Water Supply:  Los Altos de Tres Ríos Subdivision
Coronado, Costa Rica

For:  Bill Batkin & Partners

By:  Paul Collar

Osa Water Works. S.A.

Puerto Jimenez Costa Rica

www.osawaterworks.com

506 735-5702

February 11, 2007

Executive Summary

The 64-hectare Los Altos de Tres Ríos subdivision has a large stream that bounds one portion of its property, small streams that bound other portions of the property, and a 40-gpm spring located on the adjacent property.  Development of the spring water is 20% less expensive than development of the stream water above the location of the waterfall and is the preferred alternative for providing potable water supply to the property.  Neither source is completely adequate for irrigation, but if irrigation capacity is important, then the stream has an environmentally sustainable capacity  of 120-150 gpm, capable of modest irrigation supply.  While a number of factors make a firm bid preliminary at this point, the system is expected to cost around $96,000. 

Figure 1.  Los Altos de Tres Rios site plan showing lot divisions and roads both in place and projected.

Introduction

Los Altos de Tres Ríos (Figure 1) is a 64-hectare property that has been subdivided for resale into 23 lots.  Osa Water Works, S.A. (OWW) was contracted to provide a technical feasibility study for supplying water to the lots and to compare the alternatives of using a spring located on a neighboring property versus using stream water from above the waterfall.  This document summarizes the findings of the resource evaluation.

Objectives

The objectives of this report were:  1) to evaluate the spring and the stream as water supply sources; 2) to elaborate an engineering and design plan for providing potable water supply to all the lots on the property; and 3) to compare the alternatives to determine which is the technically and economically favored solution. 

Data Collection

Field Survey

The commercial model of Los Altos de Tres Ríos subdivision (Figure 1) is that of a fully outfitted residential subdivision comprising ocean view lots with ready building sites.  To date three quarters of the proposed sites have been accessed with a network of roads with drainage connecting building sites cleared by bulldozer.

On Feburary 6, a field survey was undertaken in which two possible water sources and all the lots that have been developed to date were visited.  For each site the location was determined using a Garmin 76CX global positioning system and the elevation was tracked with the same instrument and also with a Casio digital barometric altimeter.  The GPS tracks taken during the field day are superimposed upon the base map in Figure 2, and the elevations determined are provided for each lot number in Table 1.

Water Demand

Figure 1 reveals that Tres Ríos has four “threes” and five “fifteens.”  Adding these lot repetitions brings the number of lots to a total of 23 projected building sites.  If we assume a peak occupancy of four persons in each of the houses and a per capita water consumption rate of 100 gallons per day (the standard value for American residential water supply estimation), then the total peak demand is one of 9200 gpd, which is equivalent to 6.4 gallons per minute (gpm).   The instantaneous peak facility demand can be approximated by calculating water consumption when all houses are using the shower, washing machine, and kitchen sink at the same time, a draw of about ten gpm if we throw in some dishwasher and some garden hose use.  Instantaneous peak use can be considered about 75% of this estimated total, or approximately 175 gpm.  

If the ownership seeks to provide water for irrigation of the grounds, then it is necessary to budget an additional 5 mm daily in rain-equivalent irrigation.  Across the total 64 hecs, this adds up facility-wide to the equivalent of 590 gpm, or approximately one hundred times as much as is needed for peak residential water demand.  Clearly it is not feasible to design a system with such delivery capacity, and

Figure 2.  Superimposition of GPS tracking on base map, with flags representing way stations.

it will be adviseable no matter which water source is ultimately developed to establish a community water use policy restricting irrigation to levels shown to be acceptable within the eventual community’s water use policy. 

Table 1.  Summary of areas, elevations, prices, and pricing per square meter.  WF BASE = waterfall base; WF CAPT = proposed stream water capture location.

Water Supply

Two sources of water were identified during this study.  Their locations are given in Figure 3 and photographs of the water supplies are shown in Figures 4 and 5.

Figure 3.  Locations of a large spring with an estimated flow rate of 40 gpm, which is not located within the boundaries of the property, a stream with a flow from which 125 gpm of water can be easily removed without adversely impacting the environment and a third surface water source (UNKNOWN) that was not observed during the field investigations and is shown simply because it appears on the map as a surface water source bounding the property that may warrant further scrutiny.

