology,
effective factors in the gully forming, developing and rate of gully erosion
change in the study area. Overall, forming and developing of gully erosion
is affected with factors such as physiography, geology, soil, climatologic
and landuse (Mendonca and Marcelo, 2002). In this study, overall affecting
factors in the producing and developing of gully erosion in the study
area, are discussed. Tectonic factors (Soleimani, 1999) have been added
to other factors.
STUDY AREA DESCRIPTION
The study area is located about 100 km North of the Birjand city. The
drainage basin (Fig. 1) has a total area of 117.63 km2,
located in South Khorasan province, Iran. It is confined between the latitudes
of approximately 32° 21´ to 33° 12´ N and longitudes 59° 32´
to 59° 40´ E. The Shakhen River and its tributaries flow from the
mountain area to the outlet point. The highest point in the catchment
is a hilltop at 2500-m above sea level. The maximum difference in elevation
from the outlet to the highest point of the watershed is about 450 m.
About 2 km from outlet point in the catchment, the main valley splits
into two subvalleys. The basin has two main streams. One is in the east
of basin has 24.88 km length and the second in west of basin has 17 km
length, which are jointed the first stream about 2 km from outlet point
in the catchment. The two famous villages located in the area, VASHAN
and SERGHAN each house about 12 farmer. The mean annual rainfall is 256.9
mm and mean annual temperature is 14.2°C. The rainfall season runs
from December to April, when several precipitation events occur. Most
of these events are of frontal type, moving eastwards across the watershed.
According to Emberger Climate classification, the climate of this region
is cold semiarid.
The slope in the basin averages out to be 13%. Soils are generally shallow.
According to the Soil Taxonomy classification major soil orders in the
region are aridisols and entisols. One of the main characteristics of
the area is the dissection of the landscape by a deep network of gullies.
Large gullies are characterised by vertical sidewalls and are 10 or more
meters deep in many places. Geomorphologically, the area can be divided
into three parts (Fig. 1); (1) the southern part is covered
by an undulating plateau, this upland area mainly used for agriculture.
(2) Valley mountainous in the central part. (3) The Northern part (near
the outlet of basin) consists of aggraded clay narrow flood plain. The
natural vegetation is mainly pasture land with sparse bushes in the semi-arid
Structural and geological setting: Three major tectonic units
(Turanian, Iranian and Arabian plates) recognized by Lensch et al.
(1984) in Iran, are separated from each other by ophiolitic complexes
(Stocklin, 1977). These are subdivided into smaller elements, such as
Central Iran. The Central Iran comprises Sanandaj-Sirjan Belt, Orumiyeh-Dokhtar
Belt, Central-East-Iran microplate (Davoudzadeh and Schmidt, 1981); the
latter is subdivided into Yazd, Tabas and Lut blocks. The studied area
is situated in eastern Iran within the Sistan Ocean that is between two
blocks: the Lut in the west and Afghan in the east.
The structure of the mapped area (Fig. 2) broadly consists
of an assemblage of different blocks which are separated by major faults,
frequently of the transcurrent type. There are several phases of faulting,
resulting in a network of fault systems with different trends. In addition
to (or in combination with) the strike slip faults, thrust and overthrust
are present in some areas. Polyphased folding is also apparent. The juxtaposition
of blocks which may have been displaced over significant distances accounts
for the existence of sharp contacts between formations of strikingly different
facies, from typical shelf or playa facies, to flysch-like or ophiolitic
facies. A detailed and reliable analysis of the relative block movements
would not be possible, at this stage of the study within the limit of
||Situation of the study area
||Geological characteristics of the study area
Geological cross-section through the middle part of
the Basin. Location in Fig. 2
. Showing: (1) the relationship
between the stratigraphic units identified in the succession and (2)
the relationship with the fault bordering the basin
The geological and geochemical observations in the region indicate the
presence of igneous rocks of varying lithologies, intrusions and volcanic
rocks (Fig. 2). Rocks of several groups belonging to
the Oligo-Miocene constitute the mountain systems in the study area.
