Introduction to Lake Surveys Field Techniques Unit 3 Module 8 Part A Lake Morphometry

Introduction to Lake SurveysField Techniques

Unit 3 Module 8 Part A Lake Morphometry

Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s2

Objectives

Students will be able to: determine common morphometric characteristics of

lakes identify characteristics of a bathymetric map. describe methods used to create bathymetric maps. use bathymetric maps to determine areal

characteristics for lakes. determine the importance of lake volume and mean

depth in lakes. calculate lake volume and mean depth. interpret hypsographic and volumetric curves of lakes. explain the hydraulic residence time of a lake. calculate the hydraulic residence time of a lake.


Basic water quality assessment

These slides focus on learning basic field techniques used by limnologists:

• Morphometry - estimating critical lake basin measurements

• Field Profiles - physical and chemical parameters measured from top to bottom of the water column

• Sampling – collecting water, sediments, and aquatic organisms


Part A Lake morphometry


Part A Lake morphometry

Goal

This module will help you:– Learn how to determine common morphometric

characteristics of lakes


Lake morphometry

Morphometry defines a lake’s physical dimensionsand involves the quantification and measurement of lake basin shape.

These dimensions influence the lake’s water quality and productivity levels.


Determining lake morphometry

• First Step is to obtain or develop a bathymetric map-a bathymetric map is essentially a topographic map of the lake bottom that shows depth contours within the lake basin

-used to estimate morphometric and many hydrological parameters

-the reliability of your morphometric data will depend on the accuracy of the bathymetric map


Working with a bathymetric map

Many lakes have already been mapped and a good map will contain the following: name, county, and geographic location of the

water body an outline of the shoreline drawn to a known

scale depth contours drawn to a known interval geographic orientation (which way is north?) date of map and data collectors



Creating a bathymetric map

Lake mapping prior to 1950-MN DNR photo

• Making your own map requires measuring depths at precise locations:

• Sounding weight (through the ice works well)

• A secchi disk will work if weighted

• Fish finder, echo sounders (= sonar)



Bathymetric maps can be made by:

-Drawing a general outline of the lake or finding an aerial photo or map

-Measuring and recording water depths at a number of locations

-Then connecting the depth “dots” to develop simple contour lines



The most commonly used method today involves using precise echo sounders, on board computers and GPS systems.

There are several components to lake bathymetric mapping;

-the GPS equipment which will work in tandem with -the depth sounding equipment, and -the data collection process



Soundings are taken during as the boat follows transects across the lake surface

Location of transects and frequency of sounding measurements depends on: Map scale Basin complexity


Drawing the map

Transect location and direction is recorded on a hardshell which is a drawing of the lake outline and surrounding features.

Hardshells are drawn from orthorectified aerial photos or USGS quadrangle maps

MN DNR photo


Areal characteristics

characteristic units

elevation metersfeet

surface area(Ao)

hectares (ha)acres (ac)

maximum depth (z max)

metersfeet

Shoreline length(L)

metersfeet

Shoreline development(DL)

metersfeet

FetchMax widthMax length

metersfeet

Littoral Area %

A bathymetric map allows determination of these areal characteristics



Example:Morphometric (and watershed) characteristics for Ice Lake



Maximum length (fetch)

Maximum width

Z max

•Several areal characteristics and measurements can be taken directly off the

bathymetric map


Areal characteristics: surface area

• Other measurements, such as lake surface area, require more work

• There are several methods for determining lake surface area:

• Cut and weigh method

• Planimetry

• Grid method

• Digitized map


Surface area: cut and weigh method

You’ll need:

A photocopy of bathymetric map (as large as possible and be sure to include map scale)

An analytical balance


Surface area: planimetry method

You’ll need:

-a lake map

-polar compensating planimeter ($200-$600)

Image from : http://lakewatch.ifas.ufl.edu


Surface area: grid method

You’ll need:

• Bathymetric map

• Grid paper

Method:

• Count up the number of squares that fall within the shoreline of the lake

• Use the map scale to determine area represented by each square

scale

Area = # squares counted X area of one square


Surface area: digitized lake maps

You’ll need:

• Bathymetric map

• Digitizing software (e.g., ArcPad)

• Digitizing pad

Method:

• Software dependent

www.remetrix.com


Areal characteristics: shoreline length

Shoreline length (L) = circumference or perimeter of lake

• The linear measurement of the lake’s entire perimeter at a given water level

• Provides a measurement of the amount of interface between the lake and surrounding land


Areal characteristics: shoreline development

Shoreline development (SLD) = a measure of how much the lake’s surface shape deviates from being a perfect circle.

