hydrology – turning rainfall in to river flow ross woods niwa christchurch r.woods@niwa.co.nz

Hydrology – Turning Rainfall in to River Flow

Ross Woods

NIWA Christchurch

r.woods@niwa.co.nz

Context

• Second of 3 river-related talks

• Hydrology Stats (“how much - how often?”)

• From Rainfall to River Flow (“how does it work?”)

• Inundation (“how deep?”)

Outline

1. Introduction: Hydrological Cycle

2. How Floods Happen: Hydrological Processes

3. Not All Floods are the same– Different types of rivers – classification– Storms change over time, and in space

4. Flood Analysis for Design and Forecasting

Queentown, NZ.

(Water in both photos is nearly at the windowsills of the hotel in the centre of the photos.)

29 September 1878

16 November 1999(photo: Otago Daily Times. Dunedin)

Clutha River

November 1999

Insurance Payout: $46

million

If we understand

floods, we can plan better, and forecast better

Manawatu Feb 2004

http://www.ourregion.co.nz

Horizons RC, FRST

1. Introduction: Hydrological Cycle and Catchment

• Concept of a catchment

• Most of the time, things happen slowly

• During a flood, some parts of the cycle are very active

Annual Runoff and Rainmm/year

0 - 400400 - 800800 - 12001200 - 16001600 - 24002400 - 32003200 - 48004800 - 20000No Data

River Runoff

Rainfall

Month of the Year with Most Flow

Data sources: NIWA, RCs, Energy industry

2. Hydrological Processes for Floods

• Can look at this at several scales (kilometres, metres, mm) – we will think at kilometres and metres.

• Kilometre scale – headwaters generate flood runoff, and the lower reaches of the river transport these floods

WaimakaririKilometer-scale Hydrological Processes

Kilometre-scale Hydrological Processes Floodplain fed by Mountain Runoff

Orographic Rainfall Processes

Flood Generation & Routing

Upstream

TimeTime

TimeFlow

(m3/s)

Flow

(m3/s) Downstream

Heavy rain falls for long enough, over a wide enough area, some of it runs off into streams, and travels down the river network

Metre-scale Hydrological Processes for Floods - 1

• Infiltration Excess Runoff – It rains so hard that water can’t get into the soil, and instead produces flood runoff

• Affected by soil type, land cover, urbanisation

• Quantified by soil hydraulic conductivity

Hillside Stream

Cross-section

Metre-scale Hydrological Processes for Floods - 2

• Saturation Excess Runoff – there’s so much rain that there’s no room in the soil to store it, so it produces runoff

• Affected by time of year, vegetation, soil thickness, position on slope

• Quantified by soil water deficit, and locally by soil-topographic indexHillside Stream

Cross-section

Metre-scale Hydrological Processes for Floods - 3

• Subsurface stormflow – water moves through the soil so quickly that it can make floods.

• Affected by steepness, vegetation, soil depth

StreamHillside

Cross-section

3. Not All Floods are the same – River Types

• The two most important factors controlling flood size are the climate and the area of catchment (km2)

• Catchment area – Small vs Large, from a few hectares up to more than 10,000 km2

• Climate - Dry vs wet, ranging from 400 mm/y in Central Otago to more than 10,000 mm/y in Alps

• These two factors are important in many flood estimation methods.

• Other factors can come into play, depending on location: geology, urban drainage, vegetation, topography, river network, seasonality

Mountain – Large, WetAt kilometre scale, orographic rainfall processes are very important

At metre scale, subsurface stormflow is the key runoff generation process here

Lowland – Smaller, DrierMore likely to be affected by convective rain

At metre scale, saturation excess runoff is the key runoff generation process here

3. Not All Floods are the same – Time variation

• Within-storm variability

• Between-storm variability – (seasonal differences, also mention storm depth )

0

5

10

15

20

25

Ra

in m

m/1

5m

ins

30-Apr-1999 1-May 2-May0

1280

2560

3840

5120

6528

Rainfall-Runoff Response

Flow is more damped than rain because of catchment averaging Delay between rain peak and flow peak because of travel distances in catchment

Rain (mm/ 15 mins)

Flow (l/s)

3. Not All Floods are the same – Space variation

• Within-catchment variability of rain, slope, vegetation, location in river network

Rain Varies From Place to PlaceFloods are very senstitive to this

Weather radar image, 200km across Weather radar image,

15 km across

Waipaoa Catchment Soil Properties

Soils that hold less water make bigger floods

Waipaoa Catchment Land Cover

Floods tend to be smaller in forested areas, other things being equal

Waipaoa Catchment Topography and River Network

Steeper land can hold less water in the soil

River networks control timing of floods from the different sub-catchments

4. Typical Flood Analyses

• Design Flood Estimation – what is the flood that will typically only occur once in 50 (100) years? Used for planning, especially flood protection measures. Two commonly-used methods

1. Rainfall-runoff models (e.g. Rational Method)2. Regional Flood Estimation (Charles covered this)

• Flood Forecasting – how big will tomorrow’s flood be? Used for forecasting, and also as a mitigation measure where protection is tricky

