VINCENT COOPER Flood Hazard Mapping Consultant

Transcription

1 FLOOD HAZARD MAPPING OF GRENADA FINAL REPORT (NON-TECHNICAL) By VINCENT COOPER Flood Hazard Mapping Consultant For the Caribbean Development Bank January, 2006

2 TABLE OF CONTENTS INTRODUCTION 1 METHODOLOGY FOR MAP PREPARATION 1 Structure and Content of Flood Hazard Maps 1 Island-wide flood hazard map 1 Detailed flood hazard map 2 Method for Preparing the Island-wide flood hazard map 3 Method for Preparing the Island-wide flood hazard map 3 HAZARD MAPPING RESULTS Island-wide flood hazard map 4 Detailed flood hazard map 5 MAP USE AND LIMITATIONS 5 Island-wide flood hazard map 6 Detailed flood hazard map 7 DETAILED PROCESS FOR MAP UPDATING 7 RECOMMENDATIONS FOR FUTURE WORK. 9 REFERENCES 10 i

3 1. Introduction Flooding is the major natural hazard affecting the entire Caribbean region, it causes significant interruption to economic progress and inflicts recognizable social and psychological scars mostly on the economically disadvantaged peoples of the region. Despite such prevalence there is only now concerted efforts at mapping the extent and frequency, but these attempts have almost all been frustrated by the decrepit state of the databases required to adequately and effectively produce flood hazard maps. Maps can be produced with the existing databases, but their limitations must be declared and more importantly, such undertakings should detail the data required to improve the reliability of these maps. This report is submitted as required in the Contract for Development of Flood Hazard Maps for Grenada and St. Lucia. According to the Terms of Reference (TOR) at Section 3.04 (g), a technical report of the hazard assessment process is to be prepared documenting the following: (i) the hazard assessment methodology, to include descriptions of the structure and content of the hazard maps; methodology employed in map preparation (including data collected, analysis method, list of digital data layers used and final map preparation); and limitations of the methodology; (ii) hazard mapping results in terms of the distribution of the hazard zones; (iii) map use and limitations, including explanations of how the maps may be used for development planning purposes; (iv) detailed process for map updating; (v) metadata and information sources; and (vi) recommendations for future work. 2. METHODOLOGY FOR MAP PREPARATION Structure and Content of Flood Hazard Maps Island-wide Flood Hazard Map There are two sections in the island-wide flood hazard map for Grenada, one showing the location of the hazard zones, the other section providing information about and map and 1

4 its use. The map defines three hazard zones, high, medium, low, on the basis of the danger posed to an average human being as a result of floodwater depths. The map, which is at a scale of 1:25000, shows the important settlements, the road network and the catchments contributing runoff to the hazard zones. The other section provides explanations of the procedure for producing the map, the required assumptions and approximations and the limitations and use to which the map could be placed. Detailed Flood Hazard Map The detailed flood hazard map for the St. John River Floodplain has three sections with three major pieces of information. The major section, which is at a 1:2500 scale, describes the spatial extent of floodwaters at various rainfall return periods that are distinguished by colour. It shows important features on the floodplain and thus informs on the likelihood of flooding of these features. There is not significant variation in the spatial extent of the flood owing to the steep sides on the fringes of the floodplain, but the floods differ in their depth. This cannot be well represented on such a map. The inset indicates the location of the St. John River Catchment on the island. The maps on the right indicate the hazard posed by floods of the 2 year, 5 year, 10 year and 25 year return periods. The hazard takes into account not only that due to high water, but it also includes the effect of water velocity (see Methodology for preparing flood hazard maps below). Each depiction is at a scale of 1:8500, the largest scale possible for the chosen map size. The third major piece of information shown in the bottom right of the map, provides a measure of how well the output of the chosen analytical procedures simulate observed flood levels. This reports on an actual, recent flood in which the experiences from those caught in the flood will still be fresh. It is important information as it would validate the approach used in generation of the flood hazard map shown in the major section. If such real occurrences are well represented by the procedures, then users will more readily accept the predictions for the extreme flood events and may be prepared to act according to the information contained therein. This section is also at a scale of 1:8500. The map contains notes that help explain the analytical procedures for the map production, the required assumptions and approximations and the use and limitations of 2

