Mountain Precipitation Analysis for the Estimation of Flood Runoff in Coastal British Columbia
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MOUNTAIN PRECIPITATION ANALYSIS FOR THE ESTIMATION OF FLOOD RUNOFF IN COASTAL BRITISH COLUMBIA By ATHANASIOS LOUKAS DipL Eng., Aristotle University of Thessaloniki, 1988 M.A.Sc., University of British Columbia, 1991 A THESIS SUBMIYED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF CIVIL ENGINEERING We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA August 1994 © ATHANASIOS LOUKAS, 1994 In presenting this thesis in partial fulfillment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without written permission. my (Si Department of The University of British Columbia Vancouver, Canada Date - —‘-I5’-I ABSTRACT A study of the precipitation distribution in coastal British Columbia is described and a technique is proposed for the reliable estimation of the frequency of rainfall generated floods from ungauged watersheds in the region. A multi-disciplinary investigation was undertaken encompassing the areas of hydrometeorology, meteorological modelling and hydrological modelling. Study components included analysis of long- and short-term precipitation in two medium sized watersheds located in southwestern coastal British Columbia; development of a 24-hour design storm for coastal British Columbia; generalization of the results over the coastal region of British Columbia; examination of the precipitation distribution during flood producing storms; identification of the applicability of a meteorological model for the estimation of short-term precipitation; and development of a physically-based stochastic- deterministic procedure for the estimation of flood runoff from ungauged watersheds of the region. Based on an assessment of the atmospheric processes which affect climate, it was found that the strong frontal storms which form over the North Pacific Ocean and travel eastward generate the majority of the precipitation during the winter and fall months, whereas convective rainshowers and weak frontal storms produce the dry summer period precipitation. Examination of the annual, seasonal, and monthly precipitation in the two study watersheds, the Seymour River and the Capilano River watersheds, showed that the variation of annual and winter and fall precipitation with elevation follows a curvilinear pattern, increasing up to middle position of the watersheds at an elevation of about 400 m and then decreasing or leveffing off at the upper elevations. The summer precipitation is more uniformly distributed over the watersheds than the winter precipitation and accounts for about 11 25% of the total annual precipitation. The Bergeron two-cloud mechanism has been identified as the dominant rainfall producing mechanism during the winter and fall months. Analysis of regional data and results of other regional studies indicate that the curvilinear pattern found in this study is more general and is similar for the whole of coastal British Columbia and the coastal Pacific Northwest. Study of the 175 storms in the Seymour River watershed showed that the individual storm precipitation is distributed in a pattern similar to that of the annual precipitation and this distribution pattern is not affected by the type of the event. Furthennore, the analysis showed that the storm time distribution is not affected by the elevation, type of the storm, its duration, and its depth. Also, analysis of the data from three sparsely located stations of coastal British Columbia indicated that the time distribution of the storms does not change significantly over the region. With regard to the development of techniques for the better estimation of flood runoff, a 24-hour design storm has been developed by using the data from the Seymour River watershed. Analysis of its spatial distribution revealed that this 24-hour design storm is distributed in a similar patter to that of the annual precipitation. Also, it was found that the 24-hour extreme raiiifall of various return periods is a certain percentage of the mean annual precipitation. Comparison with regional data and results of other regional studies showed that the developed design storm can be transposed over the whole coastal region of British Columbia. A comparative study and rainfall-runoff simulation for a real watershed showed that from the widely used synthetic hyetographs, only the Soil Conservation Service Type IA storm or the 10% time probability distribution curve of this study can accurately generate the flood runoff from watersheds of