Inland Flood Hazard Mapping for St. Kitts and Nevis:
Summary Report

OAS/USAID Post-Georges Disaster Mitigation Project

Implemented by the Organization of American States,
Unit for Sustainable Development and Environment
for USAID-Jamaica/Caribbean Regional Program

This report was prepared by Vincent Cooper, University of the West Indies, Trinidad.

Note: The final maps that accompany this report are available online: St. Kitts | Nevis

This report describes the approach to mapping the natural hazard of inland floods on St. Kitts/Nevis. It first presents a summary of the main cause of flooding on the islands. This is due not only to extreme rainfall, which provides the potential water for inundation, but also depends on the condition of the watersheds at the time of the event. The detrimental effects of flooding are then presented, which on the islands are mainly health hazards in terms of drowning and the contraction of diseases (to a small degree), and damage to economic systems including public infrastructure and private property. The data needs and adequacy for flood quantification are then discussed. For its quantification three sets of data are required, and they include rainfall data for characterizing the rainfall with potential to cause floods, the physical properties of the watershed which determine the volume and time-distribution of runoff onto low-lying areas, and the geometric features of the drainage channel which may or may not have sufficient capacity to accommodate the runoff from the watersheds. The two approaches used for flood mapping are described, followed by the presentation of the methodology implemented for producing the maps. A general summary of the findings for each island concludes this report. Among other things, it suggests what are necessary to improve the accuracy of the maps, which given the limited data must be regarded as only preliminary maps.

The nature of inland flooding being investigated on St. Kitts/Nevis is described by the following definition: "Flooding is a general and temporary condition of partial or complete inundation of normally dry land areas from overflow of inland waters or from the usual and rapid runoff of surface waters from rainfall." In these islands, the major trigger of this inundation is extreme rainfall on the watersheds. These extreme events are sometimes associated with hurricanes that produce significant quantities of rainfall during their passage over the islands, the slower the movement over the islands, the greater the quantity of rainfall. Widespread flooding may also occur from rainfall associated with the Intertropical Convergence Zone (ITCZ) weather system. However, most extreme events have been from violent thunderstorms occurring mainly because of the high relief of the watersheds. These are generally very localized rainfall events.

Whether or not floods occur within a watershed, on the occurrence of an extreme rainfall event, depends on the characteristics of the watershed. Most watersheds in St. Kitts/Nevis have the following properties:

1. Soils that are very permeable and highly erodible. The soils can permit significant infiltration amounts and has potential to significantly reduce runoff volumes;
2. Very steep watersheds causing rapid flow of water, permitting little infiltration and causing significant soil erosion;
3. Forested areas in the upper portion of the watersheds;
4. Expanding urbanization (especially in St. Kitts) in the foothills that is generally replacing sugarcane plantations and thereby replaces pervious areas with impervious surfaces such as roofs and roadways; and
5. grazing by livestock on Nevis that compacts soils and reduces infiltration rates, and removes vegetative cover leaving the soil surface exposed to full impact of the rainfall drops and vulnerable to soil erosion.

Mainly because of the steep slopes, there is reduced infiltration and little flow retardation of the travel of the flood volume toward the low-lying areas. Thus watersheds in St. Kitts and Nevis are susceptible to flooding. Fortunately, watersheds are small, in the order of a few square kilometers, and so the volume of runoff would be relatively small even for very extreme rainfall.

Floods may have detrimental effects on three aspects of the human condition—health, agriculture, and economic systems. In terms of human health, floods can cause death by drowning, impact or exposure. In St. Kitts and Nevis, fast flowing waters may sweep persons and vehicles away toward the sea as happened recently in both islands. The main danger is at various road-crossings where ghauts may overtop the culverts and bridges and flow over the road. Unaware of the danger of fast-flowing water, several lives have been lost by persons attempting to negotiate across the swift floodwaters. In St. Kitts, persons living along Lower College Street are exposed to the danger of swift moving water when the College Street Ghaut is flowing full. Likewise on Nevis, for persons in the lower Bath Ghaut area. Numerous physical injuries are possible as well, and a common effect of flooding is disease and disease transmission. Diseases commonly are associated with disruption of fresh water supply; contact with floodwaters contaminated by septic tank and wastewater treatment plant overflows; the creation of appropriate habitat conditions for certain rodents, insects and organisms that transmit diseases. Detrimental effects in Nevis on agriculture are mainly due to loss of livestock due to drowning or becoming dispersed. Destruction of crops, seeds, and stored food stocks during inundation is limited as agriculture occurs on the foothills and not on floodplains. Public infrastructure and private property may be damaged with inundation and deposition of significant quantities of silt. For Basseterre and Lower Bath Ghaut, initially ground floors of buildings and residences may be affected with rising flood stage. But with increasing stage and flow velocities, buildings and loose property can be swept away. An additional danger with such steep sloped watersheds is caused by boulders transported by the swift currents and floating debris such as cars and logs that can increase damage as they impact structures downstream. Properties on riverbanks may be in danger if riverbanks erode and trigger bank failure. Extreme examples of bank erosion exist along the College Street Ghaut, around Monkey Town, and Cayon Ghaut.