In the study area, volcanic rocks directly overlie the Paleocene-lower
Eocene flysch and the ophiolites. They are sharply interrupted to the
northwest, together with the ophiolites, but they are extensively exposed
in the neighboring region to the southeast. Absolute age measurements
made on two samples collected in these regions, using the K-Ar method
on whole rock, gave respective ages of 26.6±1.1 and 11.6±0.4
M.Y., indicating the upper Oligocene to Miocene. The base of the volcanic
sequence is composed of flows and pyroclastics, mostly of acidic composition
(Fig. 3). The flows consist of light-colored andesites
and dasites, interlayered with tuff breccia and ignimbritic tuffs. They
are overlain by volcanic of more basic composition, consisting of andesite-basalt
flows with hornblende and pyroxene (Fig. 3). Mostly towards
their base, these flows contain interlayer of tuff and tuff breccia with
an andesitic matrix.
MATERIALS AND METHODS
In this research, different data have been used as following:
data observation with GPS system
maps 1:25000 scales
maps 1:25000 scales
photos 1:20000 scales
maps 1:25000 scales
unit maps 1:25000 scales
cover maps 1:25000 scales
Daily rainfall recorded during the period 1971-2003 was analyzed from
data collected at many meteorological stations in the region. The frequency
distribution of daily rainfalls and the average annual rainfall recorded
during this period were evaluated. This frequency distribution was compared
with that recorded in the area during the 20th century, which allowed
the classification of the study years as wet, normal or dry years, taking
into account the percentiles 0.9, 0.5 and 0.25, respectively. In addition,
using the STORMGEM program (Mulligan, 1996) and taking into account the
rainfall characteristics of the area, recorded at 1 min intervals during
the last decade, storms were modeled and the information on intensities
recorded during the whole period was obtained.
The GPS used to collect points which can use for morphological analysis
of gully in the GIS environment. Another aspect of using GPS technology
is that there is the possibility to create geomorphologically-specific
Digital Elevation Models (DEMs). Further, GPS technology was used to map
a series of gullies by walking around their perimeter. Therefore, positions
of gully erosion and its morphological characteristics by creating the
geomorphologically DEMs and 3d models in the GIS environment were analyzed.
The accuracy of GPS-derived data was later assessed visually by overlaying
the GPS data on the geometrically-corrected aerial photography. This confirmed
the ability of using GPS to delineate geographical position of features
(x, y and z) with confidence, while also emphasizing the highly spatially
accurate of the GPS data. The GPS that was used in the study is accurate
to 5 m approximately.
Other thematic maps such as: slope, elevation, profile, lithology, vegetation
cover, hydrology, geomorphology, landuse, land units, erosion rate and
field studies were added to GIS environment.
Head cuts migrations have been indicated by experimental methods such
as regression analysis of Vandekrckhove et al. (2001) which calculated
for 46 gullies in Spain:
Vc = -2.844+ 0.285(log Ap) + 0.173 Hhc + 0.017CN
In this equation: Log Vc the rates of head cut migration in cubic meter
per year (m3 year-1); Ap is drainage basin area
in sq meter (m2). Hhc is height of the head cut
in meter; CN is runoff curve number values of Soil Conservation System
of America (SCS). In this equation, for evaluation of gully head cut retreat
11 information layer have been used which are as following: slope, elevation,
profile, lithology, vegetation cover, hydrology, geomorphology, landuse,
land units and erosion rate on the base of MPSIAC method.
Regression rate of head cut of gully have been calculated after fitting
layers in GIS. In this study, the main attention is to tectonics activities,
which unconsidered in mentioned equation.
Morphotectonics and neotectonics indexes have been used for evaluation
the role of tectonics activities in increasing the intensity of gully
erosion. This factor did not considered in Vandekrckhove method. These
indexes are as following:
of mountain front
morphology of valleys including V-ratio, ratio of valley to depth,
asymmetry of streams, topography cross section symmetry of the region.
