Document Type : مقاله پژوهشی
The gravel extraction from the bottom of rivers constitutes an important source of aggregate for the construction of buildings, roads and other structures. The problems of fluvial erosion generated by these activities can affect properties and human works located near the river, and have become a severe problem in many regions of the world. The environmental costs of instream gravel extraction have generally not been factored into production costs, making instream sources more economically attractive than alternatives such as dry terrace mines, quarries or distant sources such as reservoir deltas. Gravel extraction can have both immediate and long-term consequences for channel stability. Changes to channel morphology are initiated through the lowering of the riverbed during extraction. As geomorphically effective sediment transporting events are infrequent in many gravel-bed rivers, instream mining activities may operate for several years without obvious effects upstream or downstream. However, responses may be manifest during high flows many years later.
2. Study area
This paper examines the significance of within-channel gravel extraction on the morphometery of the River Lavij in northern Alborz. The study site at Khashpel Park is 2 km in length, and is located in the lower River Lvaij catchment in Nour Township, Mazandaran province. The altitude of the Lavij catchment varies from a maximum of 3359 m to 90 m on the valley floor. Mean annual rainfall in the catchment is around 650 mm. Mean daily discharge is 1.7 m3 s−1 with a maximum mean daily discharge of 175 m3 s−1. The River Lavij is a gravel-bed channel dominated by cobble size material in the study reaches. The upper study area reaches to an altitude ranging between 120 m and 90 m with a mean slope of 0.02m/m.
3. Material and methods
In this study different types of data have been collected: (a) hydrological data, (b) channel morphometric data concerning the time evolution of the longitudinal profile, cross section and planform (c) bed grain size distributions of the study reach. The discharge records at the Aghozketi gauging station and discharge-area relationship are appropriate for the study reach. Regional catchment area-discharge plots are produced for a range of flood recurrence intervals (2, 5, 10, 50, 100, 200 years). Study area divided into two reaches. The first reach A has been mined before 2008 and the reach B has been mining to present.
The data of reach A was surveyed in 2012. Then a comparison is made between the river character (longitudinal bed profiles, cross section and planform) of the intact reach and the reach A. Information on the recent evolution of the river character was collected from surveys made along the reach B at scale 1:1000 between 2008 and 2011. The grain size distribution of the gravel surface was obtained from Wolman pebble counts sampling of bed. An estimate of annual sediment delivery from the catchment to study reach calculated using an empirical regression equation of river suspended load. Then, we calculated gravel extraction on the fluvial sediment balance by comparing the extracted yield with an estimate of the catchment sediment yield.
4. Results and Discussion
Long Profile: a general tendency towards incision four main changes observed in reach A. The changes in bed elevation were locally greater than 6.5 m, 3.3 m and 2.5 m below river’s normal slope. Three drops were built in order to limit regressive erosion, reduce channel slope, and dissipate fluvial energy. The average river slope is greatly decreased from 0.02 m/m to 0.003 m/m between drop1 and drop 2. Local slopes increased to 0.027 m/m between drop 2 and drop 3. The average river slope shows low changes in reach B, except reach 3.
Cross section: the average river width shows a 10 m change between 2008 and 2011 in reach B. The bankfull depth increased up to 0.5 m in this period. A decrease occurs in amount of stream power in both reach A and reach B. This apparent decreasing trend in stream power is not surprising because we observed a decrease in slope and increase in width.
Planform: Channel planform analysis has revealed that both the reaches A and B are changed. The straight planform formed where the river has been artificially transformed in a series of steps and Gradient is steepened as flow follows a shorter path. Braided pattern has observed in reaches A2 and A3 that river gradient decreased. In reach B, river pattern has been straight or meandering.
Sediment: the sediment average size in upper reach is 32 mm and it decreased in lower reach (B and A to 25 mm and 8 mm, respectively). A sediment budget by comparing the volume of sediment supplied from the catchment, which an estimate of that released during extraction. The volume extracted unallowable have estimated 80000 m3.
Gravel extraction in study reach has changed the slope, width, depth, stream power, planform, and sediment size of channel. Comparing to reference conditions, reach A river altered irreversibly direct human disturbance and a wholesale shift in the geomorphic unit structure, planform, and bed material texture, so that the river has operated in a fundamentally different manner to its former state. In reach B, river altered reversibly direct human disturbance. Channel geomorphic analysis indicated that management interventions, such as Drop, only delay morphological adjustment.