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Scaling the Washington Creek Restoration Project 

to the Chalk Point Oil Spill Diamondback Terrapin Injury 

By Heath Byrd, Eric English, Norman Meade and Ted Tomasi1

March 12, 2002 

The injury to Diamondback Terrapins resulting from the Chalk Point oil spill was presented in 

Estimate of Total Injury to Diamondback Terrapins from the Chalk Point Oil Spill (Byrd et. al., 

2002). The loss was estimated to be 5,244.6 discounted terrapin-years. 

This document presents a description of the methodology and results of an analysis of the 

proposed Washington Creek Restoration Project (WCRP), with reference to scaling 

compensatory restoration for the estimated terrapin injury. The WCRP and the project site are 

summarized, followed by a description of the scaling calculations. The project, as proposed, is 

more than sufficient to compensate for the terrapin injury resulting from the Chalk Point oil spill. 

Project / Site Description

The proposed WCRP will convert 5.6 acres of agricultural land at the northeast corner of the 

mouth of Washington Creek to functioning intertidal marsh. The site will be excavated to an 

appropriate elevation and planted with marsh vegetation. Two breakwaters will be installed to 

protect the site from wave energy. The sand that is excavated from the site will be used to 

augment a nearby eroding beach, which will improve the quality of diamondback terrapin nesting 

habitat on the beach. 

Diamondback Terrapins in the Patuxent River nest, primarily, on narrow (< 10 m wide) sandy 

beaches and upland areas with sparse vegetation some 0 .5 to 1.0 m above the mean high-tide 

waterline (Roosenburg, 1994). The beach at the northeast corner of Washington Creek is quite 

narrow because of erosion. As the beach has eroded over time, the bank has been undercut by 

wave action, leaving an obstruction between the beach and some portions of the upland area on 

top of the bank. This obstruction and the dense vegetation in the upland impede terrapins from 

utilizing the upland area for nesting. Overall, the beach is relatively poor nesting habitat. 

Moreover, there is a possibility that the beach is completely inundated at high tide, in which case 

the beach itself is not suitable terrapin nesting habitat. 

Sand to be excavated as part of the WCRP will be deposited on this beach, resulting in a wider 

beach with a gentle slope from the beach to the area of dense vegetation. Sparse vegetation will 

be planted on the slope to mitigate its erosion. The two breakwaters will be constructed to help 

reduce future erosion of the beach. The restoration project will increase the suitability of terrapin 

nesting habitat available on this portion of the Patuxent River. 

Discounted Terrapin-Years for the WCRP

The credit that will be produced by the restoration project was estimated by calculating the 

discounted terrapin-years the project area is expected to produce in its post-implementation 

condition versus its baseline condition. The estimation proceeded in four steps. First, appropriate 

terrapin nesting densities were estimated for the beach in its baseline and post-implementation 

conditions. Second, the number of hatchlings produced by the project was estimated over the 

 

1 Byrd and Tomasi: ENTRIX Inc.; English and Meade: NOAA 
 

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anticipated duration of the project. Third, the number of discounted terrapin-years per hatchling 

was applied to each hatchling produced, yielding the credit generated each year. Finally, these 

credits were discounted to the time of the injury, and summed to find the total discounted 

terrapin-years credit for the project. 

Nesting Densities 

Roosenburg (1994) reported that the range of nesting densities in the Patuxent River were 240 

nests/ha to 1,125 nests/ha. As discussed above, the current condition of the beach render it to be 

poor quality nesting habitat. Therefore, 240 nests/ha is an appropriate baseline nest density for 

the beach. 

Upon completion, the WCRP is expected to provide high quality terrapin nesting habitat. 

However, it is conservatively assumed that it will provide “average” quality nesting habitat when 

it reaches full project maturity (after five years). Therefore, the midpoint of the nest density 

range, 682.5 nests/ha, was used for the beach in its post-implementation, mature condition. Thus, 

the maximum nesting credit provided by the restoration project in any given year is estimated to 

be 682.5 – 240 = 442.5 nests/ha. 

Hatchlings Produced over the Project Lifespan 

The expected number of hatchlings produced in each year of the project lifespan was estimated 

by multiplying the hatchlings credited during a year of full project maturity ("full services") by 

the percent of full services the project is expected to provide in that year. For example, if 1,000 

hatchlings were produced in a year of full services, then 500 hatchlings would be produced in a 

year of 50% services. The number of hatchlings expected at full services was calculated based on 

parameter values found in Byrd, et al. (2002), which are summarized in Table 1:

Table 1: Diamondback Terrapin Parameters 

Number of Eggs per Nest 13 

Egg to Hatchling Survivorship 20% 

Discounted Terrapin Years per Hatchling 2.095 

 

The expected number of eggs produced in a year (at full services) is the nests/ha (442.5) 

multiplied by the number of eggs per nest (13), which equals 5,752.5. The expected number of 

hatchlings produced in a year is the number of eggs (5,752.5) multiplied by the probability that an 

egg will hatch (0.20), which equals 1,150.5. The expected total number of terrapin-years 

produced is the number of hatchlings (1,150.5) multiplied by the discounted terrapin years per 

hatchling (2.095), which equals 2,410.3. 

