Spatial–temporal fluvial morphology analysis in the Quelite river: It’s impact on communication systems

During 2008 and 2009 heavy rainfall took place around the Mazatlan County in the Sinaloa state, Mexico, with a return period (Tr) between 50 and 100 years. As a result, the region and its infrastructure, such as the railways and highways (designed for a Tr = 20 years) were severely exposed to floods and, as a consequence damage caused by debris and sediments dragged into the channel. One of the highest levels of damage to the infrastructure was observed in the columns of Quelite River railway’s bridge. This is catastrophic as the railway is very important for trade within the state and also among other states in Mexico and in the USA. In order to understand the impact of the flooding and to avoid the rail system being damaged it is necessary to analyse how significant the changes in the river channel have been. This analysis looks at the definition of the main channel and its floodplain as a result of the sediment variability, not only at the bridge area, but also upstream and downstream. The Quelite River study considers the integration of Geographic Information Systems (GIS) and remote sensing data to map, recognise and assess the spatio-temporal change channel morphology. This increases the effectiveness of using different types of geospatial data with in situ measurements such as hydrological data. Thus, this paper is an assessment of a 20 years study period carried out using historical Landsat images and aerial photographs as well as recent Spot images. A Digital Elevation Model (DEM) of local topography and flow volumes were also used. The results show the Quelite River is an active river with a high suspended sediment load and migration of meanders associated to heavy rainfall. The river also has several deep alluvial floodplain channels which modified the geometry and other morphological characteristics of the channel in the downstream direction. After the identification of the channel changes, their causes and solutions to control, the channel migration and the dynamics structure, a river management plan was projected not only to protect the bridge but also to provide a flood risk awareness in order to reduce the social–economical impact during a flood event. Keywords Morphology change Flood Remote sensing-GIS El Quelite River management 1 Introduction Flooding is a natural event that occurs along rivers, streams and lagoons and they are present in coastal and alluvial zones, in reservoirs and in areas with inappropriate drainage, amongst others. Flooding is highly variable in space and time, as it does not always impact in the same area and with the same intensity. This results in different levels of damage along the river according to the degree of the development of the region. Recently, it has been observed that every year there has been a major presence of floods in both developing and developed countries with different regional and local impacts but resulting in common economical and social losses. Authors such as van Alphen and van Beek (2006) associated the flood increase frequency to the climate change. Although, the climate change is related to water shortage in arid regions and excess in tropical regions, there are different natural events, for instance, low pressure systems like hurricanes that generate severe rainfall in both arid and tropical areas alike, giving place to extreme events. In tropical zones, floods are quite frequent, 1–3 times per year, with predictable losses at the end. However, in arid zones this kind of event causes huge devastation because their occurrence is unusual. In arid river basins, a floodplain is characterised as “transitory fluvial landforms subject to frequent and rapid changes” ( Graf, 1988a,b ). This is because precipitation and temperature, which define drainage and vegetation, are linked to the erosive process that affects the river channel and its floodplain. The result is a non clear distinction between the river channel and its floodplain creating a width area for water flow. Additionally, the differentiation between upstream and downstream may not change significantly, but modifications such as urbanisation can generate instability in the fluvial system forming morphological adjustments ( Biedenharn et al., 2008; DRDE, 1991 ). As a flood can affect large areas, the impact is bigger than other natural hazards and can paralyse the social life and economy of the region. For example, Richardson and Richardson (2008) pointed out that the damage to highway bridges can cost millions of dollars since they have both direct (e.g. replacement or restoration of bridges) and indirect (e.g. disruption of transportation facilities) costs. A recent example of these costs were presented in Nuevo León city, México, where after intense and prolonged rains produced by a hurricane, severe flooding took place devastating an important part of the city and stopping almost 40% of export trades to the USA. This was due to the destruction of bridges and resultant paralysation of highway and railway transit. The majority of the