the region. The above results of the short-term precipitation distribution with elevation and in time were tested for extreme storms. Five periods of historical large flood producing storms were 111 analyzed and it was shown that the fmdiiigs of the short-term precipitation analyses are valid for these extreme storms. The BOUNDP meteorological model was used for the estimation of storm precipitation in the mountainous area which covers the two study watersheds, but the results showed that this particular model is not capable of simulating the precipitation observed in the area. As a result, the initial intention of coupling the model with a hydrological model for the estimation of the runoff was abandoned. The above results of the analysis of precipitation in coastal British Columbia and the findings of previous research on the watershed response of coastal mountainous watersheds have been combined and used for the development of a physically-based stochastic- deterministic procedure. The procedure uses the method of derived distributions and Monte Carlo simulation to estimate the flood frequency for ungauged watersheds of the region. The procedure has been tested with data from eight coastal British Columbia watersheds and compared with the results of other widely used regional techniques. This comparison showed that the method is reliable and efficient, and requires very limited data, which can be found from a topographical map and the Rainfall Frequency Atlas for Canada. iv TABLE OF CONTENTS ABSTRACT ii LIST OF TABLES xi LIST OF FIGURES xiii ACKNOWLEDGMENT xxii 1. INTRODUCTION 1 2. STUDY AREA AND DATA SETS 6 2.1 Regional Climate 6 2.2 The Study Watersheds 10 2.2.1 Topography 10 2.2.2 Interaction of weather systems with the local topography 12 2.3 Data Sets 13 3. ANNUAL AND SEASONAL PRECIPITATION DISTRIBUTION 24 3.1 Introduction 24 3.2 Spatial Distribution of Precipitation 25 3.2.1 Annual and seasonal precipitation distribution in the Seymour River valley 25 3.2.2 Annual and seasonal precipitation distribution in the Capilano River valley 27 3.2.3 Monthly precipitation distribution in the two study watershed valleys 29 v 3.2.4 Comparison of mountain and valley precipitation 32 3.3 Temporal Variation of Precipitation 35 3.3.1 Seymour river watershed 35 3.3.2 Capilano river watershed 36 3.4 Spatial Variation of Precipitation 36 3.4.1 Seymour river watershed 37 3.4.2 Capilano river watershed 39 3.5 Comparison with Other Studies and Regional Data 40 3.6 Meteorological Mechanisms Affecting the Precipitation Distribution 45 3.7 Summary 47 4. STORM PRECIPITATION DISTRIBUTION 63 4.1 Introduction 63 4.2 Data Sets 64 4.3 Spatial Distribution of Storms 66 4.3.1 Storm precipitation 66 4.3.1.1 Spatial variation 67 4.3.2 Duration and average storm intensity 69 4.3.3 Maximum hourly intensity 70 4.3.4 Relative start time 70 4.4 Time Distribution of Storms 71 4.4.1 Research Procedure 72 4.4.2 Results 74 4.5 Antecedent Precipitation 78 4.6 Summary 80 vi 5. 24-HOUR DESIGN STORM FOR COASTAL BRITISH COLUMBIA 98 5.1 Introduction 98 5.2 Data Sets and Method of Analysis 100 5.3 Time Distribution 102 5.4 Spatial Distribution 106 5.5 Antecedent Rainfall 111 5.6 Simulation of Peak Storm Flow at Jamieson Creek Watershed 113 5.7 Summary 117 6. STUDY OF ifiSTORICAL LARGE STORMS 133 6.1 Introduction 133 6.2 The July 11-12, 1972 Rainstorm 134 6.2.1 Synoptic conditions 134 6.2.2 Spatial distribution 135 6.2.3 Time distribution 136 6.3 The December 13-18, 1979 Rainstorms 137 6.3.1 Synoptic conditions 137 6.3.2 The December 13-14, 1979 storm 138 6.3.2.1 Spatial distribution 138 6.3.2.2 Time distribution 140 6.3.3 The December 16-18, 1979 storm 140 6.3.3.1 Spatial distribution 140 6.3.3.2 Time distribution 141 6.4 The October 25-31, 1981 Rainstorms 142 6.4.1 Synoptic conditions 142 vii 25-28, 1981 sto. 143 6.4.2 The October 6.4.2.1 Spatial distribution 143 6.4.2.2 Time distribution 144 6.4.3 The October 28-3 1, 1981 storm 145 6.4.3.1 Spatial distribution 145 6.4.3.2 Time distribution 146 6.5 The November 8-11, 1990 Rainstorm 146 6.5.1 Synoptic conditions 146 6.5.2 Spatial distribution 147 6.5.3 Time distribution 149 6.6 The November 21-24, 1990 Rainstorm 149 6.6.1 Synoptic conditions 149 6.6.2 Spatial distribution 149 6.6.3 Time distribution 151 6.7 Summary 151 7. APPLICATION 01? A METEOROLOGICAL MODEL 160 7.1 Introduction 160 7.2 General Description of the BOUNDP Model 163 7.2.1 Overview 163 7.2.1.1 The wind model 164 7.2.1.2 The water flux model 166 7.2.1.3 Estimation of precipitation 168 7.3 Data Sets 169 7.4 Application 171 viii 7.4.1 Comp1icaons .