Engineer's Report - January 2, 2004

Foreward

Russ created a valuable summary of the guts of this report. Here is a source-to-destination representation of his diagram. 

Domestic System

  Capacity
(gpd)
Current (440 ADD)
ERUs
Target (200 ADD)
ERUs
Lake 161,280 269 269
Treatment Plant 82,000 137 234
Reservoir (current) 55,000 39 81
Reservoir (proposed) 110,000 83 176
Distribution n/a 144 247

Irrigation System

This table is meant to calculate theoretical irrigation flow potential based on the assumption that we are able to achieve our domestic consumption goals of 200/350 ADD/MDD and are granted 176 hookups.

  Capacity
(gpd)
Lake 161,280
  Less Domestic: 176 ERUs 61,600*
Water available max 99,680
Current reservoir max 50,000 gals
Current distribution max 80,000
   
Current use 25,000
Current users 5
Maximum (system capacity) 16
Maximum (water available)  19

* DOH calculates use potential based on MDD. 200 gpcd ADD is equivalent to 350 gpcd MDD


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The Report

Cover page


Water System Capacity Analysis

The purpose of this report is to access the capacity of the BIMC potable water system. The capacity of each element of the system — source, transmission, treatment, storage, and distribution — has been determined in EFRU’s (equivalent residential units). This is the rating system recommended by the State Department of Health. In general one ERU is equal to one residential connection. The total number of ERU’s assigned to each system element is based on DOH formulas established in their ‘Water System Design Manual”. While these requirements are normally applied to new systems they are also recommended for the evaluation of existing systems. They will likely be required for this system since BIMC will eventually be seeking a connection increase from 123 to 174 or higher plus the non residential uses (the marina and hangar) and any guest houses. The state considers a guest house to be a full time residential connection equal to 1 ERU unless the water purveyor demonstrates that it its water use warrants some fractional ERU value. This issue is not addressed in this report.

Average and Maximum Day Use
In order to establish the capacity of each system element in ERUs the value of one ERU must be determined. The value is Unique to each system and can change over time. It is based on the meter records of a representative number of residential units. For communities with a high proportion of part time residents like Blakely Island occupancy must also be factored in.

ERU’s are expressed in ADD (average day demand) and in MDD (maximum Day Demand). Typical values for Group A water systems like BIMC’s are: ADD = 200 gpcd (gallons per connection per day) and MDD = 350 gpcd. BIMC’s ADD and MDD are based on the Fire Hall mass flow meter which accounts for 82 of the approximately 117 existing connections. See Figure 1. The meter is read daily year round. All connections are to single family homes except the hangar area which is believed to be equivalent to one residential connection. The ADD is based on July and August use for years 2002 and 2003. The number of active connections is based on occupancy estimates by the water system manager Jim Davis. Not all 82 are active. In fact the average occupancy is estimated to be equivalent to about 36 full time connections. Note that this group of homes includes approximately 16 which irrigate regularly. It is believed that maximum water use for irrigation per lot is between 500 and 4000 gallons per day.

Analysis of the BIMC 2002 and 2003 summer water use data has yielded the following results for ADD and MDD for the BIMC system. See Table 1. (WM notes: Table 1 is on page 3)

Draft Report - Page 1


(The report used the firehouse meter for its data. This essentially draws a line from the Firehouse, down past Daveys, then down the BIMC easment to the water. The report then uses all residences South of that line for its data.)

Figure 1

Draft Report - Page 2


Table 1 - ADD and MDD for 1 ERU

 

With Irrigation

Without Irrigation

Typical for SJC

ADD

540

440

200

MDD

800

600

350

See the attached calculations titled “BIMC Water System - ADD and MDD Determination” for the method used to establish these values.

So: 1 ERU = 600 gpcd on a maximum use summer day and 440 gpcd on an average summer day.

Normally for residential communities one [RU equals one connection. Connections in other customer classes such as commercial, industrial and institutional are assigned an ERU value based on their actual water use. For the marina the highest 3 day use was during the July 4th weekend 2003. Based on this the marina is equivalent to 10 ERU’s. See attached calculations titled ‘BIMC Water System - ADD and MDD Determination

Capacity Analysis based on ERU’s
The state recommends that existing facilities be evaluated based on system ADD and MDD. In fact the rating of the overall water system is based on the facility with the lowest ERU capacity. So for instance if storage is the lowest rated facility at say 100 ERUs’ then no new connections beyond 100 would be allowed until more storage is provided. Then DOH would place the connection cap on the next lowest ranked facility.

Here are the capacity values for the BIMC facilities with some discussion for each. Where the facility rates lower than 200 ERU’s the capacity of the system will also be rated for a typical Group A system ADD and MDD (200 and 350 gpcd)

Source

The source of water for the system is Horseshoe Lake. The capacity of the lake is evaluated solely on BIMC’s legal right to withdraw water. It is believed that the physical capacity of the lake far exceeds the BIMC water rights and the Taggares water use. (see “Water Resources Report”, Alrnskaar Assoc. Nov 1 989, p 35).

BIMC water rights allow a maximum withdrawal of 112 gpm instantaneously and 52.8 million gallons annually. Based on this...

the maximum capacity of the source is 269 ERU’s.