Discussion

Technical Feasibility

The spring located just outside of the property boundaries is an attractive source of potable water.  The water is undoubtedly of high quality and will not likely require any form of water treatment.  This represents a signficant advantage over stream water, which will unavoidably require particulate filtration and disinfection.  Also, the spring already has a well-built spring box already in place, so it is ready to be connected to a water distribution system without additional construction costs.  The cons of spring water as a source are significant, however:

1)     There is not enough water flow to provide for even modest facility irrigation demands at peak occupancy,

2)     With an anticipated yield of only 30 gpm and instantaneous peak demands approaching 200 gpm, significant storage is required to ensure continuous water service during peak usage periods.

3)     The spring is located at a low elevation relative to the building sites, meaning that a significant investment is required in power to pump the water to a tank that will provide storage and pressure for gravity flow.

4)     Perhaps most importantly, the spring is not on the property and to capture it will require a tank, pump, and distribution lines that are likewise on someone else’s land.  This will likely require a negotiated arrangement to purchase the water and may leave the facility vulnerable to development plans for the adjacent property.

The waterfall is located in difficult terrain and is separated from the main part of the property by a gorge.  Development of stream water as a facility water supply provides the capacity for full potable and even modest irrigation water demands, but it has its own disadvantages, listed below:

1)     Very difficult access for intake and feeder pipeline installation

2)     Electrical power on the far side of the gorge is required to conventionally pump the water to the mother tank.

3)     Water treatment is required for removal of sediment and bacteria

4)     A water collection system must be installed in remote and rugged terrain.

Engineering and Design

Discounting the hypothetical extension of municipal water to the project site and exotic options like rainfall capture, the water supply for the Tres Ríos site would most logically come from either the spring or the stream examined during the field survey.  There are two other streams that bound the property that are shown on the map but were not visited during the field survey for this report.  If these streams run year round they may be additional candidates for facility water supply.  Lastly, the purchase of the property is reported to have included the right to mountain spring water from a separate property, and this represents an additional alternative that may be worthwhile exploring before settling on a final option.  For the purposes of this report only two options are evaluated somewhat rigorously:  1) the spring water from the neighboring property, and 2) stream water from above the waterfall.

The efficiency of any water supply is constrained by a variety of factors with the most important being the following:

1)     Adequate water feed

2)     Adequate water storage in the context of feed and demand rates

3)     Appropriate water pressure

4)     Adequate pipe diameter and network design to provide for optimal distribution

As the energy profile in Figure 6 reveals, the properties are distributed across a total of 180 meters of vertical relief.  Gravity water supply across this terrain gradient, presuming a tank at the top of the property, will vary naturally, therefore, from 0 to 254 psi, with 30-50 psi being optimal for residential water supply.  The energy gradient across the property reveals that either multiple tanks are required to ensure that pressure is optimal for the different building sites or that pressure reduction valves be distributed throughout the piping network to modulate pressure accordingly for optimal residential delivery.  Also, the spring is lower than any of the building sites visited; delivery of 30 gpm across 180 meters of head requires a 10 horsepower submersible pump. 

Figure 6.  Energy profile of the Los Altos de Tres Ríos subdivision.  The spring is the lowest elevation measured and is used as the datum of the diagram.  The highest point is Lot 8, which is located 185 meters above the spring.  WF is short for waterfall, and WF TOP is an estimate of the ideal intake location for a stream-based water withdrawal system.  Clearly, the ability to make dramatic savings in potential energy is well worth even great hardships suffered a single time.

The addition of daughter tanks provides supplemental water storage in addition to a means for regulating domestic water pressure.  Also, having tanks that directly feed houses relieves the water flow demands on the mains themselves.  For the few lots that must be fed from a main line rather than a daughter tank, the mere presence of daughter tanks protects the trunk transmissive capacity.  Since pressure-reduction valves do not provide ancillary benefits and represent a mechanical component susceptible to failure and likely to require maintenance, the distribution of small storage and pressure tanks to provide water to sets of homes at similar elevations is considered the optimal approach to pressure adjustment across the property, irrespective of which water source(s) are ultimately used.