Major disruptions of the road network may affect productivity as the workforce is delayed in reaching their workplaces.

Areas prone to flooding have the following characteristics:

• gentle slopes that are perhaps poorly drained because their drainage channels also have very gentle slopes;
• constricted natural drainage owing to, for example, heavily vegetated areas such as mangrove forests, meandering streams, or tidal influences. If communities are located around the outlet, then the hydraulic structures, such as bridges and culverts, may constrict flows owing to poor designs or because they become blocked with debris, and thus may further limit the natural carrying capacity of the channels;
• large watershed areas (relative to their area) contributing runoff from rainfall;
• so positioned within the watershed that runoff from the surrounding lands concentrate at the flood prone area;
• contributing areas having large runoff coefficients due to impervious soils, impervious ground cover, steep slopes, or/and limited abstraction losses by vegetative cover.

Data needs for flood quantification can be grouped into three categories: data for describing the meteorological event which may trigger the flood; data about the watershed which are used for determining the fraction of the rainfall that would run off the land and how it would be distributed in time; and data for describing the geometry of the drainage channels, which may or may not be able to convey all of the runoff within its banks, depending on their water carrying capacity.

The Critical Storm: The definition of a critical storm is somewhat subjective and its description may vary depending on the purpose of the investigation. In all cases it is defined by four features:

1. rainfall depth;
2. duration;
3. the time-distribution of the rainfall depth over the duration of the storm; and
4. the spatial distribution or the area over which the features in (i) to (iii) remain unchanged.

Historical daily rainfall records collected within the watersheds are vital to obtaining the rainfall depth. From the application of appropriate statistical analyses on these records, the depth of rainfall of certain return periods (see definition below) can be found. Daily rainfall records are normally available in plenty supply but more often than not there is little information to describe how that depth of rainfall occurs over the 24-hour period. The resulting runoff is very much dependent on this time distribution, and it is not sufficient to average the rainfall over the 24 hour period. Standard time distribution curves have been developed for the United States and some of these curves have been successfully used in the Caribbean (and indeed in other parts of the world). In the absence of time varying rainfall data collected from within the watersheds, these curves can be used. The storm duration is related to a characteristic of the watershed known as the time of concentration. The time of concentration, T_c, depends on the physical characteristics of the watershed. It represents the time required for water from the furthest point in the watershed to reach the place where the hydrograph is being measured. For maximum runoff generation, the critical storm must have a duration at least equal to the T_c of the watershed. The variation in space also must be considered in developing the critical storm. The amount and timing of rainfall vary considerably from one place to the other for the same storm. Therefore, the capturing of such variability requires finding a weighted average of rainfall measured at several raingauges set up throughout the watershed. The use of one raingauge may not be truly representative of the rainfall over an entire area, but in many cases, only one gauge is available.

For St. Kitts, twenty-three years of daily rainfall from seven stations distributed over the entire island were used to produce seven critical storms, one for each station. Rainfall depths for the 100-year return period for each station were obtained from the statistical analysis. These daily rainfall depths were chosen for the critical storms and the Type III time distribution curve—a designation used to describe one of the standard curves in which most of the rainfall occurs within a small fraction of the 24 hour period—was used to distribute the rainfall in time, at each station. The island was then divided into seven zones, each zone having a raingauge station at its centre. It was assumed that rainfall characteristics within a zone was everywhere the same and was identical to the critical storm formulated at the rainfall station within the zone.

Runoff: Having defined the critical storms, it would then be possible to estimate runoff properties from the surrounding watersheds. The amount of water available as runoff from the rainfall and the distribution of this runoff in time are controlled by the features of the watershed. The important features are:

• Size of the watershed;
• Soil infiltration rates and depth of storage;
• Land cover;
• The total length of all the rivers and their tributaries within the watershed;
• Shape, slope and ruggedness of the watersheds.

The first three features control the amount of water available as runoff onto the flood prone areas. The other features control the timing of the hydrograph peak. Most of the required data are available on standard maps. Most are natural conditions, but the land cover and, to some extent, the length and characteristics of the rivers and tributaries can be altered depending on the human activities occurring within the watersheds.