• Important is assessing the potential habitat available• For a lake that is a perfect circle the SLD = 1• A reservoir that impounds water in valleys may have an SLD > 4.

Round Lake LakeAmoeba

Calculating SLD


Areal characteristics: % littoral area

The littoral (shallow near shore) zone is the portion of a lake where sufficient light can penetrate to the lake bottom.

It is also sometimes defined as that portion of the lake that is less than 15 feet in depth.

The littoral zone is where the majority of the aquatic plants are found and is a primary area used by young fish.


Volumetric characteristics

Bathymetry also allows determination of several volumetric characteristics:

characteristic units

Volume (V) Cubic metersacre feet

Stratum volume Cubic metersacre feet

Mean depth(z mean or z)

metersfeet

Hypsographic curve Graph (depth vs area)

Volumetric curve Graph (depth vs volume)

Hydraulic retention time (HRT)

years


Volumetric characteristics: volume

Importance

1) Total lake volume can influence a lake’s dilution capacity.

2) Allows the determination of mixed layer (epilimnion) volume.

3) Or hypolimnion; e.g. determining available trout habitat with temperatures from 4 to 25 oC and DO > 5 mg/L.


Volumetric characteristics: volume

A top

A bottomepilimnion

hypolimnion


A top

A bottomzzz

Calculating Lake Volume

Atop = the area at the top of the layer

Abottom = the area at the bottom of the layer

z = the distance between contour lines

V = the volume of one layer

3

bottomtop AAAAzV

bottomtop


Mean depth (z)

Mean depth (z) = volume surface area

Mean depth is important for the following reasons: Shallow lakes are generally more productive than deep

lakes and mean depth is a quick way of assessing overall depth

Also indicates the potential for waves and mixing events to disrupt bottom sediments

If volume is not available you could collect numerous lake depth measurements and average them. Of course this is not as accurate and only practical for small lakes.


Hypsographic curves

Hypsographic curve = Area as a function of depth To estimate the amount of potential bottom spawning

habitat for brook trout or bass (perhaps defined by a range of temperature and dissolved oxygen)

To estimate the littoral zone area potentially available for macrophyte growth (limnologically defined as the depth to 1% of surface light). Often the epilimnion volume is used as an approximation. Often related to secchi depth by fisheries folks.

To estimate the area of sediments exposed to low oxygen. This allows you to predict internal phosphorus release (Nurenberg 1985).


Ice Lake Hypsographic Curve

0

2

4

6

8

10

12

14

16

18

0 2 4 6 8 10 12 14 16 18

Area (hectares)d

ep

th (

m)

Maximum depth

Lake bottom


Volumetric Curve

Volumetric Curve = volume as a function of depth When used in conjunction with temperature and DO

profiles, this curve can be used to estimate fisheries habitat.


Ice Lake Volumetric Curve

0

2

4

6

8

10

12

14

16

0 0.5 1 1.5 2 2.5 3 3.5

volume (x10^5 m3)

de

pth

(m

)


Hydraulic residence time (HRT)

HRT is the time required to refill an empty lake with its natural inflow.

• A large deep lake with a moderate inflow will have a longer HRT than a small, shallow lake with the same inflow.


HRT - importance

• HRT is needed to determine annual lake budgets for water, nutrients, heat, oxygen contaminants, and herbicides.

• It also provides an estimate of the turnover time for water in a lake, or “flushing time”


Calculating HRT

• A lake’s residence time is calculated by dividing the lake’s volume by its average annual water outflow.

• Lake managers calculate outflow on an annual basis so that seasonal variation doesn’t unduly influence results.

• Volume (V) is usually expressed in acre-feet, and mean outflow is expressed as acre-feet/year.


Calculating HRT cont.

So the formula looks like this:HRT (years) = lake volume (acre-ft) / mean outflow (acre-ft/yr)


rLo 2If A = lake area, then a circle with area “A” has a perimeter:

(1)2rA

A

r

AoL

2

(2)

(3)

(4)

AoL

2

A r

AoL 2

oLLSLD

A

LSLD

2

(5)

(6)

(7)

AoL

2

(8)

(Formula for area based on radius)

Substituting eq.(3) into eq. (1)

(By definition)

Substituting eq(6) into eq. (7)

(collect terms)

Calculating SLD BACK

Documents

Introduction to Lake Surveys Field Techniques Unit 3 Module 8 Part A Lake Morphometry