ENVE 300 Intro to Env Engr Fall 07 Homework #1
Due 10/9/07
A critical skill in protecting the environment in connection with an infrastructure project or restoring
a habitat (e.g. bringing urban streams back to aquatic productivity) is focusing discussion
from general
environmental principles and issues (whales, rainforests) to those relevant to the specific site in question.
We not only need data inventory, but to understand and model reactions and processes. The most basic
analysis is mass balance (Ex 3-1). There are three basic reactor models in common use: batch (Fig 3-2),
complete mix (Fig 3-3, Ex 3-6) and plug-flow (Fig 3-1, 3-4, Ex 3-7). Look through Ch 3 to get the drift.
The prime indicators of aquatic water quality are dissolved oxygen (D.O.) and nutrients (C, N & P). The
oxygen required for complete biodegradation is the sum of the theoretical carbonaceous and nitrogenous
biochemical oxygen demands, TBOD, Ex 8-1 and NBOD Eq 8.12, respectively. But, not all organic
material is biodegradable. The lumped indicator of practical impact from an untreated discharge is the
BOD from a closed batch test (p 272-281), from which we extract a 5 day (or longer) demand (BOD
ult
<
THBOD) and a rate constant (Example 8-2). Plug flow is moving a diluted batch through flowing water
(Fig 8.9).The oxygen-sag analysis (Fig 8-7) accounts for re-aeration to determine if the discharge can
have significant impact (Fig 8-8). The simplest model is complete-mix, for water bodies with high
hydraulic detention time (Eq 3-27,
θ
A 20’ high dam impounds a 20 acre reservoir at the outlet of Wendy Run watershed shown on H.O. #1,
page 1-19, Fig #24. The tributary area is 4.0 mi
2
(640 acres/ mi
2
). The contours are in meters above a zero
datum on the stream bottom at the outlet of the watershed, i.e, the base of the dam. Water leaves the pond
over the dam crest (weir). From here on, the reservoir will be referred to as the pond.
1. The watershed receives an average of 45” of precipitation yearly. 25” evaporates or is transpired by
vegetation or critters, 15” is runoff, and 5” is infiltration that recharges groundwater (See Fig 40, p 1-25).
No confined aquifers recharge within the watershed and discharge (Figs 44 & 46) out-of-basin, so all
runoff and groundwater recharge (20” total) eventually passes through the lake and over the dam.
a. Estimate average dry-weather base flow (Fig 39, p 1-25), Q
base
, a variation of Q = Vol/time = v x A,
but v= recharge rate (5”/yr), A= surface area. Express in ft
3
/sec (cfs). Also, estimate the combined
average annual stream flow from both runoff and groundwater (Q
in
= Q
base
+ Q
runoff
) in cfs. Over a year, the
pond stays full, so
Σ
Q
in
–
Σ
Q
out
=
∆
storage = 0. See Example 3-2.
b. The 20 acre area pond is 5X longer than its width. For average flow (Q