Step2: Hydraulic Analysis
Steady Flow and Subcritical Flow
HEC-RAS is a software package, developed by USACE and designed for executing one-dimensional water surface calculations. HEC-RAS system is composed of a graphical user interface, separate hydraulic analysis components, data storage and management capabilities, and graphing and reporting facilities, and it is currently capable to supports one-dimensional steady flow calculations, one and two-dimensional unsteady flow calculations, sediment transport computations, and water temperature analysis (USACE, 2002 b). Besides, It can handle the presence of bridges, culverts, and Manning's roughness coefficients, flow rate, depth, and velocity as well. Flow in an open channel is steady if the channel depth, discharge, and average velocity of flow at a particular location does not vary with time, or if it can be assumed constant during a given time period (Sredojevic, D., & Simonovic, S. P., 2009). Conversely, if the depth, discharge and velocity of flow at some point changes with time, the flow is considered as the unsteady flow (Sredojevic, D., & Simonovic, S. P., 2009). Within steady flow analysis, it can be resembled subcritical, supercritical or mixed flow regime. For this project, the stream in Chilliwack River is assumed as the steady flow and is analyzed with subcritical flow regime.
Historic Discharge Data Source
The discharge data for flood frequency analysis were downloaded from three different hydrometric stations, CHILLIWACK RIVER AT VEDDER CROSSING (08MH001), SLESSE CREEK NEAR VEDDER CROSSING (08MH056), and CHILLIWACK RIVER ABOVE SLESSE CREEK (08MH103) at the Canada Wateroffice Historic Data site (BC MFLNRO, 2016). The figure below indicates the location of three hydrometric stations. Based on the tendency of historic flood, flow rates for different food return periods can be approximately estimated. Since the trend of climate change Consequently, for this project, 100-year flood and 200-year flood will be calculated, which is able to involve various climate scenarios. The table below provides a summary of available flow data for different parts of the river.
Discharge Data List
Geographical Location of Hydrometric Stations
Flood Return Period Analysis
Flood return period is the concept of the recurrence interval of occurrence of a given flood in a given year (Gumbel, E. J., 1941). It’s rigorous to estimate flood recurrence interval with latest data so that results can eliminate the potential error from climate changes on discharge rates, since the most recent estimate of the same flood is significantly higher (Drew Brayshaw, 2016). Recurrence interval (T) can be calculated according to the Gringorten plotting position formula (Hirsch, R. M., 1987):
where:
T= recurrence interval
n = total number of years of record used
m = magnitude or rank
Based on the equation mentioned above, the flood frequency analysis of each station is able to be done, and the line graph generated by the frequency analyses can estimate the discharge rates of corresponding flood return period. The results of the historic flood frequency analysis on each year are presented in the Recurrence Analysis Page including the discharge data collect by the government for corresponding station. Line Graph 1,2,3 converted by the historic flood frequency Analysis results in Recurrence Analysis Page indicate the relationship between the recurrence interval and peak flow rates and forecast the tendency of changing in the future on each station.
R2 of each trend line of Flood Frequency Analysis is approximately equal 1, which statistically shows a strong relationship between recurrence interval and the peak flow. According to the three trend lines of each hydrometric station, estimated 100-year and 200-year magnitude in Chilliwack River upper reach, lower reach and Slesse Creek are shown on the table below:
Estimated Flow Rate of 100-year Flood and 200-year Flood for three Hydrometric Stations
Flow Data Input and Boundary Conditions
The calculation results in Table 4 was inputted into HEC-RAS as estimated flow data. For this project, simulation was performed under two flood return periods.
Defining boundary conditions was the next step after flow data input. Since no investigated flow data is available for water surface of study area, it was necessary to choose appropriate Steady Flow Boundary Conditions. Usually, normal depth is an alternative option if there is no observed data (Sredojevic, D., & Simonovic, S. P., 2009). Normal depth is the depth of flow in a channel or culvert when the water surface slope equals the channel bottom slope and the water depth keeps in constant (Swamee, P. K., 1994). For this project, normal depth is able to be calculated by input flow discharge, bottom width, side slope, bottom slope, and Manning's n. Discharge rates can be calculated by the trend line of lower reach when 1-in-2 year flood is assumed equivalent to or approximates the baneful flood, which is about 440 m3/s. Channel width data can be accurately measured from the aerial photo, and side slope along the channel can be determined from the topography. Consequently, normal depth is approximately 0.362 meters.
HEC-RAS Computation and Data Export
After setting up all data and conditions, it is the time to run HEC-RAS. In the steady flow analysis, subcritical flow regime was resembled. This step will automatically check any error in the input data, and it is necessary to fix them, and then run the simulation again. After successful simulation, the final step involves export of the simulation results back to GIS. After checking all reaches are selected to be exported under all scenarios, the computation results can be exported. The figure below indicates the configuration of GIS export window in HEC-RAS.