5 the information contained therein. The legend contains a graph that depicts the categorization of the flood hazard according to velocity and depth. Methodology for Preparing the Island-wide Flood Hazard Map The objective of this exercise is the production of a medium-scale flood hazard map over the entire main island of Grenada. A model was developed for classification of hazard zones over the island into high, medium and low. Zones were based on the likelihood of occurrence of flood depths above threshold values, and they were as follows: High refers to depths in excess of 1370 mm (4.5 ft); medium refers to depths between 1370 mm and 910 mm (4.5 ft and 3.0 ft); and low refers to depths less than 910 mm. Estimation of the depths of flooding to be expected over naturally flood prone areas (those considered to have very flat slopes) was based on assessing the extent to which flood-causing features was present. Flood-causing features are: the land cover; soil type; daily rainfall depths; and size of catchments draining into the flat lands. Extensive use was made of GIS to identify such features. Methodology for Preparing the Detailed Flood Hazard Map Production of detailed flood hazard maps requires completion of two types of studies, namely, hydrologic studies and hydraulic studies. The former, which requires detailed rainfall data, estimates runoff from the catchment area upstream of the St. John River floodplain (see Figure 1). Runoff, which depends on the catchment s land cover, soil type, topography and rainfall amounts, may or may not cause flooding on the floodplain. Using the estimates of runoff, subsequent hydraulic studies, which are based on detailed features of the floodplain and the river passing through it, produce values of flood levels and the extent of the flooded area. The studies are performed for different rainfall amounts, and each time values for the corresponding flood level and the flood area on the floodplain are made. Subsequently, maps are produced with the aid of GIS that show how flooding varies with rainfall amounts. 3

6 For the St. John River there are particular problems with the rainfall data that have made the accuracy of the estimates. The problems can be easily solved by the acquisition of automatic rainfall gauges to replace the manual ones currently in use. N W E S Meters Figure 1. The St. John River Catchment 3. HAZARD MAPPING RESULTS Island-wide Flood Hazard Map As would be expected for steep terrain such as is the island of Grenada, few areas are prone to flooding and so most of the island falls in the low hazard category. But it should be remembered that this category does not cover flash flooding that might be experienced in several places over the island. Several areas are designated as high because they have very small mean slopes and are downstream of areas having potential for high runoff amounts. 4

7 Detailed Flood Hazard Map The map suggests that flooding occurs along the reach of the St. John River on the floodplain. Most vulnerable appears to be the area in the vicinity of the two sharp bends, and around the stadium by the Humpback Bridge, both of which flood even for the frequent 2-year return period rainfall. The reason in both cases is due to inadequacies in the drainage channel. The two sharp bends always will pose a problem, but this unfavourable alignment is compounded by the poor maintenance of the river. The flooding is further aggravated by the presence of the sandbar at the river mouth and siltation from around the Humpback Bridge. The danger posed by flooding from the 2-year rainfall event generally is low to medium except in one area around the Humpback Bridge where it is high. For the 5-year rainfall event, the danger increases with a substantial portion of the inundated area designated as high. For the higher rainfall events, the 10-year and the 25-year rainfall events, persons should avoid almost all of the inundated area as there is strong threat to life. The removal of vegetation is likely to make some impact, but if possible, consideration should be given to river training works, including lining with concrete and realigning at the bends, of course if such an option is favoured by the necessary environmental and social impact assessments. Flood mitigation measures must include debris traps, especially in the upper catchment, to minimize the occasions of blockages at the bridges by logs, as this would further exacerbate flooding. 4. Map Use and Limitations Flood hazard maps are important devices for the economic and social development of Caribbean countries. They can aid in optimal use of land resources by minimizing the risk of losses as damage to personal property, crops, injury or loss of human life, by averting, in some instances, the cost for infrastructure, by reduction of the amount and the frequency of grant payments as compensation to victims, by the declaration of the expected risk and so alerting builders of the requirements for measures to circumvent flood damage. The main objective of this project was to produce detailed and island- 5

8 wide flood hazard maps, but an equally important objective was to assess the level of confidence that could be placed on such maps and, if required, the steps needed for their improvement. Island-wide Flood Hazard Map Island-wide maps by nature do not provide detailed information of flood water levels or velocities and cannot provide locations of specific hazardous places. Rather they identify the likely regions to be flooded for extreme rainfall and they give a mean depth of floodwaters within each region. They are based on general land characteristics and rainfall patterns over the island but at this scale, they cannot incorporate detailed hydraulic properties of drainage facilities in the regions. The production of the island-wide flood hazard map of Grenada requires adequate information on rainfall patterns over the island. Unfortunately, there was insufficient information for describing the spatial patterns of rainfall over the entire island, only a few stations being available around the St. John River Catchment on the southwestern side of the island. But for rainfall at these locations, the lengths of the records were too short for making reliable estimates for extreme rainfall events. Even if the land cover, soils and topography precondition a region for flooding, its realization depends on the occurrence of high extreme rainfall values. Without sufficient rainfall information, therefore, the usefulness of the map will be limited to mainly an identification of flood prone areas across the island. Such is the case with the Grenada island-wide map. As a result of these limitations this map is surely preliminary but it can be used as outlined below: i. The map provides information on areas of focus during large rainfall events. The map also shows roadway sections likely to be under water during these events; this is not exhaustive information, as other localized flooding may occur on other roadway sections. ii. The map is useful for broad predictions about the areas likely to be inundated on forecasting of particular extreme rainfall events. 6