This figure is based on MDD = 600 gpcd and the water rights instantaneous allowance which is more restrictive than the annual allotment. See attached "ERU - System Capacity Analysis Review” for details.

Draft Report - Page 3


Transmission Main

Since the practical withdrawal from the source could be limited by the flow rate of the 3500 foot long raw water transmission main its capacity has also been determined. At present the maximum flow rate of the transmission main is limited by the capacity of the pumps at the lake. The pump capacity is 60 gpm. With larger pumps the practical capacity of the 3” PVC main is about 160 gpm. For purposes of this evaluation the existing pumping rate of 60 gpm will be used to determine transmission main capacity. The maximum capacity of the transmission main is shown in Table 2 below. See attached “ERU - System capacity Analysis Review” for details.

Table 2— Transmission Main Capacity

Flow rate
(in gpm)

MDD
(in gpm)

Capacity in
ERU’s

60

600

144

60

350

247

Of course a larger pump at the lake would increase the flow rate and the facility’s rating in ERU’s. The treatment capacity would also have to be increased to keep pace, however.

Treatment

The maximum capacity of the treatment plant is shown in Table 3 below. The values are based on the current maximum daily treatment volume of 82,000gallons (after backwash). Note that the maximum sustainable treatment rate is 62gpd. See attached “ERU - System capacity Analysis Review” for details.

Table 3 — Treatment Capacity

MDD (in gpcd)

capacity in ERU’s

600

137

350

234

Storage

The system was evaluated for storage in two ways. 1. - How many ERU’s can the system storage support? and 2. How much storage is needed to support the existing and future connections?

1. The existing system storage of 50,000 gallons can support connections (ERU’s) according to Table 4 at current MDD of 600 gpcd and for comparison at the typical SJC MDD of 350 gpcd. The table also includes the system capacity if

Draft Report - Page 4


an additional 55,000 gallon tank were to be dedicated to the potable water system. Clearly there is a storage deficiency in the system.

Table 4— Storage Evaluation

Storage Volume
(in gallons)

MDD
(in gpcd)

Capacity in
ERU’s

50,000

600

39

50,000

350

81

105,000

600

83

105,000

350

176

* Assumes adding a 55,000 gallon tank

2. In order to determine the amount of storage needed the system was also evaluated for 117, 123 and 200 ERU’s at current MDD of 600 gpcd and at the future target MDD of 350 gpcd. See Table 5 below for results. Though there is now about 50,000 gallons of total storage the actual effective storage (without dead storage and air space) is actually about 48,000 gallons. Note that in all cases there is a storage shortfall. The volume of storage needed is based on the “DOH Water System Design Manual”.

 

Table 5— Storage Needs Summary

Number of
ERU’s

MDD *
(in gpcd)

Storage Requirement
(in gallons)

Existing
Effective
Storage

Storage
Shortfall
(in gallons)

117

600

143,930

48,000

95,930

117

350

68,994

48,000

20,994

123

600

150,960

48,000

102,960

123

350

72,415

48,000

24,415

200

600

217,745

48,000

169,745

200

350

104,913

48,000

56,913

* MDD — Maximum Day Demand for one ERU

The storage requirement shown in tables 4 and 5 include the following elements:

  1. Standby storage is for use during power outages and other emergencies. It equals twice the ADD and so provides about 2 days storage.
  2. Equalizing storage is for “on-call demand”. This volume equals about 150 minutes of peak hourly demand minus what the treatment plant can deliver over the same period of time.
  3. Treatment storage. This is the amount of stored water need to provide enough time for chlorine contact during peak demand periods. Normal unbaffled tanks require more volume than baffled ones like BIMC’s.

Volume for fire flow is not included in the storage evaluations. Usually rural residential areas provide about 10,000 gallons for fire flow storage. In San Juan

Draft Report - Page 5


County fire flow storage can be “nested” in the standby storage so in effect BIMC has sufficient fire flow storage.

Distribution system

The system has been modeled using WaterCAD software from Haestad Methods of Waterbury, Conn. It has been modeled for the build-out condition. See Table 6 for the assumptions used.

Table 6— Modeling Assumptions

Customer Class

Lots

ERU’s

Residential

172

174

Hangar Area

6

1

Marina

N/A

10

Total

178

185

* Assumes 3 ERU’s for Taggares Lot 1

The model was run for PHD (peak hourly demand) calculated at each node. MDD of 600 gpcd was used to calculate the PHD. In some cases the nodes shows negative demand. This is necessary to balance the accumulated demands at an upstream node so the flow at the node accurately represents the PHD flow there. Note that a fire flow scenario was not run since fire flow is not required for this system. The results of the simulation are attached. It is clear that the distribution system will have some reserve capacity at buildout. The maximum number of ERU’s that could be served by the system has not been determined but it is certain that it exceeds 200 ERU’s.

JPH
January 2, 2004

Attachments:
BIMC Water System - ADD and MDD Determination
ERU - System Capacity Analysis Review
Storage Capacity Analysis
WaterCAD Hydraulic Modeling Software Output

Draft Report - Page 6