scholarly journals Review: Continuous simulation modelling for design flood estimation – a South African perspective and recommendations

Water SA ◽  
2018 ◽  
Vol 44 (4 October) ◽  
Author(s):  
TJ Rowe ◽  
JC Smithers
Keyword(s):  
South Africa ◽  
Rainfall Variability ◽  
Simulation Modelling ◽  
Economic Losses ◽  
Design Flood ◽  
National Scale ◽  
Extreme Rainfall Events ◽  
Continuous Simulation ◽  
Flood Estimation ◽  
Further Development

A number of severe flooding events have occurred both in South Africa and internationally in recent years. Furthermore, changes in both the intensity and frequency of extreme rainfall events have been documented, both locally and internationally, associated with climate change. The recent loss of life, destruction of infrastructure, and associated economic losses caused by flooding, compounded by the probability of increased rainfall variability in the future, highlight that design flood estimation (DFE) techniques within South Africa are outdated and in need of revision. A National Flood Studies Programme (NFSP) has recently been initiated to overhaul DFE procedures in South Africa. One of the recommendations in the NFSP is the further development of a continuous simulation modelling (CSM) system for DFE in South Africa. The focus of this paper is a review of CSM techniques for DFE, to guide further development for application in South Africa. An introduction to DFE, and particularly the CSM approach, is presented, followed by a brief overview of DFE techniques used in South Africa, leading into a more detailed summary of CSM for DFE within South Africa to date. This is followed by a review of the development and application of CSM methods for DFE internationally, with a focus on the United Kingdom and Australia, where methods have been developed with the intention of national scale implementation. It is important to highlight that there is a plethora of CSM methods available internationally and this review is not exhaustive; it focuses on and identifies some of the strengths and weaknesses of several popular methods, particularly those intended for national scale application, as the intended outcome from this review is to identify a path towards the development of a usable national scale CSM system for DFE in South Africa. Emphasis on a usable method is important, considering the reality that, despite promising results, numerous benefits, and national scale methods being developed, it appears that the CSM method for DFE is rarely used in practice.

scholarly journals An assessment of scale issues related to the configuration of the ACRU model for design flood estimation

2011 ◽  
Vol 42 (5) ◽  
pp. 401-412
Author(s):  
K. Chetty ◽  
J. C. Smithers
Keyword(s):  
Daily Rainfall ◽  
Simulation Modelling ◽  
Research Unit ◽  
Physical Parameters ◽  
Design Flood ◽  
Critical Question ◽  
Continuous Simulation ◽  
Agricultural Catchments ◽  
Flood Estimation ◽  
Modelling Process

A continuous simulation modelling approach to design flood estimation has many advantages and overcomes many limitations of commonly used design event approaches. A major concern with continuous simulation using a hydrological model is the scale at which modelling should take place. According to researchers, the level of representation that will preserve the physical chain of hydrological processes, both in terms of scale of representation and level of description of the physical parameters for the modelling process, is a critical question which must be addressed. Objectives of this research were to determine the optimum levels of catchment discretization and soil and land cover information and to assess the optimum use of daily rainfall stations for the configuration of the Agricultural Catchments Research Unit (ACRU) agrohydrological model when used for design flood estimation. Results obtained for selected quaternary catchments in the Thukela catchment and Lions River catchment indicated that modelling at the level of hydrological response units (HRUs), using area-weighted soils information and more than one driver rainfall station where possible, produced the most realistic streamflow volume results when compared with observed streamflows. Design flood estimates from simulated peak flows did not compare well with observed data.

scholarly journals On the possibilities of watershed parameterization for extreme flow estimation in ungauged basins

2015 ◽  
Vol 370 ◽  
pp. 171-175 ◽  
Author(s):  
S. Kohnová ◽  
B. Karabová ◽  
K. Hlavčová
Keyword(s):  
Extreme Rainfall ◽  
Curve Number ◽  
The United States ◽  
Water Levels ◽  
Design Flood ◽  
Extreme Rainfall Events ◽  
Local Conditions ◽  
Ungauged Sites ◽  
Design Values ◽  
Flood Estimation