9 iii. The map can also be used for broad assessment of the flood hazard associated with development of infrastructure, including roads (and their elevations) and housing, in Grenada. It can be used as a guide for determining areas for further detailed flood studies. Detailed Flood Hazard Map The deficiencies in the database for producing detailed maps for the St. John River limit current usefulness of the map and great care must be exercised in interpreting the results. Nevertheless, the map can be used for the following purposes: i. For finalization of engineering designs for infrastructure, such as roadways and bridges, and for buildings; ii. For design of emergency plans in the event of warning of extreme rainfall; iii. For deciding on locations for critical facilities and for detailed land use planning; iv. For identification of most hazardous zones within the floodplain. 5. DETAILED PROCESS FOR MAP UPDATING Flood hazard maps by nature are not static products. They are dynamic in nature and need to be updated on a regular basis. Any of the following conditions may trigger the need to update the flood hazard maps: a. Changes in the physical characteristics of the watershed, such as land cover, construction of dams or other major water resources projects that could alter the flow regime, including river training works. b. Changes in rainfall regime, likely to be more gradual than above, but nevertheless may alter runoff hydrographs. c. Opportunity to produce more accurate maps because of one or all of the following: The easy access to more sophisticated procedures for performing the hydrologic/hydraulic analysis; 7

10 Availability of a more current spatial data layer (e.g map to 2005 map). Availability of spatial data of a higher resolution; d. Opportunity to validate analytical tools used in current flood hazard maps; e. Improvements to the hydrologic database that could yield more reliable results. Table 1 is the recommended frequency at with the input data need to be updated: The updating process requires a high-level of collaboration among the responsible agencies. The agency responsible for the production and maintenance of the flood hazard map should develop an MOU with the agencies responsible for the provision of the input datasets. The MOU should make specific reference to the supply of current and accurate data in the most efficient manner. Once the supply of data is secured, the next requirement is the adequate supply of computer hardware, software, and personnel needed for the production and maintenance of the flood hazard map. Adequate budgetary allocation should be provided for these resources so as to ensure unhindered production of the hazard maps. Table 1. The update frequency of input data used in flood hazard mapping Data Theme Topography Coastlines Soils Landuse Stream/Rivers Roads Buildings Cross-section Rainfall Streamflow Updating Frequency 5-10 years 5-10 years 5-10 years 2-3 years 2-3 years 2-3 years 2-3 years During each update Continuously Continuously 8

11 6. RECOMMENDATIONS The reliability of the maps, the detailed and the island-wide flood hazard maps, has both been adversely affected by the inadequacies of the rainfall database for flood studies. Lengths of records were too short to confidently perform the rainfall analysis for extreme daily rainfall and too few gauges existed for adequately covering the island. Consideration should be given to the establishment of some permanent rainfall stations which combined can provide coverage of the spatial rainfall over the entire island. There are relatively inexpensive, low maintenance continuous rainfall recorders that could be acquired for this network. These have the advantage of providing depth-time relations as well. Continuous streamflow data do not exist on any of the rivers making it difficult to check the results from the hydrologic study. Consideration needs to be given to installing necessary equipment for acquisition of such data. The improvement of such rainfall and streamflow information will be useful for other water resources projects, but the effort requires a proper administrative unit. The most appropriate place is within the water authority where some hydrological data collection currently occurs by trained persons. Perhaps it would not pose much challenge to have this aspect included in their mandate and to have the unit equipped by the acquisition of the suitable equipment for field collection and the computer facilities for analysis, storage and retrieval and dissemination. The island-wide flood hazard map has identified a few other areas having a high flood hazard designation. Such maps should be produced as soon as possible, bearing in mind the need for improved data and for trained persons in hazard mapping. Consideration needs to be given to identifying the agency or the committee responsible for updating the maps as well as developing maps for other flood prone areas on the island. The agency chosen should have the right tools for map updates and the development of new ones. These tools are software for: (i) the hydrologic study, perhaps a software known as Watershed Modelling 9

12 System (WMS); (ii) the hydraulic study, perhaps two hydraulic models, called HEC-RAS and FLO-2D; (iii) mapping, perhaps the ArcGIS suite, and computer facilities, including a plotter. The appropriate agency should have the required qualified staff, namely engineers and GIS specialists that have some exposure to hazard mapping. The possible agencies are the WASA through its water resources unit, and the Ministry of Works. If such cannot be accommodated by any of the two, then consideration should be given to the establishment of a map-making committee, under the guidance of NADMA, with specific duties of creation and updating of hazard maps. The training session held during this project served to expose participants to the tools and the process required for flood hazard mapping. For map creation and updates further training is needed, especially in the hydrologic and hydraulic procedures. Regarding flooding on St. John River floodplain, its frequency and severity can be reduced by a well planned and executed maintenance schedule involving frequent clearing of vegetation on the riverbank, desilting at the mouth and along the channel at least up to the Humpback Bridge, the inspection and clearing if required of all the bridges immediately after every rainfall. In undertaking river training works, serious consideration needs to be given to the installation of debris traps especially for woody debris, as this frequently is the sole cause of flooding. 8. REFERENCES Bureau of Reclamation, Downstream Hazard Classification Guidelines, Acer Technical Memorandum, No. 11. Denver, CO. 10