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Department of Geography and Environmental Studies, Wilfrid Laurier University
Exercise 4 in the Laboratory
Flood Frequency, Channel Characteristics, and Floodplain Landforms in Applied Geomorphology
Meandering, straight, and braided patterns can all be found in natural stream channels. Stream channels tend to take a meandering shape where sediment loads are moderate and valley gradients are gentle. The flow velocities in the course of a natural meandering stream are not uniform. Entrainment, transport, and deposition of bed and bank materials are all affected by changes in flow velocity. As they flow across the floodplain, meanders progressively modify their position laterally, resulting in a diversity of floodplain landforms. There is a sequence of regularly spaced bars and pools, as well as changes in sediment distribution, within a natural channel. A meandering channel is largely stable in its natural condition and provides a range of habitats for creatures, as well as having a high value related to the ecosystem services it delivers.
The hydrology and sediment supply to a stream channel can be affected by changes in land use. When forest cover in a watershed is reduced, we often see an increase in sediment supply and greater runoff into stream channels. The channel forms will adapt to these changes in processes, with common modifications including channel straightening and deepening, as well as a decrease in channel stability. We may try to limit or offset the detrimental effects of land-use changes on channel morphology in a watershed because we appreciate stream channels in their natural state. Fluvial geomorphology aids in the understanding of natural channel shapes and the mapping of potential problem areas that may occur when a natural channel is impacted by human activity.
We will investigate the channel patterns and fluvial landforms that emerge along with a variety of stream channels in southern Ontario during this two-week laboratory experiment. We’ll start with a look at the Nith River, which is a tributary of the Grand River, in the first week. Students will investigate topographic features along the Nith River, as well as discharge data and calculations based on that information. We’ll also employ a uniform flow formula to have a better understanding of the channel flow and geometry
Flooding and Bankfull Discharge
The discharge (Q) in a given stream at a given site will change with time. In general, the stream channel itself contains the great majority of ordinary daily flows, and very high discharge (high water flooding) events occur infrequently. The water level (stage) and stream depth rise in a channel as the discharge (flow volume) rises. Because few natural channels have a rectangular cross-section, the breadth of the channel expands as the discharge and water level rise. A stream’s bank full discharge (Qb) is the volume of water that entirely fills the channel up to the tops of its banks. During a flood, flow volumes that exceed a stream’s bankfull discharge cause water to spill out onto the floodplain, which is known as an overbank occurrence. The bankfull width of a channel is the channel width at the bankfull discharge flow volume. Several parameters relating to the bankfull channel condition and the surrounding floodplain are depicted in the diagram below. The channel is shaded dark blue because the discharge is normally moderate to low, and the active channel width (wetted width) and average depth are tiny. The water level rises to the top of the banks as the discharge increases. The water level at the bankfull discharge is indicated by the light blue coloring. Within the bank’s full stage, the hydrologic channel is contained. Water will pour from the channel onto the neighboring floodplain if an overbank event (flood) happens. The width of the floodplain that is activated and the depth of water that flows across it will be determined by the magnitude of the flood event. Floods of very high magnitude cause more widespread flooding, whilst floods of low size affect the floodplain and nearby areas less.
Figure 1: The floodplain, bankfull width, and bankfull depth (adapted from FISRWG (10/1998)). Principles, Processes, and Practices for Stream Corridor Restoration https://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/national/water/manage/restoration/?cid=stelprdb1043244 by the Federal Interagency Stream Restoration Working Group (FISRWG).
In the field, a noticeable break in the slope at the top of the banks and a change in the vegetation can often be used to identify the top of the banks (bankfull state). Accurate bank full width and depth measurements necessitate field effort. Aerial photographs or high-resolution satellite photography, o the other hand, can be used to determine bankfull width.
Frequency of Floods
In general, flow volume and frequency of occurrence have an inverse relationship. The occurrence of high discharge incidents is uncommon. Overbank flow occurs when discharge exceeds the bank full capacity of a stream channel (a flood). Understanding the frequency and size of flooding events in floodplain areas is critical for floodplain planning. Aside from the apparent effects of floods on human constructions that may be located in a floodplain, flooding can also cause considerable alterations to the stream channel
Figure 2: On February 21, 2018, the Nith River in central Ayr. Flooding occurred throughout the floodplain and into the adjacent areas. On a fast warming and melting snowpack, around 50 mm of rain fell the week before. The Nith River discharge measurements at neighboring New Hamburg revealed the second highest flow in the Nith River’s 60-year history.
There can be a lot of erosion, sediment transport, and deposition during a flood. The morphology and pattern of a natural river channel are greatly altered by relatively frequent, low-magnitude flooding episodes that occur once every 1.5 to 2 years. In this lab project, we’ll look at data on discharge occurrences at a number of Nith River locations and create flood frequency curves for these stations.
Daily, monthly, and annual series can be used to study streamflow data. A Flow Duration Curve is a graph representing the frequency of discharge as a function of time for a specific stream.
Oregon State University offers an online seminar on flood duration analyses at:
http://streamflow.engr.oregonstate.edu/analysis/flow/. Flow duration curves can be created in a variety of methods, but the most usual method is to integrate all daily discharge data collected over a period of years (described in the Oregon State tutorial linked above). Also, daily discharge data that are above a certain threshold (known as a partial duration series) or daily discharge data that are separated into classes can be used. Alternatively, a flow duration curve can be created by using simply the greatest instantaneous discharge for each year throughout the recording period (an annual series). The latter information can be utilized to create a simple flood frequency curve. It is feasible to determine the return period (recurrence interval) of flood episodes of varying magnitudes using such a curve. To create such a curve, we’ll utilize a simplified method based on maximum instantaneous discharge data.
Data on Stream Discharge
The Canadian Water Survey maintains a network of stream gauging stations across the country. There are dozens of permanent gauging points in southern Ontario, the most of which are run by the Water Survey of Canada and a lesser number by the Conservation Authorities. The Water Survey of Canada compiles federal data on a regular basis and makes it available in an archive called HYDAT. The information is updated on a regular basis. The Water Survey of Canada offers the HYDAT product for download.
In a Microsoft Access database, the HYDAT data is organized. Environment Canada offers a data explorer tool that can be downloaded and used to create a series of files that can be accessed and altered in programs like Excel. That is how the Excel file that we will utilize in this lab was created.
Data on water levels and flows can also be found at https://wateroffice.ec.gc.ca/index e.html.
Individual station data can be accessed at https://wateroffice.ec.gc.ca/search/historical e.html.
Real-time data (the most recent data) can be found at:
https://wateroffice.ec.gc.ca/search/real time e.html
We’ll use data for the Nith River that has been pulled from the HYDAT database and updated with internet data sources.
Watershed of the Nith River
The Nith River’s watershed is depicted on the map below. The Nith River is one of the Grand River’s primary tributaries. The Water Survey of Canada operates and maintains a network of permanent measuring stations along the Nith River, with the primary stations administered and maintained by them. The Grand River Conservation Authority manages a few other stations. On the map below, the positions of the Water Survey of Canada gauging stations on the Nith River are shown. There are three stations to choose from (Table 1). The Grand River Conservation Authority also has measuring stations at Philipsburg and Ayr on the Nith River (although the Ayr station is currently non-operational). The Grand River Conservation Authority website: https://apps.grandriver.ca/waterdata/kiwischarts/rf thriver. asp? mid =1054 shows the flow volumes of the Nith River System in real-time.
Table 1: Water Survey of Canada Nith River Gauging Stations
Location Drainage Area ID Station ID Station Name Station ID Station ID Station ID Station ID Station ID Station ID Station ID (km2)
Nith River 02GA038