Gully Mapping And Change Analysis

During the early years of study, 2D measurements of headcut retreat and gully growth were accomplished by rectifying and—if required—mosaicking the photographs using GCPs installed in the surroundings of the gully. Thus, detailed mapping of the gully edges was possible despite the remaining relief displacement in the gully interior, which was not at the focus of the investigation and could not have been geocorrected accurately with polynomial rectification (see Chapter 11.2.2). In later project stages, 3D mapping in a stereoviewing environment and digital elevation model (DEM) extraction using digital photogrammetry software made it possible to map the 3D gully forms even more precisely (see Fig. 11-22) and to create 3D models for measuring gully volume (see Fig. 11-23). Change analysis from the multitemporal datasets for quantifying linear retreat and increase of gully area and volume is performed with GIS software.

Three exemplary maps of gully development illuminate the variability in headcut retreat behavior and gully evolution. Gully Gorom was monitored with SFAP over a period of three seasons in a project terminated in 2002; its change map in Figure 13-3 is complemented with smaller-scale satellite image data from 2007. Note the much coarser outlines of the last gully stage; high-resolution satellite data were a viable supplement in this case only because of the large size of the gully and the fact that it is the fastest growing in the whole dataset. The gully edges retreat along the full length of the gully, reshaping the gully form during each rainy season. The surrounding glacis area has little relief that might lead to preferential linear flow paths, and it is only in the upslope direction of the shallow main drainage line that the gully clearly grows faster.

Along the gully edges, mass wasting is the main reason for gully growth, and large clods of soil come to rest beneath the gully rim before being washed away by further erosion. Piping (subsurface erosion), often promoted by desiccation cracks at the headcut vicinity, is also involved in the retreat process. Despite the great width of the gully, incision is only shallow and the heights of the sidewalls are only between 40 and 100 cm. The maximum depth of the central drainage channel is ~1.2 m. Within the part of the mapped gully, areal growth is quite regularly around 800 m2 per year, but maximum linear headcut retreat varies depending on the spatial development of the gully's shape.

At the Barranco Rojo, situated at the verge of the Ebro Basin in northeastern Spain, the situation is quite different (Fig. 13-4). Although the gully is of a similar elongated dendritic shape with many secondary headcuts along the sidewalls, active headcut retreat occurs only locally and gully growth is much less. Piping has played an important role in the past and present gully development, as can be

FIGURE 13-2 Selected gullies monitored by SFAP in Spain (A—K), Morocco (L-N), and Burkina Faso (O-P). Kite and hot-air blimp aerial photographs by IM, JBR, and collaborators, taken between 1996 and 2008. (A) Barranco de las Lenas. (B) Barranco Rojo. (C) Bardenas 1. (D) Salada 1. (E) Salada 3. (F) Salada 4. (G) Luchena 1. (H) Freila. (I) Casablanca.

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FIGURE 13-2 Selected gullies monitored by SFAP in Spain (A—K), Morocco (L-N), and Burkina Faso (O-P). Kite and hot-air blimp aerial photographs by IM, JBR, and collaborators, taken between 1996 and 2008. (A) Barranco de las Lenas. (B) Barranco Rojo. (C) Bardenas 1. (D) Salada 1. (E) Salada 3. (F) Salada 4. (G) Luchena 1. (H) Freila. (I) Casablanca.

FIGURE 13-2—Cont'd, (K) Belerda 1. (L) Talaa. (M) Foum el Hassane. (N) Icht. (O) Gorom-Gorom. (P) Oursi.

seen from the location of changed gully areas and in the underlying airphoto from many remains of collapsed pipes in the gully interior. Within the gully, a continuous surface drainage line is not visible; its interior is divided into numerous sub-catchments drained by piping holes.

At the Barranco Rojo, the formation of the various individual headcuts and branches is obviously an alternating process rather than a continuous one as at Gully Gorom. This may be due to the processes involved and the spatiotemporal variability of the gully surroundings. Although areal change measured from the aerial photographs amounts to only a fraction of the change observed at Gully Gorom, volumetric soil loss may be comparatively larger, owing to the greater depth, up to 6 m in the part mapped, and abundant subsurface erosion features of Barranco Rojo. With piping as a major process of gully development, areal change would be evident only when the surface has caved in, and the gully can appear inactive at the surface in spite of active subsurface erosion processes beneath.

