1. Overview of Urban Drainage Flood Prediction Methods and Approaches J.Y. Chen1 and B.J. Adams  Water Survey Division, Environment Canada 2. Department of Civil Engineering, University of Toronto
Good afternoon, our topic of the presentation is “overview of urban drainage flood prediction methods and approaches”, the topic looks quite broad, but we will try to stay focused.

2. Presentation Outline Urban drainage modeling approaches
Analytical model development
Model evaluation and comparison
Conclusions
We start with an introduction to urban drainage modeling approaches, or model classification with emphasis in the development of analytical models.Since some of you may be not quite familiar with the methodologies, we try to paint a broad picture for the analytical model development by sharing our thoughts and experience with you. Model evaluation and comparison is based on a case study of the subcatchment located in Don river watershed. From the results of the case study, we attempt to draw conclusions.

3. Methods for Urban Drainage Flood Prediction
Statistical/stochastic methods
Flood frequency analysis
Regional flood frequency analysis
Time series analysis
Deterministic methods
Conceptual models
Different types of methods or models may be used for urban drainage flood prediction, these methods generally fall into two broad categories, namely, statistical methods and deterministic methods. For the statistical methods, flood frequency analysis may be used at a gauged site to estimate flood quantiles with a specified return period, regional flood frequency analysis can be used for an ungauged site; time series analysis includes auto regression analysis, stepwise regression analysis, etc. An important type of deterministic model is conceptual model, which attempts to simulate major components of rainfall-runoff physical processes

4. Deterministic Conceptual Modeling Methods
Water budget
Model inputs
Model
structure
Runoff routing
Model calibration
Conceptual models normally consist of two major components, namely water budget and runoff routing components, the water budget component establishes water balance among rainfall, evaporation, soil moisture content, and different runoff components, the routing component reflects the damping effect of catchment storage. This type of model is normally characterized by the water budget component. In model applications, significant involvement is needed in model calibration and verification.
Model verification
Prediction

5. Approaches Used for This Study
Design storm approach
Simulation approach
Continuous simulation
Event simulation
Derived probability distribution approach
Design storm, simulation and derived probability distribution approaches are used for this study. Depending on the time scale is used, simulation may be further divided into event and continuous simulation. Nevertheless, different approaches may have its advantages and disadvantages. For example design storm approach cannot consider antecedent soil moisture condition, and continuous simulation is usually considered more accurate, but time consuming.

6. Analytical Model Development
Closed-form analytical models are developed with derived probability distribution theory
Probability distributions of runoff volumes and peak flow rates can be derived from probability distributions of rainfall characteristics
The key element in the development of analytical model is the rainfall-runoff transformation, based on which the probability distributions of the rainfall characteristics are mathematically transformed to create the probability distributions of system outputs. such as, runoff volume and peak flow rates.
We will give you some highlights of these transformations

7. Rainfall-Runoff Transformation
Runoff coefficient based
Extended form

8. Rainfall-Runoff Transformation (Cont’d)
Infiltration based

9. Analytical Model Statistic analysis of rainfall records
Rainfall characteristics, e.g.,
rainfall event volume,
duration & interevent time
Overflow
Storage
facility
Exceedance probability
of a runoff spill volume
Probability distributions
of rainfall characteristics
Rainfall-runoff
transformation
Average annual
volume of spills
Average annual
number of spills
PDF or CDF of runoff
event volume
Expected value of
runoff event volume
Average annual runoff
volume
Average annual runoff
control efficiency

10. Rational Method Peak flow rate Runoff volume Storage Post-development
Pre-development
peak
Tbase=2tc or 2.67tc

11. Simulation Models Event simulation models Continuous simulation model
OTTHYMO (Canada)
HEC-HMS (US)
SWMM (US)
Continuous simulation model
SWMM

12. Case study: Don River Watershed

13. Harding Park stormwater detention facility

14. Model Calibration
Rational method
OTTHYMO

15. Model Calibration (Cont’d)
HEC-HMS
SWMM

16. Calibration of Analytical Models

17. Model Verification

18. Model Verification (Cont’d)

19. Conclusions
Peak flow rates from event simulation models appear to be lower than the results from continuous simulation model
Event simulation models appear to be more conservative than continuous simulation model for runoff volume estimation

20. Conclusions (Cont’d)
The closed-form analytical models developed with derived probability distribution theory, are capable of providing comparable results to continuous simulation results
Different models may vary not only in modeling approach, but also in the level of complexity, it can be challenging to select an appropriate model with a desired level of performance