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Waikato stormwater runoff modelling guideline

TR 2020/06
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Report: TR 2020/06

Author: Earl Shaver (Aqua Terra International Limited)

Hamilton is the fourth largest city in New Zealand and while not equivalent in size to the large metropolitans, the population is forecast to increase by 32 per cent between 2006 and 20311. The population in the Waikato Region grew faster than the national average between 2006 and 2013, with fastest growth experienced in the Waikato District (10.1%), Waipa District (9.8%) and Hamilton City (9.3%).

The region supports over 35,000 km of streams and rivers, many of which are impacted by both rural and urban land use. The level of forecasted population growth in the region makes it imperative to ensure appropriate management of urban stormwater to help to protect our region’s waterways from further degradation and to restore and enhance them.

Contents
Preface i
Acknowledgements iv
1 Background 1
1.1 Basis for the design approach 1
1.2 Hydrological modelling software 2
2 Analytical overview 3
3 Limitations of the model 5
4 Rainfall data 6
4.1 Temporal pattern 6
4.2 Design rainfall depth 7
4.3 Climate change 7
4.4 Areal reduction factors 9
5 Estimating runoff 10
5.1 Retention parameters 10
5.2 Curve numbers 11
5.3 Hydrological soil groups 13
5.3.1 Dual classification 15
5.3.2 Hydrologic soil group assignment 15
5.3.3 Site soil testing to determining hydrologic soil group 17
5.4 Land use cover parameters 19
6 Runoff calculations 20
6.1 Unit hydrograph 20
7 Time of concentration 22
7.1 Sheet and shallow concentrated flow 22
7.2 Concentrated network flow 25
7.2.1 Time of road channel flow 25
7.2.2 Time of pipe network flow 26
7.2.3 Open channel flow 28
7.3 Catchment flow 28
7.4 Alternative equations 28
8 Analysis 29
8.1 Incorporation of initial abstraction retention 30
9 Examples 32
9.1 Case study 1 32
9.2 Case study 2  51
References 68
Appendix A: Runoff curve numbers 70
Appendix B: Worksheets 75