The greater variability of the topography and land use around the Barranco Rojo also may be potentially important for the different behavior of the two gullies depicted in Figures 13-3 and 13-4. Agricultural terraces, which are also subject to piping processes, are separated by short steep slopes, and a dirt track dissects the gully catchment just upslope from the main headcut. Land use varies both spatially and temporally between cereal fields, clean summer-fallow, and older, shrub-covered fallow land. Rainfall simulations and infiltration experiments have shown a great range of runoff coefficients, infiltration and erosion rates on these surfaces (Seeger et al., 2009). Thus, runoff coefficients and flow paths in the headcut vicinity are

FIGURE 13-3 Two-dimensional change analysis of Gully Gorom-Gorom, Province of Oudalan, Burkina Faso. Gully growth quantified between rainy seasons 2000 and 2001 (SFAP) and 2007 (Quickbird). Based on kite aerial photography by IM, JBR, and K.-D. Albert; image processing and cartography by K.-D. Albert and IM.

FIGURE 13-3 Two-dimensional change analysis of Gully Gorom-Gorom, Province of Oudalan, Burkina Faso. Gully growth quantified between rainy seasons 2000 and 2001 (SFAP) and 2007 (Quickbird). Based on kite aerial photography by IM, JBR, and K.-D. Albert; image processing and cartography by K.-D. Albert and IM.

FIGURE 13-4 Two-dimensional change analysis of Barranco Rojo, Province of Zaragoza, Spain. Gully growth quantified between March 2002 and April 2006 based on hot-air blimp photographs by IM, JBR, and M. Seeger; image processing and cartography by IM. Adapted from Marzolff and Ries (2007, fig. 4).

constantly changing over the years, contributing to the spatial variability of gully development.

The assumption that gully growth is strongly related to the characteristics of topography, substratum, and land use in the near vicinity is also confirmed by the example of Gully Salada 1 (Fig. 13-5), where human interaction plays an additional role. Until 2004, this large gully cutting into the Quaternary deposits of the Guadalentin Basin in southeastern Spain was the fastest growing of the

14 Spanish gullies being monitored. The immediately adjoining almond plantation, which was established only a few years before monitoring began, is kept weed-free by regular plowing. Soil crusting results in very high runoff rates and in the formation of ephemeral gullies between the almond trees. To bar the gully from retreating farther into the plantation, the farmer had plowed up an earthen dam around the gully margin—a measure with limited success. Runoff from the plantation collected at the earthen dam

FIGURE 13-5 Two-dimensional change analysis of Gully Salada 1, Province of Murcia, Spain. Gully growth quantified between March 1998 and October 2006 based on hot-air blimp and kite photographs by IM, JBR, and M. Seeger; image processing and cartography by IM.

subsequently resulted in subsurface erosion processes, creating a growing piping hole, which drained beneath a bridge remaining of the former dam (see also Vande-kerckhove et al., 2003).

The piping hole increased rapidly in the following years, while the main gully was used as a rubbish dump, and building rubble was shoved repeatedly over its northern edge by tractors. In 2005, the upper part of the gully including the piping hole was completely filled with rubble and soil material up to the level of the former surface (sloping into the remaining gully beyond the mapped infill boundary (Fig. 13-6A). This re-established the former border of the field; the missing almond trees were replanted between 2006 and 2008. By 2006, the infilling already had begun to subside and show large settlement cracks (Fig. 13-6B), preparing the ground for resumed piping processes in the future. Ground observations in September 2009 revealed a recent dam break with a large and deep erosion rill cutting from the almond field into the infilling where the former piping hole had been.

Using a hybrid method combining automatic height-point extraction with manual 3D editing and digitizing (Marzolff and Poesen, 2009), high-resolution DEMs (5-10 cm pixel size) were created for selected gullies. Figure 13-7 shows the example of a surface model of Gully Bardenas 1 with the corresponding orthophoto (see Figs. 11-1 and 11-22).

