hydrology – turning rainfall in to river flow ross woods niwa christchurch [email protected]
TRANSCRIPT
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!