1. Rainfall-runoff models (same as above)2. Flood routing models

Rainfall-runoff Models – 1

• Rational method – just estimates the peak• Often used for small-catchment design• Q = c i A / 3.6• Q – flood peak (m3/s)• c – runoff coefficient (dimensionless – local

knowledge!)• i – “1 in T year” rainfall intensity for appropriate

duration (mm/h - HIRDS), minus “losses”• A – area of catchment (km2)

Example: Wanaka

• Need design flood of various return periods for a small stream in subdivision (area 2 sq km)

• Get duration from textbook, design rainfalls from HIRDS, & c from MWD culvert design manual

Return Period I21min (mm/hr) cQ = c I A / 3.6

(m3/s)2 19.9 0.45 5.05 24.0 0.45 6.010 30.7 0.55 9.420 37.7 0.55 11.550 51.9 0.65 18.7

100 68.1 0.65 24.6150 81.1 0.70 31.6

OtagoRC

Rainfall-runoff Models - 2

• Distributed water balance model - Topnet – estimates the hydrograph at many places – more detail coming up …

• Requires GIS data and rainfall information at many places

• Useful for large rivers, with variety of subcatchments especially those where flow measurments are sparse

• Good where rain is spatially variable, catchments are a complex spatial mixture, or physical interpretation of processes is important - e.g. climate change

TOPNET - 1

Sat. zone Subsurfaceflow

Recharge

Throughfall Surfaceflow Sub-basin

outflow River Network

Other sub-basins: each one is

unique

Root zone

Canopy

Topo. controls

Rain, Temperature, Evap

Snowpack

Snowmelt

TOPNET - 2

• Sub-basin outflows are connected to river network routing (kinematic waves)

• Model gives results for every sub-basin and every river reach, every day

• Preprocessing software builds the model– uses DEM to automatically define river network and

sub-basins at user’s chosen level of detail– calculates average values within each subcatchment

for soil, vegetation & climate parameters

NorthlandRunoff

Hydrographs & Maps

Model uses hourly/daily rainfall: e.g. NZLAM, RAMS, telemetered raingauge, radar, CLIDB

TopNet: River Basin Flow Forecasts – EcoConnect Component

• EcoConnect is Environmental Forecasting: weather, tide, storm surge, currents, sea surface height, river flow, inundation

• Rain forecast from weather model

• Specific rivers with Env BoP, Gisborne DC, Marlborough DC, Otago RC

Flood Forecasting: TopNet

Waipaoa – Mon 2am

Waipaoa – Mon 8am

Waipaoa – Mon 2pm

Waipaoa – Tue 2am

Waipaoa – Tue 8am

Waipaoa – Tue 2pm

Flood Routing Models

• Translation Routing – uses measurements of flow at upstream site(s), and estimate of travel times. The simplest – there are more complex options (kinematic waves, etc)

• Q(t) = a.U(t-da) + b.V(t-db)• Useful for large rivers which are not very flat,

provided upstream flow measurments are adequate• Many alternatives, if suitable data are available –

kinematic waves, St Venant equations

Translation Routing

Translation Routing on the Mataura River

November 1999

0

500

1,000

1,500

2,000

2,500

15/11/199900:00

15/11/199912:00

16/11/199900:00

16/11/199912:00

17/11/199900:00

17/11/199912:00

18/11/199900:00

18/11/199912:00

19/11/199900:00

Time

Flo

w [

m3 /s

]

Forecast for Gore

Measured at Gore

77564 Mataura at Cattle Flat

77561 Waikaia at Mahers Beach Rd

77528 Waikaka at Willowbank

77525 Waimea at Mandeville Bridge

16 hours lead time

Provides 16 hours reliable warning because • Most floods begin in the headwaters & are measured • Travel times from headwaters are long

Env. Southland

Models, models, models

• There are lots (too many!) hydrological models. Hydrologists are gradually reducing this as the science matures

• Model selection depends on river type, and on data availability, and on your goal!

Important Issues that Affect Floods• Climate Change may cause increased flood

magnitude over next 30-100 years, and may change the balance of rain vs snow (more rain, less snow)

• Climate Variability does cause changes in flood risk between decades (risk goes up and down, differently in different parts of NZ)

• Urbanisation of a catchment does increase flood risk, because it prevents rain from infiltrating, and it transports water very quickly

• Removal of forests from large portion of a catchment may increase flood risk, because forests can keep the soils drier, and so provide more space in the soil to hold rain during storms.

Buller - Climate Change & Floods

Now

2080

Now

2080vsNow

Storm Rainfall Increases Flood Peak Almost Doubled

MfE, BullerDC

Issues I Haven’t Covered

• Urban Drainage Modelling

• Reservoir modelling (floods and dams)

• Effect of Flood Mitigation – stopbanks etc

• Modelling of soil water, landslides

• Effects of floods on contaminants (sediments, heavy metals)

May all your rivers be well-behaved!