Cut-and-fill operations in a GIS environment enable determining the total soil loss at the gully site as well as volumetric changes between monitoring dates; 3D modelling from SFAP is clearly superior here to traditional

FIGURE 13-6 Gully Salada 1; kite aerial photograph taken by IM and JBR in October 2006. (A) Overview of gully after refilling of the upper part (compare with Figs. 13-2D and 13-5). Arrow indicates former position of piping hole. (B) Detail of large settlement crack near former gully edge.

FIGURE 13-7 Orthophoto draped over photogrammetrically derived DEM of Gully Bardenas 1, Province of Navarra, Spain. Based on kite aerial photographs taken by JBR, M. Seeger, and S. Plegniere in March 2007; photogrammetric processing by IM. Until February 2009, this gully increased its 35 m length shown here by 2.5 m, eroding just over 20 m3 on an area of 16.6 m2.

FIGURE 13-7 Orthophoto draped over photogrammetrically derived DEM of Gully Bardenas 1, Province of Navarra, Spain. Based on kite aerial photographs taken by JBR, M. Seeger, and S. Plegniere in March 2007; photogrammetric processing by IM. Until February 2009, this gully increased its 35 m length shown here by 2.5 m, eroding just over 20 m3 on an area of 16.6 m2.

terrestrial measurement methods. Modelling the complete form rather than taking sample measurements of gully extent and depth allows the stratification of volumetric change in erosion and deposition aspects, yielding results for the gully's own sediment balance. The example of Gully Salada 3—a typical bank gully of the simplest, single-headcut U-profile form (Fig. 13-8)—illustrates the complexity of the patterns often simplified as ''headcut retreat.''

Between 1998 and 2002, ~4.4 m area, corresponding to

13 m soil material, was lost to backward erosion at the headcut. In the same period, the surface height within the gully increased as 45% of the material eroded at the headcut was deposited on the gully bottom. Most of this came to lie close to the headcut, but some was washed farther down-slope at the gully bottom. Limited erosion only took place on the gully floor, and its longitudinal profile, which can be estimated from the 3D models shown in Figure 13-8A and B, obviously experienced an increase of gradient not due to downslope erosion, but due to upslope deposition.

Figure 13-8C also shows some of the difficulties associated with creating surface models for highly variable terrain (see Chapter 3.4.3). Along the sidewalls, which contributed to the gully growth with another 0.7 m2, the volume change visible in the change map must be considered somewhat inaccurate (note the improbable gain at the sidewall to the lower right of the calculation area limit). Here, the camera's perspective eye could not obtain an unobstructed view of the narrow and deep corridor carved by the gully, causing less inclined slopes in the 2002 model.

More details about the gully development rates and their relation with local topography, substratum, and runoff and infiltration behavior in the gully headcut surroundings are given by Geißler (2007), Marzolff and Ries (2007), Ries and Marzolff (2007), and Seeger et al. (2009). In summary, the development rates of the gullies vary between individual study areas along the transect, and particularly the Spanish gullies retreat more slowly than expected. Maximum linear retreat per year (Rmax/a) varies by a factor of 100, ranging from below 0.1 m/a to nearly 10 m/a. Within the same study area, the variability is much lower with a factor of only 10. When ranking the study regions according to soil erosion parameters assessed by experimental measurements, a clear association exists between the resulting order, both for maximum and minimum values of runoff coefficients and material output with the maximum retreat rates observed at the gully headcuts.

26.3.1998

26.3.1998

Surface height change [m]

Limit of interior gully

Surface height change [m]

Limit of interior gully

3D change quantification

Area loss headcut: 4.37 m2 Volume loss headcut: 12.98 m3

Volume gain gully bottom: 5.87 m3

FIGURE 13-8 Three-dimensional change quantification of Gully Salada 3, Province of Murcia, Spain. Based on hot-air blimp photographs taken by IM, JBR, and M. Seeger. All heights are given relative to local datum. (A) DEM in March 1998. (B) DEM in March 2002. (C) Surface difference map. (D) Orthophoto draped over 1998 DEM with summary of change measurements within the interior gully area. Adapted from Marzolff and Poesen (2009, fig. 9).

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