bridge failures are highly associated to extreme events such as flooding because of the erosive action of flowing water, which can remove the bed material of the channel around the columns. Also, debris or sedimentation materials can be carried by the flow damaging the structure of the columns themselves generating instability. A bridge emplacement needs to be fluvial stable, this means that a braided, meandered or movement free river will need some channel control in the future. However, a narrow channel is not always synonymous with stability. Thus, there is a necessity to know the river interactions considering the hydrology, the hydraulics and the morphological aspects of it. The fluvial system is complex since it responds to flow channel changes and sediment regimes. Thus, natural and manmade disturbances such as floods and flood protection, road construction, riparian vegetation change, in-channel gravel mining, logging, dredging, etc. need to be considered in order to identify the boundary conditions to restore or simulate the natural condition and stability of the river ( Biedenharn et al., 2008 ). However, this stability is highly compromised since mans activities modify the return period (Tr) design of the hydraulic infrastructure. In particular, rivers present sediments banks that are converted into agricultural land or urban places reducing the Tr and increasing the flood impact. The river recognition links the form of the river to the fluvial process in order to obtain a morphology classification and determine the flooding extension. For this, the main characteristics to be considered are the topography in order to determine the slopes, the geomorphology to define the type and quality of soils, in particular for non-consolidate fluvial material, and the hydrology to analyse water bodies and the river network. Thus, the story of the river could provide important information to the remote sensing techniques which could offer a tool to determine the morphologic changes. The use of temporal images can show how the river and its floodplain dynamics move. Also, these images help to predict how the mobility will continue or which kind of measures need to be considered in order to restore the stability. The advantages of using remote sensing techniques are the use of synoptic data covering both spatial and temporal aspects. These advantages could be maximised by using ground information and other geographical data, all of them integrated in a Geographical Information System (GIS) ( Rogan and Miller, 2007 ). This paper aims to establish the channel and floodplain response to extreme events (floods) looking at the geography of the basin, the process in the fluvial system, the sedimentary features and the channel morphology. After analysing the response, the causes of the instability are settled and some actions to restore the equilibrium are proposed. 2 Description of the river basin The Sinaloa State, Mexico, has a railway network of 1168 km from north to south being the most important system of the Chihuahua-Pacifico network with 26 stations linking 56 towns and a connection to the USA. The railway bridge has a longitude of 140 m and crosses the Quelite River. The bridge is located at km 1146 + 07 in the geographical coordinates 23.44 N/106.54 W at the north of the Mazatlan County at the south of Sinaloa. The geology of Mazatlan County is based on sedimentary rocks which are associated to the crop up of marine fragments and consolidates of continental, volcanic and metamorphic rocks. The majority of these rocks are tonalities and monsonites from the Tertiary, crop up of rhyodacites, rhyolites and ignimbrites with tobaceous sediments in the base, and esites and phelsites rocks from the Late Cretacic. Limestone, sandstone, slate and quartz from the carboniferous, as well as gravel and conglomerates are part of the alluvial fan and tallus deposits. Two types of soils can be identified: podzols and lateric. Podzols have a strongly white eluvial horizon and an external brown layer with organic detritus; the main characteristic is a sporadic horizon which can be water saturated in one period of the year. Lateric soils are common in rainy tropical areas with the presence of two small mosaics (red and yellow) as characteristic of humid temperate zones in a subtropical system. The Occidental mountain chains at the SW and N define the Quelite chains at 50–700 masl. Also, at this point, the Zapote stream is originated in the W as a tributary of the Quelite River. In the N in the San Marcos chains at 50–700 masl in the SE and NW direction, the Copal stream and other tributaries of the Quelite River are originated. The watershed of the Quelite River has an area of 835 km 2 with an average runoff of 107 Mm 3 . The Quelite River has a longitude of 110 km from its point of origin in the Silla hill until its discharge into the Pacific sea ( Fig. 1 ). Mazatlan is characterised as being a rainy tropical place in the summer with strong droughts. In mountain areas the climate is semi-warm, sub-humid with an annual average temperature of 24 °C, whereas in the coastal areas it is semi-warm semi-dry with annual average temperature of 25 °C. The annual average rainfall between 1940 and 1980 was 478 mm with a maximum value of 215.4 mm in 24 h and 90.4 mm in 1 h. During the same period the annual average evaporation was 2146.8 mm, the dominant wind direction was NW and the average wind speed was 5.0 m s −1 ( GES, 2010 ). The main economical activity in the area is the agriculture favoured by the Quelite river. 