Territory Stories

Alice Springs Flood Investigation and Floodplian Mapping Study

Details:

Title

Alice Springs Flood Investigation and Floodplian Mapping Study

Creator

WRM Water & Environment Pty Ltd; Northern Territory. Department of Lands, Planning & the Environment

Editor

Roads, Greg

Collection

E-Publications; PublicationNT; E-Books; EXTA1463

Date

2015-10-12

Location

Alice Springs

Description

Made available via the Publications (Legal Deposit) Act 2004 (NT).; WRM Water & Environment (WRM) was commissioned by the Department of Lands & Planning (DLP) to undertake a flood study of the Todd River in the Alice Springs area. The aim of the study is to develop a 2 dimensional flood model, which allows flood flow in a complex flood plain to be modelled, to determine the extent and severity of riverine flooding in the town of Alice Springs and the Alice Springs rural area south of Heavitree Gap. The purpose of the study is to produce floodplain maps of the Alice Springs and its rural area for land use planning and counter – disaster purposes. This study is an extension of the Alice Springs Farms Area Floodplain Mapping Study (WRM, 2006), undertaken by WRM on behalf of the Department of Planning & Infrastructure in 2006 in order to develop detailed Todd River floodplain maps for the rural area south of Alice Springs.

Table of contents

1. INTRODUCTION -- 2. CATCHMENT DESCRIPTION -- 3. AVAILABLE DATA -- 4. CALIBRATION AND AND VERIFICATION EVENTS -- 5. HYDROLOGIC MODEL CALIBRATION AND VERIFICATION -- 6. HYDRAULIC MODEL DEVELOPMENTLOPMENT -- 7. HYDRAULIC MODEL CALIBRATION -- 8. FLOOD FREQUENCY ANALYSIS -- 9. DESIGN EVENT HYDROLOY -- 10. DESIGN FLOOD LEVELS AND EXTENTS -- 11. CONCLUSION -- 12. REFERENCES

Language

English

Subject

Flood control -- Northern Territory -- Alice Springs; Flood damage prevention -- Northern Territory -- Alice Springs; Floodplain management -- Northern Territory -- Alice Springs; Alice Springs Flood Investigation; Floodplain Mapping

Publisher name

WRM Water & Environment Pty Ltd

Place of publication

Darwin (N.T.)

Series

EXTA1463

Format

vii, 95 pages : colour maps ; 30 cm.

File type

application/pdf.

Copyright owner

Check within Publication or with content Publisher.

Parent handle

https://hdl.handle.net/10070/259732

Citation address

https://hdl.handle.net/10070/494349

Page content

0269-02-C1 12 October 2015 49 Both studies also found that the bed level tends to return to the pre-flood level after the flood subsides. It is likely that sediment loads in the reach are generally in equilibrium with upstream sediment loads replenishing lost sediment along the reach. During calibration and verification of the hydraulic model, it was determined that minimal bed mobility is likely to occur during events with peak flood discharges of less than 450m3/s at Anzac Oval. This threshold was determined by comparing hydraulic model predictions with recorded stream gauge data and surveyed debris marks for calibration and verification events, as well as an assessment of predicted maximum velocities and bed shear stress. For flood events with peak discharges at Anzac Oval greater than 450m3/s a sliding scale of bed scour was adopted for the Todd River and Charles River channels, based on the scour depths recommended in GHD (1996) for flood events of different ARIs. Table 6.5 lists the range of bed scour adopted for flood events with peak discharges greater than 450m3/s. Spatial variation in scour depth within the channel was determined based on predicted bed shear stresses for each event. It should be noted that the adopted scour depths are applied to the river bed within the hydraulic model at the beginning of each event, rather than varied throughout the event based on discharge. This methodology allows an accurate calibration to be made against peak flood levels, but results in underestimation of flood levels during the rising and falling limbs of the flood. Table 6.5 Adopted Todd River and Charles River Bed Scour Depths Peak Event Discharge at Anzac Oval (m3/s) Scour Depth (m) BSS <70 N/m2 70 N/m2 < BSS < 80 N/m2 80 N/m2 < BSS < 90 N/m2 90 N/m2 < BSS < 100 N/m2 BSS >100 N/m2 <450 0.0 0.0 0.0 0.0 0.0 450 850 0.2 0.3 0.4 0.5 0.6 850 1250 0.3 0.5 0.8 1.0 1.3 1250 1650 0.4 0.7 1.1 1.4 1.9 >1650 0.5 1.6 3.3 4.5 6.2 BSS Bed Shear Stress 6.2.7 Todd River Infiltration The Todd River flows are a major contributor to groundwater recharge in the Alice Springs Area (Knapton, 2005). Previous investigations by Quinlan and Wooley (1969) cited in Knapton (2005) suggest initial infiltration rates of up to 218m/day through the river bed falling to 0.6m/day after a day or so of flow in the river. Investigations undertaken by Knapton (2005) for the June 2004 flood suggest that Todd River infiltration rates were about 0.6m/day with no high initial infiltration rate. The 2004 flood had a peak flow of only 5.3m3/s at the Anzac Oval gauge and petered out about 9km downstream. It is quite possible that antecedent rainfall and local stormwater runoff from the town may have removed the initial infiltration loss. Braybrook (1968), sited in Knapton (2005), measured Todd River infiltration rates to vary from 55m/day up to 97.5m/day using a dual ring infiltrometer. This infiltration rate is not expected to be representative of infiltration rates during a flood because there would be significant lateral movement of the pumped inflows into the surrounding sand using this test. A study on the expected infiltration rates in proposed treated effluent basins located on the Todd River floodplain undertaken by Knapton (2006) indicates that the infiltration rates in test basins flooded with potable water over a two month period were in the order of 3.5m/day in silty sand, and up to 11m/day in gravel. It is anticipated that infiltration rates during a flood event would be lower than those observed in the test basins due to lateral movement of water.