These indexes have been used as information layers with other information layers
for evaluation of influenced factors in gully erosion changes. A GIS was used
to bring together all spatial data sets. GIS benefit geographical feature detection
by allowing the integration of many deference data source; including spatially
referenced field surveys, map-based information and DEMs. The use of several
data sets such as thematic maps information maximizes the information available
RESULTS AND DISCUSSION
Gully erosion evaluation: Figure 4 shows the
zones (in grey) where gully erosion occurred, using GPS by walking around
their perimeter. The intensity of water run off and thick layer of sediments
prepared good condition for gully creating and development, especially
near to outlet of the drainage basin (Fig. 4). The length
of this permanent gully is 2004 m.
of region with gully erosion
Base on the field study and aerial photos (1:20000), the length of gully
have been changed in the interval of photography (dated 1984) and field
study (achieved in 2004) (Fig. 5). In the aerial photos,
gully length is 1200 m, while in the field studies and measurement with
GPS, the measured length of gully is 2004 m.
The rate of permanent gully head cut migration in Shakhen basin is 2.55
m3 year-1 using Vandekrckhove et al. (2001)
model. From point of Vandekrckhove model in the drainage basin with 127.6
km2 areas, head cut will be active if the migration rates in
gully erosion due to the model equal to 4 m3 year-1.
Vandekrckhove head cut migration threshold for Shakhen basin with 117.63
km2 areas is lower than from critical threshold.
By comparing the annual rainfall recorded in the study period with a
32 year record in the area, one can observe that the period was mostly
normal-semi aired. Four years received rainfalls >0.75 percentile,
ranging between 280 and 300 mm. During that 32-year period, 162 rainfall
events were recorded. Of those events, 21 were erosive, according to the
criteria of Wischmeier and Smith (1978).
Thus, results of rainfall characteristic analysis show that it is not
critical variables in head cut erosion of permanent gully in the study
area and head cut migration rate were computed for permanent gully in
the basin using Vandekrckhove et al. (2001) model, show that the
gully head cut migration is inactive. While, comparing of gully length
by aerial photo and field measurement by GPS, Show that 804 m change in
the length of permanent gully occurred in the outlet of basin. For this
condition there are two possible answers: 1) after regional tectonics
erosion accelerated and after time spending reduced. (2) Thresholds of
Vandekrckhove model are lower than.
of the gully change in the study area
Ongoing tectonic deformation in the area is manifested by (1) prominent
seismic activity, (2) displacement of artificial structures, (3) formation
of tectonic landforms (including shutter ridges, displaced gullies and
active landslides) and (4) formation of the convex longitudinal profile
of the Shaken River Fig. 6. The present morphotectonic
configuration in the area is shaped by the northwest-southeast-striking
and northeast-southwest-striking fault lines, which have a major sinisterly
In continuation of this part, new tectonic evolutions of the area discussed
using morphotectonic factors.
Evaluation of morphotectonic factors: Tectonic faults are often
associated with characteristic geomorphologic features such as linear
valleys, ridgelines and slope breaks that can be identified as lineaments
in Digital Elevation Models (DEMs). Lineaments are defined as straight
linear elements visible at the earth`s surface and which are the representations
of geological and/or geomorphologic phenomena (Clark and Wilson, 1994).
In geomorphometric analysis, a linear feature can have geometric
origin only and represent a change in terrain elevation, such as a valley
or ridgeline, slope break or in flex line. In terms of digital terrain
modeling, a lineament is a continuous series of pixels having similar
terrain values (Koike et al., 1998).
convex in longitudinal profile of Main Rivers in the two sub-basins
of valleys (Table 1)
Sinuosity of mountain front: Length of mountain front in study
area is 54.778 km and the length of straight front is 46.672 km. Therefore,
sinuosity is equal with 1.17, which near to one and has been shown new
Morphology of valleys: The result of reviewing indexes of valleys
shows that they are deep and V form (Table 1, Fig.
|| Results of Valleys
of the study area with eastward tendency
parameters for calculating topographic cross conjunction
of area and length of sub-streams on the beside of main streams
topographic cross conjunction
indexes results of width to depth ratio across the mainstream indicated
that, there is wide bed only in the mouth of drainage basin.