2.1 Natural events in the zone The watershed has the Quelite hydrometric station with records from 1960 to 2001. The station presents extreme events in 1965, 1968, 1972, 1981 and 1986. 1ss981 was the year with the average maximum and instantaneous annual flow rates of 785 and 1743 m 3 s −1 respectively. In general, the area is subject to hurricanes and tropical depressions that turn into hurricanes ( Table 1 , NHC, 2009 ). In 1981, two tropical depression systems and two hurricanes, Norma and Otis ( Fig. 2 ) took place which explains the maximum values that were registered in the Quelite station. The socio-economical impact is used to define how much resource is expended in order to protect urban areas and the infrastructure. This becomes considerable since there is evidence of a major frequency of extreme events. Until 2002 the impact of these events had an average of 5 years, but after 2006 an extreme event has occurred every year with heavy storms and faster winds which has maximised the flooding and landslide (highway slides) in different places generating both human and economical losses. 2.2 The Quelite river characteristics The Quelite river is considered to be dynamic and very active with high suspended sediment load as a result of the heavy rainfall in extreme events (tropical storms and hurricanes). The river did not generate an important delta since its main channel is divided and distributed into the beach zone and wetlands where its discharge is significant to maintain the ecosystems. The main channel is characterised by a slow movement, gravel and sand bed and its alluvial coastal plain which provides large meanders as a result of the sediment transport. This condition can be associated to a roughness coefficient of 0.028 (Manning coefficient). The meanders have been used as agricultural zones intensifying the land use and river banks changes throughout this time. The columns of the bridge show scour ( Fig. 3 ) as a result of the erosion of the bed material around them. Material is also removed from the stream bed and neighbouring bank not only at the bridge crossing, but also through upstream migration. Fig. 3 shows the action of the debris carried in the channel, which generated damage to the columns. It also accumulates around the columns to levels that can be equal to the column height (during the last flood event debris accumulation reached 5 m height). In order to reinforce the columns, provisional protection was used in particular for the second column close to the left bank ( Fig. 3 ) but there is a necessity to avoid future bridge failures. 3 Methods and data River morphology is determinant since it considers the interaction between different variables which can be grouped into four categories: dynamic flow (rate, discharge, roughness and shear load), shape and characteristics of the channel (width, depth, slop, shape, pattern, etc.), load of sediments and bed, and bank material (shape and characteristics) ( Biedenharn et al., 2008; Martín Vide, 1997 ). The methodology determines those characteristics that define the river and floodplain morphology, firstly describing the form of the channel and floodplain changes, then using a hydraulic analysis to define the river profile around the railway bridge using a cross-sectional survey at different sites upstream and downstream (20 m between the cross sections for a total length of 1540 m) in order to determine water depth and hydraulic variables such as hydraulic area and radium, wetted perimeter and width of the free surface, roughness coefficient and the flow discharge provided by a hydrological analysis ( Fuentes et al., 2010 ). Finally, a comparative analysis was carried out to determine the channel morphology and erosional forms, as well the floodplain erosion that defines the expansion of the flooded areas. To describe the channel and floodplain dynamics, remote sensing data and Geographic Information System (GIS) data were used in order to identify and measure channel width, sinuosity, islands, and meanders ( Best et al., 2007; Schmidt, 2007 ). There was a particular interest in the area around the railway bridge (km 1146 + 07), thus the river analysis was restricted to 8.5 km; 5 km upstream, where it was clear that the river movement created and removed meanders and islands, and 3.5 km downstream looking at the dynamics of the river before the coastal alluvial plain where the movement was erratic ( Fig. 4 ). As the river has been exposed to different changes such as switching of the upstream course, meander movement, increment of the discharge, floods and river capture into old river patterns, amongst others, we consider the main pattern as the principal channel and