Estimation of bankfull discharge based on stream channel characteristics in Garmabdasht Catchment, Golestan Province

Introduction
The bankfull discharge corresponds to the river level just before it starts to flow out of its main channel and over its floodplain (Navratil et al., 2006). Bankfull discharge is a deterministic discharge often used to estimate the channel-forming discharge. It is adopted that the bankfull discharge (magnitude and frequency) is one of the important concepts in the analysis of river morphology, flood events and ecological systems. River geomorphologists are particularly interested in bankfull discharge because it serves as a consistent morphological index that can be applied in river engineering and stream restoration to design a stable size and shape for a stream so that its channel will maintain its dimensions, pattern, and profile over time without degrading or aggrading (Rosgen, 1994; Knighton, 1996). Bankfull hydraulic geometry relationships vary by region along with changes in hydrology, soils, and land uses (Doll et al., 2002). This study investigates spatial changes in the bankfull hydraulic geometry such as width, depth and cross section area with bankfull discharge and catchment area.
Garmab-Dasht catchment is located in the northeastern Alborz, Golestan province and south east Gorgan. The study area geologically consists of marine and continental sequences, which deposited through the Msozoic to Cenozoic. The altitude of the Garmab-Dasht catchment varies from a maximum of 3400 m to 400 m on the valley floor. Mean annual rainfall in the catchment is around 550 mm. Mean annual discharge of main stream is measured 0.58 m³/s.
 
Materials and Methods
In this research, Garmabdasht catchment is divided into eleven reaches. Several criteria have been identified to assist in field identification of the bankfull stage. It contains significant breaks in slope, changes in vegetation, the highest scour line, and the top of the bank. Field measurements of bankfull width (W), bankfull depth (d) channel slope (S), bankfull cross-sectional area and streambed-material sizes (D_i) were conducted at each site. Two or three width measurements per channel reach were conducted and a mean width per reach was calculated. The grain sizes of surface sediments were sampled along the channel by pebble count method. After data collection, hydraulic geometry relationships calculated for bankfull discharge, cross-sectional area, width, and mean depth as functions of watershed area for the Garmab Dasht catchment. Flood frequency analysis is used to estimate the recurrence of bankfull discharge by a 30 years Annual Maximum Series (AMS).
 
Results and Discussion
The relationships between channel-morphology characteristics and bankfull discharges were examined using the power function regression equations. The high coefficients of determination indicate good agreement between the measured data and the best-fit relationships. The exponent value of width and depth were obtained 0.42 and 0.34, respectively. It shows that channel width increases more rapidly with the area than channel depth. Also, values of shear stress and stream power at bankfull condition represent a power function regression with catchment area. The study reaches show a good relationship between cross section area, bankfull discharge and drainage area, through the linear regression with a high R² (0.95). Shear stress and stream power show a power regression with catchment area. Also, stream power has a good relationship with bankfull width (R² = 0.74). The empirical equations expressing the relationships between D_i (in m) and drainage area is calculated as power regression. But, correlation results give no significant p level (P>0.05) for D_i and drainage area. Many research show that there are relationships between bankfull hydraulic geometry (width, depth, cross section area) and drainage area (Harman et al., 1999; Vianello & D'Agostino, 2007; Mulvihill & Baldigo, 2012; Modrick & Georgakakos, 2014). The results of this study verify such relations in Garmab-Dasht catchment. 
Log-Pearson Type III distributions were used to analyze annual peak discharge data for Garmab Dasht gage station. Many studies have found that bankfull discharge occurs at a recurrence interval of about 1–2 years on the basis of the annual maximum series (AMS) approach (Leopold et al., 1964; Dury, 1976; Castro & Jackson, 2001, Navratil et al., 2006; Schneider, 2011). The recurrence interval for the bankfull discharge in study area obtained 2 years.
 
Conclusion
The results of the regression analyses indicated that Bankfull width, bankfull depth, bankfull cross-sectional area, and bankfull discharge were related to drainage area using regression analysis. Bankfull hydraulic geometry relationships can be used to estimate the bankfull discharge at an ungaged site. Also, it can be toassist in field identification of bankfull stage and dimension in un-gaged watersheds. Further work is necessary to develop reliable relationships for other regions and rainfall/runoff conditions of Iran.