Asymmetry of streams: The calculated asymmetric index for Vashan
River is 32.22, which has been shown asymmetry and uplifting of left side
of river. Overall, uplifting in the left side of basin (western part of
Shakhen drainage basin) is more than east side. So, Shakhen drainage basin
inclined to the eastward (Fig. 8). With attention to
area of main drainage in Sergan sub-basin (Table 2),
area differences between left and right side of main stream is 4.66 km2,
while in Washan sub-basin it is 15.78 km2. Therefore, area
increasing in the left side of main stream in both sub-basins, result
in the length increasing of stream orders in the left side of main streams
(Table 2). Measuring of length have been certified this
subject. Therefore, that may be caused vertical displacement of left sides.
Topographic cross conjunction: This index has been calculated
by extracting 12 profiles across two main streams. The calculated results
have been shown there are conjunctions in the study area (Table
3, Fig. 9). The average rate is 0.18. Thus, displacement
and uplifting in the area result in small rate of non-conjunction condition.
This uplifting caused to divergence of streams direction eastward.
Subsequently, the strata of the region formation were affected by faulting
and folding. The uplift of the area ended the deposition in the basin
valleys and caused erosion and vertical incision of the river. The incision
of the Shaken River, in turn, caused the exposure of the very fine depositional
sediment of unconsolidated or poorly sorted materials such as gypsiferous
and salty silt marls and silt-clay deposits of Tertiary and Quaternary
age. The uplift is accompanied by subsidence of blocks toward the outside
of valley basin.
Mechanisms for gully erosion appearance and growth are still poorly understood.
In the study area, gully formation is associated with both natural drainage
features and land-management practices. In addition to the rain characteristics,
topography plays an important role in the initiation of gullies. But,
the empirical approach shows the need for information on the environment
other than topography alone. This is indicated by the weak correlation
between drainage basin area and local slope measurements at the initiation
point of gullies, even when they are obtained by standard methods.
In this area, in addition to natural factors such as: soil, vegetation,
hydrology and landuse, new tectonic activity have been considered also.
Throughout the history of the Earth, the movement of plates has resulted
in continual global environmental change. The impact of plate tectonics
can be considered on two levels. The long-term earth history indicates
significant impacts due to the agglomeration of landmasses into larger
landmasses and the subsequent changes in temperatures that these landmasses
experienced. The splitting of landmasses, together with the changes in
global water circulation that followed, also caused considerable change
in environmental conditions, both in the oceans and on land. On a smaller
time scale, the impacts of earthquakes and volcanic eruptions that have
occurred in the recorded history of humans provide insight into the slow
but inexorable change brought upon land masses by tectonic activity. This
unit allows examining the scale of change caused by both earthquakes and
volcanoes while gathering and analyzing the information that indicates
past tectonic activity. Using neotectonic indexes, new tectonic activities
in this region studied. Results have been certified in early Pleistocene
time, uplift and emergence resulted in the progressive migration and the
superposition of drainage on the beds. During the recent years, the area
was uplifted while base level falls also. In this research, the rates
of changes in gully erosion by comparison of aerial photos (1:20000) and
GPS in field study have been indicated. The results show permanently changes
in gully erosion in the recent decades. On the other hand, change in landuse
and decreasing vegetation cover and overgrazing are external factors in
gully erosion in this region.
The most active processes, leading to the largest retreat rates, were
soil fall after tension crack development (Fig. 4) and
undercutting by plunge pool erosion or by the development and consecutive
destruction of flutes (Fig. 4) and piping as a potential
mechanism for head cut retreat was active in the studied bank gullies,
but remnants of pipes observed in some gullies indicate that this process
might be more important at an earlier stage of development. Finally, the
availability of suitable topographic indices able to satisfactorily estimate
gully erosion, together with terrain digital models and geographic information
systems, would identify areas susceptible to gully erosion.