Study of the energy profile and the base map suggests a conservative distribution of tanks, like that shown in Figure 7.  The mother storage tank is shown as a rectangle and is the highest one on the property.  It receives all the water that is pumped from either the spring or the stream and distributes water via four-inch water mains to smaller tanks, subsequently referred to as daughter tanks, with some branches off the main lines to stub out on individual properties according to location and elevation. 

Figure 7.  Conceptual diagram showing key features of a facility water capture, pumping, storage, and distribution system for Los Altos de Tres Rios.

Correct tank sizing must take into account both water supply and demand to calculate what is required to “float” the system.  For small supply and large demand, a large amount of water storage is needed to ensure sustained functionality.  For large water supplies and small demands, then considerably less storage size is required.  In theory, since the stream has three times the exploitable water flow as the spring, it will require about one third of the storage size than if the spring is used, presuming pump and pipeline sizes are proportionately sized. 

The mother tank must be able to attenuate the instantaneous peak water demand of 175 gpm.  This corresponds to 11,000 gallons per hour.  Abstracting the 1800 gallons delivered hourly from a conservative 30-gpm spring feed, this means that the balance, 9200 gallons, must derive from storage capacity, replenished during periods of lower consumption.  Clearly, the more storage capacity the better, but at approximately $1 capital cost per gallon of storage, the difference between a 25,000 and a 50,000 gallon tank is around $25,000, so there is non-trivial economic value to making the tank as small as possible to still serve its purpose.

The number-crunching makes apparent that the bare minimum tank size to sustain instantaneous peak capacity is one of 10,000 gallons if the source water is the 30-gpm spring water feed.  If 120 gpm were provided from the stream, then a bare minimum storage capacity of 2600 gallons would be theoretically required to satisfy the demands of instantaneous peak consumption sustained for an entire hour.

An additional criterion of tank sizing is the water demand during peak occupancy, but spread out over an entire day, referred to as peak water demand, ommitting the adjective “instantaneous.”  Full occupancy (4 pax, 23 homes, 100 gpd/person) has a peak demand of 9200 gallons per day.  With a daily spring supply of 43.200 gallons, that means the upper practical theoretical limit of storage capacity is 40,000 gallons presuming the spring water source.  A tank of that size would provide the facility 4.3 days of offline capacity at full usage and would be arguably quite over-sized for its intended duty.  A more practical design criterion would be to provide two days of storage at peak occupancy and assume that emergency water conservation practices reduce the per capita water demand by 25%.  This would require 13,800 gallons of storage.  To provide an additional margin of safety and to provide the facility with a bit more water usage versatility, a mother tank of 20,000 gallons is recommended at a design feed rate of 30 gpm.

A smaller tank is likely to be too close to the bare minimum and create an operational environment in which water shortages are more likely.  If a feed rate of 120 gpm were delivered from the stream, the tank size could be reduced to 15,000 gallons.  However, the operational limits on water delivery under high usage rates next becomes the diameter of the water mains and point-of-use piping, and four inch diameter is at the upper limit of practical pipeline diameter for this scale of project owing to basic costs/benefits considerations.  Therefore, even if the stream water is the ultimate water supply decided upon, I recommend that the tank size be no less than 20,000 gallons in size.  If the ownership wishes to include the versatility of having modest irrigation capacity, then the tank capacity should be raised to 30-40,000 gallons depending on the extent of water usage flexibility desired.

If the daughter tanks are deployed to serve four or five houses each and are fed with a four-inch water main, then the water feed is expected to be adequate in its own right to accommodate instantaneous peak demands of 10 gpm per household easily.  Theoretically, no storage at all is required in such a circumstance.  However, it is necessary to provide some storage in order to reduce the amount of water each main must transmit during periods of peak usage.  It is also important to have backup water in storage at the point of use in case of feeder line maintenance requirements.  Commercial plastic daughter tanks of 5000 liter capacity are recommended for this duty.

Economic Analysis

Case One:  Spring Only

An approximation of installation cost for a water supply in which the spring is the sole water source is provided below.  The installation costs are calculated on the basis of a cost-plus-20% basis.  Criteria for the system include the delivery of gravity water between 35 and 60 psi (except lots 2 and 8), at least 40 psi pressure, 10 gpm for the household even during full Tres Ríos occupancy.

Table 2.  Job costing of Case One:  spring water only.