Watershed Corridors

[Bayou]

I'm working on a subwatershed masterplan.
The new lingo is "waterway corridors".
Basically these are wide "geomorphed" constructed
waterways with multiple shelves to include parks,
paths, as well as the 100-year floodplain.
The idea is to have the stream meander with a base
flow within this corridor with higher manning benchs
above it. From several books and other sources the
standard for a "base flow" is about a 2-year flow.
So initially I was trying to come up with a section
that held the 2-year in the stream, then first bench
would hold the 10-year, then the top of the channel
would contain the 100-year. My problem arose when
I realize that the 2-year flow (700 cfs) required
a base channel that was too big (7-8 ft deep).

I was wondering if anyone else has done these sorts
of waterway designs and how they approached it.

 

[dylanc]

This may be too late for your information but will hopefully be of use to somebody. The following information comes from the Brisbane (Australia) City Council - Natural Channel Design Guidelines.

When determining a bankfull flow rate they recommend (a) Matching the flow rate to existing upsteam and downstream to provide continuity. (b) a calculated flow rate from other streams in the area or (c) if no other information is available use Q1.5.

Further, their discussion as to the use of A Q2 event for sizing a channel is based on allowable velocities during early revegetation. By sizing the channel for the Q2 event it is less likely that the maximum flows/velocities will occur within the first 1-2 years while plants are establishing.

If you did choose say the Q2 event velocities could be controlled by increasing the channel length, and therefore reducing slope, with more meanders.

I understand that this may not help to reduce your channel size but that is the fun part trying to balance the hydraulics and ecology. A site visit to study other creeks in the area may give an indication as to what the finished product should look like.

 

[sta7240]

You may want to check your hydrology. The peak discharge may be too large. It sounds like you are dealing with a large watershed requiring a computer model such as HEC-HMS or TR20. If so, this model should be calibrated and verified to be sure you are getting the correct results. If you haven't done this, you may find the model is producing unusally high peaks requiring calibration.

If your hydrology is ok, I think dylanc had a good point regarding the upstream and downstream conditions. If a Q2 is out of bank upstream and downstream then you will be hard pressed to design for in-bank conditions. If upstream land use has become urbanized, it may be normal for Q2 to be out of bank.