analysed changes as a result of the sediment deposit as a branch. The dynamics of the river and its floodplain were mapped, determining areas with erosion and deposit features by using the length and width of the river. Thus the morphodynamically effective area could be established ( Hauer and Habersack, 2009 ). Meanders were measured considering the radius from the centre line of the main channel previous to the river movement ( Mertes et al., 1996 ). The importance of the need to understand the meander’s effect is related to the prediction of future sites of bank erosion. The sinuosity was established as the coefficient between the actual length of the principal channel among two points and the valley length among the same points ( Vizcaíno et al., 2003 ). The floodplain was considered because it is seasonally flooded and natural sedimentation, inundation magnitude and topography are important due to the actual land use (crop growth) in the area ( Biedenharn et al., 2008 ). The expanded floodplain when extreme events or floods take place was determined using a Digital Elevation Model (DEM), aerial photographs and satellite images allow performing an historical analysis of the area, identifying the water path which produces erosion giving place to depressions or deposits of river material. Bars areas were defined since it could modify the main channel, thus its area and expansion were considered. The information available for the morphological temporal analysis of the river and its floodplain is shown in Table 2 . The hydrological analysis was performed ( Domínguez et al., 2010 ) considering the bridge as the exit of the watershed in order to establish the river flow expected in minor and extreme events and the vulnerability of the infrastructure. The computation method is based on a statistical analysis using the precipitation records registered in the El Quelite hydrometric station that controls 88.4% (835 km 2 ) of the bridge basin. This statistical analysis considers adjustments to several probability functions and the extrapolation of the precipitation values to several return periods (Tr) years. In particular, a return period (Tr) of 100 years was used as a critical condition in a hydraulic analysis to compute the water depth and flow rate to different Tr ( Fuentes et al., 2010 ). The vulnerability of the infrastructure was analysed looking at the bridge failure according to a comparative analysis proposed by Vazquez-Fernandez and Gracia-Sanchez (2005) . The authors established two nondimensional parameters: NQB = Q / ( g B 2.5 ) and NHB = H / B , where Q is the design discharge for a given Tr (m 3 s −1 ), g = gravity (m s −2 ), B = bridge length (m) and H = bridge height measured from the riverbed to the lowest beam in the span (m). In particular, the NQB parameter permits to differentiate better the risk of failure and it can be relate the Froude’s number (Fr = Q / ( g BH 1.5 ) ). In general, the bridge failure was associated more to the bridge length rather than to its height. 4 Results The river’s path are shown in Fig. 5 . Five zones can be identified as those with a major movement in the main channel as a result of a complex pattern (braided and meandered) generated from the sediment deposits and transport. In particular, zone 1 showed that in the dry period a small flow is carried out by the main channel but in the rainy period another channel is available as well as old branches generating a free movement of the river and, in consequence, a branched configuration ( Fig. 6 ). Moreover, the extreme event in 2007 (H – Henriette) modified the defined trajectory after Kenna in 2002. Zones 2, 3 and 5 are examples of the meander changes, creation, migration and removal, modifying the land use. Zone 5 showed an erratic divagation or free movement of the river highly related to the coastal alluvial zone. At the same time important agricultural areas were defined or lost since they depend on the presence of these meanders. For instance, Fig. 7 shows an old meander pattern where agriculture was developed but the crop coverage is irregular. The meander length and amplitude are 800 m and 250 m, respectively, clearly indicating a strong influence at the variation of the discharge and high sediment load that determined its configuration. In this case, a spatial erodibility of the bank materials was found responsible for the migration. In 2007, the river movement generated important divagations on the right bank giving rise to new branches as a result of the sediment deposition forming middle bars. In fact, after 1986 the island formation was significant in zone 2. Zone 4 showed an important variation along the upstream left bank presented in 1999 probably as a consequence of the extreme event in 1996. This variation reopened an old branch of the river that was not used in the post events in the area. In the area of the bridge there is a presence of islands or middle bars downstream, mainly in the centre bridge’s columns, formed from fine sand sediment ( Fig. 8 ). The condition of the bridge in this zone also presents an erosive process in the columns additional to the one observed in the banks, thus an instability condition of the infrastructure is produced. This was confirmed due to the measure applied in the central column using gabions as reinforcement (see Fig. 3 ). The local instability can be also referred to the riparian vegetation along the banks and the condition of the floodplain. In the first case, the main channel changed land use into agricultural areas. After some time the sediment deposits resulted in land gain but the activity has been lost in several points along the river. Upstream of the bridge at approx. 