The service levels provided each year were estimated based on the following assumptions. The 

lifetime of the project is expected to be 25 years. It will be completed by the end of 2002, but 

credit will not begin to accrue until 2003. The service level will increase 20% per year over five 

years, reaching 100% services (full project maturity) at the end of 2007. Full services will then 

be provided for 20 years, through the year 2027. For purposes of these calculations, it is assumed 

there are no services provided after year 2027. 
 

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Credit Produced Each Year 

Table 2 summarizes the estimation of discounted terrapin years produced on each hectare of the 

shoreline improved by the WCRP. The average service level provided each year of the project is 

contained in column (3). The hatchlings/ha per year in column (4) are the multiplication of the 

services in column (3) by the expected number of hatchlings at the full service level, 1,150.5. 

Column (5), the discounted terrapin years provided each year, is column (4) multiplied by 2.095, 

the discounted terrapin-years per hatchling from the injury assessment. Column (6) contains the 

discount factor, used to express the value of the discounted terrapin years provided each year in 

terms of 2000, the year of the injury. The standard discount rate for performing Natural Resource 

Damage Assessments, 3.0%, was used for these calculations. Column (7) contains the terrapin 

years each year discounted to 2000, and is the multiplication of columns (5) and (6). The total 

credit that is provided by one hectare of the restoration project is the sum of column (7), which 

equals 34,233.4 discounted terrapin-years. 

Table 2: Discounted Terrapin-Years Produced per Hectare 

(1) (2) (3) (4) (5) (6) (7) 

Year Service Level 

(End of Year) 

Service Level 

(Average for Year)

Hatchlings 

per ha per 

Year 

DTYs per 

Year 

Discount 

Factor 

(end 2000) 

Discounted 

Terrapin 

Years per 

ha 

2002 0% 0% 

2003 20% 10% 115.05 241.010 0.915 220.558 

2004 40% 30% 345.15 723.031 0.888 642.403 

2005 60% 50% 575.25 1205.051 0.863 1039.488 

2006 80% 70% 805.35 1687.071 0.837 1412.896 

2007 100% 90% 1035.45 2169.092 0.813 1763.670 

2008 100% 100% 1150.5 2410.102 0.789 1902.557 

2009 100% 100% 1150.5 2410.102 0.766 1847.143 

2010 100% 100% 1150.5 2410.102 0.744 1793.342 

2011 100% 100% 1150.5 2410.102 0.722 1741.109 

2012 100% 100% 1150.5 2410.102 0.701 1690.397 

2013 100% 100% 1150.5 2410.102 0.681 1641.162 

2014 100% 100% 1150.5 2410.102 0.661 1593.361 

2015 100% 100% 1150.5 2410.102 0.642 1546.953 

2016 100% 100% 1150.5 2410.102 0.623 1501.896 

2017 100% 100% 1150.5 2410.102 0.605 1458.151 

2018 100% 100% 1150.5 2410.102 0.587 1415.681 

2019 100% 100% 1150.5 2410.102 0.570 1374.448 

2020 100% 100% 1150.5 2410.102 0.554 1334.415 

2021 100% 100% 1150.5 2410.102 0.538 1295.549 

2022 100% 100% 1150.5 2410.102 0.522 1257.814 

2023 100% 100% 1150.5 2410.102 0.507 1221.179 

2024 100% 100% 1150.5 2410.102 0.492 1185.611 

2025 100% 100% 1150.5 2410.102 0.478 1151.078 

2026 100% 100% 1150.5 2410.102 0.464 1117.552 

2027 100% 100% 1150.5 2410.102 0.450 1085.002 

sum 34233.415 
 

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Area of Beach Required to Fully Compensate for Terrapin Injury

The terrapin injury was estimated to be 5,244.6 discounted terrapin years. It is estimated that the 

WCRP will produce 34,233.4 discounted terrapin years/hectare. Therefore, the area of terrapin 

nesting habitat needed to fully compensate for the terrapin injury is 5,244.6 / 34,233.4 = 0.1532 

ha, or 1532 m2. Assuming the nesting area will be 10 meters wide, 153.2 meters along the 

shoreline would be necessary for compensation. The actual length of shoreline that will be 

improved is approximately 600 meters (the exact length of the beach is dependent on the 

configuration of the breakwaters, which has not been finalized to date). Thus, the WCRP, under 

conservative assumptions, more than compensates for the diamondback terrapin injury.

References Cited

Byrd, et al. 2002. Estimate of Total Injury to Diamondback Terrapins from the Chalk Point Oil 

Spill. 

Roosenburg, W.M. 1994. “Nesting Habitat Requirements of the Diamondback Terrapin: A 

Geographic Comparison”. Wetland Journal 6(2).