River Diversity and Hydrology, Flow Alteration, and Environmental Flow Assignment

Assignment Task

Background

A myriad of diversity of river forms is evident in the 'natural' world. In simple terms these can be differentiated into confined (bedrock) rivers, rivers within partly-confined valleys (with pockets of floodplain), and rivers with continuous floodplains. Most emphasis in the fluvial geomorphology literature has been placed on the latter category, which are referred to as alluvial streams. The term ‘alluvial’ refers to rivers that flow upon their own sediments (i.e. rivers that are able to deform their boundaries), as opposed to bedrock controlled rivers which are laterally and/or vertically constrained. The most common procedure used to describe the morphology of a river is its planform. This refers to the configuration of a river in plan view. Distinction of planform type is normally made on the basis of sinuosity, number of channels and lateral stability of the channels(s). Primary examples of river planform types include:

The aim of this exercise is to explore the diversity of river forms and processes, show how to differentiate among these forms in a meaningful manner, and start to develop an appreciation of why these rivers have developed in the way they have, in the places they are found. Start by trying to group the above river types into high and low energy systems – this will help you to understand the resulting planforms.

PART A. identification and characterisation of River typeS

For Questions 1 and 2 , use the Google Earth (kmz) file on iLearn to take a tour of a number of different river systems with different river planforms, channels, sinuosity, confinement, energy and (in)stability. Change the view and angle to get a 3D image, move up and down the river, etc., to gain a different perspective.

Question

1. You are presented with a series of locations on a Google Earth tour. Interpret the following set of attributes for each site and complete the following table:

2. For representative reaches of each river on the Google Earth tour, determine the following planform attributes and complete the following table:

Part B. Flood Hydrology And Geomorphology

Background

Fluvial landscapes are those that are ‘shaped by running water’. Although rivers account for only 0.03% of the total global freshwater involved in the hydrological cycle, rivers are major agents of water and sediment transfer from continents to the coast. 

A plot of basin runoff response to a precipitation event is referred to as a hydrograph. Stream discharge, Q, is measured as width x depth x velocity (i.e. Q = w d v). Variations in discharge are measured as magnitude-frequency relations to floods. Of particular importance is extreme event analysis (Gumbel analysis), which determines the likely recurrence interval of flood events of differing magnitude. Expressed differently, the magnitude of floods with differing recurrence intervals can be determined (i.e. the magnitude of the 1 in 10 year flood vs the 1 in 100 year flood). When analysed in conjunction with the true form of a river, this sort of analysis allows geomorphologists to determine the likely morphological changes that will result on different types of rivers under different flood events. 

Estimates of flood frequency are derived primarily from historical records. In most instances, this record is so short that statistical procedures must be applied to undertake analysis of extreme flood events. Most procedures of extreme event analysis are based on an approach to statistics terms Gumbel analysis. A step-by-step guide to Gumbel analysis of discharge data is provided below.

Question

1.  The flooding events have been ranked according to magnitude; determine the recurrence interval for each flood event (r _i ):

r _i = (m + 1) / n

where m = total number of years with records

n = the rank of the flood event

2.  Create a graph of the data in MS excel. Show the magnitude of the flood event against the recurrence interval using a log scale for the recurrence interval. Insert a line of best fit through the data and then answer the following questions:

a. What is the magnitude of the mean annual flood (what is discharge (Q) when r _i = 1.58)?

b) What is the magnitude of statistical bankfull discharge (what is discharge (Q) when r _i = 2.33)?

c) How often would you expect a flood of magnitude of 3000 m ³ s ^-1 to occur (what is r _i at this Q)?

Part C. stream power analysis at a cross-section

The morphology of a river reflects the balance of erosion and deposition (i.e the balance between water and sediment movement). In rivers, potential and kinetic energy can perform mechanical ‘work’ (Wolman and Miller, 1960). ‘Work’ takes various forms:

• Work done against viscous shear and turbulence (internal friction of the flow)
• Work done against friction at the channel boundary
• Work done in eroding the channel boundary
• Work done in transporting the sediment load

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