500 m, the main change was observed in the curve were the left bank is higher than the right one reducing the water depth to 2 m for a 2260 m 3 s −1 discharge. The pattern of the river at the railway bridge is important, since the recent motorway bridge has modified the profile of the stream bed escalating the flow velocity and hence the scour and bank erosion. After the railway bridge contraction the flow velocity is reduced and bars are created as well as new branches redefining the floodplain ( Fig. 9 ). The floodplain delimitation was established as a function of the extreme events footprint in the alluvial zone. The floodplain showed symmetry on each side of the river, thus the two of them has the same dynamics. The precipitation statistical analysis in the hydrological study considers adjustments to several probability functions. In this case a Double Gumbel distribution provides the best fitting method to extrapolate values applied to several Tr years. The relationship between instantaneous and average flow rates was around 2.3. This suggests that avenues are variable in the same day with a relatively slow discharge. A Tr of 100 years was used as a critical condition in a hydraulic analysis to compute the water depth and discharge ( Fuentes et al., 2010 ). Results show an instantaneous discharge of 2261 m 3 s −1 ; 23% higher than the one registered in 1981 as the annual maximum instantaneous value. The bridge height was 9 m and the maximum water depth was 6 m, thus the bridge was not damaged since it has enough free surface. The bridge width is 140 m and the hydraulic length was 106 m. Upstream the railway bridge, the El Quelite River has a channel width of 171 m whereas downstream is 200 m. The Fr was 0.88 implying that the regime of the river is subcritical. This means that the hydraulic regime is permanent in time, a slow type with small slope and granular bed. This granular bed could indicate that the velocity is high but 5.5 m s −1 was higher, thus this velocity could generate scour at the piles at around 15 m explaining their failure. Fig. 10 illustrates the El Quelite River railway bridge at risk, since the NQB is too small, thus it is necessary to review the relationship between the bridge length and width. 5 Conclusions Remote sensing techniques and GIS are key tools to map and monitor the geomorphological change in both the channel and floodplain of the Quelite River providing detailed historical and present analysis of the changes. Static and dynamic aspects associated to the river were covered, thus the identification of erosive and sediment deposit processes were observed and measured. There is an important movement of the river detected at the creation, migration or removal of meanders, and islands or bars favoured by the land use change. The main river and floodplain changes observed were associated to extreme events (hurricanes and tropical storms) but also the human intervention was important transforming riparian areas into agricultural ones. These changes were definitive until the next event, thus the river is highly dynamic and subject to instability. A very important issue is that the instability of the system is the result of the stream and its observed characteristics. In the case of the railway bridge, it did not represent a critical hydraulic problem since it was evaluated for a Tr of 100 years giving sufficient area for divagation and free capacity for the maximum water level. However, the water speed caused problems because it was close to the critical flow. This also implies a local erosive process at the columns, in particular, at the central columns where the undermining can reach up to 15 m. Thus, the railway bridge is at high failure risk and it is necessary to take some measurements to recover the stability of the system (river, floodplain and bridge). In order to recover the stability of the system it is highly recommended to investigate and perform corrections to the floodplain, but this implies a large period to produce a change. However, some adjustments can be made in order to protect the pile’s foundation against erosion such as collars or gabions. There are many ways to protect the local erosion at the piles but the selection depends upon the different factors that impact the erosion process. Moreover, it is important to review the relationship length and width bridge, especially the first one since NQB was too small. Acknowledgment This research was supported by Ferrocarril Mexicano, S.A. de C.V. (FERROMEX). 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