CVEN9612 –Assignment 1 Rainfall-Runoff Modeling and Routing

Assignment Task

Question 1 – Runoff routing 

There was a large flood on the Richmond and Wilsons Rivers in February 2024, which led to extensive damage in Lismore and surrounding areas. The flood hydrograph on the Richmond River was measured at Wiangaree (EL. 91m AHD) and we would like to be able to model the hydrograph at the town of Kyogle, 21km further downstream (EL 71m AHD). The observed streamflow hydrographs at Wiangaree and Kyogle are available on Moodle.

Using Manning’s Equation calculate a representative wave velocity for flow down the channel assuming that the hydraulic radius of the channel is R=5.2 and it has a Manning roughness n of 0.05. Assume the wave celerity is 1.7 times the Mannings velocity. Employ the Muskingum method to determine the best estimate of the flood hydrograph at Kyogle, assuming that an approximate value of K can be calculated by dividing the reach length by the wave speed. Calibrate your model to estimate the best value for x by minimizing the sum of squared error of the prediction.

Provide a short report (less than 2 pages), with appropriate figures to answer the following questions. Show your working where appropriate and justify any assumptions that are required.

a) What time step did you use for your calculations (in hours) and how do you know if it is appropriate? 

b) What is the peak of the calculated flow hydrograph at Kyogle?

(c) What value of x did you use and how did you know this was the optimum value? Demonstrate with the use of an appropriate figure. 

d) What is the RMSE of the calculated outflow hydrograph compared to the observed data at Kyogle?

e) What is the impact of changing the value of x to 0? What is the practical meaning of x = 0? 

f) If there was interest in investigating Nature Based Solutions in the Richmond catchment and riparian zone was revegetated and as a result Mannings n was 0.1, what is the new peak flow at Kyogle? Explain how revegetation affects the routing of flows. 

Question 2 – Model Calibration 

Model aim In this part of the assignment, you need to model the runoff and Lake Werri Berri in the Thirlmere Lakes National Park wetland system using GR4J. The model is to be set up to understand

1. how often the lake is suitable for water sports and

2. the risk of bushfires affecting the sensitive peat ecosystem of the lake.

Water sports can only occur when the lake is more than 2 m deep and bushfire risk is too high if water levels in lakes are less than 0.1 m for more than 30 consecutive days. The management plan for the lakes requires decisions to be supported with 50 years of data hence why a model is required as flow data is only available since the last decade. The management plan requires the frequency of these two events (i.e. water sports and bushfire risk) to be documented.

The aim of the modelling exercise is twofold:

(i) to understand the implications of choices in modelling such as objective function and data transformation on lake water level predictions and what the most appropriate choices are given the aims of the model.

(ii) To assess an alternative GR4J model for the same catchment which uses not observed streamflow but satellite retrieved surrogates as has been described in your week 3 lecture using the SRM methodology of Yoon et al., 2022.

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