All required data can be extracted from standard maps and land capability reports, and these were all readily available, some in digital format.

Geometric Features of the Drainage Channels The geometric properties of the drainage channels and the nature of their lining determine the flow rate that can be accommodated within their banks. Meanders on channels reduce their capacity, so too would reduced slopes. Capacity is also reduced with increasing roughness of the channels. Carrying capacity is affected by man-made hydraulic structures, such as bridges, weirs, and sluice gates culverts, that may be constructed within the channels.

Normally, detailed surveys are required to gather the geometric data for describing the drainage channels. Some surveys were carried out within the College Street Ghaut and Wash Ghaut watersheds on St. Kitts, and the Bath Ghaut and the Camps River watersheds in Nevis.

Two approaches to flood mapping, differing mainly in the degree of accuracy in mapping floodwater levels, were applied. The first approach was done at the floodplain scale and the results, though still approximate for lack of sufficient data, provide information on flood levels within the plain. This traditional approach required hydrological inputs and detailed hydraulic data. It is concerned with bank overflow as it considers the carrying capacity of the drainage facilities on the flood prone areas and the extent to which water from surrounding watersheds remains as "within-bank" flow. For the results to be meaningful careful analysis on the hydraulics of the floodplain is needed and the analyst must take into account all the significant features within the flood prone area that could affect water levels.

If all required information is available, and most importantly if floodwater levels have been collected from actual flood events, then the hydraulic analysis can be fine-tuned by making adjustments to uncertain inputs until the water levels from the analysis match the levels from the recorded flood events. Typically, the accuracy is within 150 mm (6 inches).

The second approach was applied because of lack of detailed information about channel geometries in other flood prone areas not subject to detailed surveys. Whereas the same runoff information as above is needed to quantify the amount of water within the flood prone area, this approach does not attempt any hydraulic analyses. Instead, the method estimates a flood depth over the entire flood prone area, which is essentially an average value derived from a simple division of the total volume of water from all runoff flowing into the flood prone area over its surficial area. This depth does not provide any information about how water depths vary within the flood prone area as it assumes that the area has the same mild slope throughout. Furthermore, this value may overestimate actual flood levels in some places, and underestimate them in other places. Such approaches may be necessary when the type of information required in the first approach is missing or unavailable, but the approach is very useful for providing basic ranking of the extent to which flood prone areas over the island is susceptible. Its approximations must be well understood so that it can be properly used. Most of the requirements can be extracted from standard topographical maps.

Island-wide Scale Mapping:

1. Identify areas, called flood prone areas, having mild slopes. This can be done either via manual inspection of topographical maps or with the aid of DEMs and computer software;
2. for each flood prone area, demarcate the watershed draining into the area and then determine the ratio between the watershed area and the flood prone area (from (1) above);
3. for a particular 24-hour return period rainfall, generate runoff hydrographs discharging into the flood prone area, based on the shape, the drainage network, the soil properties and the landuse;
4. from the hydrographs, determine the volume flowing into the flood prone area and estimate an average discharge rate over the hydrograph period;
5. estimate a discharge rate from the flood prone area and use it to determine the maximum volume of water (expressed as depth of water) that is likely to be stored on the flood prone area;
6. determine the hazard category of each flood prone area according to water depth within the plain from the 100 year return period storm using the following classification:

Table 1 Hazard classification according to mean depth of water.

The categorization of the hazard in this way provides a measure of the magnitude of flooding to be expected from such an event. It also provides a measure, certainly by rank if not in absolute terms, of the severity of flooding among the identified zones.

• Perform statistical analyses on the daily rainfall data and obtain amounts for various return periods.
• Derive the critical storm for a 24-hour duration, using standard time curves, and assuming that the rainfall was the same throughout the watershed;
• Obtain contour maps of St. Kitts and Nevis and demarcate the watersheds draining into the four areas—Lower College Street Ghaut, Wash Ghaut, Lower Bath Ghaut, and Camp River—conduct field trips to confirm the boundaries;
• Determine the existing land use within the watersheds from topographical maps and field visits;
• Obtain soils maps of St. Kitts/Nevis;
• Digitize information from the map and field visit;
• Conduct interviews with residents within the four areas to determine specific information about recent floods—the highest water level reached, the direction from which the flood waters came, the time it reached its highest level, the length of time it stayed at that level, the time it took to subside to some remembered level;
• Survey the river or drain from which the reported flooding originated, paying attention to: (i) any natural constrictions such as a narrowing of the river cross-section, or manmade constrictions due to hydraulic structures such as culverts and bridges, or to construction of property within the waterway; (ii) the free clearance of hydraulic structures and bends; (iii) the lining of the channels; (iv) the level of maintenance of the channels (v) the nature of the flood prone area from the river bank in terms of its coverage—pastures, wooded, urbanized;
• Identify critical river sections based on its impact on flooding with potential for significant losses;
• Conduct detailed surveys of these areas at critical cross-sections, hydraulic structures and any other important features.
• Examine the discharge point of the section to determine outlet control conditions—whether, for example flow was unhindered or under the influence of tides.
• Perform detailed hydraulic analyses using appropriate computer software.
• Produce a flood map showing variation of water depths within the flood prone area.