Abstract. The estimation of design discharges and water levels of extreme floods is one of the most important parts of the design process for a large number of engineering projects and studies. Design flood estimates require a consideration of the hydrological, meteorological and physiographical situation, the legal requirements, and the available estimation techniques and methods. In the last decades changes in floods have been observed (Hall et al., 2014) which makes design flood estimation particularly challenging. Methods of design flood estimation can be applied either locally or regionally. A significant problem may arise in small catchments that are poorly gauged or when no recorded data exist. To obtain the design values in such cases, many countries have adopted procedures that fit the local conditions and requirements. One of these methods is the Soil Conservation Service – Curve number (SCS-CN) method which is often used in design flood estimation for ungauged sites, including those in Slovakia. Since the method was derived on the basis of the specific characteristics of selected river basins in the United States, it may lead to significant uncertainties in other countries with different hydrological conditions. The aim of this study was to test the SCN-CN method and derive regional runoff curve numbers based on rainfall and discharge measurements for selected region in Slovakia. The results show that the classical CN method gives too high estimates of event runoff depths and is not valid in the study area. To avoid the overestimation of runoff caused by extreme rainfall events, the use of the empirically derived regional runoff curves was tested and finally proposed for practical application in engineering hydrology.

scholarly journals Methods for design flood estimation in South Africa

Water SA ◽  
2012 ◽  
Vol 38 (4) ◽  
Author(s):  
JC Smithers
Keyword(s):  
South Africa ◽  
Design Flood ◽  
Flood Estimation

scholarly journals Development and assessment of rules to parameterise the ACRU model for design flood estimation

Water SA ◽  
2018 ◽  
Vol 44 (1 January) ◽  
Author(s):  
JC Smithers ◽  
TJ Rowe ◽  
MJC Horan ◽  
RE Schulze
Keyword(s):  
South Africa ◽  
Land Cover ◽  
Land Management ◽  
Management Practices ◽  
Peak Discharge ◽  
Future Research ◽  
Design Flood ◽  
Time Step ◽  
Hydrological Condition ◽  
Flood Estimation

Design flood estimation (DFE) is essential in the planning and design of hydraulic structures. In South Africa, outdated methods are widely applied for DFE. In this paper the potential of a continuous simulation modelling (CSM) approach to DFE in South Africa, using the daily time-step ACRU agrohydrological model, is investigated. The paper focuses on the links and similarities between the SCS-SA and ACRU models and the subsequent preliminary investigations that were undertaken to account for and incorporate the land cover classes, including land management practices and hydrological condition, of the SCS-SA model into the ACRU CSM approach. The approach to this study was to investigate how design volumes simulated by the SCS-SA model for various land management practices or conditions could be simulated by the ACRU model. Since peak discharge estimation in both models is directly dependent on simulated volumes, this preliminary study focused only on design runoff volumes, with subsequent investigations on peak discharge required in future research. In the absence of observed data, design runoff volumes and changes in design runoff volumes, as simulated by the SCS-SA model, were used as a substitute for observed data, i.e., as a reference, to achieve similar design runoff volumes and changes in design volumes in the ACRU model. This was achieved by adjusting relevant input parameters in the ACRU model to represent the change in management practice or hydrological condition, as represented in the SCS-SA model. Following a sensitivity analysis of relevant ACRU parameters, calibration of 2 selected parameters against SCS-SA CN values for selected land cover classes was performed. A strong linear relationship (R2 = 0.94) between these ACRU parameters and SCS-SA CNs for selected land cover classes was found and consequently specific rules and equations were developed to represent SCS-SA land cover classes in ACRU. Recommendations are made to further validate and verify the approach and to further the development of a CSM system for DFE in South Africa.

scholarly journals An Investigation of Modelling Accuracy Needs for Urban Design Flood Estimation

2020 ◽  
Author(s):  
James E. Ball
Keyword(s):  
Peak Flow ◽  
Continuous Model ◽  
Flood Management ◽  
Urban Environments ◽  
Flood Frequency ◽  
Flood Frequency Analysis ◽  
Design Flood ◽  
Continuous Simulation ◽  
Flood Estimation ◽  
Quantile Estimates

Flood Management remains a major problem in many urban environments. Commonly, catchment models are used to generate the data needed for estimation of flood risk; event-based and continuous-based models have been used for this purpose. Use of catchment models requires calibration and validation with a calibration metric used to assess the predicted catchment response against the recorded catchment response. In this study, a continuous model based on SWMM using the Powells Creek catchment as a case study is investigated. Calibration of the model was obtained using 25 selected events from the monitored data for the catchment. Assessment of the calibration used a normalised peak flow error. Using alternative sets of parameter values to obtain estimates of the peak flow for each of the selected events and different accuracy criteria, the best datasets for each of the accuracy criteria were identified. These datasets were used with SWMM in a continuous simulation mode to predict flow sequences for extraction of Annual Maxima Series for an At-Site Flood Frequency Analysis. From analysis of these At-Site Flood Frequency Analyses, it was concluded that the normalised peak flow error needed to be less than 10% if reliable design flood quantile estimates were to be obtained.

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