swales in the city...

Id love to get some peoples ideas on this project I want to do.

I live on a 1/5 acre city lot in central Texas.  We get a lot of heavy but short downpours.  Not a lot of room for a traditional swale, but water does run off my property.  Id rather it stayed here instead of going into the street.  Ive been watching how the water flows off the front yard.  It sort of pools up in the center of the yard before it overflows onto the front walk and flows out into the street.  Where it pools up is also along a line that would be a great path to the driveway. 

My idea was to dig a 2' deep trench,  2' wide and it would be 25' feet long.  100 cubic feet or 748 gallons... if i made it 3' deep I it would be over 1100 gallons, thats a lot of water to me.  Fill the entire trench with wood mulch and make it a path from the front walk to the front drive.  I can get the mulch for free from the city and as it decays keep adding more mulch.  The mulch would reduce the amount of water the swale could hold but the wood would also absorb water and release it slowly.  After some time the swale could be dug out and probably get some pretty decent dirt.  Then just refill it.

I have a few concerns.  Do ya think dirt from the edges of the trench would slowly erode into the trench in between the large mulch pieces and cause a depression in the center of the yard.  I suppose I could alleviate this by having the trench walls sloped like a traditional swale instead of 2' wide down to 2'.  That cuts its water holding ability in half though.

Is this amount of wood mulch (not like logs in hugelbeds) in the ground going to suck nitrogen out of the surrounding ground?  From how far away? 

Is the mulch going to float out when the trench fills!  That could be that awful but funny sort of thing.

I can't answer your exact questions, but if you have not yet been to Midwestpermaculture.com, you can find some info where swales and small ponds were placed into an urban front yard to collect over 1200 gallons of water from their roof.

Here's a direct link: http://www.midwestpermaculture.com/permaculture-125crescent-stelle.php

They do talk about putting leaves in the small ponds but it looks as though nothing else were put in the swales.   

I can tell you this,  I have about 4 inches of mulch in an area about 5 feet wide directly off of my patio.  When it rains heavily and the mulch is dry, a small stream gets created and all that mulch goes with the flow of water.   When the mulch is already wet, it doesn't move with the flow of water as much.  But, it does float most of the time.  What I'm trying to get at, is if you have a sudden rush of water thru your swale, alot of your mulch will flow to the end of your swale.  Not really that big of a deal, as you can rake it back as I do by my patio or if it overflows some of your mulch will end up in the grass.

I also have some beds where I have edged deeply with a shovel, kinda like a very wide tilted V.  I don't usually put mulch into the void, but it ends up there over time.  These edges need to be redug every other year to keep them sharp, and I would imagine you will also need to do that with your swale.


I hope this helps some.

Brad Lancaster has quite a bit of information about rainwater harvesting in the city.

http://www.harvestingrainwater.com/

Personally I wouldn't put sharp edges on a swale - I would have only gradual slopes.  You don't want to have to be redigging it all the time, that's just unnecessary labor.