7. Summary Results

St. Kitts

• The watersheds are mountainous and they frequently experience high intensity localized rainfall. Runoff from these watersheds are flashy as their times of concentration are very short, in all cases being less than one hour. Several watersheds are undergoing some developmental pressure especially in the foothills, which are currently used for sugarcane cultivation. Generally, however, the watersheds are well covered with vegetation especially in the headwaters, which comprise largely primary forests. Soils are very erodible and are washed away by runoff where they are deposited in the ghauts, especially at road crossings via bridge or culvert. Flooding often occurs at these crossings owing to infrequent clearing of the silt. In some instances flooding has occurred due to reduction of the waterway by encroachment of residential property within the waterway area of the ghauts. To a lesser extent, flooding is caused by obstruction of the waterway due to dumping of solid waste in the ghauts.
• The most extensive flooding occurs in the lower reaches of College Street Ghaut where it passes through Basseterre. The potential for major damage is great because the flooding occurs within the commercial centre.
• Detailed survey data picked up within the lower College Ghaut watershed and Wash Ghaut provided sufficient information for conducting detailed flood plain mapping.
• An adequate database of daily rainfall exists, which includes records from gauges covering the entire island. The twenty-three years of records provided for the project were used for estimating the 100-year return period storm. No continuous rainfall records were available and so use was made of standard time distribution curves for determining peak flows of the catchments.
• Limitations in river flow data prevented full-scale calibration of the model output of water levels within College Street and Wash ghauts. The limited data acquired from field data provided validation of the output, but recommendations for improving the reliability of the output are provided within the technical report.
• Erosion is a major problem within the vicinity of the ghauts and should be addressed. Some preliminary consideration has been given to it but further information needs to be gathered for generating accurate maps.

• Similar to St. Kitts, the watersheds in Nevis also are mountainous, well vegetated in the headwaters, have very short times of concentration and therefore have flashy responses to rainfall. They also will be subject to random, violent rainfall. Their soils are highly erodible and heavy silt deposition blocks culverts and bridges at road crossings. Infrastructure is inadequate owing to poor design and construction and infrequent maintenance. Very little attention is given to enforcing land use zoning and this has been the sole cause of flooding in several areas of the island.
• The two years of daily rainfall records provided for the project were insufficient for producing reliable rainfall frequency curves and so records from St. Kitts were used in the interim to generate the rainfall frequencies. The one year of Nevis rainfall data that overlapped with records from St. Kitts could not be used to do any reliable rainfall comparison.
• Detailed flood maps were produced for the lower Bath Ghaut using detailed survey data. The output, however, should only be regarded as representative until a rainfall frequency analysis using Nevis data has been complete, and water levels acquired from within the catchment are available for validating the water levels shown in the map.
• Urgent attention is required at addressing the drainage inadequacies around Newcastle Airport, and so avoid frequent closure of the airport due to flooding on the runway.

Definitions

Flood: A general and temporary condition of partial or complete inundation of normally dry land areas from overflow of inland waters or from the usual and rapid runoff of surface waters from rainfall.

Hydrograph: A flood wave passing a particular place in the river. It represents how the stage (water elevation) or the flow in a river changes with time.

Return Period: This is an expression of the probability of a particular rainfall depth of a given duration occurring. The rainfall depth corresponding to a specified return period will be equaled or exceeded at a probability equal to the inverse of the return period, that is the relation can be expressed as follows: Return Period = 1 / (Probability of Occurrence). For a 10 year return period: So the probability that the rainfall depth for a 10-year return period will be equaled or exceeded is 0.1 (1/10)

Critical Storm: The rainfall input used for producing the hydrograph. It represents what is viewed by the analyst to be most important in predicting the effects of the rainfall on runoff. The rainfall depth, the duration of the storm, the type of distribution of the rainfall depth in time and the uniformity of the distribution of the storm over the watershed must all be specified for complete specification of the critical storm.

Time of Concentration: The time elapsed between the start of the rain and the runoff from the most remote point in the watershed to reach the place where the hydrograph is being measured.