Flood Risk Assessment: Flood Hydrograph Simulation - Unit Hydrograph Flashcards
What does a unit hydrograph (UH) represent?
The direct runoff response to 1 unit of effective rainfall over a catchment in unit time.
Simulation models estimate flood frequency from ______ frequency.
rainfall
True/False
Flow simulation provides only peak flows.
False (it provides the full hydrograph)
Event-based simulation → __
Continuous simulation → __
- Simulates response to design storm
- Simulates decades of flows for frequency analysis
What is the effective rainfall hyetograph?
Rainfall after accounting for losses (net rainfall)
What does the unit hydrograph method transform?
Effective rainfall into surface runoff hydrograph
In the UK, unit depth = ___ mm and unit duration = ___ h
10 mm, 1 h
Unit Hydrograph (UH): Hydrograph resulting from ____ mm of rainfall over D hours.
10 mm (or unit depth)
Unit Hydrograph Principles
Time-invariance →
Proportionality →
Superposition →
- same response regardless of timing
- response ∝ rainfall
- add responses for multiple rain pulses
What is the discrete convolution equation?
Qₙ = ∑ (from j = 1 to m) of [Pⱼ × Uₙ₋ⱼ₊₁]
What is the matrix inversion solution?
u = (PᵀP)⁻¹ Pᵀ q
For m rainfall pulses and n storm ordinates, UH has __ ordinates.
r = n − m + 1
What three steps are involved in synthesising a storm hydrograph?
1) Estimate net rainfall
2) Convolve with UH
3) Add baseflow
What does UH of 0.5 h mean for applying to 1 h rainfall pulses?
You may need to adjust duration using S-curve method
What matrix method gives least squares solution to convolution?
Matrix inversion
True/False
The unit hydrograph’s depth equals the sum of ordinates.
True
What are the three steps to derive the UH from observed data?
1) Estimate net rainfall
2) Remove baseflow
3) Deconvolute
Rainfall Loss Models
φ-index method →
PR method →
- constant loss
- % runoff constant
True/False
Volume of stormflow = volume of net rainfall under UH assumptions.
True
What is the difference between uniform and proportional loss models?
Uniform subtracts a constant; proportional scales total rainfall
S-Curve Method
Formula
u(ΔT₂, t) = S(t) − S(t − ΔT₂)
To convert a 1h UH to a 0.5h UH, scale ordinates by ___.
ΔT1 / ΔT2 = 1 / 0.5 = 2
What is the Shortcut Method?
u(ΔT₂, t) = [u(t) + u(t + ΔT₁)] / 2
What are the Triangular Unit Hydrographic Equations?
For t ≤ t₁:
- u(t) = t / t₁
For t₁ < t ≤ t₂:
- u(t) = (t₂ − t) / (t₂ − t₁)
Why are triangular UHs used?
They simplify the UH using only two parameters (t1, t2)
True/False
You can directly apply a 0.5 h UH to 1 h rainfall pulses without adjustment.
False
When is the shortcut method valid?
When UH durations are exact multiples (e.g. 2h = 2×1h)
Urbanisation Effects on UH
Urbanisation →
Baseflow separation →
UH shape →
- Increases peak, reduces time to peak
- More required
- Sharper, faster
Strengths of UH:
- Simple
- Low data needs
- Gives full hydrograph
- Can be regionalised
Limitations of UH:
- Assumes linearity & stationarity
- Requires baseflow estimation
- Requires effective rainfall estimation
True/False
Continuous simulation models help overcome UH limitations.
True
